Patent Publication Number: US-8986059-B1

Title: Marine propulsion device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a marine propulsion device. 
     2. Description of the Related Art 
     Outboard motors and other such marine propulsion devices include exhaust passages for discharging exhaust from the engine. A honeycomb structure composed of, for example, a catalyst carrier is disposed inside the exhaust passage. In a marine propulsion device, exhaust is discharged from the engine into the water via the exhaust passage. Because of this, it is possible that the honeycomb structure may become wet due to water infiltrating from the exhaust passage. Moreover, in cases where the honeycomb structure is retained within the exhaust passage via a retainer mat, the retainer mat deteriorates if it becomes wet. If the retainer mat deteriorates, retention of the honeycomb structure becomes looser, and the honeycomb structure can no longer be retained. 
     According to a catalytic converter disclosed in Japanese Laid-open Patent Application No. 2003-020939, a stopper is disposed downstream from the retainer mat of the catalyst carrier, and the catalyst carrier is prevented from shifting towards the downstream end by the stopper. However, because the stopper is extended inwardly in the radial direction past the outside peripheral face of the catalyst carrier, the flow channel cross section of the catalyst carrier is constricted. For this reason, the exhaust gas cleaning performance of the catalyst is diminished. 
     According to an exhaust gas treatment device disclosed in Japanese Laid-open Patent Application No. 2006-070886, there is provided a stopper portion which is spaced apart from the retaining mat, and which protrudes by a protrusion amount less than the thickness of the retainer mat. Consequently, in the event that the retainer mat and the catalyst carrier move in unison, the movement of the retainer mat and the catalyst carrier can be stopped by the stopper portion. Moreover, owing to the small protrusion amount of the stopper portion, constriction of the flow channel cross section of the catalyst carrier by the stopper portion is prevented. 
     However, with the exhaust gas treatment device disclosed in Japanese Laid-open Patent Application No. 2006-070886, in the event that only the catalyst carrier has moved, the catalyst carrier cannot be caught by the stopper portion. It is conceivable that in a marine propulsion device, the retainer mat may become wet and deteriorate due to water infiltrating from the exhaust passage in the manner discussed above. In such a case, with the stopper portion of Japanese Laid-open Patent Application No. 2006-070886, when the retaining power of the retainer mat declines owing to deterioration, the catalyst carrier may slip out from the retainer mat. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide a marine propulsion device that prevents a porous body such as a honeycomb structure from slipping out from a retainer mat, without constricting a flow channel cross section of the porous body. 
     A marine propulsion device according to a preferred embodiment of the present invention includes an engine, an exhaust passage, a porous body, a retainer mat, and a stopper portion. The engine includes an exhaust port. The exhaust passage connects to the exhaust port. The porous body is disposed in the exhaust passage. The retainer mat covers the outside peripheral surface of the porous body. The retainer mat retains the porous body. The stopper portion is disposed inside the exhaust passage. The stopper portion is disposed downstream of the porous body, and spaced apart from a downstream-side end portion of the porous body. The stopper portion extends inwardly in the radial direction, past the outside peripheral surface of the porous body. 
     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 side view of a marine propulsion device according to a preferred embodiment of the present invention. 
         FIG. 2  is a rear view of the marine propulsion device according to a preferred embodiment of the present invention. 
         FIG. 3  is a side view of an engine unit including in a marine propulsion device according to a preferred embodiment of the present invention. 
         FIG. 4  is a cross-sectional view along line IV-IV in  FIG. 1 . 
         FIG. 5  is a cross-sectional view along line V-V in  FIG. 1 . 
         FIG. 6  is a cross-sectional view along line VI-VI in  FIG. 2 . 
         FIG. 7  is an enlarged cross-sectional view of a water capture member and surrounding structures thereof. 
         FIG. 8  is a cross-sectional view of a catalyst unit. 
         FIG. 9  is an enlarged cross-sectional view of a catalyst member and surrounding structures thereof. 
         FIG. 10  is a cross-sectional view of a catalyst unit according to another preferred embodiment of the present invention. 
         FIG. 11  is a cross-sectional view of an engine unit according to another preferred embodiment of the present invention. 
         FIG. 12  is an enlarged cross-sectional view of a water capture member, a catalyst member, and surrounding structures thereof according to another preferred embodiment of the present invention. 
         FIG. 13  is an enlarged cross-sectional view of a water capture member and surrounding structures thereof according to another preferred embodiment of the present invention. 
         FIG. 14  is an enlarged cross-sectional view of a water capture member, a catalyst member, and surrounding structures thereof according to another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a side view showing a marine propulsion device  1  according to a preferred embodiment of the present invention.  FIG. 2  is a rear view showing the marine propulsion device  1  according to a preferred embodiment of the present invention. While the marine propulsion device  1  according to present preferred embodiment of the present invention preferably is an outboard motor, it is possible to implement the present invention in other types of marine propulsion devices as well, such as an inboard/outboard motor, for example. As shown in  FIGS. 1 and 2 , the marine propulsion device  1  according to the present preferred embodiment includes an upper casing  2 , a lower casing  3 , an exhaust guide section  4 , and an engine unit  5 . For ease of understanding, the upper casing  2  is shown in cross section in  FIGS. 1 and 2 . The upper casing  2 , the lower casing  3 , and the engine unit  5  are fixed to the exhaust guide section  4 . 
     The engine unit  5  is disposed inside the upper casing  2 . Consequently, the upper casing  2  corresponds to the engine cover according to a preferred embodiment of the present invention, which covers the engine unit  5 . A drive shaft  11  is disposed inside the lower casing  3 , as shown in  FIG. 1 . The drive shaft  11  is disposed in the vertical direction inside the lower casing  3 . The drive shaft  11  is connected to a crankshaft  26  of an engine  6 . A propeller  12  is disposed on the bottom portion of the lower casing  3 . The propeller  12  is disposed below the engine  6 . The propeller  12  includes a propeller boss  13 . A propeller shaft  14  is disposed inside the propeller boss  13 . The propeller shaft  14  is connected to the propeller boss  13 . The propeller shaft  14  is disposed in a rear to front direction. The propeller shaft  14  is connected to the bottom portion of the drive shaft  11  via a bevel gear  15 . 
     In the marine propulsion device  1 , the drive force generated by the engine  6  is transmitted to the propeller  12  via the drive shaft  11  and the propeller shaft  14 . The propeller  12  is thereby rotated forward or in reverse. As a result, a propulsion force will be generated to cause the vessel equipped with the marine propulsion device  1  to move forward or backward. 
     The marine propulsion device  1  also includes an exhaust passage  16 . The exhaust passage  16  connects to the exhaust port of the engine  6 . The exhaust passage  16  is provided so as to extend from intake ports  25   a  to  25   d  (see  FIG. 5 ) of the engine  6  through the exhaust guide section  4  and the lower casing  3  to the propeller boss  13  of the propeller  12 . The exhaust passage  16  communicates with the interior of the propeller boss  13 . The exhaust discharged from the engine  6  is discharged into the water from the exhaust passage  16  through the internal space of the propeller boss  13 . The construction of the exhaust passage  16  will be described in detail later. 
       FIG. 3  is a side view of the engine unit  5 . The engine unit  5  includes an engine  6 , an exhaust manifold  31 , and a catalyst unit  32 , as shown in  FIG. 3 . 
     The engine  6  includes a cylinder block  21 , a cylinder head  22 , and a crankcase  23 . The cylinder block  21  is disposed above the exhaust guide section  4  and fixed to the exhaust guide section  4 .  FIG. 4  is a cross-sectional view along line IV-IV in the marine propulsion device  1  in  FIG. 1 . As shown in  FIG. 4 , the cylinder block  21  preferably includes four cylinders  21   a  to  21   d , for example. The four cylinders  21   a  to  21   d  are disposed preferably in a line in a vertical direction. 
     As shown in  FIG. 3 , the cylinder head  22  is disposed behind the cylinder block  21 .  FIG. 5  is a cross-sectional view along line V-V in the marine propulsion device  1  in  FIG. 1 . As shown in  FIG. 5 , intake ports  24   a  to  24   d  and exhaust ports  25   a  to  25   d  are disposed inside the cylinder head  22 . The intake ports  24   a  to  24   d  and the exhaust ports  25   a  to  25   d  are connected to the cylinders  21   a  to  21   d , respectively. The intake ports  24   a  to  24   d  are connected to a fuel supply system not shown in the drawing. The exhaust ports  25   a  to  25   d  extend in a lateral direction and are connected to a first passage  33  of an exhaust manifold  31 , discussed below. 
     The crankcase  23  is disposed at the front of the cylinder block  21 , as shown in  FIG. 3 . The crankshaft  26  (see  FIG. 1 ) is disposed inside the crankcase  23 . The crankshaft  26  extends in a vertical direction. The top end portion of the above-described driveshaft  11  is linked to the bottom end portion of the crankshaft  26 . The movement of a piston (not shown) disposed inside the cylinders  21   a  to  21   d  is transmitted to the driveshaft  11  via the crankshaft  26 . 
     The exhaust manifold  31  is disposed on the side of the cylinder head  22 , as shown in  FIG. 3 . The exhaust manifold  31  is preferably integral with the cylinder head  22 . 
     A catalyst unit  32  is preferably formed separately from the cylinder head  22  and the cylinder block  21 . The catalyst unit  32  is also preferably separate from the exhaust manifold  31 . The catalyst unit  32  is attached to the cylinder head  22  and the cylinder block  21 . As shown in  FIG. 4  and  FIG. 6 , the catalyst unit  32  includes a catalyst member  44 , a second retainer mat  42 , and a pipe  45 . The catalyst member  44  is disposed inside the pipe  45 . The second retainer mat  42  is wrapped onto the catalyst member  44 , and contacts the catalyst member  44 . The second retainer mat  42  covers the outside peripheral surface of the catalyst member  44 . The outside peripheral surface of the second retainer mat  42  contacts the inside peripheral surface of the pipe  45 . Because of this, the second retainer mat  42  retains the catalyst member  44  in the interior of the pipe  45 . The second retainer mat  42  directly retains the catalyst member  44 . The second retainer mat  42  preferably is a non-expanding mat, and is preferably composed of alumina fibers, for example. The catalyst member  44  is positioned above the bottom end portion of the cylinder  21   d , which is the lowest-positioned of the four cylinders  21   a  to  21   d . The pipe  45  houses the catalyst member  44 . Consequently, the catalyst member  44  is disposed inside the upper casing  2 . The catalyst member  44  preferably includes a catalyst carrier which supports a catalyst to clean exhaust. A three-way catalyst, for example, can be used as the catalyst. The catalyst member  44  preferably includes a cylindrical member having a honeycomb structure. The catalyst member  44  is disposed such that the flow channel is oriented in the vertical direction. In other words, the pipe  45  is disposed so that the axis line extends in the vertical direction. The catalyst member  44  is preferably made of a ceramic. The exhaust passing through the exhaust passage  16  passes through the catalyst member  44  inside the pipe  45 , and is thereby cleaned. 
     Next, the configuration of the exhaust passage  16  is described. The exhaust passage  16 , shown in  FIG. 1 , includes a first passage  33 , a second passage  34 , and a third passage  35 , shown in  FIG. 6 , and a fourth passage  38 , shown in  FIG. 1 . The first passage  33  and the second passage  34  are disposed inside the exhaust manifold  31 , as shown in  FIG. 6 . The first passage  33  is connected to the above-described plurality of exhaust ports  25   a  to  25   d . The first passage  33  is disposed on the side of the cylinder head  22  and extends in a vertical direction. A plurality of first openings  36   a  to  36   d  is formed in the first passage  33 , as shown in  FIG. 5 , and each of the exhaust ports  25   a  to  25   d  is connected to the first passage  33  via each of the first openings  36   a  to  36   d . The first passage  33  collects the exhaust discharged from the exhaust ports  25   a  to  25   d.    
     The second passage  34  is connected to the first passage  33 . As shown in  FIG. 5 , the portion connecting the second passage  34  and the first passage  33  is positioned between the top end of the cylinder  21   a  positioned at the uppermost portion of the plurality of cylinders  21   a  to  21   d , and the bottom end of the cylinder  21   d  positioned at the lowermost portion of the plurality of cylinders  21   a  to  21   d . Specifically, the vertical center portion of the portion connecting the second passage  34  and the first passage  33  is positioned higher than the vertical central portion of the first passage  33 . More specifically, the portion connecting the second passage  34  and the first passage  33  is positioned at roughly the same height as the second highest cylinder  21   b  of the four cylinders  21   a  to  21   d . The second passage  34  extends in a rear to front direction from the first passage  33 . The second passage  34  is substantially parallel to the central axis lines of the cylinders  21   a  to  21   d . In other words, the second passage  34  extends in a roughly horizontal direction. The second passage  34  also includes a second opening  37 . The catalyst unit  32  is connected to the second opening  37 . 
     The third passage  35  includes the pipe  45  of the catalyst unit  32  and a first lower passage  51 , as shown in  FIG. 6 . The third passage  35  is connected to the second passage  34 . The third passage  35  extends downward from the second passage  34 . In other words, the third passage  35  corresponds to the vertical direction passage according to a preferred embodiment of the present invention, which extends in the vertical direction. Therefore, the third passage  35  is disposed substantially parallel to the crankshaft  26  (see  FIG. 1 ). The third passage  35  is disposed on the side of the cylinder block  21 . The first lower passage  51  is disposed inside the cylinder block  21 . The first lower passage  51  includes a first lower opening  54 . The first lower opening  54  is located on the lower portion of the lateral face of the cylinder block  21 . The first lower passage  51  is connected to the catalyst unit  32  via the first lower opening  54 . 
     A water capture member  47  is disposed downstream of the catalyst member  44  inside the third passage  35 . Stated another way, the catalyst member  44  is disposed upstream from the water capture member  47 . Consequently, the water capture member  47  corresponds to the first porous body according to a preferred embodiment of the present invention. The catalyst member  44  corresponds to the second porous body according to a preferred embodiment of the present invention. The water capture member  47  is disposed in the first lower passage  51 , and is positioned below the catalyst member  44 . The water capture member  47  preferably includes a tube-shaped member having the same honeycomb structure as the catalyst member  44 . In other words, the water capture member  47  preferably is a catalyst carrier the same as the catalyst member  44 , but does not support a catalyst. The water capture member  47  is preferably made of a ceramic. The water capture member  47  is disposed inside the upper casing  2 . The outside diameter of the water capture member  47  is smaller than the outside diameter of the catalyst member  44 . The water capture member  47  is retained by the first retainer mat  48 . The first retainer mat  48  is wrapped onto the water capture member  47  and contacts the water capture member  47 . The first retainer mat  48  covers the outside peripheral surface of the water capture member  47 . The outside peripheral surface of the first retainer mat  48  contacts the inside peripheral surface of the first lower passage  51 . Because of this, the first retainer mat  48  retains the water capture member  47  in the first lower passage  51 . The first retainer mat  48  directly retains the water capture member  47 . The first retainer mat  48  preferably is a non-expanding mat, and is composed of alumina fibers, for example. 
     The third passage  35  includes a coolant passage  46 . The coolant passage  46  is disposed at a minimum surrounding the water capture member  47  and the catalyst member  44 . In  FIG. 4 , a plurality of sections included in the coolant passage  46  are depicted as being dispersed, but these sections are connected to and in communication with one another. A coolant supplied from the coolant supply portion, not shown, circulates in the coolant passage  46 . 
     The fourth passage  38 , as shown in  FIG. 1 , guides the exhaust from the exhaust ports  25   a  to  25   d  below the engine  6  and discharges the exhaust to the outside via the propeller boss  13 . The fourth passage  38  is positioned below the engine  6 . The fourth passage  38  includes a second lower passage  52  and a third lower passage  53 . The second lower passage  52  is disposed inside the exhaust guide section  4 . The second lower passage  52  is connected to the first lower passage  51 , as shown in  FIGS. 4 and 6 . The third lower passage  53  is disposed inside the lower casing  3 , as shown in  FIG. 1 . The third lower passage  53  is connected to the second lower passage  52 . The third lower passage  53  is also connected to the propeller boss  13 . 
     In the marine propulsion device  1  according to the present preferred embodiment, the exhaust from the exhaust ports  25   a  to  25   d  of the engine  6  is collected in the first passage  33 . The exhaust flows from the first passage  33  through the second passage  34  to the third passage  35 . The exhaust is cleaned by being passed through the catalyst member  44  in the third passage  35 . The exhaust flows from the third passage  35  to the fourth passage  38 . The exhaust is sent downward from the engine  6  by being passed through the fourth passage  38 . Then, the exhaust passes through the inside section of the propeller boss  13  from the fourth passage  38  and is discharged outside. 
     A linking passage  43  is also connected to the bottom end portion of the first passage  33 , as shown in  FIGS. 5 and 6 . The linking passage  43  passes through the wall section of the cylinder block  21  and is linked to the first lower passage  51  as shown in  FIG. 4 . Therefore, the linking passage  43  links the bottom end portion of the first passage  33  and the first lower passage  51 . More specifically, the linking passage  43  links the section of the first lower passage  51  positioned downstream of the water capture member  47  and the bottom end portion of the first passage  33 . The linking passage  43  has a smaller cross-sectional area than the cross-sectional area of the second opening  37 . Because of this, the exhaust discharged from the linking passage  43  is negligible in comparison with the second opening  37 . By contrast, the condensed water generated inside the first passage  33  flows to the first lower passage  51  via the linking passage  43 . Then, the condensed water passes through the fourth passage  38  and is discharged outside via the propeller boss  13 . The linking passage  43  thus functions as a condensed water removal passage whereby the condensed water generated inside the first passage  33  is removed from the first passage  33 . 
     The catalyst unit  32  also includes a first oxygen sensor  55  and a second oxygen sensor  56  arranged to detect an oxygen concentration in the exhaust, as shown in  FIGS. 3 and 4 . The first oxygen sensor  55  is disposed in the exhaust passage  16  upstream from the catalyst member  44 . Specifically, the first oxygen sensor  55  is disposed above the catalyst member  44  in the pipe  45 . The second oxygen sensor  56  is disposed below the catalyst member  44  in the pipe  45 . The second oxygen sensor  56  is disposed in the exhaust passage  16  downstream from the catalyst member  44 . Specifically, the second oxygen sensor  56  is disposed between the catalyst member  44  and the water capture member  47  in the exhaust passage  16 . That is, the water capture member  47  is disposed between the second oxygen sensor  56  and the linking passage  43  in the exhaust passage  16 . A detection signal from the first oxygen sensor  55  and the second oxygen sensor  56  is supplied to an ECU (not shown). The ECU controls the engine  6  on the basis of the detection value from the first oxygen sensor  55  and the second oxygen sensor  56 . 
     As shown in  FIG. 4 , a first stopper portion  61  and a second stopper portion  62  are provided inside the exhaust passage  16 . The configurations of the first stopper portion  61  and the second stopper portion  62  are described below. 
     The first stopper portion  61  is disposed in the third passage  35 , and downstream from the water capture member  47 . Specifically, the first stopper portion  61  is disposed inside the first lower passage  51 .  FIG. 7  is an enlarged cross-sectional view showing the water capture member  47  and portion of the lower passage  51  shown in  FIG. 4 . The first stopper portion  61  is preferably integral with the inner surface of the first lower passage  51 . The first stopper portion  61  is disposed so as to prevent downward movement of the water capture member  47 . Specifically, the first stopper portion  61  is defined by an inner surface of the first lower passage  51 , by extending a section thereof situated downstream from the water capture member  47  inwardly in a radial direction past the outside peripheral surface of the water capture member  47 . The first stopper portion  61  is disposed downstream of the water capture member  47 , and spaced apart from the downstream-side end portion of the water capture member  47 . The first stopper portion  61  faces the first retainer mat  48 . The first stopper portion  61  extends to a point inward in the radial direction past the outside peripheral surface of the water capture member  47 . The distance L1 between the end portion at the downstream side of the water capture member  47  and the first stopper portion  61  is smaller than the thickness W1 of the first retainer mat  48 . The distance L1 between the end portion at the downstream side of the water capture member  47  and the first stopper portion  61  is greater than the distance D1 in the radial direction between the inward end section in the radial direction of the first stopper portion  61  and the outside peripheral surface of the water capture member  47 . Stated another way, this distance D1 is a distance for which the water capture member  47  and the first stopper portion  61  overlap in the direction of exhaust flow. In other words, the distance L1 between the end portion of the downstream side of the water capture member  47  and the first stopper portion  61  is greater than the distance D1 of overlap of the water capture member  47  and the first stopper portion  61  in the direction of exhaust flow. 
     The second stopper portion  62  is disposed in the third passage  35 , and downstream from the catalyst member  44 . Specifically, the second stopper portion  62  is included in the pipe  45 .  FIG. 8  is a cross-sectional view of the pipe  45 . The pipe  45  includes a first pipe  45   a  and a second pipe  45   b . The second pipe  45   b  is disposed downstream of the first pipe  45   a  in the third passage  35 . The second pipe  45   b  is disposed below the first pipe  45   a . The upper end portion of the second pipe  45   b  is linked to the lower end portion of the first pipe  45   a . The catalyst member  44  and the second retainer mat  42  are disposed inside the second pipe  45   b . The second stopper portion  62  is preferably integral with the inner surface of the second pipe  45   b.    
       FIG. 9  is an enlarged view of portion of the second pipe  45   b  and the catalyst member  44  shown in  FIG. 8 . The second stopper portion  62  is disposed so as to prevent downward movement of the catalyst member  44 . The second stopper portion  62  is disposed below the catalyst member  44 . The second stopper portion  62  is defined by an inner surface of the second pipe  45   b , by extending a section thereof situated downstream from the catalyst member  44  inwardly in radial direction past the outside peripheral surface of the catalyst member  44 . The second stopper portion  62  is disposed downstream of the catalyst member  44 , and spaced apart from the downstream-side end portion of the catalyst member  44 . The second stopper portion  62  faces the second retainer mat  42 . The second stopper portion  62  extends to a point inward in the radial direction past the outside peripheral surface of the catalyst member  44 . The distance L2 between the end portion at the downstream side of the catalyst member  44  and the second stopper portion  62  is smaller than the thickness W2 of the second retainer mat  42 . The distance L2 between the end portion at the downstream side of the catalyst member  44  and the second stopper portion  62  is greater than the distance D2 in the radial direction between the inward end section in the radial direction of the second stopper portion  62  and the outside peripheral surface of the catalyst member  44 . Stated another way, this distance D2 is a distance for which the catalyst member  44  and the second stopper portion  62  overlap in the direction of exhaust flow. In other words, the distance L2 between the end portion of the downstream side of the catalyst member  44  and the second stopper portion  62  is greater than the distance D2 of overlap of the catalyst member  44  and the second stopper portion  62  in the direction of exhaust flow. 
     The marine propulsion device  1  according to the present preferred embodiment preferably includes the following characteristics. 
     Because the water capture member  47  and the first stopper portion  61  are spaced apart from each other by a certain distance, constriction of the flow channel cross section of the water capture member  47  by the first stopper portion  61  is prevented. Moreover, because the first stopper portion  61  extends inwardly in the radial direction past the outside peripheral surface of the water capture member  47 , even if the first retainer mat  48  has deteriorated and lost retaining power and the water capture member  47  has moved downward, the first stopper portion  61  contacts the water capture member  47 . Because of this, downward movement of the water capture member  47  is restricted by the first stopper portion  61 . Because of this, the water capture member  47  is prevented from slipping out from the first retainer mat  48 . 
     If the water capture member  47  strikes against the first stopper portion  61  in the event that the water capture member  47  is forcibly inserted into the first lower passage  51  during manufacture or in other circumstances, it is possible for the water capture member  47  to be damaged. Particularly where the water capture member  47  is made of a ceramic, it is easily damaged by impact. Consequently, the present invention is even more effective where the water capture member  47  is made of a ceramic. Also, if the water capture member  47  is made of a ceramic, the lighter weight as compared with a water capture member  47  made of metal helps to prevent the water capture member  47  from dropping out. 
     Because the catalyst member  44  and the second stopper portion  62  are spaced apart from each other by a certain distance, constriction of the flow channel cross section of the catalyst member  44  by the second stopper portion  62  is prevented. Because of this, a decline in the exhaust gas cleaning ability due to the second stopper portion  62  is prevented. 
     Because the second stopper portion  62  extends inwardly in the radial direction past the outside peripheral surface of the catalyst member  44 , even if the second retainer mat  42  has deteriorated and lost retaining power and the catalyst member  44  has moved downward, the second stopper portion  62  contacts the catalyst member  44 . Because of this, downward movement of the catalyst member  44  is restricted. As a result, the catalyst member  44  is prevented from slipping out from the second retainer mat  42 . In particular, because an engine of large size is disposed inside the upper casing  2  of the marine propulsion device  1 , the layout of the catalyst unit  32  is limited. Accordingly, the catalyst member  44  is disposed such that the direction of the flow channel thereof faces in the vertical direction, like the pipe  45  of the catalyst unit  32 . With this unique arrangement, in the unlikely event that the second retainer mat  42  becomes wet, the catalyst member  44  readily moves downward due to the effect of gravity. Consequently, the present preferred embodiment of the present invention is even more effective for an outboard motor that is provided with a catalyst unit  32  disposed such that the direction of the flow channel of the catalyst member  44  faces in the vertical direction. 
     If the catalyst member  44  strikes against the second stopper portion  62  in the event that the catalyst member  44  is forcibly inserted into the second pipe  45   b  during manufacture or in other circumstances, it is possible for the catalyst member  44  to be damaged. Particularly where the catalyst member  44  is made of a ceramic, it is easily damaged by impact. Consequently, the present invention is even more effective where the catalyst member  44  is made of a ceramic, as in the present embodiment. Also, if the catalyst member  44  is made of a ceramic, the lighter weight as compared with a catalyst member  44  made of metal helps to prevent the catalyst member  44  from dropping out. 
     A preferred embodiment of the present invention was described above, but the present invention is not limited to the above-described preferred embodiment and can be modified in a variety of ways within a range that does not depart from the scope of the present invention. 
     In the preferred embodiment described above, the second stopper portion  62  is preferably integral with a pipe that retains the second stopper portion  62 , namely, the second pipe  45   b . However, the second stopper portion  62  may instead be defined by a member that is separate from the portion of the exhaust passage that retains the second stopper portion  62 , as shown in  FIG. 10 . Specifically, the second pipe  45   b  includes an upstream second pipe  451  and a downstream second pipe  452 . The upstream second pipe  451  retains the catalyst member  44  and the second retainer mat  42 . In other words, the upstream second pipe  451  corresponds to the retaining portion according to a preferred embodiment of the present invention. The downstream second pipe  452  is a member that is separate from the upstream second pipe  451 . The downstream second pipe  452  is situated downstream from the upstream second pipe  451  in the exhaust passage  16 . Also, the downstream second pipe  452  is situated below the upstream second pipe  451 . The upper end portion of the downstream second pipe  452  is linked to the lower end portion of the upstream second pipe  451 . The inside diameter of the upper end portion of the downstream second pipe  452  is smaller than the outside diameter of the catalyst member  44 . Because of this, the upper end portion of the downstream second pipe  452  is arranged to extend from the inside peripheral surface of the upstream second pipe  451 , to a location inward in the radial direction past the outside peripheral surface of the catalyst member  44 . In this way, the second stopper portion  62  is defined by the upper end portion of the downstream second pipe  452 . 
     The first retainer mat  48  and the water capture member  47  may be disposed inside the pipe  45  of the catalyst unit  32  as shown in  FIG. 11 . Also, the first stopper portion  61  may be included in a first outer tube  67  which is disposed inside a first pipe  66 . The pipe  45  of the catalyst unit  32  includes the first pipe  66 , a second pipe  68 , and a third pipe  70 . The first pipe  66 , the second pipe  68 , and the third pipe  70  are respectively separate members, and are linked to one another by fastening members such as bolts or the like. The first outer tube  67  is disposed inside the first pipe  66 . A second outer tube  69  is disposed inside the second pipe  68 . 
     The first pipe  66  is situated downstream of the second pipe  68 . The upper end portion of the first pipe  66  is linked to the lower end portion of the second pipe  68 . The upper portion of the first pipe  66  has a linear shape extending in the vertical direction. The lower portion of the first pipe  66  has a shape that curves towards a first lower opening  54 . The second pipe  68  has a linear shape extending in the vertical direction. The third pipe  70  is situated above the second pipe  68 . The lower end portion of the third pipe  70  is linked to the upper end portion of the second pipe  68 . The third pipe  70  has a shape that curves towards the second opening  37  mentioned above. 
     The first outer tube  67  contacts the outside peripheral surface of the first retainer mat  48  described above, and retains the first retainer mat  48  and the water capture member  47 . The first outer tube  67  has a round tube shape. The first outer tube  67  includes the first stopper portion  61 . The first stopper portion  61  is situated at the lower end portion of the first outer tube  67 . The first stopper portion  61  is preferably defined by a flange extending inwardly in the radial direction of the first outer tube  67 .  FIG. 12  is an enlarged view showing portion of the water capture member  47  and the catalyst member  44  shown in  FIG. 11 , and of surrounding structures thereof. The inside diameter of the first stopper portion  61  is smaller than the outside diameter of the water capture member  47 . In other words, the first stopper portion  61  is formed by extending the end portion at the downstream side of the first outer tube  67  inwardly in the radial direction past the outside peripheral surface of the water capture member  47 . The first outer tube  67  also includes a first flange portion  71 . The first flange portion  71  is situated at the upper end portion of the first outer tube  67 . The first flange portion  71  is preferably defined by a flange extending to the outside in the radial direction of the first outer tube  67 . The outside diameter of the first flange portion  71  is larger than the inside diameter of the first pipe  66 . By virtue of the first flange portion  71  being held between the first pipe  66  and the second pipe  68 , the first retainer mat  48  and the water capture member  47  are retained inside the first pipe  66 . 
     As shown in  FIG. 11 , the second outer tube  69  is disposed inside the second pipe  68 . The second outer tube  69  is situated above the first outer tube  67 . The second outer tube  69  contacts the outside peripheral surface of the second retainer mat  42  described above and retains the second retainer mat  42  and the catalyst member  44 . The second outer tube  69  has a round tube shape. The second outer tube  69  includes the second stopper portion  62 . The second stopper portion  62  is situated at the lower end portion of the second outer tube  69 . The second stopper portion  62  is preferably defined by a flange extending inwardly in the radial direction of the second outer tube  69 . As shown in  FIG. 12 , the inside diameter of the second stopper portion  62  is smaller than the outside diameter of the catalyst member  44 . In other words, the second stopper portion  62  is formed by extending the end portion at the downstream side of the second outer tube  69  inwardly in the radial direction past the outside peripheral surface of the catalyst member  44 . Also, as shown in  FIG. 11 , the second outer tube  69  includes a second flange portion  72 . The second flange portion  72  is situated at the upper end portion of the second outer tube  69 . The second flange portion  72  is preferably defined by a flange extending to the outside in the radial direction of the second outer tube  69 . The outside diameter of the second flange portion  72  is larger than the inside diameter of the second pipe  68 . By virtue of the second flange portion  72  being held between the second pipe  68  and the third pipe  70 , the second retainer mat  42  and the catalyst member  44  are retained inside the second pipe  68 . 
     Even where the water capture member  47  is disposed inside the first lower passage  51  as in the preferred embodiment described above, the water capture member  47  and the first retainer mat  48  may be retained by the first outer tube  67  which includes the first stopper portion  61 , as shown in  FIG. 12 . 
     The first stopper portion  61  may also be defined by a pipe  73  separate from the first lower passage  51 , as shown in  FIG. 13 . The pipe  73  is disposed downstream of the water capture member  47 . The pipe  73  has an inside diameter smaller than the outside diameter of the water capture member  47 . The second stopper portion  62  may also be defined by a pipe that, like the pipe  73  that defines the first stopper portion  61 , is separate from the pipe  45 . 
     In the preferred embodiment described above, the water capture member  47  does not support a catalyst, but a water capture member having a catalyst supported thereon may be provided. In the preferred embodiment described above, both the first stopper portion  61  and the second stopper portion  62  are preferably provided in the exhaust passage  16 , but optionally, only one may be provided. However, of the water capture member  47  and the catalyst member  44 , the water capture member  47 , which is situated downstream, is more likely to become wet. Because of this, it is preferable to provide the first stopper portion  61  at least downstream of the water capture member  47 . 
     The catalyst member  44  is not limited to being made of a ceramic as in the preferred embodiments described above; a metal one is also acceptable, for example. However, from the standpoint of lighter weight, it is preferable for the catalyst member  44  to be made of a ceramic. Also, the water capture member  47  is not limited to being made of a ceramic as in the preferred embodiments described above; a metal one is also acceptable, for example. However, from the standpoint of lighter weight, it is preferable for the water capture member  47  to be made of a ceramic. 
     The catalyst member  44  is not limited to a honeycomb structure as in the preferred embodiments described above; a porous body including openings through which exhaust may pass is acceptable, for example. The water capture member  47  is not limited to a honeycomb structure as in the preferred embodiments described above; a porous body including openings through which exhaust may pass is acceptable, for example. 
     The first stopper portion  61  may be defined by a member separate from the first outer tube  67 , as shown in  FIG. 14 . The first stopper portion  61  is attached to the downstream-side end portion of the first outer tube  67 . A circlip, for example, can be used as the first stopper portion  61 . The second stopper portion  62  may be defined by a member separate from the second outer tube  69 . The second stopper portion  62  is attached to the downstream-side end portion of the second outer tube  69 . A circlip, for example, can be used as the stopper portion  62 . 
     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.