Patent Publication Number: US-2023150635-A1

Title: Vessel propulsion apparatus, vessel, vessel engine, and exhaust structure of vessel engine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2021-185987 filed on Nov. 15, 2021. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vessel propulsion apparatus, a vessel, a vessel engine, and an exhaust structure of a vessel engine. 
     2. Description of the Related Art 
     U.S. 2014/0242859 A1 discloses an outboard motor that is an example of a vessel propulsion apparatus. An engine for use in the outboard motor includes a plurality of cylinders, a cylinder head attached to the cylinders, and an exhaust pipe. The cylinder head includes a plurality of combustion chambers respectively corresponding to the plurality of cylinders, and a pair of exhaust ports connected to each of the combustion chambers. The exhaust pipe is connected to each of the exhaust ports. The exhaust pipe guides exhaust gases discharged from each of the combustion chambers through the exhaust port. 
     SUMMARY OF THE INVENTION 
     In a vessel propulsion apparatus, such as the outboard motor disclosed in U.S. 2014/0242859 A1, there is a case in which the exhaust port and the exhaust pipe are integral with each other in order to make the engine compact. In this case, there is a possibility that exhaust pressure loss will increase because of a structure in which exhaust gases that have flowed out from each of the combustion chambers to each of the exhaust ports immediately reach the exhaust pipe. An increase in exhaust pressure loss is a factor that lowers engine performance, and therefore it is preferable to achieve a decrease in exhaust pressure loss. 
     Preferred embodiments of the present invention provide vessel propulsion apparatuses each able to achieve a decrease in exhaust pressure loss of an engine, vessels including the vessel propulsion apparatuses, vessel engines included in the vessel propulsion apparatuses, and exhaust structures of the vessel engines. 
     The inventor of preferred embodiments of the present invention described and claimed in the present application conducted an extensive study and research regarding a vessel propulsion apparatus, a vessel, a vessel engine, and an exhaust structure of a vessel engine, such as the one described above, and in doing so, discovered and first recognized new unique challenges and previously unrecognized possibilities for improvements as described in greater detail below. 
     In order to overcome the previously unrecognized and unsolved challenges described above, a preferred embodiment of the present invention provides a vessel propulsion apparatus including an engine and a propulsion unit to be driven by the engine. The engine includes a plurality of cylinders arranged in series and each including a combustion chamber, a plurality of exhaust ports respectively connected to the combustion chambers of the plurality of cylinders and curved in a predetermined downstream direction to allow exhaust gases to flow out from the combustion chambers, and a collecting exhaust pipe. The collecting exhaust pipe is integral with the plurality of exhaust ports and extends in the downstream direction to allow exhaust gases in the exhaust ports to flow in the downstream direction. At least one of a plurality of connector portions that individually connect the plurality of exhaust ports to the collecting exhaust pipe includes a concave portion at an inner surface thereof. 
     With this structural arrangement, in the vessel propulsion apparatus, the propulsion unit is driven when the engine generates a driving force, and therefore the vessel propulsion apparatus generates a thrust. In the engine, the plurality of exhaust ports respectively connected to the combustion chambers of the plurality of cylinders arranged in series and the collecting exhaust pipe are integral with each other, thus making it possible to make the engine compact. The exhaust ports allow exhaust gases discharged from the corresponding combustion chambers to flow in the same direction, i.e., in a downstream direction, and the collecting exhaust pipe allows exhaust gases in each of the exhaust ports to continuously flow in the downstream direction. The concave portion provided at the inner surface in the at least one of the plurality of connector portions that individually connect the exhaust ports to the collecting exhaust pipe allows exhaust gases in the at least one of the plurality of connector portions to flow in the downstream direction, thus enabling the exhaust gases from each of the exhaust ports to smoothly flow toward the collecting exhaust pipe in the downstream direction. This makes it possible to achieve a decrease in exhaust pressure loss of the engine. 
     In a preferred embodiment of the present invention, each of the plurality of exhaust ports includes an upstream portion connected to the combustion chamber and a downstream portion that is curved from the upstream portion in the downstream direction and that is connected to the collecting exhaust pipe. The concave portion includes a first concave portion located at an inner surface of the downstream portion. 
     With this structural arrangement, the first concave portion provided at the inner surface of the downstream portion allows exhaust gases in the downstream portion to flow in the downstream direction, and therefore it is possible to allow exhaust gases from each of the exhaust ports to smoothly flow toward the collecting exhaust pipe in the downstream direction. 
     In a preferred embodiment of the present invention, the first concave portion is located in a region of the inner surface of the downstream portion that is more distant from a center of curvature of the downstream portion than a central axis of the downstream portion. 
     With this structural arrangement, the first concave portion allows exhaust gases in the downstream portion of the exhaust port to effectively flow in the downstream direction, and therefore it is possible to allow exhaust gases from each of the exhaust ports to flow toward the collecting exhaust pipe in the downstream direction more smoothly. 
     In a preferred embodiment of the present invention, the exhaust port includes a pair of exhaust ports for each of the cylinders. The pair of exhaust ports include an upstream exhaust port and a downstream exhaust port located at a more downward location in the downstream direction than the upstream exhaust port. The downstream portion of the upstream exhaust port and the downstream portion of the downstream exhaust port are merged together. The first concave portion is located in a region of an inner surface of the downstream portion of the upstream exhaust port that is more distant from a center of curvature of the downstream portion of the upstream exhaust port than a central axis of the downstream portion of the upstream exhaust port. 
     With this structural arrangement, the first concave portion allows exhaust gases in the merging portion of the downstream portion of the upstream exhaust port and the downstream portion of the downstream exhaust port to effectively flow in the downstream direction, and therefore it is possible to allow exhaust gases from each of the exhaust ports to flow toward the collecting exhaust pipe in the downstream direction more smoothly. 
     In a preferred embodiment of the present invention, the plurality of cylinders include a first cylinder located at a top in a direction opposite to the downstream direction and a second cylinder adjacent to the first cylinder. The first concave portion located at the exhaust port connected to the combustion chamber of the second cylinder is larger than the first concave portion located at the exhaust port connected to the combustion chamber of any cylinder other than the second cylinder of the plurality of cylinders. 
     With this structural arrangement, the first concave portion provided at the exhaust port connected to the combustion chamber of the second cylinder allows exhaust gases in the downstream portion of this exhaust port to effectively flow in the downstream direction so as not to flow toward the exhaust port side connected to the combustion chamber of the first cylinder, i.e., so as not to flow in the opposite direction. Therefore, it is possible to allow exhaust gases from the exhaust port connected to the combustion chamber of the second cylinder to flow toward the collecting exhaust pipe in the downstream direction more smoothly. 
     In a preferred embodiment of the present invention, each of the plurality of exhaust ports includes an upstream portion connected to the combustion chamber and a downstream portion that is curved from the upstream portion in the downstream direction and connected to the collecting exhaust pipe. The concave portion includes a second concave portion located in a region of an inner surface of the collecting exhaust pipe that is adjacent to the downstream portion. 
     With this structural arrangement, the second concave portion provided in a region of the inner surface of the collecting exhaust pipe that is adjacent to the downstream portion of each of the exhaust ports allows exhaust gases in this region to effectively flow in the downstream direction so as not to flow toward the exhaust port located farther downstream than this region. Therefore, exhaust gases that have flowed into the collecting exhaust pipe from each of the exhaust ports are allowed to continuously flow to the collecting exhaust pipe, thus enabling the exhaust gases to smoothly flow in the downstream direction. 
     In a preferred embodiment of the present invention, the second concave portion is located in a region of the inner surface of the collecting exhaust pipe that is closer to the exhaust port than a central axis of the collecting exhaust pipe. 
     With this structural arrangement, the second concave portion allows exhaust gases existing in a region of the inner surface of the collecting exhaust pipe that is adjacent to the downstream portion of each of the exhaust ports to flow in the downstream direction more effectively so as not to flow toward the exhaust port located farther downstream than this region. Therefore, exhaust gases that have flowed into the collecting exhaust pipe from each of the exhaust ports are allowed to continuously flow to the collecting exhaust pipe, thus enabling the exhaust gases to flow in the downstream direction more smoothly. 
     In a preferred embodiment of the present invention, the plurality of cylinders include a first cylinder located at a top in a direction opposite to the downstream direction of any other cylinder. The second concave portion, located in a region of the inner surface of the collecting exhaust pipe that is adjacent to the downstream portion of the exhaust port connected to the combustion chamber of the first cylinder, is larger than the second concave portion located in a region adjacent to the downstream portion of an exhaust port connected to the combustion chamber of each of the other cylinders. 
     With this structural arrangement, the second concave portion provided in a region of the inner surface of the collecting exhaust pipe that is adjacent to the downstream portion of the exhaust port connected to the combustion chamber of the first cylinder allows exhaust gases in this region to effectively flow in the downstream direction so as not to backwardly flow to the exhaust port of the first cylinder. Therefore, exhaust gases that have flowed into the collecting exhaust pipe from the exhaust port of the first cylinder are allowed to continuously flow to the collecting exhaust pipe, thus enabling the exhaust gases to smoothly flow in the downstream direction. 
     A preferred embodiment of the present invention provides a vessel propulsion apparatus that includes an engine and a propulsion unit to be driven by the engine. The engine includes a plurality of cylinders arranged in series and each including a combustion chamber, a plurality of exhaust ports respectively connected to the combustion chambers of the plurality of cylinders and curved in a predetermined downstream direction to allow exhaust gases to flow out from the combustion chambers, and a collecting exhaust pipe. The collecting exhaust pipe is integral with the plurality of exhaust ports and extends in the downstream direction to allow exhaust gases in the exhaust ports to flow in the downstream direction. At least one of a plurality of connector portions that individually connect the plurality of exhaust ports to the collecting exhaust pipe includes a guide at an inner surface thereof to direct exhaust gases in the at least one of the plurality of connector portions in the downstream direction. 
     With this structural arrangement, in the vessel propulsion apparatus, the propulsion unit is driven when the engine generates a driving force, and therefore the vessel propulsion apparatus generates a thrust. In the engine, the plurality of exhaust ports respectively connected to the combustion chambers of the plurality of cylinders arranged in series and the collecting exhaust pipe are integral with each other, thus making it possible to make the engine compact. The exhaust ports allow exhaust gases discharged from the corresponding combustion chambers to flow in the same direction, i.e., in a downstream direction, and the collecting exhaust pipe allows exhaust gases in each of the exhaust ports to continuously flow in the downstream direction. The guide provided at the inner surface in the at least one of the plurality of connector portions that individually connect the exhaust ports to the collecting exhaust pipe allows exhaust gases in the at least one of the plurality of connector portions to flow in the downstream direction, thus enabling the exhaust gases from each of the exhaust ports to smoothly flow toward the collecting exhaust pipe in the downstream direction. This makes it possible to achieve a decrease in exhaust pressure loss of the engine. 
     In a preferred embodiment of the present invention, the engine includes a crankshaft extending along a vertical direction. The vessel propulsion apparatus includes an outboard motor including a drive shaft, a propeller shaft, a propeller, and a transmission. The drive shaft is connected to the crankshaft and extends along the vertical direction. The propeller shaft extends along a horizontal direction. The propeller functions as the propulsion unit, and is connected to the propeller shaft. The transmission is configured to transmit rotation of the drive shaft to the propeller shaft. 
     With this structural arrangement, in the outboard motor, the rotation of the crankshaft of the engine is transmitted to the propeller shaft through the drive shaft and the transmission, and, as a result, the propeller shaft rotates together with the propeller, and therefore the propeller generates a thrust. In the thus provided outboard motor, it is possible to achieve a decrease in exhaust pressure loss of the engine as described above. 
     A preferred embodiment of the present invention provides a vessel including a hull and the vessel propulsion apparatus mounted to the hull to provide a thrust to the hull. 
     With this structural arrangement, in the vessel propulsion apparatus included in the vessel, it is possible to achieve a decrease in exhaust pressure loss of the engine as described above. 
     A preferred embodiment of the present invention provides a vessel engine including a plurality of cylinders arranged in series and each including a combustion chamber, a plurality of exhaust ports respectively connected to the combustion chambers of the plurality of cylinders and curved in a predetermined downstream direction to allow exhaust gases to flow out from the combustion chambers, and a collecting exhaust pipe. The collecting exhaust pipe is integral with the plurality of exhaust ports and extends in the downstream direction to allow exhaust gases in the plurality of exhaust ports to flow in the downstream direction. At least one of a plurality of connector portions that individually connect the plurality of exhaust ports to the collecting exhaust pipe includes a concave portion at an inner surface thereof. 
     With this structural arrangement, in the vessel engine, it is possible to achieve a decrease in exhaust pressure loss as described above. 
     A preferred embodiment of the present invention provides an exhaust structure of a vessel engine including a plurality of cylinders arranged in series and each including a combustion chamber, a plurality of exhaust ports respectively connected to the combustion chambers of the plurality of cylinders and curved in a predetermined downstream direction to allow exhaust gases to flow out from the combustion chambers, and a collecting exhaust pipe. The collecting exhaust pipe is integral with the plurality of exhaust ports and extends in the downstream direction to allow exhaust gases in the exhaust ports to flow in the downstream direction. At least one of a plurality of connector portions that individually connect the plurality of exhaust ports to the collecting exhaust pipe includes a concave portion at an inner surface thereof. 
     With this structural arrangement, in the vessel engine, it is possible to achieve a decrease in exhaust pressure loss as described above. 
     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 plan view of a vessel according to a preferred embodiment of the present invention. 
         FIG.  2    is a schematic side view of an outboard motor included in the vessel. 
         FIG.  3    is a schematic view shown to describe an air intake/exhaust system of the outboard motor. 
         FIG.  4    is a rear view of a main portion of an exhaust structure in the air intake/exhaust system. 
         FIG.  5    is a plan view of a main portion of the exhaust structure. 
         FIG.  6    is a cross-sectional view taken along line A-A in  FIG.  5   . 
         FIG.  7    is an enlarged view of a portion of  FIG.  6   . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG.  1    is a schematic plan view of a vessel  1  according to a preferred embodiment of the present invention. The vessel  1  includes a hull  2 , a vessel operation device  3 , and an outboard motor  4 , and the hull  2  is equipped with both the vessel operation device  3  and the outboard motor  4 . An example of the vessel operation device  3  includes a steering wheel  6  and a throttle lever  7  both of which are provided at an operational platform  5  around a vessel operation seat of the hull  2 , and a communication bus  9  by which an ECU (electronic control unit)  8  built into the outboard motor  4 , a steering wheel  6 , and the throttle lever  7  are connected together. A vessel operator turns the steering wheel  6  in a left-right direction to steer. The vessel operator turns the throttle lever  7  in a front-rear direction to adjust the output of the outboard motor  4 . A joystick  10  that is operated by the vessel operator to steer and adjust the output of the outboard motor  4  may be provided at the operational platform  5 . 
     The outboard motor  4  is an example of a vessel propulsion apparatus that provides a thrust to the hull  2 , and is provided as a single outboard motor or as a plurality of outboard motors. The single outboard motor  4  is attached to a transom stern  2 A on a virtual center line C along the front-rear direction through the transom stern  2 A and a bow  2 B of the hull  2 . The plurality of outboard motors  4  are attached to the transom stern  2 A at bilaterally symmetrical positions with respect to the center line C. 
       FIG.  2    is a schematic right side view of the outboard motor  4 . The left side in  FIG.  2    is the front side of the outboard motor  4 , and the right side in  FIG.  2    is the rear side of the outboard motor  4 . The upper side in  FIG.  2    is the upper side of the outboard motor  4 , and the lower side in  FIG.  2    is the lower side of the outboard motor  4 . An up-down direction is also a vertical direction. A direction perpendicular to the plane of paper of  FIG.  2    is the left-right direction of the outboard motor  4 . In the following description, a leftward or rightward direction of the outboard motor  4  is determined based on a direction given when the outboard motor  4  is seen from the front side. Therefore, the near side in the direction perpendicular to the plane of paper of  FIG.  2    is the right side of the outboard motor  4 , and the far side in the direction perpendicular to the plane of paper of  FIG.  2    is the left side of the outboard motor  4 . 
     The outboard motor  4  includes a mount  11  to attach the outboard motor  4  to the transom stern  2 A and an outboard motor main body  12 . The mount  11  includes a clamp bracket  13  fixed to the transom stern  2 A and a swivel bracket  15  coupled to the clamp bracket  13  through a tilt shaft  14  horizontally extending in the left-right direction. The swivel bracket  15  is coupled to the outboard motor main body  12  through a steering shaft  16  extending in the up-down direction. Thus, the outboard motor main body  12  is attached to the transom stern  2 A by the mount  11  in a vertical or substantial vertical attitude. 
     The outboard motor main body  12  and the swivel bracket  15  are turnable in the up-down direction around the tilt shaft  14  with respect to the clamp bracket  13 . The outboard motor main body  12  is turned around the tilt shaft  14 , and, as a result, the outboard motor main body  12  is tilted with respect to the hull  2  and the clamp bracket  13 . The outboard motor main body  12  is turnable in the left-right direction together with the steering shaft  16  with respect to the clamp bracket  13  and the swivel bracket  15 . When the outboard motor main body  12  turns in the left-right direction, the vessel  1  is steered. 
     The outboard motor main body  12  includes a box-shaped engine cover  17 , a hollow casing  18  extending downwardly from the engine cover  17 , and a plate-shaped exhaust guide  19  attached to a lower end portion of the engine cover  17  so as to close an internal space of the casing  18  from above. A lower end portion of the casing  18  is a lower case  18 A. The outboard motor main body  12  includes an engine  20  mounted on an upper surface of the exhaust guide  19  in the engine cover  17 , a drive shaft  21  extending along the up-down direction in the casing  18 , and a propeller shaft  22  and a transmission  23  both of which are located in the lower case  18 A. 
     The engine  20  is a vessel engine, and includes an internal combustion engine that burns fuel, such as gasoline, and generates power. The engine  20  includes a cylinder block  25  including a single or a plurality of cylinders  24 , a piston  26  located in the cylinder  24  one by one, and a crankshaft  27  extending along the up-down direction in the cylinder block  25  and that is coupled to the piston(s)  26 . The engine  20  in the present preferred embodiment is a straight-type four-cylinder engine in which four cylinders  24  are arranged in series along the up-down direction. 
     An internal space of each of the cylinders  24  includes a circular cylindrical shape extending along the front-rear direction. A combustion chamber  28  is defined in a region behind the piston  26  in the internal space of each of the cylinders  24 . A front portion that houses the crankshaft  27  in the cylinder block  25  is a crank case  25 A. 
     The engine  20  includes a cylinder head  29  attached to the cylinder block  25  from behind and a head cover  30  attached to the cylinder head  29  from behind. The cylinder head  29  and the head cover  30  may be regarded as elements of the cylinder block  25 . Concave portions  29 A each of which is rearwardly hollowed as a portion of the combustion chamber  28  are provided one by one at a portion, which faces the combustion chamber  28  of each of the cylinders  24 , of a front surface of the cylinder head  29 . The engine  20  includes an intake valve  31  and an exhaust valve  32  that are exposed to each of the concave portions  29 A and a camshaft  33  that extends along the up-down direction and that is rotatably supported by the head cover  30 . The camshaft  33  may be provided as a pair of camshafts in accordance with each of the intake valve  31  and the exhaust valve  32 . 
     The crankshaft  27  has a crankshaft axis  27 A extending in the up-down direction. An upper end portion of the crankshaft  27  protrudes upwardly from the crank case  25 A. A lower end portion of the crankshaft  27  is connected to the upper end portion of the drive shaft  21 . The engine  20  includes a flywheel magneto  34  fixed to the upper end portion of the crankshaft  27  and a cam chain  35  connecting the lower end portion of the crankshaft  27  and a lower end portion of the camshaft  33 . The flywheel magneto  34  is located at a higher position than the crank case  25 A. The cam chain  35  is located below the four cylinders  24  in the cylinder block  25 . 
     The piston  26  is rectilinearly reciprocated in the front-rear direction perpendicular to the crankshaft axis  27 A by combustion of an air-fuel mixture in each of the combustion chambers  28 . When the piston  26  is rectilinearly reciprocated, the crankshaft  27  is driven and rotated around the crankshaft axis  27 A along with the drive shaft  21 . In accordance with the rotation of the crankshaft  27 , the flywheel magneto  34  rotates and generates electricity, and the cam chain  35  moves in a circular motion. The camshaft  33  is rotated in accordance with the circular movement of the cam chain  35 . The intake valve  31  and the exhaust valve  32  are actuated interlockingly with the rotation of the camshaft  33 . Thus, intake/exhaust is performed in each of the combustion chambers  28 . 
     The propeller shaft  22  horizontally extends along the front-rear direction in the lower case  18 A. A lower end portion of the drive shaft  21  is coupled to a front end portion of the propeller shaft  22  by the transmission  23 . A rear end portion of the propeller shaft  22  protrudes rearwardly from the lower case  18 A. A propeller  36  as an example of a propulsion unit that is an element of the outboard motor  4  is connected to the rear end portion of the propeller shaft  22 . The propeller shaft  22  rotates together with the propeller  36  around a rotational axis  22 A that extends in the front-rear direction. 
     The transmission  23  is used to transmit the rotation of the drive shaft  21  to the propeller shaft  22 . The transmission  23  includes a driving gear  38  fixed to the lower end portion of the drive shaft  21  and a rotary body  39  and a dog clutch  40  both of which are attached to the front end portion of the propeller shaft  22 . The driving gear  38  is a bevel gear. The propeller shaft  22  is located below the driving gear  38 . The rotary body  39  includes a first rotary body  41  and a second rotary body  42  that are located side by side in the front-rear direction along the propeller shaft  22 . The first rotary body  41  and the second rotary body  42  are, for example, cylindrical bevel gears, respectively. 
     In the present preferred embodiment, the first rotary body  41  is located at a more forward position than the driving gear  38 , and the second rotary body  42  is located at a more rearward position than the driving gear  38 , and yet the front-rear positional relationship between the first rotary body  41  and the second rotary body  42  may be opposite to that of the present preferred embodiment. In a rear surface of the first rotary body  41 , a tooth portion  41 A is provided at a tapered outer peripheral portion, and a claw portion  41 B is provided at an inner peripheral portion. In a front surface of the second rotary body  42 , a tooth portion  42 A is provided at a tapered outer peripheral portion, and a claw portion  42 B is provided at an inner peripheral portion. 
     The first rotary body  41  surrounds a portion, which is at a more forward position than the driving gear  38 , of the front end portion of the propeller shaft  22 , and the second rotary body  42  surrounds a portion, which is at a more rearward position than the driving gear  38 , of the front end portion of the propeller shaft  22 . The first rotary body  41  and the second rotary body  42  are located so that their tooth portions  41 A and  42 A face each other at a distance from each other in the front-rear direction, and engage with the driving gear  38 . When the driving gear  38  rotates together with the drive shaft  21  in response to the driving of the engine  20 , the rotation of the driving gear  38  is transmitted to the first rotary body  41  and to the second rotary body  42 . Thus, the first rotary body  41  and the second rotary body  42  rotate around the rotational axis  22 A of the propeller shaft  22  in mutually opposite directions. 
     The dog clutch  40  is located between the first rotary body  41  and the second rotary body  42 . The dog clutch  40  is, for example, cylindrical, and surrounds the front end portion of the propeller shaft  22 . A first claw portion  40 A is provided at a front end surface of the dog clutch  40 , and a second claw portion  40 B is provided at a rear end surface of the dog clutch  40 . The dog clutch  40  is coupled to the front end portion of the propeller shaft  22  by, for example, a spline. Therefore, the dog clutch  40  rotates together with the front end portion of the propeller shaft  22 . Additionally, the dog clutch  40  is movable in the front-rear direction with respect to the front end portion of the propeller shaft  22 . In other words, the dog clutch  40  is rotatable together with the propeller shaft  22 , and is movable along the front-rear direction relatively with the propeller shaft  22 . 
     The transmission  23  also includes a shifter  43  located at a more forward position than the propeller shaft  22  in the lower case  18 A. The shifter  43  includes, for example, a shift rod  44  extending in the up-down direction and an electric shift actuator  45  connected to the shift rod  44 . A lower end portion of the shift rod  44  is coupled to the dog clutch  40 . When the shift actuator  45  is operated by the control of the ECU  8  (see  FIG.  1   ), the shift rod  44  turns around an axis of the shift rod  44 . The shift rod  44  turns, and, as a result, the dog clutch  40  is moved along the front-rear direction between a disconnection position and a connection position. 
     The disconnection position is a position in which the dog clutch  40  is spaced apart from the first rotary body  41  and the second rotary body  42 , and does not engage with either of these rotary bodies of the rotary body  39  as shown in  FIG.  2   . In a state in which the dog clutch  40  is located in the disconnection position, each of the rotary body  39  to which the rotation of the drive shaft  21  is transmitted runs idle, and therefore the rotation of the drive shaft  21  is not transmitted to the propeller shaft  22 . In the following description, the shift position of the outboard motor  4  at this time is referred to as “neutral.” 
     The connection position is a position in which the dog clutch  40  engages with either one of the first rotary body  41  or the second rotary body  42 . The connection position includes a first connection position in which the first claw portion  40 A of the dog clutch  40  engages with only the claw portion  41 B of the first rotary body  41  and a second connection position in which the second claw portion  40 B of the dog clutch  40  engages with only the claw portion  42 B of the second rotary body  42 . The disconnection position is a position between the first connection position and the second connection position. The first connection position is more forward than the disconnection position, and the second connection position is more rearward than the disconnection position. 
     In a state in which the dog clutch  40  is located in the first connection position and is coupled to only the first rotary body  41 , the rotation of the first rotary body  41  is transmitted to the propeller shaft  22 , and therefore the shift position of the outboard motor  4  is shifted into “forward.” Thereupon, the rotation of the drive shaft  21  is transmitted to the propeller shaft  22  through the first rotary body  41  and the dog clutch  40 , and, as a result, the propeller  36  rotates in a forward rotational direction (for example, a clockwise direction when seen from the rear side). Thus, the propeller  36  is driven by the engine  20 , and a forward thrust is generated. 
     In a state in which the dog clutch  40  is located in the second connection position and is coupled to only the second rotary body  42 , the rotation of the second rotary body  42  is transmitted to the propeller shaft  22 , and therefore the shift position of the outboard motor  4  is shifted into “reverse.” Thereupon, the rotation of the drive shaft  21  is transmitted to the propeller shaft  22  through the second rotary body  42  and the dog clutch  40 , and, as a result, the propeller  36  rotates in a reverse rotational direction opposite to the forward rotational direction. Thus, the propeller  36  is driven by the engine  20 , and a reverse thrust is generated. As thus described, in the present preferred embodiment, the first rotary body  41  is a gear for forward movement, and the second rotary body  42  is a gear for reverse movement. Of course, the first rotary body  41  may be a gear for reverse movement, and the second rotary body  42  may be a gear for forward movement. 
     The outboard motor main body  12  includes an exhaust passage  46  provided inside the outboard motor main body  12  and connected to the engine  20 . The exhaust passage  46  passes through the exhaust guide  19  in the up-down direction, and extends downwardly in the casing  18  and rearwardly in the propeller  36 . The exhaust passage  46  includes an outlet  46 A provided at a rear end surface of the propeller  36 . In a state in which the vessel  1  is floating on water and in which the propeller  36  is located below a water surface, the outlet  46 A is located in the water, and therefore water that has passed through the outlet  46 A enters a downstream portion of the exhaust passage  46 . On the other hand, when the engine  20  rotates at a high speed, water in the exhaust passage  46  is pushed by the pressure of an exhaust gas emitted from the engine  20 , and is discharged from the outlet  46 A together with the exhaust gas. Thus, the exhaust gas generated by the engine  20  is discharged into the water. 
     A steering rod  47  that forwardly extends is fixed to the outboard motor main body  12 . An electric steering actuator  48  that is controlled by the ECU  8  is connected to the steering rod  47 . The outboard motor main body  12  is able to turn around the steering shaft  16  by allowing the steering actuator  48  to operate, thus making it possible to perform steering. 
       FIG.  3    is a schematic view shown to describe an air intake/exhaust system  49  of the outboard motor  4 . The air intake/exhaust system  49  includes the engine  20 , a pressure charger  50  that compresses air and supplies the air to the engine  20 , and an intercooler  51  that cools air compressed by the pressure charger  50 . 
     With respect to the air intake/exhaust system  49 , the engine  20  includes the exhaust passage  46 , an air intake passage  52 , and an electric throttle valve  53  located in the air intake passage  52 . The exhaust passage  46  is connected to each of the combustion chambers  28  through a plurality of exhaust ports  54  provided in the cylinder head  29  of the engine  20 . The air intake passage  52  is connected to each of the combustion chambers  28  through a plurality of intake ports  55  provided in the cylinder head  29 . An inlet  52 A is provided at an end portion, which is opposite to the intake port  55 , of the air intake passage  52 . The ECU  8  controls the throttle valve  53 , and, as a result, the opening degree of the throttle valve  53  is adjusted. 
     The pressure charger  50  is interposed between the ends of the air intake passage  52 . The pressure charger  50  is a supercharger driven by the rotation of the crankshaft  27  of the engine  20 . The pressure charger  50  includes a housing  50 A including an internal space defining a portion of the air intake passage  52 , a compressor wheel  50 B located in the housing  50 A, and a rotational shaft  50 C coaxially fixed to the compressor wheel  50 B. An end portion, which is spaced apart from the compressor wheel  50 B, of the rotational shaft  50 C is located outside the housing  50 A, and a rotor  56  is coaxially fixed to this end portion. 
     The air intake/exhaust system  49  includes a power transmission by which the crankshaft  27  and the pressure charger  50  are connected together. An example of the power transmission includes the rotor  56 , another rotor  57  attached to the crankshaft  27 , and a belt  58  by which the rotor  56  and the rotor  57  are connected together. An example of each of the rotors  56  and  57  is a pulley. The rotor  57  is attached to a portion, which is located at a higher position than the flywheel magneto  34 , of the upper end portion of the crankshaft  27  (see  FIG.  2   ). 
     When the crankshaft  27  rotates, the rotor  57  rotates together with the crankshaft  27 . The rotation of the rotor  57  is transmitted to the rotor  56  through the belt  58 . Thereupon, the rotational shaft  50 C rotates together with the compressor wheel  50 B, and, as a result, the pressure charger  50  is driven. A sprocket may be used as each of the rotors  56  and  57  instead of the pulley, and a chain may be used instead of the belt  58 . 
     When the pressure charger  50  operates in a state in which the throttle valve  53  has been opened, air that has been taken from the inlet  52 A and that flows through the air intake passage  52  is compressed by the compressor wheel  50 B rotating in the housing  50 A. Another arrangement, such as a Lysholm-type device, may be used as the pressure charger  50  without being limited to the centrifugal-type device shown in  FIG.  3   . 
     The intercooler  51  is interposed between each of the intake ports  55  of the engine  20  and the pressure charger  50  in the air intake passage  52 . The intercooler  51  includes a housing  51 A including an internal space defining a portion of the air intake passage  52  and a cooling fin (not shown). Either of an air-cooled intercooler or a water-cooled intercooler may be used as the intercooler  51 . The intercooler  51  includes an intake manifold  51 B that extends from the housing  51 A and is connected to the intake port  55 . The intake manifold  51 B is integral with the housing  51 A. 
     Air compressed by the compressor wheel  50 B in the housing  50 A of the pressure charger  50  continuously flows through the air intake passage  52 , and thus is guided to the intercooler  51 , and is cooled by heat exchange with the cooling fin in the housing  51 A of the intercooler  51 . The air cooled by the intercooler  51  flows through the intake manifold  51 B, and then is turned into an air-fuel mixture, supplied from the intake port  55  to the combustion chamber  28  in the cylinder  24 , and combusted. Exhaust gas generated by the combustion flows from the exhaust port  54  through the exhaust passage  46 , and then is discharged from the outlet  46 A into the water as described above. 
       FIG.  4    is a rear view of a main portion of an exhaust structure  70  included in the air intake/exhaust system  49  of the engine  20 . The exhaust structure  70  includes the plurality of exhaust ports  54  and a collecting exhaust pipe  71  located on the left side of the exhaust ports  54 . The cylinder head  29  may be provided as a single cylinder head so as to straddle between the plurality of (in the present preferred embodiment, four) cylinders  24  arranged side by side in the up-down direction, and the concave portions  29 A (a portion of the combustion chamber  28 ) whose number is equal to that of the cylinders  24  may be provided at the cylinder head  29 . Alternatively, the cylinder head  29  may be provided as a plurality of cylinder heads so as to respectively correspond to the cylinders  24  on a one-to-one basis as shown in  FIG.  4   . In this case, the plurality of cylinder heads  29  are arranged side by side in the up-down direction, and the concave portions  29 A are respectively provided at the cylinder heads  29  one by one. 
     In the following description, the uppermost cylinder  24  among the four cylinders  24  in the up-down direction will be referred to as a first cylinder  24 A if necessary, and the cylinder  24  downwardly of and next to the first cylinder  24 A will be referred to as a second cylinder  24 B if necessary. The cylinder  24  downwardly of and next to the second cylinder  24 B will be referred to as a third cylinder  24 C if necessary, and the cylinder  24  downwardly of and next to the third cylinder  24 C, i.e., the lowermost cylinder  24  will be referred to as a fourth cylinder  24 D if necessary. In the present preferred embodiment, a downward direction Z 1  is an example of a predetermined downstream direction, and the first cylinder  24 A is located at the head or top in a direction opposite to the downstream direction (i.e., upward direction Z 2 ). 
     A pair of intake openings  72  and a pair of exhaust openings  73  are provided in a region of a rear surface of the cylinder head  29  that coincides with the combustion chamber  28  in a rear view. The intake opening  72  and the exhaust opening  73  are each a round hole that extends through the cylinder head  29  in the front-rear direction or substantially in the front-rear direction. A single intake valve  31  is located at a single intake opening  72 , and a single exhaust valve  32  is located at a single exhaust opening  73  (not shown). 
     For the intake opening  72  and the exhaust opening  73  that correspond to a single combustion chamber  28 , the pair of exhaust openings  73  arranged side by side along the up-down direction are located at a more leftward location than the pair of intake openings  72  arranged side by side along the up-down direction. The pair of intake openings  72  and the pair of exhaust openings  73  communicate with the combustion chamber  28  from the rear. The intake port  55  is provided as a plurality of (in the present preferred embodiment, eight) intake ports whose number is equal to that of the intake openings  72 , and the intake ports  55  are each a circular tubular pipe passage, and the single intake port  55  is connected to the single intake opening  72  (not shown). 
     The exhaust port  54  is a circular tubular pipe passage, and is provided as a pair of exhaust ports for each cylinder  24 , i.e., for each combustion chamber  28 , and eight exhaust ports  54  in total are provided in the present preferred embodiment. Each of the exhaust ports  54  is connected to a single exhaust opening  73 . Thus, the plurality of exhaust ports  54  are connected to the plurality of combustion chambers  28 , respectively. 
     In the following description, the pair of exhaust ports  54  connected to the combustion chamber  28  of the first cylinder  24 A will be referred to as a pair of first exhaust ports  54 A if necessary, and the pair of exhaust ports  54  connected to the combustion chamber  28  of the second cylinder  24 B will be referred to as a pair of second exhaust ports  54 B if necessary. The pair of exhaust ports  54  connected to the combustion chamber  28  of the third cylinder  24 C will be referred to as a pair of third exhaust ports  54 C if necessary, and the pair of exhaust ports  54  connected to the combustion chamber  28  of the fourth cylinder  24 D will be referred to as a pair of fourth exhaust ports  54 D if necessary. The pair of second exhaust ports  54 B are located downwardly of and next to the pair of first exhaust ports  54 A, and the pair of third exhaust ports  54 C are located downwardly of and next to the pair of second exhaust ports  54 B, and the pair of fourth exhaust ports  54 D are located downwardly of and next to the pair of third exhaust ports  54 C. 
     The pair of exhaust ports  54  connected to each of the combustion chambers  28  include an upstream exhaust port  54 E and a downstream exhaust port  54 F located at a more downward location in the downward direction Z 1  than the upstream exhaust port  54 E. Referring to the pair of first exhaust ports  54 A, each of the upstream exhaust port  54 E and the downstream exhaust port  54 F includes an upstream portion  54 G connected to the combustion chamber  28  and a downstream portion  54 H connected to the collecting exhaust pipe  71  while being curved in the downward direction Z 1  from the upstream portion  54 G. 
     The upstream portion  54 G is provided with a valve guide  74  that supports the exhaust valve  32  (see  FIG.  2   ). In  FIG.  4    and the figures subsequent to  FIG.  4   , the intake valve  31  and the exhaust valve  32  are omitted and are not shown. The downstream portion  54 H of the upstream exhaust port  54 E and the downstream portion  54 H of the downstream exhaust port  54 F are merged together. 
     The collecting exhaust pipe  71  extends in the downward direction Z 1  as an upstream portion of the exhaust passage  46 . The inner surface  71 A of the collecting exhaust pipe  71  has a cylindrical shape (for example, a circular cylindrical shape or a substantially circular cylindrical shape) extending in the up-down direction. The collecting exhaust pipe  71  is integral with each of the exhaust ports  54 . This makes it possible to make the engine  20  compact (particularly in the left-right direction in the present preferred embodiment). The downstream portions  54 H of the pair of first exhaust ports  54 A are connected to an upper end of the collecting exhaust pipe  71  from the upward direction Z 2 . The downstream portions  54 H of the pair of second exhaust ports  54 B are connected to the collecting exhaust pipe  71  from the right side. The downstream portions  54 H of the pair of third exhaust ports  54 C are connected to the collecting exhaust pipe  71  from the right side. The downstream portions  54 H of the pair of fourth exhaust ports  54 D are connected to the collecting exhaust pipe  71  from the right side. The downstream portion  54 H of each of the exhaust ports  54  and a portion of the collecting exhaust pipe  71  that is adjacent to the downstream portion  54 H define a connector portion between the exhaust port  54  and the collecting exhaust pipe  71 . 
       FIG.  5    is a plan view of a main portion of the exhaust structure  70 .  FIG.  6    is a cross-sectional view taken along line A-A in  FIG.  5   . A plurality of concave portions  75  are provided at an inner surface in the connector portion between the exhaust port  54  and the collecting exhaust pipe  71 . More specifically, the plurality of concave portions  75  include a first concave portion  76  provided at an inner surface  54 I of the downstream portion  54 H that is a root of the exhaust port  54  and a second concave portion  77  provided in a region of the inner surface  71 A of the collecting exhaust pipe  71  that is adjacent to the downstream portion  54 H in the downward direction Z 1 . Each of the first and second concave portions  76  and  77  may be a hemispherically hollowed concave portion, or may be a groove extending along a circumferential direction of the exhaust port  54  or along a circumferential direction of the collecting exhaust pipe  71 . The first concave portion  76  and the second concave portion  77  are each provided as three concave portions in the present preferred embodiment. 
     The three first concave portions  76  include a first concave portion  76 A provided at the second exhaust port  54 B, a first concave portion  76 B provided at the third exhaust port  54 C, and a first concave portion  76 C provided at the fourth exhaust port  54 D. The first concave portion  76  is not provided at the first exhaust port  54 A. 
     Each of the first concave portions  76 A,  76 B, and  76 C is provided in a region  54 J, which is more distant from a center of curvature Q of the downstream portion  54 H than a central axis P of the downstream portion  54 H, of the inner surface  54 I of the downstream portion  54 H of the corresponding upstream exhaust port  54 E (see  FIG.  7   ). The central axis P is a virtual line extending through the center of the inner surface  54 I of the downstream portion  54 H (i.e., the center of a flow-passage cross section of the downstream portion  54 H). The center of curvature Q of the downstream portion  54 H may be a center of curvature of the central axis P of the downstream portion  54 H. Regarding at least either one among a depth dimension R, an opening area, and a volume of the first concave portion  76  (see  FIG.  7   ), the first concave portion  76 A may be larger than the first concave portions  76 B and  76 C provided at the other exhaust ports  54 . The depth dimension R is about several millimeters, for example. 
     The second concave portion  77  includes a second concave portion  77 A adjacent to the pair of first exhaust ports  54 A, a second concave portion  77 B adjacent to the pair of second exhaust ports  54 B, and a second concave portion  77 C adjacent to the pair of third exhaust ports  54 C. The second concave portion  77  is not provided in a region located at a more downward location in the downward direction Z 1  than the pair of fourth exhaust ports  54 D. 
     The second concave portion  77 A is provided in a region  71 B of the inner surface  71 A of the collecting exhaust pipe  71  that is adjacent to the downstream portion  54 H of the downstream exhaust port  54 F of the pair of first exhaust ports  54 A in the downward direction Z 1 . The second concave portion  77 B is provided in a region  71 C of the inner surface  71 A that is adjacent to the downstream portion  54 H of the downstream exhaust port  54 F of the pair of second exhaust ports  54 B in the downward direction Z 1 . The second concave portion  77 C is provided in a region  71 D of the inner surface  71 A that is adjacent to the downstream portion  54 H of the downstream exhaust port  54 F of the pair of third exhaust ports  54 C in the downward direction Z 1 . 
     The regions  71 B,  71 C, and  71 D are regions of the inner surface  71 A of the collecting exhaust pipe  71  that are closer to each of the exhaust ports  54  than a central axis S of the collecting exhaust pipe  71 . The central axis S is a virtual line extending through the center of the inner surface  71 A of the collecting exhaust pipe  71  (i.e., the center of a flow-passage cross section of the collecting exhaust pipe  71 ). Regarding at least either one among a depth dimension T, an opening area, and a volume of the second concave portion  77  (see  FIG.  7   ), the second concave portion  77 A may be larger than the second concave portions  77 B and  77 C provided in the other regions  71 C and  71 D. The depth dimension T is about several millimeters, for example. 
     A convex portion  78  that protrudes between each of the concave portions  75  is provided at each outer surface of the exhaust port  54  and of the collecting exhaust pipe  71 . 
     The pair of first exhaust ports  54 A allow exhaust gases to flow out from the combustion chamber  28  of the first cylinder  24 A. The pair of second exhaust ports  54 B allow exhaust gases to flow out from the combustion chamber  28  of the second cylinder  24 B. The pair of third exhaust ports  54 C allow exhaust gases to flow out from the combustion chamber  28  of the third cylinder  24 C. The pair of fourth exhaust ports  54 D allow exhaust gases to flow out from the combustion chamber  28  of the fourth cylinder  24 D. The exhaust ports  54  allow exhaust gases discharged from the corresponding combustion chambers  28  to flow in the same direction, i.e., in a downstream direction (in the present preferred embodiment, in the downward direction Z 1 ), and the collecting exhaust pipe  71  allows exhaust gases in each of the exhaust ports  54  to continuously flow in the downward direction Z 1 . The collecting exhaust pipe  71  receives exhaust gases from each of the exhaust ports  54 , and allows the exhaust gases to flow in the downward direction Z 1 . 
     The concave portion  75  provided at a connector portion between each of the exhaust ports  54  and the collecting exhaust pipe  71  functions as a guide that allows exhaust gases at this connector portion to flow in the downward direction Z 1 . More specifically, each of the first concave portions  76  allows exhaust gases at a merging portion of the downstream portion  54 H of the upstream exhaust port  54 E and the downstream portion  54 H of the downstream exhaust port  54 F to effectively flow in the downward direction Z 1  (see the thick solid arrow Y 1  in  FIG.  6   ). Therefore, it is possible to allow exhaust gases from each of the exhaust ports  54  to smoothly flow toward the collecting exhaust pipe  71  in the downward direction Z 1 . This makes it possible to achieve a decrease in exhaust pressure loss of the engine  20  and to achieve a decrease in pumping loss resulting from this decrease. 
     Particularly in a case in which the first concave portion  76 A is larger than the other first concave portions  76 , the first concave portion  76 A allows exhaust gases in the downstream portion  54 H of the second exhaust port  54 B to effectively flow in the downward direction Z 1  so as not to flow toward the first exhaust port  54 A side, i.e., so as not to flow in the upward direction Z 2 . Therefore, it is possible to allow exhaust gases from the second exhaust port  54 B connected to the combustion chamber  28  of the second cylinder  24 B to flow toward the collecting exhaust pipe  71  in the downward direction Z 1  more smoothly. 
     Each of the second concave portions  77  allows exhaust gases in the collecting exhaust pipe  71  to flow in the downward direction Z 1  (see the thick solid arrow Y 2  in  FIG.  6   ). More specifically, the second concave portion  77 A provided in the region  71 B, which is adjacent to the first exhaust port  54 A in the downward direction Z 1 , separates exhaust gases flowing along the region  71 B from the region  71 B so as not to flow toward the second to fourth exhaust ports  54 B to  54 D located farther downward in the downward direction Z 1 , and allows the exhaust gases to flow in the downward direction Z 1 . The second concave portion  77 B provided in the region  71 C, which is adjacent to the second exhaust port  54 B in the downward direction Z 1 , separates exhaust gases flowing along the region  71 C from the region  71 C so as not to flow toward the third and fourth exhaust ports  54 C and  54 D located farther downward in the downward direction Z 1  than the region  71 C, and allows the exhaust gases to flow in the downward direction Z 1 . The second concave portion  77 C provided in the region  71 D, which is adjacent to the downstream portion  54 H of the third exhaust port  54 C in the downward direction Z 1 , separates exhaust gases flowing along the region  71 D from the region  71 D so as not to flow toward the fourth exhaust port  54 D located farther downward in the downward direction Z 1  than the region  71 D, and allows the exhaust gases to flow in the downward direction Z 1 . 
     Therefore, exhaust gases that have flowed into the collecting exhaust pipe  71  from each of the exhaust ports  54  are allowed to continuously flow into the collecting exhaust pipe  71 , thus enabling the exhaust gases to smoothly flow in the downward direction Z 1 . Particularly in a case in which the second concave portion  77 A is larger than the other second concave portions  77 B and  77 C, the second concave portion  77 A allows exhaust gases in the region  71 B to effectively flow in the downstream direction so as not to backwardly flow toward the first exhaust port  54 A. Therefore, exhaust gases that have flowed into the collecting exhaust pipe  71  from the first exhaust port  54 A are allowed to continuously flow into the collecting exhaust pipe  71 , thus enabling the exhaust gases to smoothly flow in the downstream direction. 
     The pressure charger  50  includes only the supercharger in the above-described preferred embodiments. The outboard motor  4  may be provided with a turbocharger (not shown) driven by exhaust gases passing through the exhaust passage  46 . The pressure charger  50  may include only a turbocharger, or may include both a supercharger and a turbocharger. 
     Additionally, an inboard/outboard motor, or an inboard motor, or a waterjet drive may be used as an example of a vessel propulsion apparatus other than the outboard motor  4 . In the inboard/outboard motor, a vessel engine arranged in the same way as the engine  20  is located inside the vessel, and a drive unit including a propulsion unit (propeller  36 , etc.) and a steering assembly is located outside the vessel. The inboard motor is a vessel engine and a drive unit that are built into the hull  2  and in which the propeller  36  is attached to a propeller shaft extending from the drive unit and outwardly from the vessel. In this case, the steering assembly is separately provided. The waterjet drive is arranged to accelerate water taken in from a vessel bottom by a pump, and jet the water from a jet nozzle of a transom stern, and thus obtain a thrust. In this case, the pump is driven by the vessel engine, and the steering assembly includes the jet nozzle and a mechanism that turns the jet nozzle along a horizontal plane. In the vessel engine, a plurality of cylinders  24  may be arranged in series along the horizontal direction, etc., so that the crankshaft  27  extends in the front-rear direction. 
     Various features described above may be appropriately combined together. Additionally, an arrangement may be used in which only either one of the first concave portion  76  and the second concave portion  77  described above is provided. At least one of the connector portions that individually connect the exhaust ports  54  to the collecting exhaust pipe  71  may include the concave portion  75  which includes at least one of the first concave portion  76  and the second concave portion  77 . 
     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.