Patent Publication Number: US-9840317-B2

Title: Cooling water passage structure of outboard motor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-026188, filed Feb. 13, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a cooling water passage structure of an outboard motor for cooling an engine mounted on the outboard motor by using cooling water. 
     Description of the Related Art 
     As shown in  FIG. 9 , a cooling water passage structure of an outboard motor disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2013-124592) includes a cooling water passage which causes sea water, river water, or the like taken in through an intake  101  of the outboard motor and led to a water passage  103  by a water pump  102  to flow as cooling water through a cylinder head water jacket  104  of a cylinder head, exhaust passage periphery water jacket  105  of an exhaust passage containing an exhaust purification catalyst, and a cylinder periphery water jacket  106  of a cylinder block in order. 
     In the cylinder head water jacket  104 , a first combustion chamber periphery water jacket  107 A and a second combustion chamber periphery water jacket  107 B located on an upstream side are communicated with an exhaust manifold periphery water jacket  108  located on a downstream side. The first combustion chamber periphery water jacket  107 A, second combustion chamber periphery water jacket  107 B, and exhaust manifold periphery water jacket  108  are so designed as to pass cooling water upward from a lower side. 
     In the case of the cooling water passage structure of an outboard motor described in Patent Document 1, in a peripheral portion of a combustion chamber in the cylinder head, a temperature of a portion in the first combustion chamber periphery water jacket  107 A and second combustion chamber periphery water jacket  107 B which corresponds to a lowermost cylinder and into which cooling water flows first is lower than that of portions corresponding to the other cylinders, and hence, is overcooled. 
     Furthermore, in a peripheral portion of an exhaust manifold of the cylinder head, cooling water yet to be cooled in the first combustion chamber periphery water jacket  107 A and second combustion chamber periphery water jacket  107 B flows into that peripheral portion of the exhaust manifold periphery water jacket  108  which corresponds to the lowermost cylinder, and accordingly, a temperature of such portion is lower than portions corresponding to the other cylinders, and hence, is overcooled. 
     Consequently, exhaust gas flowing through the exhaust manifold is cooled excessively, steam in the exhaust gas condenses, and droplets are produced in the exhaust manifold. 
     SUMMARY OF THE INVENTION 
     The present invention was conceived in consideration of the circumstances mentioned above, and an object thereof is to provide a cooling water passage structure of an outboard motor, capable of preventing steam in exhaust gas from being condensed by excessive cooling of the exhaust gas flowing through an exhaust passage, preventing an oxygen sensor installed in the exhaust passage from getting wet, and thereby improving durability of the oxygen sensor. 
     The above and other objects can be achieved according to the present invention by providing, in one preferred embodiment, a cooling water passage structure of an outboard motor which includes a four-stroke engine, an intake unit having an intake port configured to take in water from an underwater, and a water passage configured to supply the water taken in through the intake unit to the four-stroke engine as cooling water, wherein the four-stroke engine includes a cylinder block in which a cylinder is formed by extending in a horizontal direction, a cylinder head fixed to the cylinder block so as to cover the cylinder, configured to form a combustion chamber together with the cylinder, and provided with an exhaust port configured to discharge exhaust gas in communication with the combustion chamber, and an exhaust passage connected to the exhaust port so as to lead the exhaust gas to outside the engine, wherein a combustion chamber periphery water jacket through which the cooling water flows around the combustion chamber and an exhaust port periphery water jacket through which the cooling water flows around the exhaust port are formed in the cylinder head, a cylinder periphery water jacket through which the cooling water flows around the cylinder is formed in the cylinder block, and an exhaust passage periphery water jacket through which the cooling water flows around the exhaust passage is formed around the exhaust passage, and wherein the water jackets are connected such that the cooling water from the water passage will flow through the combustion chamber periphery water jacket, the exhaust port periphery water jacket, the cylinder periphery water jacket, and the exhaust passage periphery water jacket in order. 
     According to the preferred embodiment of the present invention, the cooling water from the water passage flows through the combustion chamber periphery water jacket, the exhaust port periphery water jacket, the cylinder periphery water jacket, and the exhaust passage periphery water jacket in order. Accordingly, the cooling water heated in the combustion chamber periphery water jacket and exhaust port periphery water jacket in sequence flows through the exhaust passage periphery water jacket. Therefore, the exhaust gas flowing through the exhaust passage is not cooled excessively by the cooling water, which makes it possible to prevent condensation of the steam contained in the exhaust gas, and hence, makes it possible to prevent an oxygen sensor installed in the exhaust passage from getting wet, thereby improving the durability of the oxygen sensor. 
     The nature and further characteristic features of the present invention will be made clearer from the following description made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a left side view showing an outboard motor to which an embodiment of a cooling water passage structure for the outboard motor according to the present invention is applied; 
         FIG. 2  is a sectional view of the outboard motor of  FIG. 1  taken along the line II-II; 
         FIG. 3  is a sectional view taken along the line III-III in  FIG. 2 ; 
         FIG. 4  is a schematic diagram showing the cooling water passage structure of an engine of the outboard motor shown in  FIGS. 2 and 3 ; 
         FIG. 5  is a sectional view taken along the line V-V in  FIG. 2  for showing a combustion chamber periphery water jacket and exhaust port periphery water jacket of  FIG. 4 ; 
         FIG. 6  is a sectional view taken along the line VI-VI in  FIG. 3  for showing a manner how the exhaust port periphery water jacket is connected with a cylinder periphery water jacket and exhaust passage periphery water jacket (first exhaust manifold periphery water jacket and catalyst periphery water jacket) shown in  FIG. 4 ; 
         FIG. 7  is a sectional view taken along the line VII-VII in  FIG. 8 , for showing the cylinder periphery water jacket and exhaust passage periphery water jacket (third exhaust manifold periphery water jacket and catalyst periphery water jacket) of  FIG. 4 ; 
         FIG. 8  is a sectional view taken along the line VIII-VIII in  FIG. 7 , for showing a manner how the cylinder periphery water jacket and exhaust passage periphery water jacket (third exhaust manifold periphery water jacket) of  FIG. 4  are connected with each other; and 
         FIG. 9  is a diagram showing a conventional cooling water passage structure for an outboard motor. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will be described hereunder with reference to the accompanying drawings. It is to be noted that, in the following descriptions, terms “upper”, “lower”, “right”, “left”, “forward”, “rearward” and the like terms indicating directions are used with reference to the illustrated state of the drawings or a state mounted to a hull. 
       FIG. 1  is a left side view showing an outboard motor to which an embodiment of a cooling water passage structure of the outboard motor according to the present invention is applied. The outboard motor  10  shown in  FIG. 1  is equipped with an engine holder  12 , on which an engine  11  is mounted. The engine  11  is a vertical engine in which a crankshaft  26  (described hereinlater) is mounted substantially perpendicularly (i.e., vertically). A drive shaft housing  13  and a gear case  14  are assembled in sequence under the engine holder  12 . 
     In  FIG. 1 , an oil pan  15  is located under the engine holder  12  in which a lubricating oil is reserved. A vertically dividable engine cover  9  includes a lower engine cover  9 A and an upper engine cover  9 B so as to cover the engine  11  and engine holder  12 . 
     The outboard motor  10  is supported pivotally in a horizontal direction by means of a pilot shaft  16  pivotally supported on a swivel bracket  17 . The swivel bracket  17  is supported on a swivel shaft  18  pivotally in a vertical direction with respect to a clamp bracket  19 , which is attached to a stern (transom)  20 A of a hull  20 . Consequently, the outboard motor  10  is mounted on the hull  20  swingably in a horizontal direction (steering direction) and vertical direction (trim and tilt direction). 
     A driving force generated on the crankshaft  26  of the engine  11  is transmitted through reduction gears  21 A and  21 B to a drive shaft  22  disposed so as to extend substantially vertically in the drive shaft housing  13  and gear case  14  and is then transmitted through a shift mechanism  23  and propeller shaft  24  disposed in the gear case  14  to a propeller  25 , thereby turning the propeller  25  in a forward or reverse direction. According to such arrangement, the outboard motor  10  causes the hull  20  to move forward or backward. 
     As shown in  FIGS. 1 and 2 , the engine  11  is a V-type four-stroke-cycle engine which includes the crankshaft  26  extending in a vertical direction, a left bank  27  extending diagonally left rearward, and a right bank  28  extending diagonally right rearward. In such V-type engine, the left bank  27  is composed of a cylinder head  31  and a cylinder head cover  38  placed in sequence behind a left bank portion  30 A of a cylinder block  30 , and the right bank  28  is composed of a cylinder head  31  and a cylinder head cover  38  placed in sequence behind a right bank portion  30 B of the cylinder block  30 . Further, a crankcase  32  is placed in front of the cylinder block  30 . 
     As shown in  FIG. 2 , cylinders  33  are formed in a horizontal direction inside the left bank portion  30 A of the cylinder block  30 , extending diagonally left rearward. Cylinders  33  are also formed in a horizontal direction inside the right bank portion  30 B of the cylinder block  30 , extending diagonally right rearward. Pistons  29  are reciprocally located in the cylinders  33  and coupled to the crankshaft  26  via connection rods, not shown. 
     Along cylinder axes P of the cylinders  33  in the left bank portion  30 A and right bank portion  30 B of the cylinder block  30 , the cylinder heads  31  are fixed, respectively, to the left bank portion  30 A and right bank portion  30 B so as to cover the cylinders  33 , and concurrently, to form combustion chambers  34  in conjunction with respective cylinders  33  in the left bank portion  30 A and right bank portion  30 B. 
     Moreover, in the left bank portion  30 A and right bank portion  30 B of the cylinder block  30 , intake ports  35  communicated with the combustion chambers  34  are formed in the cylinder heads  31  inwardly of the cylinder axes P of the cylinders  33  in a width direction of the outboard motor. Furthermore, in the cylinder heads  31 , exhaust ports  36  communicated with the combustion chambers  34  are formed in the cylinder heads  31  outwardly of the cylinder axes P of the cylinders  33  in the left bank portion  30 A and right bank portion  30 B of the cylinder block  30  in the width direction of the outboard motor. 
     The crankcase  32  is coupled to the cylinder block  30 , thereby forming a crank chamber  37  in conjunction with the cylinder block  30 , and the crankshaft  26  is housed in the crank chamber  37 . Herein, in each of the above-mentioned left bank  27  and right bank  28 , plural cylinder assemblies (i.e., piston-cylinder assemblies)  40  each equipped with a cylinder  33 , a combustion chamber  34 , an intake port  35 , and an exhaust port  36  are arranged side by side in a vertical direction as shown in  FIGS. 3 and 4 . According to the present embodiment, three cylinder assemblies  40  are arranged side by side in the vertical direction in each of the right bank  28  and left bank  27 , thus constituting the engine  11  as a V-type six-cylinder four-stroke engine. 
     As shown in  FIGS. 2 and 3 , the respective exhaust ports  36  of the plural cylinder assemblies  40  in the left bank  27  are connected with a left exhaust passage  41  adapted to lead the exhaust gas from the exhaust ports  36  out of the engine  11 . The left exhaust passage  41  is formed integrally with the left bank portion  30 A of the cylinder block  30 . Further, the respective exhaust ports  36  of the plural cylinder assemblies  40  in the right bank  28  are connected with a right exhaust passage  42  adapted to lead the exhaust gas from the exhaust ports  36  out of the engine  11 . The right exhaust passage  42  is formed integrally with the right bank portion  30 B of the cylinder block  30 . Each of the left exhaust passage  41  and the right exhaust passage  42  includes an exhaust manifold  43  serving as a first exhaust passage section and a catalyst storage space  44  serving as a second exhaust passage section. 
     The exhaust manifold  43  is mounted on at least one of both the lateral sides, in the present embodiment, on both sides, of the cylinder block  30  in the width direction. That is, the exhaust manifold  43  of the left exhaust passage  41  is provided in lateral part of the cylinder block  30  on the left side in the width direction (left bank portion  30 A), corresponding to the left bank  27 , while the exhaust manifold  43  of the right exhaust passage  42  is provided in lateral part of the cylinder block  30  on the right side in the width direction (right bank portion  30 B), corresponding to the right bank  28 . 
     Furthermore, as shown in  FIG. 3 , in particular, the exhaust manifolds  43  collect the exhaust gas discharged from the respective exhaust ports  36  of the plural cylinder assemblies  40 . 
     In addition, a plurality of exhaust guiding portions  46  are provided for the respective exhaust manifolds  43  of the left exhaust passage  41  and right exhaust passage  42  so as to guide the exhaust gas discharged from the respective exhaust ports  36  of the plural cylinder assemblies  40  to connecting portions  45  between the exhaust manifolds  43  and catalyst storage chambers  44 . Each of the exhaust guiding portions  46  is formed as a vertical plane opposed to a joint surface (parting plane)  47  between the cylinder block  30  and the cylinder head  31 . The exhaust gas flowing in the exhaust port  36  is guided upward by the exhaust guiding portion  46  in the exhaust manifold  43 , and the exhaust gas then reaches the connecting portion  45  between the exhaust manifold  43  and the catalyst storage chamber  44 . 
     As shown in  FIG. 2 , the catalyst storage chamber  44  in the left exhaust passage  41  is formed integrally on the left bank portion  30 A of the cylinder block  30  and the catalyst storage chamber  44  in the right exhaust passage  42  is formed integrally on the right bank portion  30 B of the cylinder block  30 , both being, for example, substantially circular in passage section. 
     As shown in  FIG. 3 , the catalyst storage chambers  44  are communicated with both the connecting portions  45  of the exhaust manifolds  43  and an exhaust passage  51  of the engine holder  12 , thereby connecting exhaust manifolds  43  with an exhaust silencing chamber (i.e., muffler), not shown, inside the drive shaft housing  13  installed outside the engine  11 . Then, catalytic converters  53  having, for example, a circular shape in section for purifying the exhaust gas are installed and housed in the catalyst storage chambers  44 . 
     Each catalytic converter  53  is configured such that a catalyst carrier  54  formed into, for example, a columnar shape and equipped with an exhaust purification function is housed in a catalyst tube  55 , having a cylindrical shape, for example. When the catalyst carrier  54  comes into contact with exhaust gas, it chemically changes toxic substances such as carbon monoxide, hydrocarbon, nitrogen oxides, and the like contained in the exhaust gas into water, carbon dioxide, nitrogen or the like via oxidation-reduction reactions to thereby purify the exhaust gas. 
     Accordingly, the exhaust gas produced in the combustion chambers  34  of the plural cylinder assemblies  40  in the left bank  27  and right bank  28  of the engine  11  shown in  FIG. 2 , flows through the exhaust ports  36  of the cylinder assemblies  40  in the left bank  27  and right bank  28  and into the respective exhaust manifolds  43  of the left exhaust passage  41  and right exhaust passage  42 . 
     As shown in  FIG. 3 , the exhaust gas flowing into each exhaust manifold  43  ascends by being guided by the exhaust guiding portion  46 , and then reaches the connecting portion  45  between the exhaust manifold  43  and the catalyst storage chamber  44 . The exhaust gas flows downward in the connecting portion  45  into the catalytic converter  53  in the catalyst storage chamber  44  in order to be purified. 
     The exhaust gas purified by the catalytic converters  53  flows downward into the exhaust silencing chamber of the drive shaft housing  13 , thereby being expanded and silenced (muffled) therein. Subsequently, the exhaust gas flows in an exhaust passage, not shown, formed around the propeller shaft  24  in the gear case  14  shown in  FIG. 1  and is discharged into water from a center of the propeller  25 . 
     In  FIG. 2 , reference numeral  57  denotes an intake manifold connected to the intake port  35  of the engine  11  and adapted to introduce fuel/air mixture into the combustion chamber  34  through the intake port  35 . 
     Herein, as shown in  FIG. 4 , the outboard motor  10  shown in  FIG. 1  is provided with a cooling water passage structure  60  to cool the engine  11  by leading water, as cooling water, to the engine  11  from a sea or a river on which the hull  20  with the outboard motor  10  installed thereon navigates. The cooling water passage structure  60  includes an intake port  61  formed in the gear case  14 , a water passage  62  installed in the drive shaft housing  13  and provided with a water pump  63 , a combustion chamber periphery water jackets  65  and an exhaust port periphery water jackets  66  formed in the cylinder heads  31  on the left bank  27  and right bank  28  of the engine  11 , and cylinder periphery water jackets  67  and exhaust passage periphery water jackets  68  formed in the left bank portion  30 A and right bank portion  30 B of the cylinder block  30 . 
     As the gear case  14  is located in water during the use and operation of the outboard motor  10 , the intake port  61  is formed in the gear case  14  so as to be able to take in water by being located underwater. Furthermore, the water passage  62  installed in the drive shaft housing  13  includes the water pump  63  and has its lower end and upper end connected to the intake port  61  and a cooling water passage  64  of the engine holder  12 , respectively. The water pump  63  is installed in the drive shaft housing  13  near a mating surface between the drive shaft housing  13  and the gear case  14  and driven by the drive shaft  22 . 
     The water passage  62  takes in water through the intake port  61  when the water pump  63  operates, and supplies the water as cooling water to cooling water inlet ports  69  formed in the left bank portion  30 A and right bank portion  30 B of the cylinder block  30  through the cooling water passage  64  of the engine holder  12 . 
     The cooling water supplied to the cooling water inlet ports  69  in the left bank portion  30 A and right bank portion  30 B flows first through the combustion chamber periphery water jackets  65  in the cylinder heads  31  on the left bank  27  and right bank  28  as shown in  FIGS. 4 and 5  without cooling the cylinder block  30  to thereby cool the peripheries of the combustion chambers  34  of the plural cylinder assemblies  40  in the cylinder heads  31  on the left bank  27  and right bank  28 . Then, the cooling water flows through the exhaust port periphery water jackets  66  in the cylinder heads  31  on the left bank  27  and the right bank  28  to thereby cool the peripheries of the exhaust ports  36  of the plural cylinder assemblies  40  in the cylinder heads  31  on the left bank  27  and the right bank  28 . 
     Then, as shown in  FIGS. 4 and 7 , the cooling water flows simultaneously through the cylinder periphery water jackets  67  and the exhaust passage periphery water jackets  68  in the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  in parallel to thereby cool the peripheries of the cylinders  33  of the plural cylinder assemblies  40  in the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  as well as the peripheries of the left exhaust passage  41  and the right exhaust passage  42  (especially, peripheries of the exhaust manifolds  43  in the left exhaust passage  41  and the right exhaust passage  42  as well as the catalyst  53 ). 
     Subsequently, the cooling water is discharged out of the engine  11  through a thermostat case  70  of the engine  11 . In order for the cooling water to flow as described above, the combustion chamber periphery water jacket  65 , the exhaust port periphery water jackets  66 , the cylinder periphery water jacket  67 , and the exhaust passage periphery water jacket  68  are connected in sequence. 
     The combustion chamber periphery water jackets  65  formed in the cylinder heads  31  on the left bank  27  and the right bank  28  are formed around the combustion chambers  34  of the plural cylinder assemblies  40  in the cylinder heads  31  by being communicated with each other as shown in  FIGS. 4 and 5 . In the combustion chamber periphery water jacket  65 , as indicated by arrow “A”, the cooling water flows in from the side of the lowermost cylinder assembly  40 , ascends, and flows around the combustion chambers  34  in sequence to the side of an uppermost cylinder assembly  40 . 
     Consequently, the peripheries of the combustion chambers  34  in the cylinder heads  31  are cooled in sequence from the lowermost cylinder assembly  40  to the uppermost cylinder assembly  40 . 
     The exhaust port periphery water jackets  66  formed in the cylinder heads  31  on the left bank  27  and the right bank  28  are formed around the exhaust ports  36  of the plural cylinder assemblies  40  in the cylinder heads  31  by being communicated with each other as shown in  FIGS. 3, 4, and 5 . As indicated by arrow “B”, on the side of the uppermost cylinder unit  40 , the cooling water from the combustion chamber periphery water jacket  65  flows into the exhaust port periphery water jacket  66 , descends, and flows around the exhaust ports  36  in sequence to the side of the lowermost cylinder assembly  40 . 
     Consequently, the peripheries of the exhaust ports  36  in the cylinder heads  31  are cooled in sequence from the uppermost cylinder assembly  40  to the lowermost cylinder assembly  40 . 
     As shown in  FIGS. 3, 4, and 6 , a lowermost portion (on the side of the lowermost cylinder unit  40 ) of the exhaust port periphery water jacket  66  formed in the cylinder head  31  on each of the left bank  27  and the right bank  28  is communicated with a first exhaust manifold periphery water jacket  73 A (described hereinlater) of the exhaust passage periphery water jacket  68  in the corresponding one of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30 . 
     The first exhaust manifold periphery water jackets  73 A are communicated with lowermost portions of the cylinder periphery water jackets  67  in the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  through a communicating path  71  between the left bank portion  30 A and the right bank portion  30 B and communicated with catalyst periphery water jackets  74  (described hereinlater) of the exhaust passage periphery water jackets  68  in the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  through communicating paths  72  of the left bank portion  30 A and the right bank portion  30 B. 
     Further, as shown in  FIGS. 4, 6, and 7 , the cylinder periphery water jackets  67  formed in the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  are formed around the cylinders  33  of the plural cylinder assemblies  40  in the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  by being communicated with each other. The cooling water flowing into the cylinder periphery water jackets  67  from the lowermost portions (on the side of the lowermost cylinder units  40 ) of the exhaust port periphery water jackets  66  through the first exhaust manifold periphery water jackets  73 A and communicating path  71  flows into the side of the lowermost cylinder assemblies  40 , ascends, and flows around the cylinders  33  in sequence to the side of the uppermost cylinder assembly  40 , as indicated by arrow “C”. 
     Consequently, the peripheries of the cylinders  33  in the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30 , are cooled in sequence from the lowermost cylinder assembly  40  to the uppermost cylinder assembly  40 . 
     As shown in  FIGS. 2 to 4, 6, and 7 , the exhaust passage periphery water jacket  68  formed in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  includes the first exhaust manifold periphery water jacket  73 A, a second exhaust manifold periphery water jacket  73 B, a third exhaust manifold periphery water jacket  73 C, and the catalyst periphery water jacket  74 , which are communicated with one another. 
     Furthermore, the exhaust passage periphery water jacket  68  (actually, the second exhaust manifold periphery water jacket  73 B, the third exhaust manifold periphery water jacket  73 C, and the catalyst periphery water jacket  74 ) is configured into a separate circuit by being connected in parallel with the cylinder periphery water jacket  67 . 
     That is, as shown in  FIGS. 3 and 6 , in particular, the first exhaust manifold periphery water jacket  73 A is formed around a lower portion of the exhaust manifold  43  formed in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  and is communicated with the catalyst periphery water jacket  74  through the communicating path  72  as described above. The catalyst periphery water jacket  74  is formed by a gap  59  provided between an inner wall surface of the catalyst storage chamber  44  in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  and an outer lateral surface of the catalyst tube  55  of the catalytic converter  53  and is provided around the catalytic converter  53 . 
     As shown in  FIGS. 2, 3, and 7 , in particular, the second exhaust manifold periphery water jacket  73 B is formed around that part of the exhaust manifold  43  formed in each of the left bank portion  30 A and right bank portion  30 B of the cylinder block  30  which is located on the side of the cylinders  33 , with a lower part of the water jacket  73 B being communicated with the catalyst periphery water jacket  74  through a communicating path  75  and an upper part of the water jacket  73 B being communicated with the third exhaust manifold periphery water jacket  73 C through a communicating path  76 . 
     As shown in  FIGS. 3 and 7 , in particular, the third exhaust manifold periphery water jacket  73 C is formed around an upper portion (connecting portion  45 ) of the exhaust manifold  43  formed in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30 . The third exhaust manifold periphery water jacket  73 C is communicated with the second exhaust manifold periphery water jacket  73 B through the communicating path  76  as described above as well as communicated with the catalyst periphery water jacket  74  through a communicating path  77 , a communicating path  80  between an exhaust passage lid  78  and the water jacket lid  79 , and a communicating path  81  as shown in  FIG. 8 . 
     Thus, in the exhaust passage periphery water jacket  68  described above, as indicated by arrow “D” in  FIGS. 4, 6, 7, and 8 , the cooling water from the lowermost portions of the exhaust port periphery water jackets  66  flows into the first exhaust manifold periphery water jacket  73 A, and then flows into the catalyst periphery water jacket  74  through the communicating path  72  and ascends therein while flowing into and then ascending in the second exhaust manifold periphery water jacket  73 B through the communicating path  75  in parallel to the flow in the catalyst periphery water jacket  74 . The cooling water in the catalyst periphery water jacket  74  flows into the third exhaust manifold periphery water jacket  73 C through the communicating paths  77 ,  80 , and  81 . 
     The cooling water in the second exhaust manifold periphery water jacket  73 B also flows into the third exhaust manifold periphery water jacket  73 C through the communicating path  76 . The cooling water flowing through the exhaust passage periphery water jacket  68  in this way cools the periphery of the exhaust manifold  43  in each of the left exhaust passage  41  and the right exhaust passage  42  as well as the catalyst  53  in each of the left exhaust passage  41  and the right exhaust passage  42 . 
     As shown in  FIGS. 4 and 8 , the cooling water flowing into the third exhaust manifold periphery water jacket  73 C flows through a communicating path  82  formed in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  and joins with the cooling water flowing through the uppermost portion (on the side of the uppermost cylinder unit  40 ) of the cylinder periphery water jacket  67 . The joined cooling water flows into the thermostat case  70  as indicated by arrow “E” in  FIG. 4  and is discharged out of the engine  11  when a thermostat contained in the thermostat case  70 , not shown, opens. 
     Then, as shown in  FIG. 3 , a portion of the exhaust manifold  43  in each of the left exhaust passage  41  and the right exhaust passage  42 , i.e., a portion of the exhaust manifold  43  which is close to the exhaust guiding portion  46 , is positioned adjacent to the catalyst periphery water jacket  74  of the exhaust passage periphery water jacket  68 . 
     Consequently, the cooling water flowing through the catalyst periphery water jacket  74  has a function to cool not only the catalytic converter  53 , but also that portion, mentioned above, of the exhaust manifold  43  which is close to the exhaust guiding portion  46 . 
     Furthermore, as shown in  FIGS. 4 and 6 , the cylinder periphery water jacket  67  and the exhaust passage periphery water jacket  68  are formed in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30 , and in these water jackets, only the catalyst periphery water jacket  74  of the exhaust passage periphery water jacket  68  is formed and the cylinder periphery water jacket  67  is not formed in a region outside the cylinders  33  in the cylinder block  30  in the width direction of the outboard motor, i.e., in a portion “M” corresponding to the largest diameter portion of the catalytic converter  53  along a radial direction of the cylinders  33 . 
     Consequently, in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30 , the catalyst storage chamber  44  is formed close to the cylinders  33 . 
     Furthermore, as shown in  FIG. 3 , in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30 , an oxygen sensor  83  adapted to measure oxygen concentration in the exhaust gas is installed at that position, mentioned above, of the exhaust manifold  43  in each of the left exhaust passage  41  and the right exhaust passage  42  which faces a neighborhood of the connecting portion  45 . 
     The oxygen sensor  83  is intended to measure the oxygen concentration in the exhaust gas to detect whether the engine  11  achieves a theoretical air fuel ratio needed for the catalytic converter  53  to efficiently oxidize or reduce nitrogen oxides, hydrocarbons, and carbon monoxide. However, the oxygen sensor  83  is made of ceramics, and thus could be broken if the oxygen sensor  83  gets wet repeatedly at a time of being activated under high temperature conditions. 
     According to the structure and arrangement of the embodiment of the present invention described above, the following advantageous features (1) to (7) can be achieved. 
     (1) As shown in  FIG. 4 , since the cooling water taken in through the intake  61  and led to the water passage  62  flows through the combustion chamber periphery water jacket  65 , the exhaust port periphery water jackets  66 , the cylinder periphery water jacket  67 , and the exhaust passage periphery water jacket  68  in sequence, the cooling water heated in the combustion chamber periphery water jacket  65  and the exhaust port periphery water jackets  67  in sequence flows through the exhaust passage periphery water jacket  68 . Because of this reason, as shown in  FIG. 3 , the exhaust gas flowing through the exhaust manifold  43  in each of the left exhaust passage  41  and the right exhaust passage  42  is not cooled excessively by the cooling water, and as a result, it becomes possible to prevent condensation of the steam contained in the exhaust gas in the exhaust manifold  43 , and this fact in turn makes it possible to prevent the oxygen sensor  83  installed in the exhaust manifold  43  from getting wet and to thereby improve the durability of the oxygen sensor  83 . 
     (2) As shown in  FIG. 4 , in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30 , the cylinder periphery water jacket  67  and the exhaust passage periphery water jacket  68  (the second exhaust manifold periphery water jacket  73 B, the third exhaust manifold periphery water jacket  73 C, and the catalyst periphery water jacket  74 ) are connected in parallel and constitute a separate circuit. Consequently, as shown in  FIG. 7 , even in a case when the required temperature characteristics are different between a periphery of the cylinders  33  and peripheries of the left exhaust passage  41  and the right exhaust passage  42  (peripheries of the exhaust manifolds  43  in the left exhaust passage  41  and the right exhaust passage  42  as well as the catalyst  53 ), if the cooling water flow rate is adjusted by changing flow channel diameters of, for example, the cylinder periphery water jacket  67  and the exhaust passage periphery water jacket  68  according to the required temperature characteristics, the temperatures around the cylinders  33  as well as around the left exhaust passage  41  and the right exhaust passage  42  can be managed optimally. 
     (3) As shown in  FIGS. 4 and 5 , in the combustion chamber periphery water jacket  65  formed in the cylinder head  31 , when the cooling water flows in from the side of the lowermost cylinder unit  40 , ascends, and flows to the side of the uppermost cylinder unit  40 , the temperature of the portion of the combustion chamber periphery water jacket  65  which corresponds to the cylinder head  31  located on the side of the lowermost cylinder unit  40  into which the cooling water flows first, the periphery of the exhaust port  36  becomes lower than the temperature of the other cylinder assemblies  40  because the periphery of the combustion chamber  34  is cooled by the cooling water having low temperature. At this time, in the exhaust port periphery water jacket  66  formed in the cylinder head  31 , the cooling water from the combustion chamber periphery water jacket  65  on the side of the uppermost cylinder assembly  40  flows into the side of the uppermost cylinder assembly  40  and flows downward to the side of the lowermost cylinder assembly  40 , making it possible to prevent the temperature form falling around the exhaust port  36  on the side of the lowermost cylinder assembly  40  in the cylinder head  31 , and as a result, the cylinder head  31  can be cooled uniformly. 
     (4) As shown in  FIG. 3 , a portion of the exhaust manifold  43  in each of the left exhaust passage  41  and right exhaust passage  42 , i.e., a portion of the exhaust manifold  43  which is close to the exhaust guiding portion  46 , is positioned adjacent to the catalyst periphery water jacket  74  of the exhaust passage periphery water jacket  68 . Accordingly, the cooling water flowing through the catalyst periphery water jacket  74  cools not only the catalytic converter  53 , but also the portion, mentioned above, of the exhaust manifold  43  which is close to the exhaust guiding portion  46 , thereby eliminating the need for a water jacket used to cool that portion of the exhaust manifold  43  which is close to the exhaust guiding portion  46 , and therefore, an opening diameter of the catalytic converter  53  can be expanded, thereby decreasing pressure loss of the exhaust gas, and improving the power of the engine  11 . 
     Furthermore, since the exhaust gas flowing along the exhaust guiding portion  46  in the exhaust manifold  43  is cooled by the cooling water in the catalyst periphery water jacket  74  warmed by the catalytic converter  53 , the temperature of the exhaust gas flowing through the exhaust manifold  43  can be suppressed more securely from excessively falling down, which makes it possible to prevent the oxygen sensor  83  installed in the exhaust manifold  43  from getting wet and thus, further improving the durability of the oxygen sensor  83 . 
     (5) As shown in  FIGS. 3 and 6 , the catalyst periphery water jacket  74  of the exhaust passage periphery water jacket  68  is formed by the gap  59  between the inner wall surface of the catalyst storage space  44  in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30  and the outer lateral surface of the catalyst tube  55  of the catalytic converter  53  and is provided around the catalytic converter  53 . Accordingly, since the catalyst tube  55  of the catalytic converter  53  comes into direct contact with the cooling water in the catalyst periphery water jacket  74 , it becomes possible to improve the cooling efficiency of the catalytic converter  53 , and the volume of the cooling water flowing through the catalyst periphery water jacket  74  can be hence reduced. It also becomes possible to reduce a cross sectional area of a flow channel of the catalyst periphery water jacket  74  to thereby downsize the cylinder block  30  in which the catalyst periphery water jacket  74  is formed. 
     (6) The catalytic converter  53  is made up of the catalyst carrier  54  contained in the catalyst tube  55 . For example, as described in Patent Document 2 (Japanese Patent Laid-Open No. 2010-242744), when a catalyst storage portion is formed integrally with a housing which makes up an exhaust passage, the catalyst storage portion has a thick-walled structure. In contrast, according to the present embodiment, since the catalyst carrier  54  is contained in the catalyst tube  55  of the thin-walled structure, the catalytic converter  53  can be downsized accordingly. Thus, if the catalytic converter  53  is not downsized, the opening diameter of the catalyst carrier  54  can be increased and the pressure loss of the exhaust gas flowing through the catalyst carrier  54  decreases. As a result, the power of the engine  11  can be improved. 
     (7) As shown in  FIG. 6 , in a region outside the cylinders  33  in the cylinder block  30  in the width direction of the outboard motor, i.e., in the portion “M” corresponding to the largest diameter portion of the catalytic converter  53  along the radial direction of the cylinders  33 , only the catalyst periphery water jacket  74  of the exhaust passage periphery water jacket  68  is formed and the cylinder periphery water jacket  67  is not formed. As a result, the catalyst storage chamber  44  can be formed close to the cylinders  33  in each of the left bank portion  30 A and the right bank portion  30 B of the cylinder block  30 , thereby reducing the size of the outboard motor  10  in the width direction. Further, when a plurality of the outboard motors  10  are installed side by side on the transom  20 A of the hull  20 , the plural outboard motors  10  can be clustered in a center of the hull  20  in the width direction by reducing installation intervals of the plural outboard motors  10 . 
     It is to be noted that the present invention is not limited to the embodiments described above as preferred examples, and many other changes, modifications, and alternations may be made without departing from the sprits of the present invention and scope of the appended claims. 
     For example, although in the embodiment described above, the engine  11  mounted on the outboard motor  10  is a V-type multi-cylinder four-stroke engine, an in-line multi-cylinder four-stroke type or single-cylinder four-stroke type may be adopted as the engine  11 .