Patent Publication Number: US-8978372-B2

Title: V-type engine, outboard motor, and vessle

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a V-type engine, an outboard motor powered by the V-type engine, and a vessel propelled by the outboard motor. 
     2. Description of the Related Art 
     A vessel described in each of Japanese Unexamined Patent Publication No. 2008-31868, Japanese Unexamined Patent Publication No. 2008-31897, and Japanese Unexamined Patent Publication No. 2008-31898 includes an outboard motor powered by a V-type eight-cylinder engine. Each engine is equipped with an in-bank exhaust system that discharges exhaust gas to an inner side of two cylinder banks. 
     The exhaust device of Japanese Unexamined Patent Publication No. 2008-31868 includes eight upstream exhaust pipes connected to the two cylinder banks, four midstream exhaust pipes, by which the eight upstream exhaust pipes are merged into four pipes, and a single downstream exhaust pipe, by which the four midstream exhaust pipes are merged into a single pipe. 
     The exhaust device of Japanese Unexamined Patent Publication No. 2008-31897 includes eight upstream exhaust pipes connected to the two cylinder banks, four midstream exhaust pipes, by which the eight upstream exhaust pipes are merged into four pipes, and two downstream exhaust pipes, by which the four midstream exhaust pipes are merged into two pipes. 
     As with the device of Japanese Unexamined Patent Publication No. 2008-31897, the exhaust device of Japanese Unexamined Patent Publication No. 2008-31898 includes eight upstream exhaust pipes connected to the two cylinder banks, four midstream exhaust pipes, by which the eight upstream exhaust pipes are merged into four pipes, and two downstream exhaust pipes, by which the four midstream exhaust pipes are merged into two pipes. 
     With these conventional vessels, each midstream exhaust pipe of the exhaust device is connected to an upstream exhaust pipe connected to one of the cylinder banks and connected to an upstream exhaust pipe connected to the other cylinder bank. The two upstream exhaust pipes branching from the midstream exhaust pipe in common are connected to two cylinders that differ in ignition timing. It is described that exhaust interference, which lowers the engine output, is thus prevented. 
     With an outboard motor, the restrictions of the space in which the engine is disposed are more severe than those of an automobile and it is thus preferable for the engine to be compact. However, with the V-type engine equipped with the in-bank exhaust system, the plurality of exhaust pipes meander in the width direction of the engine and the width of the exhaust pipes as a whole is wide. The engine is thus large in the width direction. 
     SUMMARY OF THE INVENTION 
     In order to overcome the previously unrecognized and unsolved challenges described above, a preferred embodiment of the present invention provides a V-type engine including two cylinder banks that include N (where N is an integer not less than 2) first cylinders, aligned in a direction parallel or substantially parallel to a crank axis direction, and N second cylinders, aligned in the direction parallel or substantially parallel to the crank axis direction, and are disposed along V-shaped lines defined by a first plane passing through center lines of the N first cylinders and a second plane passing through center lines of the N second cylinders, N first exhaust ports disposed at an inner side of the V-shaped lines when viewed from the crank axis direction and respectively connected to the N first cylinders, N second exhaust ports disposed at the inner side of the V-shaped lines when viewed from the crank axis direction and respectively connected to the N second cylinders, a first exhaust manifold that includes N first branch pipes, respectively connected to N exhaust ports that include at least one of the first exhaust ports and at least one of the second exhaust ports, and a first collecting pipe, disposed adjacent to the N first cylinders and extending from one end to the other end of the N first cylinders aligned in the direction parallel or substantially parallel to the crank axis, and a second exhaust manifold that includes N second branch pipes, respectively connected to N exhaust ports that include at least one of the first exhaust ports and at least one of the second exhaust ports, and a second collecting pipe disposed adjacent to the N second cylinders and extending from one end to the other end of the N second cylinders aligned in the direction parallel or substantially parallel to the crank axis. 
     With this arrangement of the present preferred embodiment of the present invention, the N first cylinders that are aligned in the direction parallel or substantially parallel to the crank axis direction are provided in one of the cylinder banks and the N second cylinders that are aligned in the direction parallel or substantially parallel to the crank axis direction are provided in the other cylinder bank. The N first exhaust ports are respectively connected to the N first cylinders and the N second exhaust ports are respectively connected to the N second cylinders. The first exhaust ports and the second exhaust ports are disposed at the inner side of the V-shaped lines. The exhaust generated in combustion chambers are thus collected to the inner sides of the two cylinder banks disposed in a V-shape. 
     The N first branch pipes of the first exhaust manifold are connected to the two cylinder banks via the first exhaust ports and the second exhaust ports. Similarly, the N second branch pipes of the second exhaust manifold are connected to the two cylinder banks via the first exhaust ports and the second exhaust ports. The N first branch pipes are thus connected to N cylinders that differ in ignition timing and the N second branch pipes are connected to N cylinders that differ in ignition timing. Occurrence of exhaust interference is thus prevented and the engine has an increased output. 
     Further, the first collecting pipe of the first exhaust manifold extends from one end to the other end of the N first cylinders that are aligned in the direction parallel or substantially parallel to the crank axis direction. Similarly, the second collecting pipe of the second exhaust manifold extends from one end to the other end of the N second cylinders that are aligned in the direction parallel or substantially parallel to the crank axis direction. The first collecting pipe and the second collecting pipe are thus long in the crank axis direction. The first exhaust manifold and the second exhaust manifold are thus decreased in width while securing the length (passage length) of the exhaust passage. The engine is thus compact in the width direction. 
     Further, the first collecting pipe of the first exhaust manifold is disposed adjacent to the N first cylinders and the second collecting pipe of the second exhaust manifold is disposed adjacent to the N second cylinders. Therefore, in comparison to a case where the first exhaust manifold and the second exhaust manifold are disposed adjacent to a common cylinder, the N first branch pipes and the N second branch pipes are arranged efficiently. Therefore, not only are the shapes of the first exhaust manifold and the second exhaust manifold prevented from becoming complicated but each individual exhaust manifold is compact to enable further reduction of the widths of the first exhaust manifold and the second exhaust manifold. The engine is thus compact in the width direction. 
     In the present preferred embodiment, at least one of the N second branch pipes may intersect at least one of the N first branch pipes when viewed in the crank axis direction. 
     With this arrangement of the present preferred embodiment of the present invention, the second branch pipe intersects the first branch pipe when viewed in the crank axis direction and, therefore, the entirety of the two exhaust manifolds (the first exhaust manifold and the second exhaust manifold) is compact. The engine thus is even more compact. 
     In the present preferred embodiment, the first collecting pipe may be integral and unitary with the N first branch pipes and the second collecting pipe may be integral and unitary with the N second branch pipes. 
     With this arrangement of the present preferred embodiment of the present invention, each of the first branch pipes extends from the first collecting pipe to a cylinder bank because the first collecting pipe is integral and unitary with the N first branch pipes. The first exhaust manifold is thus more compact than in a case where another exhaust pipe is interposed between the first branch pipes and the first collecting pipe. Similarly, the second collecting pipe is integral and unitary with the N second branch pipes and thus the second exhaust manifold is more compact than in a case where another exhaust pipe is interposed between the second branch pipes and the second collecting pipe. The engine is thus even more compact. 
     The present preferred embodiment may further include an exhaust pipe that is integral and unitary with the first exhaust manifold and the second exhaust manifold. 
     With this arrangement of the present preferred embodiment of the present invention, the first exhaust manifold and the second exhaust manifold are provided in the exhaust pipe and the number of parts of the engine is thus reduced. 
     In the present preferred embodiment, the V-type engine may further include a catalytic unit, at least a portion of which is disposed at the same position as the exhaust pipe in regard to the crank axis direction and which purifies the exhaust discharged from the first exhaust manifold and the second exhaust manifold. 
     With this arrangement of the present preferred embodiment of the present invention, the exhaust discharged from the first exhaust manifold and the second exhaust manifold is purified by the catalytic unit. At least a portion of the catalytic unit is disposed at the same position as the exhaust pipe in regard to the crank axis direction. The length of the engine in the crank axis direction is thus reduced more in comparison to a case of adopting a configuration where the entire catalytic unit does not overlap with the exhaust pipe when viewed in a direction orthogonal or substantially orthogonal to the crank axis direction. The engine is thus compact. 
     In the present preferred embodiment, the catalytic unit may include a catalyst case, into which the exhaust discharged from the first exhaust manifold and the second exhaust manifold flows, and a catalyst housed in the catalyst case. The catalyst case may extend from one end to the other end of the N first cylinders aligned in the direction parallel or substantially parallel to the crank axis direction. 
     With this arrangement of the present preferred embodiment of the present invention, the exhaust discharged from the first exhaust manifold and the second exhaust manifold flows into the catalyst case of the catalytic unit. The catalyst is disposed inside the catalyst case. The exhaust that is discharged into the catalyst case from the first exhaust manifold and the second exhaust manifold is thus purified. Further, the catalyst case extends from one end to the other end of the N first cylinders aligned in the direction parallel or substantially parallel to the crank axis direction. The catalyst case is thus long in the crank axis direction. The catalyst case defines a portion of the exhaust passage. The catalyst case are thus reduced in width while securing the length of the exhaust passage. The engine is thus compact in the width direction. 
     In the present preferred embodiment, the exhaust pipe may be provided with an exhaust relay passage that is independent of the first exhaust manifold and the second exhaust manifold and guides the exhaust, purified by the catalytic unit, from the catalytic unit to the two cylinder banks. 
     With this arrangement of the present preferred embodiment of the present invention, the exhaust purified by the catalytic unit is discharged from the catalytic unit into the exhaust relay passage and thereafter discharged from the exhaust relay passage to the two cylinder banks. The exhaust relay passage is independent of the first exhaust manifold and the second exhaust manifold. That is, the internal space of the exhaust relay passage is separated from the internal spaces of the first exhaust manifold and the second exhaust manifold and do not intersect with the internal spaces of the first exhaust manifold and the second exhaust manifold. The pre-purification exhaust in the first exhaust manifold and the second exhaust manifold are thus prevented from flowing into the exhaust relay passage. Further, as with the first exhaust manifold and the second exhaust manifold, the exhaust relay passage is provided in the exhaust pipe and the number of parts of the engine are thus reduced. 
     In the present preferred embodiment, the exhaust pipe may include a fixed portion fixed to one of the two cylinder banks and a floating portion movably connected to the other of the two cylinder banks. 
     With this arrangement of the present preferred embodiment of the present invention, the fixed portion provided in the exhaust pipe is fixed to one of the two cylinder banks and the floating portion provided in the exhaust pipe is movably connected to the other of the two cylinder banks. The respective parts of the engine have dimensional tolerances and, therefore, if the exhaust pipe is fixed to the two cylinder banks at all locations, gaps due to dimensional variations may occur between the exhaust pipe and the cylinder banks. Therefore, by connecting a portion (the floating portion) of the exhaust pipe to the other cylinder bank in a manner enabling movement, the dimensional variations are absorbed. The sealing property between the exhaust pipe and the cylinder banks is thus improved and leakage of exhaust is prevented. 
     In the present preferred embodiment, N may be 4 and the two cylinder banks may include four of the first cylinders and four of the second cylinders. The first exhaust manifold may be connected to four cylinders, including two of the first cylinders and two of the second cylinders, via four exhaust ports including two of the first exhaust ports and two of the second exhaust ports. The second exhaust manifold may be connected to four cylinders, including two of the first cylinders and two of the second cylinders, via four exhaust ports including two of the first exhaust ports and two of the second exhaust ports. 
     In this case, the four first cylinders may be allocated to NO. 1, NO. 3, NO. 5, and NO. 7, respectively, and the four second cylinders may be allocated to NO. 2, NO. 4, NO. 6, and NO. 8, respectively. The V-type engine may further include eight spark plugs respectively corresponding to the eight cylinders including the four first cylinders and the four second cylinders and a controller igniting the eight spark plugs in the order of NO. 1, NO. 8, NO. 4, NO. 3, NO. 6, NO. 5, NO. 7, and NO. 2. 
     With this arrangement of the present preferred embodiment of the present invention, the first exhaust manifold may be connected via the four exhaust ports to the four cylinders to which NO. 1, NO. 5, NO. 6, and NO. 8 are respectively allocated. The second exhaust manifold may be connected via the four exhaust ports to the four cylinders to which NO. 2, NO. 3, NO. 4, and NO. 7 are respectively allocated. 
     Or the first exhaust manifold may be connected via the four exhaust ports to the four cylinders to which NO. 1, NO. 4, NO. 6, and NO. 7 are respectively allocated. The second exhaust manifold may be connected via the four exhaust ports to the four cylinders to which NO. 2, NO. 3, NO. 5, and NO. 8 are respectively allocated. 
     Another preferred embodiment of the present invention provides an outboard motor including the V-type engine, an engine supporting member disposed below the engine and supporting the engine in an attitude such that the rotational axis of the engine is vertical or substantially vertical, and a power transmission device transmitting a power of the V-type engine to a propeller. 
     Yet another preferred embodiment of the present invention provides a vessel including the outboard motor and a hull propelled by the outboard motor. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of a vessel according to a first preferred embodiment of the present invention. 
         FIG. 2  is a partial sectional view of a portion of an engine as viewed from above. 
         FIG. 3  is a side view of a rear portion of the engine. 
         FIG. 4  is an exploded perspective view of an exhaust pipe and a catalytic unit. 
         FIG. 5  is a front view of the exhaust pipe. 
         FIG. 6  is a rear view of the exhaust pipe. 
         FIG. 7A  is a plan view of an internal structure of the exhaust pipe. 
         FIG. 7B  is a side view of the internal structure of the exhaust pipe. 
         FIG. 7C  is a rear view of the internal structure of the exhaust pipe. 
         FIG. 8  is a rear view of a main body of the engine. 
         FIG. 9  is a perspective view of the front of the exhaust pipe as viewed from obliquely upward to the left. 
         FIG. 10  is a partial sectional view of the integration of the exhaust pipe and the engine main body. 
         FIG. 11  is a longitudinal sectional view of the exhaust pipe and the catalytic unit as viewed in the direction of arrows XI shown in  FIG. 10 . 
         FIG. 12A  is a plan view of an internal structure of the catalytic unit. 
         FIG. 12B  is a side view of the internal structure of the catalytic unit. 
         FIG. 12C  is a rear view of the internal structure of the catalytic unit. 
         FIG. 13  is a rear view of a lower portion of the exhaust pipe. 
         FIG. 14  is a rear view of a gasket disposed between the lower portion of the exhaust pipe and a lower portion of the catalytic unit. 
         FIG. 15A  is a plan view of an engine exhaust passage. 
         FIG. 15B  is a side view of the engine exhaust passage. 
         FIG. 15C  is a rear view of the engine exhaust passage. 
         FIG. 16  is a rear view of the engine exhaust passage from which a catalyst housing passage is omitted. 
         FIG. 17  is a schematic view of a connection of eight cylinders and two exhaust manifolds. 
         FIG. 18  is a graph of ignition timings, exhaust periods, and intake periods of the respective cylinders. 
         FIG. 19  is a schematic side view of an outline of a cooling device of a vessel propulsion apparatus. 
         FIG. 20  is a schematic view of a cooling water passage provided in the engine. 
         FIG. 21  is a perspective view of upper portions of the exhaust pipe and the catalytic unit. 
         FIG. 22  is a sectional view of an internal structure of a restriction valve. 
         FIG. 23  is a rear view of an engine exhaust passage according to a second preferred embodiment of the present invention from which a catalyst housing passage is omitted. 
         FIG. 24  is a schematic view of a connection, according to the second preferred embodiment of the present invention, of eight cylinders and two exhaust manifolds. 
         FIG. 25  is a partial sectional view of a connection, according to a third preferred embodiment of the present invention, of an exhaust pipe and an engine main body. 
         FIG. 26  is a sectional view of a portion of an engine according to a fourth preferred embodiment of the present invention as viewed from above. 
         FIG. 27  is a schematic view of a cooling water passage provided in an engine according to a fifth preferred embodiment of the present invention. 
         FIG. 28  is a perspective view of a rear of an exhaust pipe according to a sixth preferred embodiment of the present invention as viewed from obliquely rearward to the left. 
         FIG. 29  is a rear view of an internal structure of an exhaust pipe according to the sixth preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, examples in which a crank axis direction (direction in which a rotational axis Ac of a crankshaft  25  extends) is the vertical direction and a rotational axis Ac of an engine  9  (rotational axis Ac of the crankshaft  25 ) extends in the vertical direction shall be described. However, the rotational axis Ac of the engine  9  may extend in the horizontal direction or in a direction inclined with respect to the horizontal direction or the vertical direction. The crank axis direction D1 may thus be the horizontal direction or a direction inclined with respect to the horizontal direction or the vertical direction. 
       FIG. 1  is a schematic side view of a vessel according to a first preferred embodiment of the present invention.  FIG. 2  is a partial sectional view of a portion of the engine as viewed from above.  FIG. 3  is a side view of a rear portion of the engine.  FIG. 4  is an exploded perspective view of an exhaust pipe and a catalytic unit. In  FIG. 2 , the hatching that indicates a cross-section is omitted. The cross-sections of two cylinder banks  22  shown in  FIG. 2  preferably differ in height at the right side and the left side of a center C1 (a vertical plane passing through the crank axis Ac and orthogonal or substantially orthogonal to the right/left direction) of the outboard motor  4 . 
     As shown in  FIG. 1 , the vessel  1  includes a hull H1 that floats on a water surface and a vessel propulsion apparatus  2  that propels the hull H1. The vessel propulsion apparatus  2  includes a suspension device  3 , mountable to a rear portion (stern) of the hull H1, and an outboard motor  4  coupled to the suspension device  3 . 
     As shown in  FIG. 1 , the suspension device  3  includes a pair of right and left clamp brackets  5  to be mounted on the hull H1, a tilting shaft  6  supported by the pair of clamp brackets  5  extending in the right/left direction, and a swivel bracket  7  mounted on the tilting shaft  6 . The suspension device  3  further includes a steering shaft  8  supported by the swivel bracket  7  extending in the up/down direction. 
     As shown in  FIG. 1 , the outboard motor  4  is mounted on the steering shaft  8 . The steering shaft  8  is supported by the swivel bracket  7  in a manner enabling rotation around a steering axis (center line of the steering shaft  8 ) extending in the up/down direction. The swivel bracket  7  is supported by the clamp brackets  5  via the tilting shaft  6 . The swivel bracket  7  is rotatable around a tilt axis (center line of the tilting shaft  6 ) extending in the right/left direction with respect to the clamp brackets  5 . The outboard motor  4  is rotatable to the right and left with respect to the suspension device  3  and is rotatable up and down with respect to the suspension device  3 . The outboard motor  4  is thus rotatable to the right and left with respect to the hull H1 and is rotatable up and down with respect to the hull H1. 
     As shown in  FIG. 1 , the outboard motor  4  includes an engine  9  that generates power that rotates a propeller  13  and a power transmission device that transmits the power of the engine  9  to the propeller  13 . The power transmission device includes a driveshaft  10  coupled to the engine  9 , a forward/reverse switching mechanism  11  coupled to the driveshaft  10 , and a propeller shaft  12  coupled to the forward/reverse switching mechanism  11 . The outboard motor  4  further includes an engine cover  14  covering the engine  9  and a casing  17  housing the power transmission device. 
     As shown in  FIG. 1 , the engine cover  14  houses the engine  9 . The engine cover  14  includes a cup-shaped bottom cover  15  that is upwardly open and a cup-shaped top cover  16  that is downwardly open. The top cover  16  is detachably mounted on the bottom cover  15 . The opening portion of the top cover  16  is vertically overlapped with the opening portion of the bottom cover  15  via a seal (not shown). The bottom cover  15  is mounted on the casing  17  (specifically, an exhaust guide  18  to be described below). As shown in  FIG. 3 , a bottom portion of the bottom cover  15  is provided with an opening that penetrates through the bottom portion and a portion (cylinder bodies  27  to be described below) of the engine  9  is disposed in the opening at the bottom portion. 
     As shown in  FIG. 1 , the casing  17  includes an exhaust guide  18  disposed below the engine  9 , an upper case  19  disposed below the exhaust guide  18 , and a lower case  20  disposed below the upper case  19 . The engine  9  is mounted on the exhaust guide  18 . The engine  9  is disposed higher than the steering shaft  8 . The exhaust guide  18  that serves as an engine supporting member supports the engine  9  with the rotational axis (crank axis Ac) of the engine  9  having a vertical attitude. 
     As shown in  FIG. 1 , the engine  9  is disposed above the driveshaft  10 . The driveshaft  10  extends in the up/down direction inside the casing  17 . A center line of the driveshaft  10  may be disposed on the rotational axis of the engine  9  or may be shifted with respect to the rotational axis of the engine  9 . An upper end portion of the driveshaft  10  is coupled to the engine  9  and a lower end portion of the driveshaft  10  is coupled to a front end portion of the propeller shaft  12  via the forward/reverse switching mechanism  11 . The propeller shaft  12  extends in the front/rear direction inside the casing  17 . A rear end portion of the propeller shaft  12  projects to the rear from the casing  17 . The propeller  13  is detachably mounted on the rear end portion of the propeller shaft  12 . The propeller  13  includes an outer cylinder  13   a  surrounding the propeller shaft  12  around a center line of the propeller shaft  12  and a plurality of blades  13   b  extending outward from the outer cylinder  13   a . The outer cylinder  13   a  and the blades  13   b  rotate together with the propeller shaft  12  around a propeller axis (center line of the propeller shaft  12 ). 
     The engine  9  is preferably an internal combustion engine. The engine  9  rotates in a fixed rotation direction. The rotation of the engine  9  is transmitted to the propeller  13  by the power transmission device (the driveshaft  10 , the forward/reverse switching mechanism  11 , and the propeller shaft  12 ). The propeller  13  is thus caused to rotate together with the propeller shaft  12  and a thrust that propels the vessel  1  forward or in reverse is generated. Also, the direction of the rotation transmitted from the driveshaft  10  to the propeller shaft  12  is switched by the forward/reverse switching mechanism  11 . The rotation direction of the propeller  13  and the propeller shaft  12  is thus switched between a forward rotation direction (clockwise direction when the propeller  13  is viewed from the rear) and a reverse rotation direction (direction of rotation opposite to the forward rotation direction). The direction of thrust is thus switched. 
     As shown in  FIG. 2 , the engine  9  is, for example, a V-type eight-cylinder four-cycle engine. The engine  9  includes two cylinder banks  22  provided with a plurality of cylinders  21  and a crankcase  23  mounted on the respective cylinder banks  22 . The engine  9  further includes a plurality of pistons  24  respectively disposed inside the plurality of cylinders  21 , a crankshaft  25  rotatable around the crank axis Ac extending in the up/down direction, and a plurality of connecting rods  26  coupling the plurality of pistons  24  respectively to the crankshaft  25 . 
     As shown in  FIG. 2 , the two cylinder banks  22  are disposed along V-shaped lines V1 that are open rearward in a plan view. The two cylinder banks  22  are disposed at the right and left sides of the center C1 of the outboard motor  4 . Center lines of the four cylinders  21  provided in the cylinder bank  22  at the left side are disposed in a first plane PL that intersects the crank axis Ac. Center lines of the four cylinders  21  provided at the cylinder bank  22  at the right side are disposed in a second plane PR that intersects the crank axis Ac. The first plane PL and the second plane PR are symmetrical with respect to the center C1 of the outboard motor  4  and are disposed in a V-like shape in a plan view. The V-shaped lines V1 are defined by the first plane PL and the second plane PR. The V-shaped lines V1 extend rearward from the crank axis Ac. 
     As shown in  FIG. 2 , the two cylinder banks  22  include cylinder bodies  27  of a rearwardly opened V-shape in a plan view, two cylinder heads  28  respectively mounted on the two rear end portions of the cylinder bodies  27 , and two head covers  29  respectively mounted on the two cylinder heads  28 . 
     As shown in  FIG. 2 , the cylinder bodies  27  extend along the V-shaped lines V1 in a plan view. Together with the two cylinder heads  28 , the cylinder bodies  27  define the plurality of cylinders  21 . The two cylinder heads  28  are disposed behind the cylinder bodies  27  and the crankcase  23  is disposed in front of the cylinder bodies  27 . The crankcase  23  is mounted on a front end portion of the cylinder bodies  27 . The crankshaft  25  is housed in the interiors of the crankcase  23  and the cylinder bodies  27 . 
     As shown in  FIG. 2 , the two cylinder heads  28  include a plurality of combustion chambers  30  respectively corresponding to the plurality of cylinders  21 , a plurality of intake ports  31  supplying air into the plurality of combustion chambers  30 , and a plurality of exhaust ports  32  discharging exhaust generated in the plurality of combustion chambers  30 . The engine  9  includes a plurality of spark plugs  33  causing combustion of a mixed gas of air and fuel inside the plurality of combustion chambers  30 , a plurality of intake valves opening and closing the plurality of intake ports  31 , a plurality of exhaust valves  34  opening and closing the plurality of exhaust ports  32 , and a valve mechanism that moves the plurality of intake valves and the plurality of exhaust valves  34 . 
     As shown in  FIG. 2 , a region between the V-shaped lines V1 in the right/left direction is the inner side of the V-shaped lines V1 and a region at the right and left of the V-shaped lines V1 is the outer side of the V-shaped lines V1. The intake ports  31  are disposed at the outer side of the V-shaped lines V1 and the exhaust ports  32  are disposed at the inner side of the V-shaped lines V1. The plurality of intake ports  31  are respectively connected to the plurality of combustion chambers  30 , and the plurality of exhaust ports  32  are respectively connected to the plurality of combustion chambers  30 . Two exhaust ports  32  are preferably provided for each cylinder  21  (see  FIG. 15B ). The number of exhaust ports  32  corresponding to the same cylinder  21  is not restricted to two and may be one, for example. 
     As shown in  FIG. 2 , the engine  9  includes an intake device  35  supplying air to the plurality of combustion chambers  30 , a fuel supplying device  36  supplying fuel to the plurality of combustion chambers  30 , and an exhaust device  37  discharging the exhaust generated in the plurality of combustion chambers  30 . The intake device  35 , the fuel supplying device  36 , and the exhaust device  37  are mounted on an engine main body that includes the cylinder banks  22  and the crankcase  23 . 
     As shown in  FIG. 2 , the intake device  35  includes an intake manifold  38  supplying air to the plurality of combustion chambers  30  via the plurality of intake ports  31  and throttle valves adjusting the flow rates of air supplied from the intake manifold  38  to the plurality of combustion chambers  30 . The intake manifold  38  is mounted on the cylinder heads  28  and the interior of the intake manifold  38  is connected to the respective intake ports  31 . The throttle valves are mounted on the intake manifold  38 . The throttle valves correspond to the respective combustion chambers  30 . The intake manifold  38  and the throttle valves are disposed at the outer side of the V-shaped lines V1. 
     As shown in  FIG. 2 , the fuel supplying device  36  includes a plurality of fuel injectors  40  supplying fuel to the plurality of combustion chambers  30 . The fuel injectors  40  are mounted respectively according to the combustion chambers  30 . Each fuel injector  40  is mounted on a cylinder head  28 . A fuel outlet of the fuel injector  40  that injects fuel is disposed inside an intake port  31 . The fuel outlet of the fuel injector  40  is not restricted to being disposed inside the intake port  31  and may be disposed inside a combustion chamber  30  instead. That is, the engine  9  is not restricted to being a port-injection engine and may instead be a direct-injection engine. 
     As shown in  FIG. 2 , the exhaust device  37  includes an exhaust pipe  41  guiding the exhaust discharged from the plurality of combustion chambers  30  via the plurality of exhaust ports  32  and a catalytic unit  42  that purifies the exhaust discharged from the exhaust pipe  41 . As shown in  FIG. 4 , the exhaust device  37  further includes an upper spacer  43  and a lower spacer  44  interposed between the exhaust pipe  41  and the catalytic unit  42 . The catalytic unit  42  is disposed behind the exhaust pipe  41  and is mounted on the exhaust pipe  41  via the upper spacer  43  and the lower spacer  44 . As shown in  FIG. 2 , the exhaust pipe  41  is disposed behind the two cylinder banks  22  and is mounted on the two cylinder heads  28 . The exhaust pipe  41  and the catalytic unit  42  are disposed at the inner side of the V-shaped lines V1. The exhaust pipe  41  and the catalytic unit  42  overlap with the center C1 of the outboard motor  4  that bisects the V-shaped lines V1 in a plan view. 
     As shown in  FIG. 3 , the exhaust pipe  41  and the catalytic unit  42  are disposed higher than the bottom cover  15 . The exhaust pipe  41  and the catalytic unit  42  are disposed further to the front than to a rear end of the bottom cover  15 . As shown in  FIG. 2 , the exhaust pipe  41  is shorter in the front/rear direction than the catalytic unit  42 . The catalytic unit  42  is shorter in the front/rear direction than the two cylinder banks  22  (see the “front/rear direction length L1 of the cylinder banks  22 ” in  FIG. 2 ). The exhaust pipe  41  is thus shorter in the front/rear direction than the two cylinder banks  22 . Also, the width (length in the right/left direction) of the catalytic unit  42  is shorter than the width of the exhaust pipe  41 . The width of the exhaust pipe  41  is shorter than the width W1 of the two cylinder banks  22 . The width of the catalytic unit  42  is thus shorter than the width of the two cylinder banks  22 . The front/rear direction length L1 of the cylinder banks  22  is the front/rear direction length from the front end (foremost portion) of the cylinder banks  22  to the rear end (rearmost portion) of the cylinder banks  22 . Also, the width W1 of the two cylinder banks  22  is the right/left direction length from the right end (rightmost portion) of the two cylinder banks  22  to the left end (leftmost portion) of the two cylinder banks  22 . 
     Each combustion chamber  30  is connected to an internal space of the exhaust pipe  41  via the corresponding exhaust port  32 . As shall be described below, the exhaust pipe  41  includes an internal passage guiding the exhaust discharged from the combustion chambers  30  to the catalytic unit  42  and an internal passage guiding the exhaust discharged from the catalytic unit  42  to the two cylinder banks  22 . The exhaust generated in each combustion chamber  30  is thus discharged into the interior of the exhaust pipe  41  via the corresponding exhaust port  32  and is discharged from the interior of the exhaust pipe  41  into the interior of the catalytic unit  42 . The exhaust discharged into the interior of the catalytic unit  42  is purified by the catalytic unit  42 . The purified exhaust is discharged from the interior of the catalytic unit  42  to the interior of the exhaust pipe  41  and discharged from the interior of the exhaust pipe  41  to the interiors of the two cylinder banks  22 . 
     In the following description, the “cylinder bank  22  at the left side with respect to the center C1 of the outboard motor  4 ” may be referred to as the “first cylinder bank  22 L” and the “cylinder bank  22  at the right side with respect to the center C1 of the outboard motor  4 ” may be referred to as the “second cylinder bank  22 R.” Also, the “cylinders  21  corresponding to the first cylinder bank  22 L” and the “exhaust ports  32  corresponding to the first cylinder bank  22 L” may be referred to respectively as the “first cylinders  21 L” and the “first exhaust ports  32 L,” and the “cylinders  21  corresponding to the second cylinder bank  22 R” and the “exhaust ports  32  corresponding to the second cylinder bank  22 R” may be referred to respectively as the “second cylinders  21 R” and the “second exhaust ports  32 R.” The first cylinder bank  22 L thus includes four first cylinders  21 L and four pairs of first exhaust ports  32 L (eight first exhaust ports  32 L) and the second cylinder bank  22 R includes four second cylinders  21 R and four pairs of second exhaust ports  32 R (eight second exhaust ports  32 R). 
       FIG. 5  is a front view of the exhaust pipe.  FIG. 6  is a rear view of the exhaust pipe.  FIG. 7A ,  FIG. 7B , and  FIG. 7C  are, respectively, a plan view, a side view, and a rear view of an internal structure of the exhaust pipe. In  FIG. 7A  to  FIG. 7C , a first exhaust manifold  53  provided in the exhaust pipe  41  is indicated in gray. 
     As shown in  FIG. 5 , the exhaust pipe  41  includes eight front exhaust inlets  45  opening at the outer surface of the exhaust pipe  41  and two front exhaust outlets  46  opening at the outer surface of the exhaust pipe  41 . The exhaust pipe  41  further includes two front cooling water inlets  47  opening at the outer surface of the exhaust pipe  41  and two front cooling water outlets  48  opening at the outer surface of the exhaust pipe  41 . 
     As shown in  FIG. 5 , the front exhaust inlets  45 , the front exhaust outlets  46 , the front cooling water inlets  47 , and the front cooling water outlets  48  define two columns extending in the up/down direction. Each column preferably includes four front exhaust inlets  45 , one front exhaust outlet  46 , one front cooling water inlet  47 , and one front cooling water outlet  48 . The front exhaust outlet  46  is disposed below the front exhaust inlets  45  of the same column, and the front cooling water inlet  47  is disposed below the front exhaust outlet  46  of the same column. The front cooling water outlet  48  is disposed above the front exhaust inlets  45  of the same column. The two columns are mutually parallel or substantially parallel and are spaced apart by an interval in the right/left direction. The front exhaust inlets  45 , the front exhaust outlet  46 , the front cooling water inlet  47 , and the front cooling water outlet  48  of the left column in  FIG. 5  open at the same plane. Also, the front exhaust inlets  45  and the front exhaust outlet  46  of the right column in  FIG. 5  open at the same plane. 
     As shown in  FIG. 6 , the exhaust pipe  41  includes two rear exhaust inlets  49  opening at the outer surface of the exhaust pipe  41  and two rear exhaust outlets  50  opening at the outer surface of the exhaust pipe  41 . The exhaust pipe  41  further includes rear cooling water inlets  51  opening at the outer surface of the exhaust pipe  41  and rear cooling water outlets  52  opening at the outer surface of the exhaust pipe  41 . 
     As shown in  FIG. 6 , the rear exhaust inlets  49  are disposed lower than the rear exhaust outlets  50 . The two rear exhaust inlets  49  are aligned in the right/left direction, and the two rear exhaust outlets  50  are aligned in the right/left direction at a height higher than the rear exhaust inlets  49 . The two rear exhaust inlets  49  are respectively disposed below the two rear exhaust outlets  50 . The rear cooling water inlets  51  are disposed at a periphery of the two rear exhaust outlets  50 , and the rear cooling water outlets  52  are disposed at a periphery of the two rear exhaust inlets  49 . The rear cooling water inlets  51  and the rear cooling water outlets  52  respectively include a plurality of openings. The rear cooling water inlets  51  and the rear exhaust outlets  50  open at the same plane, and the rear cooling water outlets  52  and the rear exhaust inlets  49  open at the same plane. 
     As shown in  FIG. 7A  to  FIG. 7C , the exhaust pipe  41  includes the first exhaust manifold  53  extending from four of the front exhaust inlets  45  to one of the rear exhaust outlets  50  and a second exhaust manifold  54  extending from the other four front exhaust inlets  45  to the other rear exhaust outlet  50 . The exhaust pipe  41  further includes a first relay pipe  59  extending from one of the front exhaust outlets  46  to one of the rear exhaust inlets  49  and a second relay pipe  60  extending from the other front exhaust outlet  46  to the other rear exhaust inlet  49 . 
     As shown in  FIG. 7A  to  FIG. 7C , the first exhaust manifold  53  includes four first branch pipes  55  and one first collecting pipe  56 . Similarly, the second exhaust manifold  54  includes four second branch pipes  57  and one second collecting pipe  58 . The first branch pipes  55 , the first collecting pipe  56 , the second branch pipes  57 , the second collecting pipe  58 , the first relay pipe  59 , and the second relay pipe  60  are provided in the exhaust pipe  41 . The pipes defining the exhaust pipe  41  are preferably integral and unitary. The first branch pipes  55 , the first collecting pipe  56 , the second branch pipes  57 , the second collecting pipe  58 , the first relay pipe  59 , and the second relay pipe  60  are thus preferably integral and unitary. 
     As shown in  FIG. 7A  to  FIG. 7C , the four first branch pipes  55  are respectively connected to four of the front exhaust inlets  45 . The first branch pipes  55  extend from the first collecting pipe  56  to the front exhaust inlets  45 . The first collecting pipe  56  connects each of the four first branch pipes  55  to a rear exhaust outlet  50 . The first collecting pipe  56  is disposed behind the four first cylinders  21 L. The first collecting pipe  56  extends in the up/down direction. The first collecting pipe  56  overlaps with the four first cylinders  21 L in a rear view. The four first branch pipes  55  are connected to the first collecting pipe  56  at respectively different heights. The first relay pipe  59  and the second relay pipe  60  are disposed lower than the first branch pipes  55 . 
     As with the first exhaust manifold  53 , the four second branch pipes  57  of the second exhaust manifold are respectively connected to the other four front exhaust inlets  45 . The second branch pipes  57  extend from the second collecting pipe  58  to the front exhaust inlets  45 . The second collecting pipe  58  connects each of the four second branch pipes  57  to a rear exhaust outlet  50 . The second collecting pipe  58  is disposed behind the four second cylinders  21 R. The second collecting pipe  58  extends in the up/down direction. The four second branch pipes  57  are connected to the second collecting pipe  58  at respectively different heights. The first relay pipe  59  and the second relay pipe  60  are disposed lower than the second branch pipes  57 . 
     As shown in  FIG. 7A  to  FIG. 7C , the first collecting pipe  56  is a first rectilinear pipe that extends rectilinearly in the direction of alignment of the four first cylinders  21 L. The first collecting pipe  56  extends from the height of the first cylinder  21 L that is disposed uppermost among the four first cylinders  21 L to the height of the first cylinder  21 L that is disposed lowermost among the four first cylinders  21 L. The first collecting pipe  56  overlaps, in a rear view, with the first cylinder  21 L that is disposed uppermost among the four first cylinders  21 L and overlaps, in a rear view, with the first cylinder  21 L that is disposed lowermost among the four first cylinders  21 L. 
     As shown in  FIG. 7A  to  FIG. 7C , the second collecting pipe  58  is a second rectilinear pipe that extends rectilinearly in the direction of alignment of the four second cylinders  21 R. The second collecting pipe  58  extends from the height of the second cylinder  21 R that is disposed uppermost among the four second cylinders  21 R to the height of the second cylinder  21 R that is disposed lowermost among the four second cylinders  21 R. The second collecting pipe  58  overlaps, in a rear view, with the second cylinder  21 R that is disposed uppermost among the four second cylinders  21 R and overlaps, in a rear view, with the second cylinder  21 R that is disposed lowermost among the four second cylinders  21 R. 
       FIG. 8  is a rear view of the engine main body.  FIG. 9  is a perspective view of the front of the exhaust pipe as viewed from obliquely upward to the left.  FIG. 10  is a partial sectional view of the integration of the exhaust pipe and the engine main body.  FIG. 11  is a longitudinal sectional view of the exhaust pipe and the catalytic unit as viewed in the direction of arrows XI shown in  FIG. 10 . The cross-sections of the exhaust pipe  41  shown in  FIG. 10  differ in height at the right side and the left side of the center C1 of the outboard motor  4 . 
     As shown in  FIG. 8 , the cylinder heads  28  include two exhaust inlets  61   b  that open at the outer surfaces of the cylinder heads  28  and eight exhaust outlets  62   b  that open at the outer surfaces of the cylinder heads  28 . The cylinder heads  28  further include two cooling water inlets  63   b  that open at the outer surfaces of the cylinder heads  28  and two cooling water outlets  64   b  that open at the outer surfaces of the cylinder heads  28 . 
     As shown in  FIG. 8 , the exhaust inlets  61   b , the exhaust outlets  62   b , the cooling water inlets  63   b , and the cooling water outlets  64   b  define two columns extending in the up/down direction. Each column preferably includes one exhaust inlet  61   b , four exhaust outlets  62   b , one cooling water inlet  63   b , and one cooling water outlet  64   b . The exhaust outlets  62   b  are disposed above the exhaust inlet  61   b  of the same column, and the cooling water inlet  63   b  is disposed above the exhaust outlets  62   b  of the same column. The cooling water outlet  64   b  is disposed below the exhaust inlet  61   b  of the same column. The two columns are mutually parallel or substantially parallel and are disposed spaced apart by an interval in the right/left direction. The exhaust inlet  61   b , the exhaust outlets  62   b , the cooling water inlet  63   b , and the cooling water outlet  64   b  of the right column in  FIG. 8  open at the same plane. Also, the exhaust inlet  61   b  and the exhaust outlets  62   b  of the left column in  FIG. 8  open at the same plane. 
     As shown in  FIG. 9 , the exhaust pipe  41  includes a fixed portion  65   p  including a plurality of openings and five cylindrical insertion portions  66  including five openings provided with five cylindrical insertion portions  66  respectively. The fixed portion  65   p  includes a flat mounting surface  67   p  extending in the up/down direction. The front exhaust inlets  45 , the front exhaust outlet  46 , the front cooling water inlet  47 , and the front cooling water outlet  48 , included in one of the columns, open at the mounting surface  67   p . The front exhaust inlets  45  and the front exhaust outlet  46 , included in the other column, open at end surfaces of the five insertion portions  66 . The five insertion portions  66  are aligned at intervals in the up/down direction. 
     As shown in  FIG. 8 , the two cylinder banks  22  include a fixed portion  68   b  including a plurality of openings and five supporting recesses  69  including five openings provided with five supporting recesses  69 , respectively. The fixed portion  68   b  is provided at the second cylinder bank  22 R and the supporting recesses  69  are provided at the first cylinder bank  22 L. The fixed portion  68   b  includes a flat mounting surface  70   b  extending in the up/down direction. The exhaust inlet  61   b , the exhaust outlets  62   b , the cooling water inlet  63   b , and the cooling water outlet  64   b  of one of the columns open at the mounting surface  70   b . The exhaust inlet  61   b  and the exhaust outlets  62   b  of the other column open at bottom surfaces of the five supporting recesses  69 . The five supporting recesses  69  are aligned at intervals in the up/down direction. 
     As shown in  FIG. 10 , the mounting surface  67   p  of the exhaust pipe  41  is disposed parallel or substantially parallel to the mounting surface  70   b  of the cylinder banks  22 . The mounting surface  67   p  of the exhaust pipe  41  is in contact with the mounting surface  70   b  of the cylinder banks  22  via a gasket (not shown). The seven openings (the front exhaust inlets  45 , the front exhaust outlet  46 , the front cooling water inlet  47 , and the front cooling water outlet  48 ) provided at the mounting surface  67   p  respectively face the seven openings (the exhaust inlet  61   b , the exhaust outlets  62   b , the cooling water inlet  63   b , and the cooling water outlet  64   b ) provided at the mounting surface  70   b . In this state, the fixed portion  65   p  is fixed to the fixed portion  68   b  preferably by a plurality of bolts, for example. The front exhaust inlets  45  and the exhaust outlets  62   b  are thus connected and the exhaust inlet  61   b  and the front exhaust outlet  46  are connected. Similarly, the front cooling water inlet  47  and the cooling water outlet  64   b  are connected and the cooling water inlet  63   b  and the front cooling water outlet  48  are connected. 
     Also as shown in  FIG. 10 , the five insertion portions  66  of the exhaust pipe  41  are respectively inserted in the five supporting recesses  69  of the cylinder head  28 . The five openings (the front exhaust inlets  45  and the front exhaust outlet  46 ) provided in the five insertion portions  66  respectively face the five openings (the exhaust inlet  61   b  and the exhaust outlets  62   b ) provided in the five supporting recesses  69 . The front exhaust inlets  45  and the exhaust outlets  62   b  are thus connected and the exhaust inlet  61   b  and the front exhaust outlet  46  are connected. As shown in  FIG. 11 , the front cooling water inlet  47  and the cooling water outlet  64   b  are connected via a cooling water pipe  71  inserted in the cylinder head  28  and the exhaust pipe  41 , and the cooling water inlet  63   b  and the front cooling water outlet  48  are connected via a cooling water pipe  71  inserted in the cylinder head  28  and the exhaust pipe  41 . 
     As shown in  FIG. 10 , the engine  9  includes a plurality of O-rings  72 , each disposed between an outer peripheral surface of an insertion portion  66  and an inner peripheral surface of a supporting recess  69 . A gap between the outer peripheral surface of the insertion portion  66  and the inner peripheral surface of the supporting recess  69  is sealed by the O-ring  72 . The insertion portion  66  that is a floating portion is movable in an axial direction of the insertion portion  66  with respect to the supporting recess  69  in the state in which the gap between the insertion portion  66  and the supporting recess  69  is sealed. The relative positions of the insertion portion  66  and the supporting recess  69  change due to assembly errors of the engine  9  and thermal expansion of the engine  9 . The insertion portion  66  and the supporting recess  69  are included in a floating mechanism that absorbs the assembly errors of the engine  9  and the thermal expansion of the engine  9 . 
       FIG. 12A ,  FIG. 12B , and  FIG. 12C  are, respectively, a plan view, a side view, and a rear view of an internal structure of the catalytic unit.  FIG. 13  is a rear view of a lower portion of the exhaust pipe.  FIG. 14  is a rear view of a gasket disposed between the lower portion of the exhaust pipe and a lower portion of the catalytic unit. 
     As shown in  FIG. 11 , the catalytic unit  42  includes a hollow catalyst case  73  connected to the exhaust pipe  41 , a catalyst  74  housed in the catalyst case  73 , an upstream sensor  75  measuring a concentration of the exhaust at an upstream side relative to the catalyst  74  in the direction of flow of the exhaust, and a downstream sensor  76  measuring the concentration of the exhaust at a downstream side relative to the catalyst  74 . The catalyst  74  is, for example, a three-way catalyst. The catalyst  74  includes a honeycomb-shaped carrier, through the interior of which the exhaust passes, and a catalytic substance held on the surface of the carrier. Also each of the upstream sensor  75  and the downstream sensor  76  is, for example, an oxygen concentration sensor. The air-fuel ratio of the mixed gas supplied to each combustion chamber  30  is adjusted based on detection values of the upstream sensor  75  and the downstream sensor  76 . 
     As shown in  FIG. 11 , the catalyst case  73  includes two exhaust inlets  77   c  opening at the outer surface of the catalyst case  73  and two exhaust outlets  78   c  opening at the outer surface of the catalyst case  73 . The catalyst case  73  further includes cooling water inlets  79   c  opening at the outer surface of the catalyst case  73  and cooling water outlets  80   c  opening at the outer surface of the catalyst case  73 . As shown in  FIG. 12A  to  FIG. 12C , the catalyst case  73  includes two upstream branch pipes  81  including the two exhaust inlets  77   c , two downstream branch pipes  83  including the two exhaust outlets  78   c , and a catalyst housing pipe  82  extending from the two upstream branch pipes  81  to the two downstream branch pipes  83 . 
     As shown in  FIG. 12A  to  FIG. 12C , the exhaust inlets  77   c  are disposed higher than the exhaust outlets  78   c . The two exhaust inlets  77   c  are aligned in the right/left direction, and the two exhaust outlets  78   c  are aligned in the right/left direction at a height lower than the exhaust inlets  77   c . The two exhaust inlets  77   c  are respectively disposed above the two exhaust outlets  78   c . As shown in  FIG. 11 , the cooling water inlets  79   c  are disposed at a periphery of the exhaust outlets  78   c  and the cooling water outlets  80   c  are disposed at a periphery of the exhaust inlets  77   c . The cooling water inlets  79   c  and the exhaust outlets  78   c  open at the same plane and the cooling water outlets  80   c  and the exhaust inlets  77   c  open at the same plane. 
     As shown in  FIG. 11 , the exhaust pipe  41  includes a flat upper mounting surface  84   p  including the rear cooling water inlets  51  and the rear exhaust outlets  50 , and a flat lower mounting surface  85   p  including the rear cooling water outlets  52  and the rear exhaust inlets  49 . The catalyst case  73  includes a flat upper mounting surface  86   c  including the cooling water outlets  80   c  and the exhaust inlets  77   c , and a flat lower mounting surface  87   c  including the cooling water inlets  79   c  and the exhaust outlets  78   c . The upper mounting surface  86   c  is disposed behind the upper mounting surface  84   p , and the lower mounting surface  87   c  is disposed behind the lower mounting surface  85   p . The upper mounting surface  84   p  is mounted on the upper mounting surface  86   c  via the upper spacer  43  and the lower mounting surface  85   p  is mounted on the lower mounting surface  87   c  via the lower spacer  44 . 
     As shown in  FIG. 11 , the upper spacer  43  includes exhaust holes  88   s  through which the exhaust passes and cooling water holes  89   s  through which the cooling water passes. Similarly, the lower spacer  44  includes exhaust holes  88   s  through which the exhaust passes and cooling water holes  89   s  through which the cooling water passes. The exhaust holes  88   s  and the cooling water holes  89   s  penetrate through the upper spacer  43  and the lower spacer  44  in the thickness direction. The rear exhaust inlets  49  and the exhaust outlets  78   c  are connected via the exhaust holes  88   s  of the lower spacer  44 , and the rear exhaust outlets  50  and the exhaust inlets  77   c  are connected via the exhaust holes  88   s  of the upper spacer  43 . Similarly, the rear cooling water inlets  51  and the cooling water outlets  80   c  are connected via the cooling water holes  89   s  of the upper spacer  43 , and the rear cooling water outlets  52  and the cooling water inlets  79   c  are connected via the cooling water holes  89   s  of the lower spacer  44 . 
     As shown in  FIG. 14 , the exhaust device  37  includes a gasket  90  disposed between the exhaust pipe  41  and the lower spacer  44  (see also  FIG. 4 ). The gasket  90  is sandwiched by the exhaust pipe  41  and the lower spacer  44  and seals a gap between the exhaust pipe  41  and the lower spacer  44 . The gasket  90  includes exhaust holes  91   g , through which the exhaust passes, and cooling water holes  92   g , through which the cooling water passes. The exhaust holes  91   g  and the cooling water holes  92   g  penetrate through the gasket  90  in the thickness direction. The cooling water holes  92   g  include a plurality of holes. The rear exhaust outlets  49  of the exhaust pipe  41  are connected to the exhaust holes  88   s  of the lower spacer  44  via the exhaust holes  91   g  of the gasket  90 . The rear cooling water outlets  52  of the exhaust pipe  41  are connected to the cooling water holes  89   s  of the lower spacer  44  via the cooling water holes  92   g  of the gasket  90 . 
     As shown in  FIG. 13 , the rear cooling water outlets  52  (gray portions), provided at the lower portion of the exhaust pipe  41 , include a plurality of holes disposed in a periphery of the two rear exhaust inlets  49 . The cooling water that flows inside the outer wall of the exhaust pipe  41  flows out from the rear cooling water outlets  52 . The cooling water discharged from the rear cooling water outlets  52  flows into the cooling water inlets  79   c  of the catalyst case  73  via the cooling water holes  92   g  of the gasket  90  and the cooling water holes  92   g  of the lower spacer  44 . 
     In  FIG. 14 , the outline of the rear cooling water outlets  52  of the exhaust pipe  41  is indicated by alternate long and two short dashed lines. As shown in  FIG. 14 , a portion of the outline of the rear cooling water outlets  52  is disposed outside the outline of the cooling water holes  92   g  (gray portion) provided in the gasket  90 . The flow passage area of the cooling water holes  92   g  of the gasket  90  is thus smaller than the flow passage area of the rear cooling water outlets  52  of the exhaust pipe  41 . Therefore, when the cooling water passes through the gasket  90 , pressure loss of the cooling water occurs and the flow rate of the cooling water supplied from the exhaust pipe  41  into the catalyst case  73  decreases. 
     The flow passage area of the gasket  90  is less than the flow passage area of the exhaust pipe  41  and, therefore, the supply flow rate of the cooling water supplied from the exhaust pipe  41  into the catalyst case  73  decreases and the supply flow rate of the cooling water is adjusted by the gasket  90 . The gasket  90  is one gasket selected from a plurality of gaskets  90  that respectively differ in the flow passage area of the cooling water holes  92   g . The supply flow rate of the cooling water supplied from the exhaust pipe  41  into the catalyst case  73  is thus adjusted by selection of the gasket  90 . 
       FIG. 15A ,  FIG. 15B , and  FIG. 15C  are, respectively, a plan view, a side view, and a rear view of an engine exhaust passage.  FIG. 16  is a rear view of the engine exhaust passage from which a catalyst housing passage is omitted. 
     As shown in  FIG. 1 , the outboard motor  4  includes an exhaust passage  93  by which the exhaust generated at the engine  9  is discharged to the exterior of the outboard motor  4 . The exhaust passage  93  is provided in the interior of the outboard motor  4 . The exhaust passage  93  includes an exhaust opening  94  that opens at a rear end portion of the propeller  13  (rear end portion of the outer cylinder  13   a ) and a main exhaust passage  95  extending from the combustion chambers  30  to the exhaust opening  94 . The exhaust passage  93  further includes an idle exhaust port  96  opening at the outer surface of the outboard motor  4  and an idle exhaust passage  97  extending from the main exhaust passage  95  to the idle exhaust port  96 . 
     As shown in  FIG. 1 , the main exhaust passage  95  extends downward from the engine  9  to the propeller shaft  12  via the exhaust guide  18  and extends rearward along the propeller shaft  12 . The main exhaust passage  95  opens rearward at the rear end portion of the propeller  13 . The exhaust opening  94  is thus disposed underwater. The idle exhaust port  96  and the idle exhaust passage  97  are disposed higher than the exhaust opening  94 . The idle exhaust passage  97  branches from the main exhaust passage  95 . The idle exhaust port  96  is disposed higher than a waterline WL (height of the water surface when the vessel  1 , equipped with the vessel propulsion apparatus  2 , is stopped). The idle exhaust port  96  thus opens into air. 
     The exhaust generated in the combustion chambers  30  is discharged into the main exhaust passage  95  and is guided toward the exhaust opening  94 . When the output of the engine  9  is high, the exhaust inside the main exhaust passage  95  is mainly discharged underwater from the exhaust opening  94 . Also, a portion of the exhaust inside the main exhaust passage  95  is guided to the idle exhaust port  96  by the idle exhaust passage  97  and is released into the atmosphere from the idle exhaust port  96 . On the other hand, when the output of the engine  9  is low (for example, when the engine  9  is idling), the exhaust pressure inside the main exhaust passage  95  is low and the exhaust inside the main exhaust passage  95  is mainly released into the atmosphere from the idle exhaust port  96 . 
     As shown in  FIG. 1 , the main exhaust passage  95  includes an engine exhaust passage  98  that is disposed higher than the exhaust guide  18 . The engine exhaust passage  98  is provided in the cylinder bodies  27 , the cylinder heads  28 , the exhaust pipe  41 , and the catalyst case  73 . The cylinder bodies  27 , the cylinder heads  28 , the exhaust pipe  41 , and the catalyst case  73  are preferably made, for example, of an aluminum alloy. The engine exhaust passage  98  is thus preferably made of a material that contains aluminum, which is an example of a light metal. 
     In  FIG. 15A  to  FIG. 15C , illustration of the combustion chambers  30  is omitted. As shown in  FIG. 15A  to  FIG. 15C , the engine exhaust passage  98  includes eight pairs of exhaust ports  32  respectively connected to the eight combustion chambers  30 , four first branch passages  99  respectively connected to four pairs of the exhaust ports  32 , and a first exhaust collecting passage  100  connected to the four first branch passages  99 . The engine exhaust passage  98  further includes four second branch passages  101  respectively connected to the other four pairs of the exhaust ports  32 , and a second exhaust collecting passage  102  connected to the four second branch passages  101 . 
     As shown in  FIG. 15A  to  FIG. 15C , the engine exhaust passage  98  further includes a catalyst housing passage  103  connected to the first exhaust collecting passage  100  and the second exhaust collecting passage  102 , and a first exhaust relay passage  104  and a second exhaust relay passage  105  connected to the catalyst housing passage  103 . The engine exhaust passage  98  further includes two head interior exhaust passages  106  respectively connected to the first exhaust relay passage  104  and the second exhaust relay passage  105 , and two body interior exhaust passages  107  respectively connected to the two head interior exhaust passages  106 . 
     The eight pairs of exhaust ports  32  are provided in the two cylinder heads  28 . As shown in  FIG. 15A to 15C , two exhaust ports  32  are provided for each cylinder  21 . A pair of exhaust ports  32  are connected to a common exhaust outlet  62   b  that opens at the outer surface of a cylinder head  28 . The pair of exhaust ports  32  merge between the combustion chamber  30  and the exhaust outlet  62   b  and extend from the combustion chamber  30  to the exhaust outlet  62   b . The eight pairs of the exhaust ports  32  are respectively connected to the eight exhaust outlets  62   b.    
     The four first branch passages  99  are respectively provided in the four first branch pipes  55  of the first exhaust manifold  53 . Each first branch passage  99  extends from a front exhaust inlet  45  opening at the outer surface of the exhaust pipe  41  to the first exhaust collecting passage  100 . As shown in  FIG. 16 , the four first branch passages  99  are connected to the first exhaust collecting passage  100  at respectively different heights. 
     As with the first branch passages  99 , the four second branch passages  101  are respectively provided in the four second branch pipes  57  of the second exhaust manifold  54 . Each second branch passage  101  extends from a front exhaust inlet  45  opening at the outer surface of the exhaust pipe  41  to the second exhaust collecting passage  102 . As shown in  FIG. 16 , the four second branch passages  101  are connected to the second exhaust collecting passage  102  at respectively different heights. 
     As shown in  FIG. 16 , two of the first branch passages  99  (the two at the upper side in  FIG. 16 ) extend toward two of the first cylinders  21 L from the first exhaust collecting passage  100  and the other two first branch passages  99  (the two at the lower side in  FIG. 16 ) extend toward two of the second cylinders  21 R from the first exhaust collecting passage  100 . Similarly, two of the second branch passages  101  (the two at the lower side in  FIG. 16 ) extend toward two of the first cylinders  21 L from the second exhaust collecting passage  102  and the other two second branch passages  101  (the two at the upper side in  FIG. 16 ) extend toward two of the second cylinders  21 R from the second exhaust collecting passage  102 . A portion of the first branch passages  99  intersects the second branch passages  101  in a rear view. Further as shown in  FIG. 15A , a portion of the first branch passages  99  intersect the second branch passages  101  in a plan view. 
     As shown in  FIG. 16 , two of the first branch passages  99  are respectively connected to the two exhaust outlets  62   b  provided in the first cylinder bank  22 L, and the other two first branch passages  99  are respectively connected to the two exhaust outlets  62   b  provided in the second cylinder bank  22 R. The four first branch passages  99  are thus respectively connected to four cylinders  21  (two of the first cylinders  21 L and two of the second cylinders  21 R). 
     Similarly, two of the second branch passages  101  are respectively connected to the two exhaust outlets  62   b  provided in the first cylinder bank  22 L, and the other two second branch passages  101  are respectively connected to the two exhaust outlets  62   b  provided in the second cylinder bank  22 R. The four second branch passages  101  are thus respectively connected to four cylinders  21  (two of the first cylinders  21 L and two of the second cylinders  21 R). 
     The first exhaust collecting passage  100  is provided in the first collecting pipe  56  of the first exhaust manifold  53 . Similarly, the second exhaust collecting passage  102  is provided in the second collecting pipe  58  of the second exhaust manifold  54 . The first exhaust collecting passage  100  is connected to a rear exhaust outlet  50  that opens at the outer surface of the exhaust pipe  41  and the second exhaust collecting passage  102  is connected to the other rear exhaust outlet  50 . 
     As shown in  FIG. 16 , the first exhaust collecting passage  100  and the second exhaust collecting passage  102  extend in the up/down direction. The first exhaust collecting passage  100  and the second exhaust collecting passage  102  are disposed parallel or substantially parallel and spaced apart by an interval in the right/left direction and are positioned at the respective sides of the center C1 of the outboard motor  4 . The first exhaust collecting passage  100  is disposed behind the four first cylinders  21 L and the second exhaust collecting passage  102  is disposed behind the four second cylinders  21 R. 
     As shown in  FIG. 16 , the first exhaust collecting passage  100  overlaps, in a rear view, with the first cylinders  21 L and the first exhaust ports  32 L, and the second exhaust collecting passage  102  overlaps, in a rear view, with the second cylinders  21 R and the second exhaust ports  32 R. The first exhaust collecting passage  100  extends from the height of the first cylinder  21 L that is disposed uppermost among the four first cylinders  21 L to the height of the first cylinder  21 L that is disposed lowermost among the four first cylinders  21 L. Similarly, the second exhaust collecting passage  102  extends from the height of the second cylinder  21 R that is disposed uppermost among the four second cylinders  21 R to the height of the second cylinder  21 R that is disposed lowermost among the four second cylinders  21 R. 
     The catalyst housing passage  103  is provided in the catalyst case  73 . The catalyst housing passage  103  extends from the exhaust inlets  77   c  opening at the outer surface of the catalyst case  73  to the exhaust outlets  78   c  opening at the outer surface of the catalyst case  73 . As shown in  FIG. 15A  to  FIG. 15C , the catalyst housing passage  103  includes an upstream portion  103   a , guiding the exhaust before purification from the first exhaust collecting passage  100  and the second exhaust collecting passage  102  to the catalyst  74 , a catalyst housing portion  103   b  housing the catalyst  74 , and a downstream portion  103   c  guiding the purified exhaust from the catalyst  74  to the first exhaust relay passage  104  and the second exhaust relay passage  105 . The catalyst housing portion  103   b  extends from the upstream portion  103   a  to the downstream portion  103   c . The flow passage area of the catalyst housing portion  103   b  is greater than the flow passage area of the first exhaust collecting passage  100  and is greater than the flow passage area of the second exhaust collecting passage  102 . 
     The first exhaust relay passage  104  and the second exhaust relay passage  105  are provided in the exhaust pipe  41 . The first exhaust relay passage  104  extends from a rear exhaust inlet  49  opening at the outer surface of the exhaust pipe  41  to a front exhaust outlet  46  opening at the outer surface of the exhaust pipe  41 . Similarly, the second exhaust relay passage  105  extends from a rear exhaust inlet  49  opening at the outer surface of the exhaust pipe  41  to a front exhaust outlet  46  opening at the outer surface of the exhaust pipe  41 . As shown in  FIG. 15B , the first exhaust relay passage  104  and the second exhaust relay passage  105  are disposed lower than the first branch passage  99  and the second branch passage  101 . The first exhaust relay passage  104  and the second exhaust relay passage  105  are respectively independent of the first branch passage  99 , the second branch passage  101 , the first exhaust collecting passage  100 , and the second exhaust collecting passage  102  and do not intersect with these passages. 
     The two head interior exhaust passages  106  are respectively provided in the two cylinder heads  28 . The two body interior exhaust passages  107  are respectively provided in the two cylinder bodies  27 . One of the head interior exhaust passages  106  extends from the exhaust inlet  61   b  provided in one of the cylinder heads  28  to the interior of the cylinder head  28 , and the other head interior exhaust passage  106  extends from the exhaust inlet  61   b  provided in the other cylinder head  28  to the interior of the cylinder head  28 . As shown in  FIG. 15B  and  FIG. 15C , each head interior exhaust passage  106  extends from the first exhaust relay passage  104  or the second exhaust relay passage  105  to a body interior exhaust passage  107 , and each body interior exhaust passage  107  extends from a head interior exhaust passage  106  toward the exhaust guide  18 . 
     The exhaust generated at two of the four combustion chambers  30  provided in the first cylinder bank  22 L is discharged into two of the first branch passages  99  via two pairs of the first exhaust ports  32 L. Also, the exhaust generated at two of the four combustion chambers  30  provided in the second cylinder bank  22 R is discharged into the other two first branch passages  99  via two pairs of the second exhaust ports  32 R. The exhaust discharged into the four first branch passages  99  is guided by the four first branch passages  99  to the first exhaust collecting passage  100  and is discharged from the first exhaust collecting passage  100  into the catalyst housing passage  103 . 
     Meanwhile, the exhaust generated at the other two combustion chambers  30  of the first cylinder bank  22 L is discharged into two of the second branch passages  101  via two pairs of the first exhaust ports  32 L. Also, the exhaust generated at the other two combustion chambers  30  provided in the second cylinder bank  22 R is discharged into the other two second branch passages  101  via two pairs of the second exhaust ports  32 R. The exhaust discharged into the four second branch passages  101  is guided by the four second branch passages  101  to the second exhaust collecting passage  102  and is discharged from the second exhaust collecting passage  102  into the catalyst housing passage  103 . 
     The exhaust discharged into the catalyst housing passage  103  is purified by the catalyst  74 . The purified exhaust is discharged from the catalyst housing passage  103  into the first exhaust relay passage  104  and the second exhaust relay passage  105  and is discharged from the first exhaust relay passage  104  and the second exhaust relay passage  105  into the two head interior exhaust passages  106 . The exhaust discharged into the two head interior exhaust passages  106  is guided by the two head interior exhaust passages  106  into the two body interior exhaust passages  107  and is discharged from the two body interior exhaust passages  107  into the interior of the exhaust guide  18 . 
       FIG. 17  is a schematic view of a connection of the eight cylinders and the two exhaust manifolds.  FIG. 18  is a graph of ignition timings, exhaust periods, and intake periods of the respective cylinders. 
     As shown in  FIG. 17 , the four first cylinders  21 L provided in the first cylinder bank  22 L are allocated, successively from the top, to NO. 1, NO. 3, NO. 5, and NO. 7. Also, the four second cylinders  21 R provided in the second cylinder bank  22 R are allocated, successively from the top, to NO. 2, NO. 4, NO. 6, and NO. 8. 
     As shown in  FIG. 17 , the engine  9  includes an engine ECU (electronic control unit)  111  as a controller that controls the engine  9 . The engine ECU  111  is connected to the eight spark plugs  33  (see  FIG. 2 ) respectively corresponding to the eight cylinders  21  (the four first cylinders  21 L and the four second cylinders  21 R). The engine ECU  111  repeats a single cycle of igniting the eight spark plugs  33  at a 90 degree interval in the ignition sequence of NO. 1, NO. 8, NO. 4, NO. 3, NO. 6, NO. 5, NO. 7, and NO. 2. 
       FIG. 18  shows the ignition timings (stars), exhaust periods (black bars), and intake periods (hatched bars) of the respective cylinders  21 . The bars in  FIG. 18  indicate crank angles (rotation angles of the crankshaft  25 ). The ignition timings, exhaust periods, and intake periods of the four cylinders  21  connected to the first exhaust manifold  53  are shown in the upper box of  FIG. 18 , and the ignition timings, exhaust periods, and intake periods of the four cylinders  21  connected to the second exhaust manifold  54  are shown in the lower box of  FIG. 18 . 
     As shown in the upper box of  FIG. 18 , the first exhaust manifold  53  is connected to the two first cylinders  21 L of NO. 1 and NO. 5 and to the two second cylinders  21 R of NO. 6 and NO. 8. As shown in the lower box of  FIG. 18 , the second exhaust manifold  54  is connected to the two first cylinders  21 L of NO. 3 and NO. 7 and to the two second cylinders  21 R of NO. 2 and NO. 4. 
     As can be understood by viewing the four stars in the upper box of  FIG. 18  sequentially from the left, with the four cylinders  21  connected to the first exhaust manifold  53 , ignition at a 90 degree interval and ignition at a 270 degree interval are repeated alternately. Similarly, as can be understood by viewing the four stars in the lower box of  FIG. 18  sequentially from the left, with the four cylinders  21  connected to the second exhaust manifold  54 , ignition at a 90 degree interval and ignition at a 270 degree interval are repeated alternately. 
     The first manifold  53  is connected to the four cylinders  21  with which an initial period of the exhaust period when the exhaust is discharged at high pressure does not overlap with an overlap period (period in which the exhaust period and the intake period overlap). Similarly, the second manifold  54  is connected to the four cylinders  21  with which the initial period of the exhaust period when the exhaust is discharged at high pressure does not overlap with the overlap period. Exhaust interference, with which the pressure of the exhaust discharged from a certain cylinder  21  interferes with the discharge of exhaust from another cylinder  21 , is thus unlikely to occur. A decrease in the output of the engine  9  due to reverse flow of intake air is thus prevented. 
       FIG. 19  is a schematic side view of an outline of a cooling device of the vessel propulsion apparatus.  FIG. 20  is a schematic view of a cooling water passage provided in the engine.  FIG. 21  is a perspective view of upper portions of the exhaust pipe and the catalytic unit.  FIG. 22  is a sectional view of an internal structure of a restriction valve. 
     As shown in  FIG. 19 , the outboard motor  4  includes a water-cooled type cooling device that cools the interior of the outboard motor  4 . The cooling device includes a water inlet  112  opening at the outer surface of the outboard motor  4 , a cooling water passage (water jacket)  113  provided in the engine  9 , a water supply passage  114  extending from the water inlet  112  to the cooling water passage  113 , and a water pump  115  that takes the water outside the outboard motor  4  into the interior of the outboard motor  4  from the water inlet  112  as the cooling water. The cooling device further includes a water outlet  116  opening inside the exhaust passage  93  and a drain passage  117  extending inside the outboard motor  4  from the cooling water passage  113  to the water outlet  116 . 
     As shown in  FIG. 19 , the water inlet  112  is disposed lower than the cooling water passage  113  and the water pump  115 . The water inlet  112  opens at the outer surface of the lower case  20 . The water inlet  112  is thus disposed underwater. The water inlet  112  is connected to the cooling water passage  113  via the water supply passage  114  provided in the interior of the outboard motor  4 . The water pump  115  is disposed in the water supply passage  114 . The water pump  115  is thus disposed in the interior of the outboard motor  4 . The water pump  115  is disposed lower than the engine  9 . 
     As shown in  FIG. 19 , the water pump  115  is mounted on the driveshaft  10 . The water pump  115  is a rotary pump that includes an impeller, rotating together with the driveshaft  10 , and a pump case housing the impeller. When the engine  9  rotates the driveshaft  10 , the impeller rotates inside the pump case and a suction force that sucks the water outside the outboard motor  4  into the water inlet  112  is generated. The water pump  115  is thus driven by the engine  9 . 
     As the cooling water, the water outside the outboard motor  4  is sucked into the water supply passage  114  from the water inlet  112  and is delivered from the water supply passage  114  to the cooling water passage  113  via the water pump  115 . High-temperature portions of the cylinder banks  22 , the exhaust device  37 , etc., are thus cooled by the cooling water. The cooling water supplied to the engine  9  is guided by the drain passage  117  to the water outlet  116  and discharged from the water outlet  116  disposed inside the exhaust passage  93 . The cooling water is thus discharged underwater from the exhaust opening  94  together with the exhaust. 
     As shown in  FIG. 20 , the cooling water passage  113  is disposed higher than the exhaust guide  18 . The cooling water passage  113  includes an upstream water passage  118  connected to the water supply passage  114 , a first parallel water passage  119  and a second parallel water passage  120  that are connected in series to the upstream water passage  118  and connected in parallel to each other, and a downstream water passage  121  connected to each of the first parallel water passage  119  and the second parallel water passage  120 . The second parallel water passage  120  includes a main parallel water passage  120   a  and a subparallel water passage  120   b  that are connected in series to the upstream water passage  118  and connected in parallel to each other, and connection water passages  120   c  that partially connect the main parallel water passage  120   a  and the sub parallel water passage  120   b  at intermediate junctions between the upstream water passage  118  and the downstream water passage  121 . 
     As shown in  FIG. 20 , the upstream water passage  118  is provided in the cylinder banks  22  and the exhaust pipe  41 . The upstream water passage  118  extends from the interiors of the cylinder banks  22  to the interior of the exhaust pipe  41 . The upstream water passage  118  extends along lower end portions of the cylinder banks  22  and the exhaust pipe  41 . At least a portion of the upstream water passage  118  is disposed lower than the cylinder  21  that is disposed lowermost among the plurality of cylinders  21 . An upstream end of the upstream water passage  118  that corresponds to the inlet of the cooling water passage  113  is disposed lower than the exhaust pipe  41  and the catalyst case  73 . 
     As shown in  FIG. 20 , the main parallel water passage  120   a  branches from the upstream water passage  118  at an upstream branch position P1. The first parallel water passage  119  and the sub parallel water passage  120   b  branch from the upstream water passage  118  at a downstream branch position P2 further downstream from the upstream branch position P1 in the direction of flow of the cooling water. The two branch positions (the upstream branch position P1 and the downstream branch position P2) are positions inside the exhaust pipe  41 . The first parallel water passage  119  and the second parallel water passage  120  thus branch from the upstream water passage  118  in the interior of the exhaust pipe  41 . The upstream water passage  118  is a water passage that extends from the interior of the cylinder banks  22  to the downstream branch position P2 via the upstream branch position P1. 
     As shown in  FIG. 20 , the first parallel water passage  119  is provided in the catalyst case  73 . The main parallel water passage  120   a  and the sub parallel water passage  120   b  are provided in the exhaust pipe  41 . The second parallel water passage  120  is thus provided in the exhaust pipe  41 . The first parallel water passage  119 , the main parallel water passage  120   a , and the sub parallel water passage  120   b  extend upward from the upstream water passage  118 . The first parallel water passage  119  is disposed along the catalyst housing passage  103 , and the main parallel water passage  120   a  and the sub parallel water passage  120   b  are disposed along the first exhaust collecting passage  100  and the second exhaust collecting passage  102 . As shown in  FIG. 11 , the first parallel water passage  119  is disposed at a periphery of the catalyst  74 . 
     As shown in  FIG. 20 , the first parallel water passage  119  and the subparallel water passage  120   b  join the downstream water passage  121  at an upstream junction position P3. The main parallel water passage  120   a  joins the downstream water passage  121  at a downstream junction position P4 further downstream from the upstream junction position P3 in the direction of flow of the cooling water. The two junction positions (the upstream junction position P3 and the downstream junction position P4) are positions inside the exhaust pipe  41 . The first parallel water passage  119  and the second parallel water passage  120  thus join the downstream water passage  121  in the interior of the exhaust pipe  41 . The downstream water passage  121  is a water passage that extends from the interior of the cylinder bank  22  to the upstream junction position P3 via the downstream junction position P4. 
     As shown in  FIG. 20 , the downstream water passage  121  is provided in the cylinder bank  22  and the exhaust pipe  41 . The downstream water passage  121  extends from the interior of the exhaust pipe  41  to the interior of the cylinder bank  22 . The downstream water passage  121  is disposed higher than the upstream water passage  118 . The downstream water passage  121  extends along upper end portions of the cylinder bank  22  and the exhaust pipe  41 . At least a portion of the downstream water passage  121  is disposed at the height of the cylinder  21  that is disposed uppermost among the plurality of cylinders  21 . The downstream water passage  121  is connected to the drain passage  117 . 
     As shown in  FIG. 20 , the cooling water sucked into the water inlet  112  by the water pump  115  flows from the water supply passage  114  into the upstream water passage  118  and flows from the upstream water passage  118  into each of the first parallel water passage  119 , the main parallel water passage  120   a , and the sub parallel water passage  120   b . The cooling water that flowed into the first parallel water passage  119 , the main parallel water passage  120   a , and the sub parallel water passage  120   b  flows from each of the first parallel water passage  119 , the main parallel water passage  120   a , and the sub parallel water passage  120   b  into the downstream water passage  121 . The downstream water passage  121  is connected to the drain passage  117  via a thermostat T1 that opens and closes in accordance with the temperature of the cooling water. The cooling water that flowed into the downstream water passage  121  flows from the downstream water passage  121  into the drain passage  117  and is discharged from a slit S1 (see  FIG. 19 ) opening at the outer surface of the lower case  20 . Also, a portion of the cooling water that flowed from the downstream water passage  121  into the drain passage  117  is discharged into the exhaust passage  93  from the water outlet  116  ( FIG. 19 ). 
     The flow passage area of the main parallel water passage  120   a  of the second parallel water passage  120  is greater than the flow passage area of the subparallel water passage  120   b  of the second parallel water passage  120 . The flow rate of the cooling water flowing from the upstream water passage  118  into the main parallel water passage  120   a  is thus greater than the flow rate of the cooling water flowing from the upstream water passage  118  into the sub parallel water passage  120   b . Further, the flow passage area of the second parallel water passage  120  (the sum of the flow passage area of the sub parallel water passage  120   b  and the flow passage area of the main parallel water passage  120   a ) that cools the first exhaust collecting passage  100  and the second exhaust collecting passage  102  is greater than the flow passage area of the first parallel water passage  119  that cools the catalyst housing passage  103 . The flow rate of the cooling water flowing from the upstream water passage  118  into the second parallel water passage  120  is thus greater than the flow rate of the cooling water flowing from the upstream water passage  118  into the first parallel water passage  119 . 
     As mentioned above, the exhaust pipe  41  includes the rear cooling water outlets  52  (see  FIG. 13 ) that discharge the cooling water. The gasket  90  (see  FIG. 14 ) is disposed between the exhaust pipe  41  and the lower spacer  44 . The cooling water holes  92   g  of the gasket  90  define a portion of the upstream water passage  118 . The flow passage area of the cooling water holes  92   g  of the gasket  90  is smaller than the flow passage area of the rear cooling water outlets  52  of the exhaust pipe  41 . A pressure loss thus occurs in the cooling water in the process of passage of the cooling water through the gasket  90  and the flow rate of the cooling water supplied from the exhaust pipe  41  to the catalyst case  73  is decreased. The flow rate of the cooling water supplied from the upstream water passage  118  to the first parallel water passage  119  is thus adjusted, and the cooling water is supplied from the upstream water passage  118  to the second parallel water passage  120  at a greater flow rate than the flow rate of the cooling water supplied to the first parallel water passage  119 . 
     The cylinder bodies  27 , the cylinder heads  28 , the exhaust pipe  41 , and the catalyst case  73  are preferably made, for example, of an aluminum alloy. The cooling water passage  113  is thus made of the aluminum alloy. In addition to being made of the aluminum alloy that is lower in heat resistance than iron, the exhaust pipe  41  is smaller in volume than the cylinder bodies  27  and the cylinder heads  28 . The exhaust pipe  41  is thus lower in heat capacity than the cylinder bodies  27  and the cylinder heads  28 . Similarly, in addition to being made of the aluminum alloy, the catalyst case  73  is smaller in volume than the cylinder bodies  27  and the cylinder heads  28 . The catalyst case  73  is thus lower in heat capacity than the cylinder bodies  27  and the cylinder heads  28 . 
     The first parallel water passage  119  is provided in the catalyst case  73  and the second parallel water passage  120  is provided in the exhaust pipe  41 . The water pump  115  supplies the water outside the outboard motor  4  that is of a substantially fixed temperature regardless of the operation circumstances of the engine  9  to the first parallel water passage  119  and the second parallel water passage  120 . The exhaust pipe  41  and the catalyst case  73  are thus cooled efficiently. Further, the exhaust pipe  41  is disposed further upstream than the catalyst case  73  in the direction of flow of the exhaust and, therefore, the exhaust having a higher temperature than the exhaust discharged into the catalyst case  73  is discharged into the exhaust pipe  41 . The flow rate of the cooling water supplied into the second parallel water passage  120  is greater than the flow rate of the cooling water supplied into the first parallel water passage  119 . The exhaust pipe  41 , which is exposed to exhaust having a higher temperature, is thus cooled efficiently. 
     As shown in  FIG. 19 , the cooling device further includes a pilot hole  122  opening at the outer surface of the outboard motor  4  and a pilot passage  123  extending from the cooling water passage  113  to the pilot hole  122 . As shown in  FIG. 20 , the cooling device further includes a plurality of vent holes  124 , connecting the interior of the cooling water passage  113  to the exterior of the cooling water passage  113 , and a restriction valve  125  allowing fluid to flow through from the interior of the cooling water passage  113  to the exterior of the cooling water passage  113  via the vent holes  124  and restricting the flow of fluid from the exterior of the cooling water passage  113  to the interior of the cooling water passage  113  via the vent holes  124 . 
     As shown in  FIG. 19 , the pilot hole  122  is disposed higher than the water inlet  112  and the water pump  115 . The pilot hole  122  opens at the outer surface of the engine cover  14 . The pilot hole  122  is disposed higher than the waterline WL. The pilot hole  122  is thus exposed to air. The pilot hole  122  is connected to the plurality of vent holes  124  via the pilot passage  123  provided in the interior of the outboard motor  4 . The plurality of vent holes  124  are connected to the cooling water passage  113 . A portion of the cooling water supplied to the cooling water passage  113  is thus guided by the pilot passage  123  to the pilot hole  122  and is discharged into air from the pilot hole  122 . A vessel operator can thus confirm that the cooling water is being supplied to the engine  9  by seeing the discharge of water from the pilot hole  122 . 
     As shown in  FIG. 21 , the plurality of vent holes  124  include two downstream vent holes  124   d  provided in the exhaust pipe  41  and two upstream vent holes  124   u  provided in the catalyst case  73 . The downstream vent holes  124   d  extend from the inner surface of the cooling water passage  113  to the outer surface of the exhaust pipe  41  and penetrate through the outer wall of the exhaust pipe  41  in its thickness direction. Similarly, the upstream vent holes  124   u  extend from the inner surface of the cooling water passage  113  to the outer surface of the catalyst case  73  and penetrate through the outer wall of the catalyst case  73  in its thickness direction. The downstream vent holes  124   d  and the upstream vent holes  124   u  thus connect the interior of the cooling water passage  113  to the exterior of the cooling water passage  113 . The flow passage area of each vent hole  124  is smaller than the flow passage area of the cooling water passage  113 . 
     As shown in  FIG. 20 , the downstream vent holes  124   d  are positioned at an uppermost portion of the exhaust pipe  41 . The downstream vent holes  124   d  are thus positioned at uppermost portions of the first exhaust manifold  53  and the second exhaust manifold  54 . Similarly, the upstream vent holes  124   u  are positioned at an uppermost portion of the catalyst case  73 . The downstream vent holes  124   d  and the upstream vent holes  124   u  are disposed at an uppermost portion of the cooling water passage  113 . The downstream vent holes  124   d  and the upstream vent holes  124   u  are disposed higher than the catalyst  74 . The downstream vent holes  124   d  and the upstream vent holes  124   u  are positioned further downstream than the catalyst  74  in the direction of flow of the cooling water. 
     As shown in  FIG. 20 , in the direction of flow of the cooling water, the upstream vent holes  124   u  are disposed between the two exhaust manifolds (the first exhaust manifold  53  and the second exhaust manifold  54 ) and the catalyst  74 . In the direction of flow of the cooling water, the downstream vent holes  124   d  are disposed further downstream than the upstream vent holes  124   u . One of the downstream vent holes  124   d  is connected to the cooling water passage  113  provided in the first exhaust manifold  53  and the other downstream vent hole  124   d  is connected to the cooling water passage  113  provided in the second exhaust manifold  53 . 
     As shown in  FIG. 21 , the pilot passage  123  includes two first passages  123   a  respectively connected to the two downstream vent holes  124   d , a second downstream passage  123   b  connected to the respective first passages  123   a , two third passages  123   c  respectively connected to the two upstream vent holes  124   u , and a fourth passage  123   d  connected to the respective third passages  123   c . The pilot passage  123  further includes a fifth passage  123   e  connected to the second passage  123   b  and the fourth passage  123   d . The cooling device includes a plurality of pilot pipings  126  mounted on the exhaust pipe  41  and the catalyst case  73 . A portion of the pilot passage  123  is defined by the plurality of pilot pipings  126 . The flow passage area of the pilot pipings  126  is smaller than the flow passage area of the cooling water passage  113 . The flow passage area of the pilot passage  123  is thus smaller than the flow passage area of the cooling water passage  113 . 
     As shown in  FIG. 21 , the restriction valve  125  is disposed in the pilot passage  123 . As shown in  FIG. 22 , the restriction valve  125  includes an internal flow passage  127 , through which a fluid (at least one of either of a gas and a liquid) flows, and a spherical valve element  129  that increases and decreases the flow passage area of the internal flow passage  127  between an inlet  127   i  of the internal flow passage  127  and an outlet  127   o  of the internal flow passage  127  by opening and closing an opening of a valve seat  128  provided in the internal flow passage  127 . 
     As shown in  FIG. 21 , the inlet  127   i  of the internal flow passage  127  is connected to the vent holes  124 . The inlet  127   i  of the internal flow passage  127  is thus connected to the cooling water passage  113  via the vent holes  124 . The pressure at the inlet  127   i  of the internal flow passage  127  is equal or substantially equal to the pressure inside the cooling water passage  113 . Also, the outlet  127   o  of the internal flow passage  127  is connected to the vent holes  124  via the inlet  127   i  of the internal flow passage  127 . The outlet  127   o  of the internal flow passage  127  is connected to the pilot hole  122  via the pilot passage  123 . The pilot hole  122  opens into air. The pressure at the outlet  127   o  of the internal flow passage  127  is thus equal or substantially equal to the atmospheric pressure. 
     When the pressure at the inlet  127   i  of the internal flow passage  127  is higher than the pressure at the outlet  1270  of the internal flow passage  127 , the valve element  129  is moved away from the valve seat  128  by the differential pressure as indicated by solid line in  FIG. 22 . The valve seat  128  is thus opened and the flow passage area of the internal flow passage  127  increases. The fluid flowing into the inlet  127   i  of the internal flow passage  127  thus flows to the outlet  127   o  of the internal flow passage  127  via the valve seat  128  and is discharged from the outlet  127   o  of the internal flow passage  127 . The fluid discharged into the vent holes  124  from the cooling water passage  113  thus flows through the pilot passage  123  toward the pilot hole  122 . 
     On the other hand, when the pressure at the inlet  127   i  of the internal flow passage  127  is lower than the pressure at the outlet  127   o  of the internal flow passage  127 , the valve element  129  is pressed against the valve seat  128  by the differential pressure as indicated by the alternate long and two short dashed lines. The valve seat  128  is thus closed and the flow passage area of the internal flow passage  127  decreases. The flow of fluid from the outlet  127   o  of the internal flow passage  127  to the inlet  127   i  of the internal flow passage  127  is thus restricted. The supplying of the fluid from the pilot passage  123  to the cooling water passage  113  is thus restricted. That is, the reverse flow of fluid from the vent holes  124  to the cooling water passage  113  is restricted. 
     The restriction valve  125  may be a check valve that completely stops the reverse flow of fluid (the flow of fluid from the outlet  127   o  of the internal flow passage  127  to the inlet  127   i  of the internal flow passage  127 ). Specifically, the restriction valve  125  may be a poppet valve or a reed valve. Also, the restriction valve  125  may be a leak valve that allows reverse flow of fluid from the outlet  127   o  of the internal flow passage  127  to the inlet  127   i  of the internal flow passage  127  at a flow rate smaller than that when the opening of the valve seat  128  is fully open. Specifically, as shown in  FIG. 22 , a leak groove  130  that is recessed more than the valve seat  128  and extends from an upstream side (side of the inlet  127   i  of the internal flow passage  127 ) relative to the opening provided in the valve seat  128  to a downstream side (side of the outlet  127   o  of the internal flow passage  127 ) relative to the opening may be provided in the internal surface of the internal flow passage  127 . 
     As shown in  FIG. 20 , the inlet (upstream end  113   u ) of the cooling water passage  113  into which the cooling water flows is disposed lower than the exhaust pipe  41  and the catalyst case  73  and, therefore, the cooling water delivered from the water supply passage  114  to the cooling water passage  113  by the water pump  115  rises inside the exhaust pipe  41  and the catalyst case  73  along the cooling water passage  113 . When the cooling water is supplied to the cooling water passage  113  in the state in which the cooling water passage  113  is empty, the pressure inside the cooling water passage  113  exceeds the atmospheric pressure and the restriction valve  125  opens. 
     When the restriction valve  125  opens, the air inside the cooling water passage  113  is discharged from the cooling water passage  113  via the plurality of vent holes  124  and the cooling water supplied by the water pump  115  fills the interior of the cooling water passage  113  smoothly. When the cooling water passage  113  is filled with the cooling water, the cooling water is discharged from the cooling water passage  113  via the plurality of vent holes  124  and is guided to the pilot hole  122  by the pilot passage  123 . A portion of the cooling water inside cooling water passage  113  is thus continuously discharged out of the outboard motor  4  from the pilot hole  122 . 
     The water inlet  112  from which the water outside the outboard motor  4  is taken in is open underwater (see  FIG. 19 ). The water inlet  112  may thus be clogged by underwater foreign matter, such as seaweed, etc. The supply flow rate of the cooling water to the cooling water passage  113  may thus decrease or the supply of cooling water to the cooling water passage  113  may stop. Similarly, when the water pump  115  malfunctions, the supply flow rate of the cooling water to the cooling water passage  113  may decrease or the supply of cooling water to the cooling water passage  113  may stop. 
     The cooling water inside the cooling water passage  113  tends to flow down inside the cooling water passage  113  due to its own weight. Therefore, when clogging of the water inlet  112  or other abnormality occurs in the cooling device, the pressure inside the cooling water passage  113  decreases and the restriction valve  125  closes. Consequently, air is unlikely to enter from the vent holes  124  into the cooling water passage  113  and the rate of discharge of the cooling water from the cooling water passage  113  decreases. Therefore, even if the supply flow rate of the cooling water to the cooling water passage  113  decreases, the engine  9  continues to be cooled by the cooling water retained inside the cooling water passage  113 . Overheating of the engine  9  is thus prevented. Further, even though the discharge rate of the cooling water decreases, nearly all of the cooling water is discharged from the cooling water passage  113  at a final stage and, therefore, occurrence of rust due to residual water inside the cooling water passage  113  during storage of the vessel propulsion apparatus  2  on land is reduced. 
     As described above, with the first preferred embodiment, the four first cylinders  21 L aligned in the up/down direction are provided in the first cylinder bank  22 L and the four second cylinders  21 R aligned in the up/down direction are provided in the second cylinder bank  22 R. The four first exhaust ports  32 L are respectively connected to the four first cylinders  21 L and the four second exhaust ports  32 R are respectively connected to the four second cylinders  21 R. The first exhaust ports  32 L and the second exhaust ports  32 R are disposed at the inner side of the V-shaped lines V1 with the V-shape in a plan view. The exhaust generated in the combustion chambers  30  is thus collected to the inner sides of the two cylinder banks  22  disposed in a V-shape. 
     The four first branch pipes  55  of the first exhaust manifold  53  are connected to the two cylinder banks  22  via the first exhaust ports  32 L and the second exhaust ports  32 R. Similarly, the four second branch pipes  57  of the second exhaust manifold  54  are connected to the two cylinder banks  22  via the first exhaust ports  32 L and the second exhaust ports  32 R. The four first branch pipes  55  are thus connected to four cylinders  21  that differ in ignition timing and the four second branch pipes  57  are connected to four cylinders  21  that differ in ignition timing. Exhaust interference is thus prevented and the engine  9  has an increased output. 
     Further, the first collecting pipe  56  of the first exhaust manifold  53  extends from the height of the first cylinder  21 L that is disposed uppermost among the four first cylinders  21 L to the height of the first cylinder  21 L that is disposed lowermost among the four first cylinders  21 L. Similarly, the second collecting pipe  58  of the second exhaust manifold  54  extends from the height of the second cylinder  21 R that is disposed uppermost among the four second cylinders  21 R to the height of the second cylinder  21 R that is disposed lowermost among the four second cylinders  21 R. The first collecting pipe  56  and the second collecting pipe  58  are thus long in the up/down direction. The first exhaust manifold  53  and the second exhaust manifold  54  are thus decreased in width while securing the length (passage length) of the exhaust passage  93 . The engine  9  is thus compact in the width direction (right/left direction). 
     Further, the first collecting pipe  56  of the first exhaust manifold  53  is disposed behind the four first cylinders  21 L and the second collecting pipe  58  of the second exhaust manifold  54  is disposed behind the four second cylinders  21 R. Therefore, in comparison to a case where the first exhaust manifold  53  and the second exhaust manifold  54  are disposed behind a common cylinder  21 , the first branch pipes  55  and the second branch pipes  57  are arranged efficiently. Therefore, not only are the shapes of the first exhaust manifold  53  and the second exhaust manifold  54  prevented from becoming complicated but the widths of the first exhaust manifold  53  and the second exhaust manifold  54  are also reduced further. The engine  9  is thus compact in the width direction. 
     Also with the first preferred embodiment, the second branch pipe  57  intersects the first branch pipe  55  in a plan view and, therefore, the entirety of the two exhaust manifolds (the first exhaust manifold  53  and the second exhaust manifold  54 ) is compact. The engine  9  is thus even more compact. 
     Also with the first preferred embodiment, the first collecting pipe  56  is integral and unitary with the four first branch pipes  55  and, therefore, each of the first branch pipes  55  extends from the first collecting pipe  56  to the cylinder bank  22 . The first exhaust manifold  53  is thus more compact than in a case where another exhaust pipe is interposed between the first branch pipes  55  and the first collecting pipe  56 . Similarly, the second collecting pipe  58  is integral and unitary with the four second branch pipes  57  and thus the second exhaust manifold  54  is more compact than in a case where another exhaust pipe is interposed between the second branch pipes  57  and the second collecting pipe  58 . The engine  9  is thus even more compact. 
     Also with the first preferred embodiment, the first exhaust manifold  53  and the second exhaust manifold  54  are disposed in the exhaust pipe  41  and the number of parts of the engine  9  is thus reduced. 
     Also with the first preferred embodiment, the exhaust discharged from the first exhaust manifold  53  and the second exhaust manifold  54  is purified by the catalytic unit  42 . The catalytic unit  42  is disposed behind the exhaust pipe  41 . That is, at least a portion of the catalytic unit  42  is disposed at the same height as the exhaust pipe  41 . The height (length in the up/down direction) of the engine  9  is thus reduced more in comparison to a case where the entire catalytic unit  42  is disposed higher or lower than the exhaust pipe  41 . The engine  9  is thus compact in the up/down direction. 
     Also with the first preferred embodiment, the exhaust discharged from the first exhaust manifold  53  and the second exhaust manifold  54  flows into the catalyst case  73  of the catalytic unit  42 . The catalyst  74  is disposed inside the catalyst case  73 . The exhaust that is discharged into the catalyst case  73  from the first exhaust manifold  53  and the second exhaust manifold  54  is thus purified. Further, the catalyst case  73  extends from the height of the first cylinder  21 L that is disposed uppermost among the four first cylinders  21 L to the height of the first cylinder  21 L that is disposed lowermost among the four first cylinders  21 L. The catalyst case  73  is thus long in the up/down direction. The catalyst case  73  defines a portion of the exhaust passage  93 . The catalyst case  73  is thus reduced in width while securing the length of the exhaust passage  93 . The engine  9  is thus compact in the width direction. 
     Also with the first preferred embodiment, the exhaust purified by the catalytic unit  42  is discharged from the catalytic unit  42  into the two exhaust relay passages (the first exhaust relay passage  104  and the second exhaust relay passage  105 ) and thereafter discharged from the first exhaust relay passage  104  and the second exhaust relay passage  105  to the two cylinder banks  22 . The first exhaust relay passage  104  and the second exhaust relay passage  105  are independent of the first exhaust manifold  53  and the second exhaust manifold  54 . That is, the internal spaces of the first exhaust relay passage  104  and the second exhaust relay passage  105  are separated from the internal spaces of the first exhaust manifold  53  and the second exhaust manifold  54  and do not intersect with the internal spaces of the first exhaust manifold  53  and the second exhaust manifold  54 . The pre-purification exhaust in the first exhaust manifold  53  and the second exhaust manifold  54  is thus prevented from flowing into the first exhaust relay passage  104  and the second exhaust relay passage  105 . Further, as with the first exhaust manifold  53  and the second exhaust manifold  54 , the first exhaust relay passage  104  and the second exhaust relay passage  105  are provided in the exhaust pipe  41  and the number of parts of the engine  9  is thus reduced. 
     Also with the first preferred embodiment, the fixed portion  65   p  provided in the exhaust pipe  41  is fixed to one of the two cylinder banks  22  and the insertion portion  66  provided in the exhaust pipe  41  is movably connected to the other of the two cylinder banks  22 . The exhaust pipe  41  is thus fixed to one of the cylinder banks  22  and is movably connected to the other cylinder bank  22 . The respective parts of the engine  9  have dimensional tolerances and, therefore, if the exhaust pipe  41  is fixed to the two cylinder banks  22  at all locations, gaps due to dimensional variations may occur between the exhaust pipe  41  and the cylinder banks  22 . Therefore, by connecting a portion (the insertion portion  66 ) of the exhaust pipe  41  to the other cylinder bank  22  in a manner enabling movement, the dimensional variations are absorbed. The sealing property between the exhaust pipe  41  and the cylinder banks  22  is thus improved and leakage of the exhaust is thus prevented. 
     Also with the first preferred embodiment, the exhaust generated in a plurality of combustion chambers  30  is discharged via the plurality of exhaust ports  32  into the first exhaust collecting passage  100  and the second exhaust collecting passage  102  and discharged from the first exhaust collecting passage  100  and the second exhaust collecting passage  102  into the catalyst housing passage  103 . The catalyst  74  that purifies the exhaust is housed in the catalyst housing passage  103 . The exhaust is thus purified in the process of flowing inside the catalyst housing passage  103 . 
     Meanwhile, the water pump  115  takes the water outside the vessel propulsion apparatus  2  into the vessel propulsion apparatus  2  and delivers the water into the upstream water passage  118  of the cooling water passage  113 . The cooling water delivered into the upstream water passage  118  is supplied respectively to the first parallel water passage  119  and the second parallel water passage  120  connected in series to the upstream water passage  118 . The first parallel water passage  119  is disposed along the catalyst housing passage  103 , and the second parallel water passage  120  is disposed along the first exhaust collecting passage  100  and the second exhaust collecting passage  102 . The first exhaust collecting passage  100 , the second exhaust collecting passage  102 , and the catalyst housing passage  103  are thus cooled by the cooling water being supplied to the first parallel water passage  119  and the second parallel water passage  120 , respectively. 
     The first parallel water passage  119  and the second parallel water passage  120  are thus connected in series to the upstream water passage  118  and connected in parallel to each other and, therefore, the resistance applied to the cooling water flowing in the cooling water passage  113  is reduced in comparison to the case where the first parallel water passage  119  and the second parallel water passage  120  are connected in series with respect to each other. The pressure loss of the cooling water that occurs in the cooling water passage  113  is thus reduced. The flow rate of the cooling water supplied to the first parallel water passage  119  and the second parallel water passage  120  is thus increased without increasing the capacity of the water pump  115 . The cooling ability of the vessel propulsion apparatus  2  is thus be increased and the exhaust passage  93  and the catalyst  74  is cooled reliably. 
     Also with the first preferred embodiment, the plurality of exhaust ports  32  connected to the two cylinder banks  22  having a V-shape are disposed at the inner side of the V-shaped lines V1. If the plurality of exhaust ports  32  are disposed at the outer side of the V-shaped lines V1, the exhaust passage must be provided at the outer side of the V-shaped lines V1 and the exhaust passage  93  thus gets longer. 
     The exhaust passage  93  is thus shortened by disposing the plurality of exhaust ports  32  at the inner side of the V-shaped lines V1. The vessel propulsion apparatus  2  is thus compact and lightweight. Further, the exhaust passage  93  is consolidated at the inner side of the V-shaped lines V1 to enable the exhaust generated in the respective combustion chambers  30  to be guided to the single catalyst  74  while preventing the increase of length of the exhaust passage  93 . The number of parts of the vessel propulsion apparatus  2  is thus reduced. 
     Also with the first preferred embodiment, the exhaust pipe  41  that guides the exhaust is mounted on the two cylinder banks  22 . The first exhaust collecting passage  100 , the second exhaust collecting passage  102 , and the second parallel water passage  120  are provided in the exhaust pipe  41 . In other words, the first exhaust collecting passage  100 , the second exhaust collecting passage  102 , and the second parallel water passage  120  are provided in a common member. The distance between the two exhaust collecting passages (the first exhaust collecting passage  100  and the second exhaust collecting passage  102 ) and the second parallel water passage  120  is thus shortened and the efficiency of heat transfer between the two exhaust collecting passages and the second parallel water passage  120  is thus improved. The first exhaust collecting passage  100  and the second exhaust collecting passage  102  are thus cooled efficiently. 
     Also with the first preferred embodiment, the catalyst case  73  that houses the catalyst  74  is mounted on the exhaust pipe  41 . The catalyst housing passage  103  and the first parallel water passage  119  are provided in the catalyst case  73 . In other words, the catalyst housing passage  103  and the first parallel water passage  119  are provided in a common member. The distance between the catalyst housing passage  103  and the first parallel water passage  119  is thus shortened and the efficiency of heat transfer between the catalyst housing passage  103  and the first parallel water passage  119  is thus improved. The catalyst housing passage  103  is thus cooled efficiently. 
     Also with the first preferred embodiment, the gasket  90  is disposed between opening portions (the rear cooling water outlets  52 ) of the exhaust pipe  41  and opening portions (the cooling water inlets  79   c ) of the catalyst case  73 . The cooling water flows from the opening portions of the exhaust pipe  41  to the opening portions of the catalyst case  73 . The gasket  90  defines a portion of the cooling water passage  113  between the opening portions of the exhaust pipe  41  and the opening portions of the catalyst case  73 . The flow passage area of the gasket  90  is smaller than the flow passage area of the opening portions of the exhaust pipe  41 . The flow rate of the cooling water supplied from the exhaust pipe  41  to the catalyst case  73  is thus reduced by the gasket  90  and the flow rate of the cooling water supplied to the exhaust pipe  41  is increased. In regard to the direction of flow of the exhaust, the exhaust pipe  41  is disposed further upstream than the catalyst case  73 . Exhaust having a higher temperature than the exhaust flowing into the catalyst case  73  thus flows into the exhaust pipe  41 . Therefore, by increasing the flow rate of the cooling water supplied to the exhaust pipe  41 , the exhaust pipe  41  is cooled reliably. 
     Also with the first preferred embodiment, the flow passage area of the second parallel water passage  120  is greater than the flow passage area of the first parallel water passage  119  and, therefore, the cooling water is supplied to the second parallel water passage  120  at a flow rate greater than the flow rate of the cooling water supplied to the first parallel water passage  119 . The first parallel water passage  119  is provided along the catalyst housing passage  103  and the second parallel water passage  120  is provided along the first exhaust collecting passage  100  and the second exhaust collecting passage  102 . The first exhaust collecting passage  100  and the second exhaust collecting passage  102  are disposed further upstream than the catalyst housing passage  103  in the direction of flow of the exhaust. Exhaust having a higher temperature than the exhaust flowing into the catalyst housing passage  103  thus flows into the first exhaust collecting passage  100  and the second exhaust collecting passage  102 . The first exhaust collecting passage  100  and the second exhaust collecting passage  102  is thus cooled reliably by increasing the flow rate of the cooling water supplied to the second parallel water passage  120 . 
     Also with the first preferred embodiment, at least a portion of the exhaust passage  93  is preferably made of a material containing aluminum, which is an example of a light metal. Similarly, at least a portion of the cooling water passage  113  is preferably made of a material containing aluminum, for example. The vessel propulsion apparatus  2  is thus light in weight. On the other hand, aluminum is lower in heat resistance than iron and, therefore, the heat resistance of the exhaust passage  93  is lower than when the entire exhaust passage  93  is made of a material having iron as the main component. However, the vessel propulsion apparatus  2  is improved in cooling ability as described above and the exhaust passage  93  is cooled reliably and, therefore, not only is the vessel propulsion apparatus  2  light in weight but melting of a portion of the exhaust passage  93  is also prevented. 
     Also with the first preferred embodiment, the exhaust generated in the plurality of combustion chambers  30  is discharged underwater from the exhaust opening  94 . The engine  9  is disposed on the exhaust guide  18  as an engine supporting member. The engine  9  is disposed higher than the water surface and, therefore, at least a portion of the exhaust guide  18  is disposed higher than the water surface. The catalyst  74  is disposed higher than the exhaust guide  18 . The catalyst  74  is thus disposed higher than the water surface and the height from the water surface to the catalyst  74  is large. Water that has entered into the exhaust passage  93  from the exhaust opening  94  that is opened underwater is thus unlikely to reach the catalyst  74 . Degradation of the catalyst  74  due to wetting by water is thus prevented. 
     Also with the first preferred embodiment, at least a portion of the cooling water passage  113  is disposed at the periphery of the catalyst  74 . The water pump  115  supplies the water outside the outboard motor  4  to the cooling water passage  113  via the water inlet  112 . The water pump  115  is disposed lower than the catalyst  74 . At least a portion of the cooling water passage  113  is disposed higher than the water pump  115 . The cooling water taken into the outboard motor  4  by the water pump  115  thus rises inside the outboard motor  4  toward the cooling water passage  113 . 
     The interior of the cooling water passage  113  is connected to the exterior of the cooling water passage  113  by the vent holes  124 . The vent holes  124  are disposed higher than the catalyst  74 . As mentioned above, the water pump  115  is disposed lower than the catalyst  74 . The vent holes  124  are thus disposed higher than the water pump  115 . The restriction valve  125  allows fluid to flow from the interior of the cooling water passage  113  to the exterior of the cooling water passage  113  via the vent holes  124 . Therefore, when the water pump  115  delivers the cooling water to the cooling water passage  113 , the air inside the cooling water passage  113  is discharged to the exterior of the cooling water passage  113  via the vent holes  124 . The cooling water passage  113  is thus rapidly filled with the cooling water. 
     When an abnormality, such as clogging of the water inlet  112 , etc., occurs in the cooling device, the flow rate of supply of the cooling water to the cooling water passage  113  decreases. In this condition, the cooling water remaining inside the cooling water passage  113  tends to flow down due to its own weight. The restriction valve  125  restricts the flow of fluid from the exterior of the cooling water passage  113  to the interior of the cooling water passage  113  via the vent holes  124 . The air outside the cooling water passage  113  is thus unlikely to enter into the cooling water passage  113  via the vent holes  124  and the cooling water is unlikely to be discharged from the cooling water passage  113 . The rate of discharge of the cooling water from the cooling water passage  113  is thus decreased and the retention time of the cooling water inside the cooling water passage  113  is lengthened. Lowering of the cooling ability is thus significantly reduced or prevented when an abnormality occurs in the cooling device. A temperature rise of the exhaust passage  93  and the catalyst  74  is thus significantly reduced or prevented. 
     Also with the first preferred embodiment, the interior of the cooling water passage  113  is connected to the exterior of the cooling water passage  113  via the vent holes  124  and, therefore, a portion of the cooling water inside the cooling water passage  113  is discharged from the cooling water passage  113  through the vent holes  124 . The flow passage area of the vent holes  124  is smaller than the flow passage area of the cooling water passage  113 . A large portion of the cooling water inside the cooling water passage  113  thus flows toward the downstream end of the cooling water passage  113  that corresponds to the outlet of the cooling water passage  113  and cools the exhaust passage  93  and the catalyst  74 . In other words, the amount of cooling water that is discharged from the cooling water passage  113  before reaching the downstream end of the cooling water passage  113  is small. The exhaust passage  93  and the catalyst  74  are thus cooled reliably. 
     Also with the first preferred embodiment, the vent holes  124  are positioned at the uppermost portion of the cooling water passage  113  and air is thus discharged reliably from the uppermost portion of the cooling water passage  113 . Therefore, not only is the cooling water passage  113  filled with the cooling water reliably but the cooling water reaches the uppermost portion of the cooling water passage  113  reliably as well. The exhaust passage  93  and the catalyst  74  are thus cooled efficiently. 
     Also with the first preferred embodiment, the vent holes  124  are positioned further downstream than the catalyst  74  in the direction of flow of the cooling water and, therefore, the cooling water that is to be discharged from the cooling water passage  113  via the vent holes  124  also passes close to the catalyst  74 . The catalyst  74  is thus cooled efficiently. 
     Also with the first preferred embodiment, the pilot passage  123  is connected to the interior of the cooling water passage  113  via the vent holes  124  and, therefore, a portion of the cooling water inside the cooling water passage  113  is discharged from the cooling water passage  113  to the pilot passage  123 . The flow passage area of the pilot passage  123  is smaller than the flow passage area of the cooling water passage  113 . A large portion of the cooling water inside the cooling water passage  113  thus flows toward the downstream end of the cooling water passage  113  and cools the exhaust passage  93  and the catalyst  74 . In other words, the amount of cooling water that is discharged from the cooling water passage  113  before reaching the downstream end of the cooling water passage  113  is small. The exhaust passage  93  and the catalyst  74  are thus cooled reliably. 
     Also with the first preferred embodiment, the exhaust is guided to the catalyst  74  by the first exhaust manifold  53  and the second exhaust manifold  54  that define at least a portion of the exhaust passage  93 . A portion of the cooling water passage  113  is provided in the first exhaust manifold  53  and the second exhaust manifold  54 , and the first exhaust manifold  53  and the second exhaust manifold  54  are thus cooled by the cooling water supplied from the water pump  115 . In the direction of flow of the cooling water, the vent holes  124  are disposed between the two exhaust manifolds (the first exhaust manifold  53  and the second exhaust manifold  54 ) and the catalyst  74 . That is, in the direction of flow of the cooling water, the vent holes  124  extend from a portion of the cooling water passage  113  positioned between the two exhaust manifolds and the catalyst  74  to the exterior of the cooling water passage  113 . A portion of the fluid present between the two exhaust manifolds and the catalyst  74  is thus discharged from the vent holes  124 . Retention of the cooling water between the two exhaust manifolds and the catalyst  74  is thus prevented. The exhaust passage  93  and the catalyst  74  is thus cooled efficiently. 
     Also with the first preferred embodiment, the vent holes  124  are positioned at the uppermost portions of the first exhaust manifold  53  and the second exhaust manifold  54  and, therefore, the air at the uppermost portions of the first exhaust manifold  53  and the second exhaust manifold  54  is reliably discharged from the vent holes  124 . A portion of the cooling water passage  113  is provided in the first exhaust manifold  53  and the second exhaust manifold  54 . The cooling water thus reaches the uppermost portions of the first exhaust manifold  53  and the second exhaust manifold  54  reliably. The exhaust passage  93  and the catalyst  74  are thus cooled efficiently. 
     Also with the first preferred embodiment, the catalyst  74  is disposed inside the engine cover  14  that covers the engine  9 , and the engine  9  and the catalyst  74  are thus close to each other. The engine  9  is disposed higher than the water surface. The catalyst  74  is thus disposed higher than the water surface and the height from the water surface to the catalyst  74  is large. Water that enters into the exhaust passage  93  from the exhaust opening  94  that is opened underwater is thus unlikely to reach the catalyst  74 . Degradation of the catalyst  74  due to wetting by water is thus prevented. 
     Although preferred embodiments of the present invention have been described above, the present invention is not restricted to the contents of the preferred embodiments and various modifications are possible within the scope of the claims. 
     For example, with the preferred embodiments, a case where the engine  9  is a V-type, eight-cylinder engine that includes eight cylinders  21  was described as a non-limiting example. However, the engine  9  may include a plurality of cylinders  21  of a number other than eight. Specifically, the engine  9  may be a V-type, six-cylinder engine, a V-type, ten-cylinder engine, or a V-type, twelve cylinder engine. Also, the engine  9  may be installed in a device other than an outboard motor  4 . 
     Also with the preferred embodiments, a case where the four cylinders  21  of NO. 1, NO. 5, NO. 6, and NO. 8 are connected to the first exhaust manifold  53  and the four cylinders  21  of NO. 2, NO. 3, NO. 4, and NO. 7 are connected to the second exhaust manifold  54  was described. However, four cylinders  21  (two of the first cylinders  21 L and two of the second cylinders  21 R) of a different combination may be connected to the first exhaust manifold  53 . 
     For example, four cylinders  21  that differ by 180 degrees each in ignition timing may be connected to each exhaust manifold. Specifically, a first exhaust manifold  253  may be connected to the two first cylinders  21 L of NO. 1 and NO. 7 and the two second cylinders  21 R of NO. 4 and NO. 6 as shown in  FIG. 23  and  FIG. 24 . A second exhaust manifold  254  may thus be connected to the two first cylinders  21 L of NO. 3 and NO. 5 and the two second cylinders  21 R of NO. 2 and NO. 8. 
     Also with the preferred embodiments, a case where the supporting recesses  69 , into which the insertion portions  66 , provided on the exhaust pipe  41 , are inserted, are integral and unitary with the cylinder head  28  was described as a non-limiting example. However, the supporting recesses  69  may instead be provided in a member other than the cylinder head  28  that is mounted on the cylinder head  28 . Specifically, as shown in  FIG. 25 , the engine  9  may include a spacer plate  331  interposed between the cylinder head  28  and the exhaust pipe  41  and the supporting recesses  69  may be provided in the spacer plate  331 . 
     Also with the preferred embodiments, a case where the upper portion of the catalytic unit  42  is mounted on the upper portion of the exhaust pipe  41  via the upper spacer  43  and the lower portion of the catalytic unit  42  is mounted on the lower portion of the exhaust pipe  41  via the lower spacer  44  was described as a non-limiting example. However, at least one of either of the upper spacer  43  and the lower spacer  44  may be omitted. 
     Also with the preferred embodiments, a case where the first exhaust manifold  53  and the second exhaust manifold  54  are provided in a member (the exhaust pipe  41 ) other than the cylinder heads  28  was described as a non-limiting example. However, as shown in  FIG. 26 , a first exhaust manifold  453  and a second exhaust manifold  454  may be provided in the cylinder heads  28 . That is, the first exhaust manifold  453  and the second exhaust manifold  454  may be integral and unitary with the cylinder heads  28 . In this case, as shown in  FIG. 26 , the exhaust pipe  41  may be omitted and the catalytic unit  42  may be mounted directly to the two cylinder heads  28 . 
     Also with the preferred embodiments, a case where the first exhaust manifold  53 , the second exhaust manifold  54 , the first relay pipe  59 , and the second relay pipe  60  are provided in a common member (the exhaust pipe  41 ) and are integral and unitary was described as a non-limiting example. However, at least one of the first exhaust manifold  53 , the second exhaust manifold  54 , the first relay pipe  59 , and the second relay pipe  60  may be provided in a member other than the exhaust pipe  41 . 
     Also with the preferred embodiments, a case where the fixed portion  65   p  provided at the exhaust pipe  41  is fixed to one of the two cylinder banks  22  and the insertion portions  66  provided at the exhaust pipe  41  are movably connected to the other of the two cylinder banks  22  was described as a non-limiting example. However, the exhaust pipe  41  may be fixed to both cylinder banks  22 . That is, the exhaust pipe  41  does not need to include the insertion portions  66 . 
     Also with the preferred embodiments, a case where the insertion portions  66  are provided at the exhaust pipe  41  and the supporting recesses  69  are provided at a cylinder bank  22  was described as a non-limiting example. However, insertion portions  66  provided at a cylinder bank  22  may be inserted in supporting recesses  69  provided at the exhaust pipe  41 . 
     Also with the preferred embodiments, a case where the supply flow rate of the cooling water supplied from the exhaust pipe  41  to the catalyst case  73  is adjusted by the gasket  90  was described as a non-limiting example. That is, a case where the flow passage area of the cooling water holes  92   g  of the gasket  90  is smaller than the flow passage area of the rear cooling water outlets  52  of the exhaust pipe  41  was described. However, the flow passage area of the cooling water holes  92   g  may be equal or substantially equal to the flow passage area of the rear cooling water outlets  52  or may be greater than the flow passage area of the rear cooling water outlets  52 . 
     Also with the preferred embodiments, a case where the flow passage area of the second parallel water passage  120  that cools the first exhaust collecting passage  100  and the second exhaust collecting passage  102  is greater than the flow passage area of the first parallel water passage  119  that cools the catalyst housing passage  103  was described as a non-limiting example. However, the flow passage area of the second parallel water passage  120  may be equal or substantially equal to the flow passage area of the first parallel water passage  119  or may be smaller than the flow passage area of the first parallel water passage  119 . Also, the flow passage area of the main parallel water passage  120   a  of the second parallel water passage  120  may be equal or substantially equal to the flow passage area of the sub parallel water passage  120   b  of the second parallel water passage  120  or may be smaller than the flow passage area of the sub parallel water passage  120   b.    
     Also with the preferred embodiments, a case where the first parallel water passage  119  provided at the catalyst case  73  and the second parallel water passage  120  provided at the exhaust pipe  41  are connected in series to the upstream water passage  118  and connected in parallel to each other was described as a non-limiting example. However, the first parallel water passage  119  and the second parallel water passage  120  may be connected in series to each other instead. Specifically, the first parallel water passage  119  as a first serial water passage may extend from the upstream water passage  118  to the second parallel water passage  120 , and the second parallel water passage  120  as the second serial water passage may extend from the first parallel water passage  119  to the downstream water passage  121 . That is, the upstream water passage  118 , the first parallel water passage  119 , the second parallel water passage  120 , and the downstream water passage  121  may be connected in series in that order from the upstream side in the direction of flow of the cooling water. 
     Also with the preferred embodiments, a case where the restriction valve  125  is opened and closed in accordance with the pressure inside the cooling water passage  113  was described as a non-limiting example. However, a solenoid valve that is opened and closed by an electromagnetic force may be used instead as the restriction valve  125 . 
     Specifically, as shown in  FIG. 27 , the engine  9  may include a temperature detecting device  532 , detecting a temperature of the engine  9 , and a restriction valve  525  (solenoid valve) opened and closed by the engine ECU  111  based on the detection value of the temperature detecting device  532 . In this case, the temperature of the outer wall of the engine  9  is detected by the temperature detecting device  532  and the detection value of the temperature detecting device  532  is input to the engine ECU  111 . Based on the detection value of the temperature detecting device  532 , the engine ECU  111  judges whether or not the engine  9  is overheated. That is, the engine ECU  111  judges whether or not the temperature of the engine  9  is not less than an overheating temperature. 
     When an abnormality occurs in the cooling device, the flow rate of the cooling water supplied to the cooling water passage  113  decreases and the temperature of the engine  9  thus increases. When the temperature of the engine  9  reaches the overheating temperature, the engine ECU  111  closes the restriction valve  525  that is normally open and maintains the state in which the restriction valve  525  is closed until the temperature of the engine  9  falls to less than the overheating temperature. Therefore, when an abnormality occurs in the cooling device, the discharge of cooling water from the cooling water passage  113  is restricted and lowering of the cooling ability is significantly reduced or prevented. The engine ECU  111  thus prevents the overheating temperature of the exhaust passage  93  and the catalyst  74 . 
     Also with the preferred embodiments, a case where the restriction valve  125  is a ball valve that includes a spherical valve element  129  was described as a non-limiting example. However, the restriction valve  125  may be a poppet valve with a conical valve element or a reed valve that includes a reed as a valve element or a valve of any other type. That is, the shape of the valve element  129  is not restricted to being spherical and may be conical or any other shape. 
     Also with the preferred embodiments, a case where the restriction valve  125  that restricts the inflow of fluid into the cooling water passage  113  is disposed in the pilot passage  123  was described as a non-limiting example. However, the engine  9  does not need to have the restriction valve  125 . 
     Also with the preferred embodiments, a case where the vent holes  124  are positioned at the uppermost portion of the cooling water passage  113  was described as a non-limiting example. However, the vent holes  124  may be disposed in a portion of the cooling water passage  113  other than the uppermost portion. 
     Also with the preferred embodiments, a case where, in the direction of flow of the cooling water, the vent holes  124  are disposed between the two exhaust manifolds (the first exhaust manifold  53  and the second exhaust manifold  54 ) and the catalyst  74  was described as a non-limiting example. However, the vent holes  124  may be disposed further upstream than the two exhaust manifolds or may be disposed further downstream than the catalyst  74 . 
     Also with the preferred embodiments, a case where the catalytic unit  42  is provided in the exhaust device  37  of the engine  9  was described as a non-limiting example. However, the engine  9  does not need to have the catalytic unit  42 . In this case, the engine  9  may include an exhaust pipe  641 , shown in  FIG. 28  and  FIG. 29 , in place of the exhaust pipe  41 . 
     As shown in  FIG. 29 , the exhaust pipe  641  includes a first exhaust manifold  653  and a second exhaust manifold  654 . The first collecting pipe  56  of the first exhaust manifold  653  extends from the respective first branch pipes  55  to the first relay pipe  59  and is connected to the first relay pipe  59 . Similarly, the second collecting pipe  58  of the second exhaust manifold  654  extends from the respective branch pipes  57  to the second relay pipe  60 . The exhaust discharged from a plurality of combustion chambers  30  into the first exhaust chamber  653  thus flows from the first exhaust manifold  653  to the first relay pipe  59  and returns from the first relay pipe  59  to a cylinder head  28 . Similarly, the exhaust discharged from a plurality of combustion chambers  30  into the second exhaust manifold  654  flows from the second exhaust manifold  654  to the second relay pipe  60  and returns from the second relay pipe  60  to a cylinder head  28 . Therefore, even if the catalytic unit  42  is omitted, the exhaust discharged into the first exhaust manifold  653  and the second exhaust manifold  654  is returned to the cylinder heads  28 . 
     Also with the preferred embodiments, a case where the vessel propulsion apparatus  2  includes the outboard motor  4  was described as a non-limiting example. However, the vessel propulsion apparatus  2  may instead be an inboard/outboard motor or an inboard motor. 
     The present application corresponds to Japanese Patent Application Nos. 2013-035066 and 2013-035067 filed on Feb. 25, 2013 in the Japan Patent Office, and the entire disclosures of these applications are incorporated herein by reference. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.