Abstract:
To provide an exhaust structure for a small watercraft that can cool exhaust gas, reduce the size and weight, and thereby cost of a backflow prevention chamber. An exhaust structure for a small watercraft is configured to be able to cool exhaust gas in the course of leading it to the outside when the exhaust gas of an engine is led to the outside for discharge. This exhaust structure includes a water muffler containing water therein. A backflow prevention chamber is disposed above the water muffler and adapted to prevent water from entering the side of the engine from the side of the water muffler. The backflow prevention chamber is provided with a cooling water injecting portion adapted to spray cooling water to the exhaust gas in an internal space. An outlet is formed in the bottom portion of the backflow prevention chamber to communicate with the water muffler.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2006-049256, filed in Japan on Feb. 24, 2006, the entirety of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to an exhaust structure for a small watercraft that can cool exhaust gas by using cooling water on the way to the outside when the exhaust gas is led from an engine to the outside for discharge. 
   DESCRIPTION OF BACKGROUND ART 
   One exhaust structure for a small watercraft is disclosed in Japanese Patent Laid-open No. 2003-176719. This exhaust structure includes an exhaust system for discharging the exhaust gas of an engine to the outside. A water muffler is provided in the middle of the exhaust system. An exhaust body (backflow prevention chamber) adapted to prevent water from entering toward the engine from the water muffler is disposed above the water muffler. In addition, a water jacket is provided in the circumferential wall of the backflow prevention chamber. 
   When causing exhaust gas from the engine to flow in the exhaust system and discharge it to the outside, such an exhaust structure for a small watercraft can cool the exhaust gas in the backflow prevention chamber by allowing cooling water to flow in the water jacket. 
   However, the exhaust structure of Japanese Patent Laid-open No. 2003-176719 is provided with the water jacket serving as a cooling water passage, in the circumferential wall of the backflow prevention chamber. It is necessary, therefore, to relatively increase the thickness of the circumferential wall of the backflow prevention chamber. This hinders the downsizing and weight saving of the backflow prevention chamber. In addition, the provision of the water jacket in the circumferential wall of the backflow prevention chamber complicates the shape of the chamber, which hinders the decrease in the cost of the chamber. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an exhaust structure for a small watercraft that can cool exhaust gas, reduce the size and weight, and thereby reduce the cost of a backflow prevention chamber. 
   A first aspect of the present invention is directed to an exhaust structure for a small watercraft that uses cooling water to cool exhaust gas on the way to the outside when the exhaust gas is led from an engine to the outside for discharge. The exhaust structure includes: a water muffler disposed in the course of leading the exhaust gas to the outside to receive the cooling water that has cooled the exhaust gas; a backflow prevention chamber disposed above the water muffler, having a bottom portion formed thereat with an outlet communicating with the water muffler, and adapted to prevent water entering a side of the engine from a side of the water muffler; and a cooling water injecting portion disposed in the backflow prevention chamber and adapted to lead and spray the cooling water to the exhaust gas in the backflow prevention chamber. 
   According to a second aspect of the present invention, the backflow prevention chamber is internally provided with a gas introduction pipe adapted to lead exhaust gas into the backflow prevention chamber, the gas introduction pipe is disposed to extend across above the outlet, and a cooling water injection port of the cooling water injecting portion is disposed in the vicinity of the outlet of the gas introduction pipe. 
   According to a third aspect of the present invention, the cooling water injecting portion includes: a cooling water passage adapted to lead the cooling water; and a cooling water injection port adapted to inject and spray the cooling water led through the cooling water passage, to the exhaust gas in the backflow prevention chamber. In the third aspect of the present invention, the cooling water passage is formed of a member independent of the backflow prevention chamber. 
   According to the first aspect of the present invention, the backflow prevention chamber is provided with the cooling water injecting portion, which sprays cooling water to the exhaust gas in the backflow prevention chamber. There is an advantage with this structure in that spraying cooling water therein can cool the exhaust gas. 
   Thus, it is not necessary to provide a water jacket serving as a cooling water passage in the circumferential wall of the backflow prevention chamber. Since this eliminates an unnecessary increase in thickness of the circumferential wall of the backflow prevention chamber, there is an advantage in that the backflow prevention chamber can be reduced in size and weight. 
   In addition, since there is no need to provide a water jacket in the circumferential wall of the backflow prevention chamber, there is an advantage in that the configuration of the backflow prevention chamber can be simplified, thereby reducing cost. 
   The cooling water sprayed in the exhaust gas flows on the bottom portion of the backflow prevention chamber. Therefore, the outlet communicating with the water muffler is provided in the bottom portion of the backflow prevention chamber. 
   The cooling water sprayed in the exhaust gas in the backflow prevention chamber flows on the bottom portion of the backflow prevention chamber and is satisfactorily led from the outlet of the bottom portion to the water muffler. This provides an advantage in that the cooling water sprayed in the exhaust gas can be prevented from entering toward the engine side. 
   According to the second aspect of the present invention, the gas introduction pipe is provided in the backflow prevention chamber and the cooling water injection port adapted to inject cooling water is provided in the vicinity of the outlet of the gas introduction pipe. Thus, when the exhaust gas is led from the outlet of the gas introduction chamber to the backflow prevention chamber, the cooling water can be sprayed in the exhaust gas thus led. 
   This makes it possible to satisfactorily spray the cooling water in the exhaust gas before the exhaust gas is led to the backflow prevention chamber for diffusion, providing an advantage of preferably cooling the exhaust gas. Furthermore the gas introduction prevention chamber is disposed across above the outlet provided in the bottom portion of the backflow prevention chamber. This enables the circumferential wall of the gas introduction to isolate the inside of the gas introduction pipe from the outlet of the backflow prevention chamber. 
   Thus, the cooling water sprayed in the exhaust gas is satisfactorily prevented from flowing in the inside of the gas introduction pipe when led from the outlet of the bottom portion to the water muffler. This provides an advantage in that the cooling water sprayed in the exhaust gas can be further satisfactorily prevented from entering toward the engine side through the inside of the gas introduction pipe. 
   According to the third aspect of the present invention, the cooling water passage is formed of a member independent of the backflow prevention chamber. Therefore, it can be an independent member. This can simplify the configuration of the cooling water passage, thereby providing an advantage of facilitating fabrication. 
   Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a side view of a small watercraft according to an embodiment of the present invention; 
       FIG. 2  is a perspective view illustrating the inside of the small watercraft according to an embodiment of the present invention; 
       FIG. 3  is a perspective view of an exhaust structure of the watercraft according to an embodiment of the present invention; 
       FIG. 4  is a cross-sectional view of a backflow prevention chamber according to the an embodiment of present invention; 
       FIG. 5  is a cross-sectional view of a second exhaust pipe of the exhaust structure according to an embodiment of the present invention; 
       FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 3 ; and 
       FIG. 7  is a diagram for assistance in explaining an example of cooling exhaust gas by means of the exhaust structure of watercraft according to an embodiment of the present invention; 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be hereinunder described with reference to the accompanying drawings. It should be noted that the terms “front”, “rear”, “left” and “right” denote the direction as viewed from an operator and the symbols Fr, Rr, L and R denotes the front, the rear or back, the left and the right, respectively. 
     FIG. 1  is a side view of a small watercraft according to an embodiment of the present invention. The small watercraft  10  is a water-jet propelling boat that includes a fuel tank  12  disposed at a front portion  11   a  of a watercraft body  11 . An engine  13  is disposed rearward of the fuel tank  12 . A pump chamber  14  is disposed rearward of the engine  13 . A water-jet pump  15  is placed in the pump chamber  14 . A drive shaft  17  is adapted to connect the water-jet pump  15  to an output shaft  16  of the engine  13 . In addition, the water-jet propelling boat includes an air intake structure  20  adapted to supply air to the engine  13 . An exhaust structure (exhaust structure for a small watercraft)  21  is adapted to discharge exhaust gas from the engine  13 . A steering handlebar  22  is disposed above the fuel tank  12 . A seat  23  is disposed rearward of the steering handlebar  22 . 
   The watercraft body  11  is configured such that a hull  25 , constituting the lower portion of the watercraft body  11 , is covered by and joined to a deck  26 , constituting the upper portion of the watercraft body  11 . With the small watercraft  10 , the engine  13  is driven to rotate the drive shaft  17 , thereby rotating an impeller  18 , which is built in the water-jet pump  15 . Rotating the impeller  18  takes in water from a suction opening  19  at the bottom of the watercraft and the water taken in is ejected rearward of the watercraft body  11  from a steering nozzle  24  through the inside of the water-jet pump  15 . Thus, the watercraft  10  is propelled (glides) forwardly. 
   On the other hand, when the watercraft  10  is moved rearward, a reverse bucket  27  disposed above the steering nozzle  24  is shifted to a reverse position located rearward of the steering nozzle  24 . Thus, the water ejected rearward from the steering nozzle  24  is led to the front of the watercraft body  11 , whereby the ejected water thus led moves the watercraft  10  backward. 
     FIG. 2  is a perspective view illustrating the inside of the watercraft according to an embodiment of the present invention. The engine  13  includes a plurality of cylinders  28  arranged in the back-and-forth direction of the watercraft body  11  and left-hand and right-hand mount portions fastened to the hull  25  with bolts  29 . In short, the engine  13  is longitudinally mounted on the watercraft body  11 . 
   The output shaft  16  projects from the rear lower end of the engine  13  toward the back of the watercraft body  11 . The drive shaft  17  is coupled to the output shaft  16  via a connection coupler  31 . The drive shaft  17  is rotatably journaled by a bearing portion  32  and is coupled at its rear end to the water-jet pump  15  (see  FIG. 1 ). A flange portion  33  provided on the bearing portion  32  is fastened to the hull  25  with bolts and nuts. 
   Since the engine  13  is mounted longitudinally with respect to the watercraft body  11 , an intake passage  35  is disposed on the left side (one side) of the plurality of cylinders  28  and an exhaust passage  36  is disposed on the right side (the other side) of the cylinders  28 . 
   The intake passage  35  constitutes, e.g. an intake manifold and the exhaust passage  36  constitutes, e.g. an exhaust manifold. The intake passage  35  is a member constituting part of the intake structure  20  and communicates with the cylinders  28 . The exhaust passage  36  is a member constituting part of the exhaust structure  21  and communicates with the cylinders  28 . 
   The air intake structure  20  includes an air cleaner  41 , a turbocharger  42 , an inter-cooler  43 , a distribution passage  44  and the intake passage  35 . The air cleaner  41  communicates with the turbocharger  42  via an air introduction pipe  46 . The turbocharger  42  communicates with the inter-cooler  43 , which in turn communicates with the distribution passage  44  via a distribution passage communicating portion  48 . The distribution passage  44  communicates with an intake port of the intake passage  35 . 
   The exhaust structure  21  of a small watercraft includes an exhaust passage  36 , a first exhaust pipe  51 , a second exhaust pipe  52 , a backflow prevention chamber  53 , a cooling water injecting portion  66 , a water muffler  54  and an exhaust hose  55 . Incidentally, the backflow prevention chamber  53  and the cooling water injecting portion  66  are detailed with reference to  FIGS. 3 through 7 . 
   The first exhaust pipe  51  is provided to extend from the rear end portion of the exhaust passage  36  toward the rearward of the watercraft body. The rear end portion  51   a  of the first exhaust pipe  51  communicates with a second exhaust pipe  52  and with the turbocharger  42 . The second exhaust pipe  52  communicates with the backflow prevention chamber  53 , which communicates with the water muffler  54 . The water muffler  54  communicates with the exhaust hose  55 , whose discharge port  55   a  faces the inside of the pump chamber  14  (see  FIG. 1 ). 
   According to the air intake structure  20  and the exhaust structure  21  for a small watercraft, exhaust gas is led from the inside of the cylinders  28  to the exhaust passage  36  and then led therefrom to the first exhaust pipe  51 . Most of the exhaust gas led to the first exhaust pipe  51  is discharged through the second exhaust pipe  52 , the backflow prevention chamber  53 , the water muffler  54  and the exhaust hose  55 . 
   On the other hand, the remaining of the exhaust gas led to the first exhaust pipe  51  is led to the turbine casing  42   a . The exhaust gas led to the turbine casing  42   a  rotates turbine impellers (not shown). The rotating turbine impellers rotate compressor impellers (not shown) in the compressor casing  42   b.    
   Air is sucked in the air cleaner  41  simultaneously with the rotation of the compressor impellers. The air sucked in the air cleaner  41  is led into the compressor casing  42   b  of the turbocharger  42  through the air introduction pipe  46 . Since the compressor impellers (not shown) in the compressor casing  42   b  are rotated as described earlier, the air led in the compressor casing  42   b  is compressed and led into the inter-cooler  43  therefrom. 
   The air cooled in the inter-cooler  43  is led to the distribution passage  44  via the distribution passage communicating portion  48 . The air led into the distribution passage  44  is led to the intake passage  35  and supplied to the cylinders  28  therefrom. 
     FIG. 3  is a perspective view illustrating the exhaust structure for a small watercraft according to an embodiment of the invention and  FIG. 4  is a cross-sectional view of the backflow prevention chamber of the exhaust structure according to an embodiment of the invention. 
   The second exhaust pipe  52  extends upward from a front end portion  52   a  thereof and a front wall portion  57  of the backflow prevention chamber  53  is joined to a rear end portion  52   b  of the second exhaust pipe  52 . A passage  58  of the second exhaust pipe  52  communicates with an opening  57   a  of the backflow prevention chamber  53 . This opening  57   a  is formed in the front wall portion  57  of the backflow prevention chamber  53 . 
   A water jacket  64  (see  FIGS. 5 and 6 ) is provided in the circumferential wall  63  of the second exhaust pipe  52 . A cooling water introduction pipe  65  is joined to the front end portion  52   a  of the second exhaust pipe  52 . The cooling water introduction pipe  65  communicates with the water jacket  64 . 
   The backflow prevention chamber  53  is such that an outer cylindrical body  56  is disposed above the water muffler  54 , the front wall portion  57  is disposed at the front end portion  56   a  of the outer cylindrical body  56 . A rear wall portion  59  is disposed at the rear end portion  56   b  of the outer cylindrical body  56 . An inner cylindrical body (hereinafter referred to as a “gas introduction pipe”)  60  is provided inside the outer cylindrical body  56 . 
   Upper and lower cooling water injection ports  67   a  and  68   a  are disposed near the rear end portion  60   a  of the gas introduction pipe  60 . The upper and lower cooling water injection ports  67   a ,  68   a  are members constituting the injection port of the cooling water injecting portion  66 . 
   The water muffler  54  is disposed in the course of directing the exhaust gas discharged from the engine  13  (see  FIG. 2 ) to the outside and receives the cooling water that cooled the exhaust gas. This water muffler  54  is the same as one used in common small watercrafts. 
   The outer cylindrical body  56  is formed almost cylindrical and is disposed parallel to the water muffler  54  so as to face in the back-and-forth direction of the watercraft body  11  (see  FIG. 1 ). The outer cylindrical body  56  is such that the front wall portion  57  is formed with the opening  57   a  and the rear end portion  56   b  is slightly tapered and closed with the rear wall portion  59 . 
   The outer cylindrical body  56  is formed with a recessed portion  61  at a portion, of the rear end portion  56   b , facing the central side of the watercraft body  11 . The recessed portion  61  is formed with upper and lower attachment holes  61   a ,  61   b . The upper and lower attachment holes  61   a  and  61   b  are adapted to be attached with upper and lower cooling water passages  67  and  68 , respectively. The upper and lower cooling water passages  67 ,  68  are members constituting the cooling water passage of the cooling water injecting portion  66  described later. 
   A back surface  62  of the recessed portion  61  is formed with the upper and lower cooling water injection ports  67   a  and  68   a . The back surface  62  of the recessed portion  61  is a wall surface, of the inner wall of the outer cylindrical body  56 , corresponding to the recessed portion  61 . 
   The upper and lower cooling water injection ports  67   a ,  68   a  are disposed in a space between the rear end portion  60   a  of the gas introduction pipe  60  and the rear wall portion  59  of the outer cylindrical body  56  and in the vicinity of an outlet  60   d  of the gas introduction pipe  60 . The upper and lower cooling water injection ports  67   a  and  68   a  communicate with the upper and lower attachment holes  61   a  and  61   b , respectively, (see  FIG. 3 ). 
   Thus, the attachment of the upper cooling water passage  67  to the upper attachment hole  61   a  causes the upper cooling water passage  67  to communicate with the upper cooling water injection port  67   a . Similarly, the attachment of the lower cooling water passage  68  to the lower attachment hole  61   b  causes the lower cooling water passage  68  to communicate with the lower cooling water injection port  68   a.    
   The outer cylindrical body  56  is formed at its bottom portion  56   c  with an outlet  71 , from which an outlet pipe  72  extends downward. The outlet pipe  72  is inserted into an inlet pipe  73  of the water muffler  54  and a gap between the outlet pipe  72  and the inlet pipe  73  is tightly closed with a communication hose  74 . 
   The gas introduction pipe  60  is a hollow cylindrical body having a diameter smaller than that of the outer cylindrical body  56  and is disposed in an internal space  76  of the outer cylindrical body  56  coaxially therewith. The gas introduction pipe  60  is formed with an inlet  60   c  in the front end portion  60   b  and with an outlet  60   d  in the rear end portion  60   a . The front end portion  60   b  of the gas introduction pipe  60  is provided on the front wall portion  57  and along the lip of the opening  57   a . In short, the opening  57   a  of the front wall portion  57  communicates with the inlet  60   c  of the front end portion  60   b.    
   The passage  58  of the second exhaust pipe  52  communicates with the internal space  76  of the backflow prevention chamber  53  through the opening  57   a , the inlet  60   c , and the outlet  60   d . In this way, the exhaust gas in the passage  58  is led in the internal space  76 , led therefrom to the outlet pipe  72  via the outlet  71  and then led therefrom to the water muffler  54 . 
   Since disposed coaxially with the outer cylindrical body  56 , the gas introduction pipe  60  is provided across above the outlet  71 . This enables the circumferential wall of the gas introduction pipe  60  to isolate the inside of the gas introduction pipe  60  from the outlet  71 . 
   If the watercraft  10  (see  FIG. 1 ) turns over, it is probable that the water provided in the water muffler  54  enters the internal space  76  from the outlet  71  via the outlet pipe  72 . For this reason, as described above, the circumferential wall of the gas introduction pipe  60  isolates the inside of the gas introduction pipe  60  from the outlet  71 , whereby the water entering the internal space  76  from the outlet  71  is interrupted by the circumferential wall of the gas introduction pipe  60 . 
   In this way, the water entering the internal space  76  from the outlet  71  can be prevented from entering the inside of the gas introduction pipe  60  from the engine  13  side shown in  FIG. 2 . In short, the backflow prevention chamber  53  has a backflow prevention function, which prevents water entering from the side of the water muffler  54  to the side of the engine  13 . 
   The backflow prevention chamber  53  is provided with the cooling water injecting portion  66  as described earlier. The cooling water injecting portion  66  leads and sprays cooling water to the exhaust gas in the backflow prevention chamber  53 . 
   The cooling water injecting portion  66  includes the upper and lower cooling water passages  67 ,  68  adapted to lead cooling water. The upper and lower cooling water injection ports  67   a ,  68   a  are adapted to inject and spay the cooing water led through the upper and lower cooling water passages  67 ,  68  to the exhaust gas in the backflow prevention chamber  53 . The upper and lower cooling water passages  67 ,  68  are formed of members independent of the backflow prevention chamber  53 . 
   The upper cooling water passage  67  is composed of a front end portion  67   b , a front pipe portion  67   c , a central pipe portion  67   d  and a rear pipe portion  67   e . The front end portion  67   b  is joined to the side wall  52   c  of the rear end portion  52   b . The front pipe portion  67   c  extends from the front end portion  67   b  toward the central side of the watercraft body  11  (see  FIG. 1 ). The central pipe portion  67   d  extends from an end portion of the front pipe portion  67   c  toward the rearward of the watercraft body  11 , passing along the side wall  56   d  of the outer cylindrical body  56 . The rear pipe portion  67   e  extends from the rear end portion of the central pipe portion  67   d  toward the bottom surface of the recess portion  61 . Thus, the upper cooling water passage  67  is formed in an approximate shallow-U-shape as viewed from above. 
   The end portion of the rear pipe portion  67   e  is attached to the upper attachment hole  61   a  so that the rear pipe portion  67   e  communicates with the upper cooling water injection port  67   a.    
   The lower cooling water passage  68  is disposed below and in parallel to the upper cooling water passage  67 . Similarly to the upper cooling water passage  67 , the lower cooling water passage  68  is composed of a front end portion  68   b , a front pipe portion  68   c , a central pipe portion  68   d  and a rear pipe portion  68   e . The front end portion  68   b  is joined to the side wall  52   c  of the rear end portion  52   b . The front pipe portion  68   c  extends from the front end portion  68   b  toward the central side of the watercraft body  11  (see  FIG. 1 ). The central pipe portion  68   d  extends from an end portion of the front pipe portion  68   c  toward the rearward of the watercraft body  11 , passing along the side wall  56   d  of the outer cylindrical body  56 . The rear pipe portion  68   e  extends from the rear end portion of the central pipe portion  68   d  toward the bottom surface of the recessed portion  61 . Thus, the lower cooling water passage  68  is formed in an approximate shallow-U-shape as viewed from above. 
   The end portion of the rear pipe portion  68   e  is attached to the lower attachment hole  61   b  so that the rear pipe portion  68   e  communicates with the upper cooling water injection port  68   a.    
   The upper and lower cooing water injection ports  67   a ,  68   a  are disposed close to the outlet  60   d  of the gas introduction pipe  60 . Therefore, cooling water can satisfactorily be sprayed to exhaust gas that does not yet diffuse in the internal space  76 . 
   As described above, the front pipe portion  67   c , the central pipe portion  67   d , and the rear pipe portion  67   e  which constitute the upper cooling water passage  67  are each made independent of the outer cylindrical body  56 . Similarly, the front pipe portion  68   c , the central pipe portion  68   d , and the rear pipe portion  68   e  which constitute the lower cooling water passage  68  are each made independent of the outer cylindrical body  56 . 
   As described above, since the upper and lower cooling water passages  67  and  68  are made independent of the outer cylindrical body  56 , the upper and lower cooling water passages  67  and  68  can be made of separate members. This can simplify the respective configurations of the upper and lower cooling water passages  67 ,  68 , thereby facilitating fabrication. 
     FIG. 5  is a cross-sectional view of the second exhaust pipe of the exhaust structure according to the present invention and  FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 3 . 
   The second exhaust pipe  52  is such that the circumferential wall  63  forms the passage  58  and the water jacket  64  is provided in the circumferential wall  63 . The passage  58  communicates with the passage of the first exhaust pipe  51  shown in  FIG. 2  at the front end portion  52   a  of the second exhaust pipe  52 . Furthermore, the passage  58  communicates with the opening  57   a  of the backflow prevention chamber  53  at the rear end  52   b  of the second exhaust pipe  52 . 
   Thus, exhaust gas led from the first exhaust pipe  51  to the passage  58  as indicated with arrows is led therefrom to the opening  57   a  of the backflow prevention chamber  53  as indicated with arrows. 
   Exhaust gas led to the opening  57   a  is led from the inlet  60   c  of the gas introduction pipe  60  into the gas introduction pipe  60  as indicated with arrows and, through the inside of the gas introduction pipe  60 , led from the outlet  60   d  to the internal space  76  of the backflow prevention chamber  53  as indicated with arrows. 
   The water jacket  64  communicates with the cooling water introduction pipe  65  via an introduction port  64   a  at the front end portion  52   a  of the second exhaust pipe  52 . Furthermore, the water jacket  64  communicates with the upper cooling water passage  67  via an upper outlet  64   b  at the rear end portion  52   b  of the second exhaust pipe  52 . In addition, the water jacket  64  communicates with the lower cooling water passage  68  via a lower outlet  64   c  at the rear end portion  52   b  of the second exhaust pipe  52 . 
   Thus, cooling water led from the cooling water introduction pipe  65  to the water jacket  64  as indicated with arrows is led to the upper cooling water passage  67  via the upper outlet  64   b  as indicated with arrows and also to the lower cooling water passage  68  via the lower outlet  64   c  as indicated with arrows. The cooling water led to the lower cooling water passage  68  is led to the lower cooling water injection port  68   a  via the lower cooling water passage  68  and is injected therefrom into the internal space  76  of the outer cylindrical body  56 . 
   In addition, similarly to the cooling water led to the lower cooling water passage  68 , the cooling water led to the upper cooling water passage  67  is led to the upper cooling water injection port  67   a  (see  FIG. 4 ) via the upper cooling water passage  67  and injected therefrom into the internal space  76  of the outer cylindrical body  56 . 
   A description will now be provided of an example in which exhaust gas is cooled by the exhaust structure  21  for a small watercraft with reference to  FIG. 7 .  FIG. 7  is a diagram for assistance in explaining the example in which exhaust gas is cooled by the exhaust structure for a small watercraft. 
   It is to be noted that for easy understanding of a flow of cooling water,  FIG. 7  illustrates that the upper and lower cooling water injection ports  67   a  and  68   a  are provided at the leading ends of the upper and lower cooling water passages  67  and  68 , respectively. 
   Exhaust gas is led from the first exhaust pipe  51  shown in  FIG. 2  to the passage  58  of the second exhaust pipe  52  as indicated with arrow A. The exhaust gas led to the passage  58  is led therethrough toward the opening  57   a  of the backflow prevention chamber  53 . The exhaust gas led to the opening  57   a  is led from the inlet  60   c  of the gas introduction pipe  60  into the gas introduction pipe  60  as indicated with arrow B. The exhaust gas led into the gas introduction pipe  60  is led therethrough from the outlet  60   d  to the internal space  76  of the backflow prevention chamber  53  as indicated with arrow C. 
   On the other hand, the cooling water led through the cooling water introduction pipe  65  as indicated with arrow D is led therefrom to the water jacket  64  (see  FIG. 5 ). The cooling water led to the water jacket  64  is led to the upper cooling water passage  67  as indicated with arrow E and also to the lower cooling water passage  68  as indicated with arrow F. 
   The cooling water led to the upper cooling water passage  67  is led therethrough to the upper cooling water injection port  67   a  and injected therefrom in the internal space  76  of the outer cylindrical body  56  as indicated with arrow G The cooling water led to the lower cooling water passage  68  is led therethrough to the lower cooling water injection port  68   a  and injected therefrom in the internal space  76  of the outer cylindrical body  56  as indicated with arrow H. 
   The upper and lower cooling water injection ports  67   a ,  68   a  are disposed in the vicinity of the outlet  60   d  of the gas introduction pipe  60 . The cooling water injected from the upper and lower cooling water injection ports  67   a ,  68   a  can be sprayed in the exhaust gas led to the internal space  76  from the outlet  60   d.    
   Thus, the cooling water can be satisfactorily sprayed in the exhaust gas that does not yet diffuse in the internal space  76 , thereby satisfactorily spraying the cooling water in the exhaust gas to preferably cool the exhaust gas. This eliminates a water jacket serving as a cooling water passage otherwise to be provided in the circumferential wall of the outer cylindrical body  56 . 
   The exhaust gas cooled by cooling water is led downward by the rear wall portion  59  (see  FIG. 4 ) as indicated with arrow I by way of example. The exhaust gas led downward is led to the bottom portion  56   c  and then led therefrom through the outlet  71  into the outlet pipe  72  as indicated with arrow J. The exhaust gas led into the outlet pipe  72  is led into the water muffler  54 . 
   On the other hand, the cooling water sprayed to the exhaust gas is led to the bottom portion  56   c  as indicated with arrow K and then led therefrom through the outlet  71  into the outlet pipe  72  as indicated with arrow L. The cooling water led into the outlet pipe  72  is led into the water muffler  54 . 
   Thus, the cooling water sprayed to the exhaust gas can be prevented from entering the second exhaust pipe  52 , thereby preventing the cooling water from entering toward the engine  13  shown in  FIG. 2 . 
   The gas introduction pipe  60  is disposed to extend across above the outlet  71  provided in the bottom portion  56   c  of the outer cylindrical body  56 . Therefore, the circumferential wall of the gas introduction pipe  60  can isolate the inside of the gas introduction pipe  60  from the outlet  71 . Thus, when led from outlet  71  to the water muffler  54 , the cooling water sprayed to the exhaust gas can be satisfactorily prevented from entering the second exhaust pipe  52  from the inside of the gas introduction pipe  60 . 
   In this way, the cooling water sprayed to the exhaust gas can be further satisfactorily prevented from entering toward the engine  13  through the inside of the gas introduction pipe  60 . In short, the backflow prevention chamber  53  has a backflow prevention function of preventing the cooling water sprayed to the exhaust gas from entering toward the engine  13  through the inside of the gas introduction pipe  60 . 
   As described above, the exhaust structure  21  of the small watercraft can satisfactorily cool exhaust gas by using cooling water in the course of leading the exhaust gas to the outside when the exhaust gas is led from the engine  13  shown in  FIG. 2  to the outside for discharge. 
   Incidentally, the present embodiment describes the example in which the cooling water injecting portion  66  is provided with the two, upper and lower cooling water passages  67 ,  68 . However, the number of the cooling water passages can be selected arbitrarily. 
   In addition, the present embodiment describes the example in which the backflow prevention chamber  53  is provided with the two, upper and lower cooling water injection ports  67   a ,  68   a . However, the present invention is not limited to this embodiment. The upper and lower cooling water injection ports  67   a  and  68   a  may be directly provided at the upper and lower cooling water passages  67  and  68 , respectively. 
   Furthermore, the shape of the upper and lower cooling water passages  67 ,  68  exemplified in the embodiment is modifiable appropriately. 
   The present invention can be preferably applicable to an exhaust structure of a small watercraft that can cool exhaust gas on the way to the outside when the exhaust gas is led from an engine thereto. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.