Patent Publication Number: US-6659821-B2

Title: Exhaust structure for jet propulsion boat

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2001-249277, filed Aug. 20, 2001, the entire contents of are which hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust structure for a jet propulsion boat in which exhaust gas from the engine is discharged into the pump chamber by providing a jet propulsion machine inside a pump chamber of the hull and connecting an exhaust pipe to the pump chamber. 
     2. Description of Background Art 
     The jet propulsion boat is a vessel provided with a jet pump mounted at the rear portion of the hull, and propelled by drawing in water from the vessel bottom by driving the jet pump by the engine, and forcing the water rearwardly. 
     An exhaust structure for a jet propulsion boat has been disclosed in Japanese Patent Laid-Open No. 282840/2000 entitled “exhaust structure for a jet propulsion boat”. The means to lower the exhaust noise generated in the jet propulsion boat is disclosed in the same publication. According to this technology, a resonator for sound-deadening is provided on the exhaust pipe connected to the engine. Exhaust noise is resonated by means of the resonator, so that the exhaust noise is reduced. 
     In the above disclosure, a part of the exhaust pipe is formed into a substantially U-shape which is upwardly convex in order to prevent water from entering from the outlet port of the exhaust pipe into the engine side. Forming a part of the exhaust pipe into a substantially U-shape makes the length of the exhaust pipe relatively long. Thus, in order to attenuate the noise in the elongated exhaust pipe, the length of the resonator must be long relative to the exhaust pipe. 
     Therefore, in order to mount the elongated resonator inside the hull, a sufficient storage space must be secured. 
     However, the space in the hull is limited. Thus, the layout of the required accessories of the boat which are also to be mounted inside the hull must be considered, in order to secure the relatively large storage space required for the resonator in this limited space. In other words, securing a storage space for and mounting the resonator, which is required for reducing the exhaust noise of the jet propulsion boat, has proved to be difficult. 
     Accordingly, it is an object of the present invention to provide an improved exhaust structure for a jet propulsion boat that addresses the above problem. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     In order to solve the problem, the present invention provides an exhaust structure for a jet propulsion boat in which a tunnel-shaped pump chamber is provided at the rear portion of the hull, a jet propulsion machine is provided in the pump chamber, an engine is connected to the jet propulsion machine for driving the boat, and an exhaust port of the exhaust pipe extending from the engine is faced toward the pump chamber. Further, a resonator for sound-deadening is disposed in the pump chamber, and the exhaust pipe is brought into communication with the resonator. 
     In this case, since the jet propulsion machine is disposed at the center of the tunnel-shaped pump chamber, a space is left in the vicinity of the wall surfaces of the top wall and the left and right walls of the pump chamber as a dead space. 
     Therefore, the resonator for sound-deadening can be mounted effectively in the dead space in the pump chamber. Therefore, the difficulty of securing a storage space in the boat for the resonator is eliminated. 
     The invention also includes an exhaust port disposed in the resonator by passing the exhaust pipe through the peripheral wall of the resonator, and an opening provided on the bottom wall of the resonator at the location facing upwardly toward the exhaust pipe, and downwardly toward pump chamber. 
     The exhaust port is disposed in the resonator and an opening is formed on the bottom wall of the resonator at the location facing toward the pump chamber. Therefore, exhaust gas discharged from the exhaust port and cooling water discharged together with exhaust gas can be conducted effectively out through the opening on the bottom of the resonator. 
     The opening is divided into a first and a second opening by a supporting beam, and a valve body is attached on the supporting beam so that the first and the second openings can be opened and closed by a pair of flaps provided on the valve body. 
     The first and the second openings in the resonator can be open and closed individually by the flaps. By providing separate individual flaps, the size of the flaps may be reduced, which allows the first and the second openings to be quickly closed by the respective flaps. Quick closing of the first and the second openings by the flaps helps to prevent water from entering the openings. 
     Further, the supporting beam of the present invention is provided with a guide portion of V-shaped or substantially V-shaped cross section projecting toward the exhaust port. 
     Since the supporting beam is provided with a guide portion of V-shaped or substantially V-shaped cross section projecting toward the exhaust port, exhaust gas and cooling water discharged together with exhaust gas can be guided via the guide portion and conducted smoothly to the first and second openings. 
     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 the jet propulsion boat provided with an exhaust structure according to the present invention (first embodiment); 
     FIG. 2 is a side view of the exhaust structure for a jet propulsion boat according to the present invention (first embodiment); 
     FIG. 3 is an exploded perspective view of the exhaust structure for a jet propulsion vessel according to the present invention (first embodiment); 
     FIG. 4 is an exploded perspective view showing a principal portion of the exhaust structure for a jet propulsion boat according to the present invention (first embodiment); 
     FIG. 5 is a cross sectional view taken along the line  5 — 5  of FIG. 3; 
     FIG. 6 is a cross sectional view taken along the line  6 — 6  in FIG. 2; 
     FIG. 7 is an explanatory drawing illustrating how the first embodiment of the present invention prevents water from entering the valve body; 
     FIG. 8 is a cross sectional view showing a principal portion of the exhaust structure for a jet propulsion boat according to the present invention (second embodiment); and 
     FIG. 9 is an explanatory drawing showing how to the second embodiment of the present invention prevents water from entering the valve body. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a side view of a jet propulsion boat provided with an exhaust structure (first embodiment) according to the present invention. 
     The jet propulsion boat  10  includes a fuel tank  14  mounted at the front portion  11   a  of the hull  11 , an engine  15  provided rearwardly of the fuel tank  14 , a pump chamber  16  provided rearwardly of the engine  15 , and a Jet pump (jet propulsion machine)  20  provided in the pump chamber  16 . Also included are an exhaust structure  30  for a jet propulsion boat attached to the engine  15  on the air intake side and to the pump chamber  16  on the exhaust side, a steering handle  25  mounted above the fuel tank  14 , and a seat  27  mounted rearwardly of the steering handle  25 . 
     The jet pump  20  includes a housing  21  extending rearward from the opening  13  of the vessel bottom  12 , and an impeller  22  rotatably mounted in the housing  21  and connected to the drive shaft  23  of the engine  15 . 
     With the jet pump  20 , water drawn in through the opening  13  of the vessel bottom  12  can be forced from the steering pipe (steering nozzle)  24  disposed at the rear of the hull  11  by driving the engine  15  and rotating the impeller  22  via the rear end opening of the housing  21 . The water forced from the steering nozzle  24  can be guided toward the front by moving the reverse bucket  26  to the position rearwardly of the steering nozzle  24 . 
     The vessel  10  can be propelled by supplying fuel from the fuel tank  14  to the engine  15  to drive the engine  15 , transmitting a driving force of the engine  15  to the impeller  22  via the drive shaft  23 , drawing in water through the opening  13  of the vessel bottom  12  by rotating the impeller  22 , and forcing the water drawn in through the rear end of the housing  21  from the steering nozzle  24 . 
     FIG. 2 is a side view of the exhaust structure for a jet propulsion boat according to the present invention (first embodiment). 
     The exhaust structure  30  for a jet propulsion boat is such that an exhaust pipe,  31  is connected to an exhaust manifold (not shown) of the engine  15 , and the end  32  of the exhaust pipe  31  is passed through the top wall  17  of the pump chamber  16 , the end  32  of the exhaust pipe  31  in turn passes through the resonator  40  disposed on the top wall  17 . The opening  46  of the bottom wall  41  of the resonator  40  (See FIG. 4) faces toward the internal space  16   a  of the pump chamber  16 . 
     The exhaust pipe  31  includes an exhaust pipe  34  connected to the exhaust manifold, an exhaust body  35  connected to the exit of the exhaust pipe  34 , a muffler  36  connected to the exit side of the exhaust body  35 , a connecting pipe  37  connected to the exit  36   a  of the muffler  36 , and a tail pipe  38  connected to the exhaust port of the connecting pipe  37 . The end  32  of the tail pipe  38  (cf. the end of the exhaust pipe  31 ) is attached to the top wall  17  of the pump chamber  16 . 
     The connecting pipe  37  is a pipe bent so that the convex portion  37   a  comes to the top. By disposing the convex portion  37   a  of the connecting pipe  37  on top, in the unlikely event that water enters from the tail pipe  38  to the connecting pipe  37 , the entered water cannot flow over the convex portion  37   a  of the connecting pipe  37 , thereby preventing water from entering into the engine  15  side. That is, the connecting pipe  37  has a “water locking” capability. 
     The pump chamber  16 , being formed into the shape of a tunnel, the internal space  16   a  of which extends in the fore-and-aft direction, includes a jet pump  20  at the center, and a reverse bucket  26  provided in the vertical direction on the rear end opening side via a bracket  11   a.  A steering pipe (steering nozzle)  24  is provided at the rear end of the housing  21  of the jet pump  20 , the steering pipe  24  being capable of swinging in the lateral direction. 
     The steering direction of the hull  11  can be controlled by operating the steering cable by the steering handle shown in FIG.  1  and swinging in the lateral direction. The hull  11  can be reversed by operating the reverse cable  28   a  by the lever of the steering handle  25  to dispose the reverse bucket  26  rearwardly of the steering nozzle  24 . 
     FIG. 3 is an exploded perspective view of the exhaust structure for a jet propulsion vessel according to the present invention (first embodiment). 
     The resonator  40  is a member bent in the meandering shape, and each bent portion is placed adjacent with each other so that the entire resonator  40  forms a substantially flat plate. 
     The resonator  40  includes a base  42  to be mounted at the end  32  of the tail pipe  38  and a resonator body  50  integrally formed with the base  42 . 
     The base  42  is a substantially rectangular frame body provided with a hollow portion  43  therein, including a mounting port  44  (shown in FIG. 5) formed on the upper wall  42   a  of the frame body (that is, on the peripheral wall of the resonator), and a packing  45  attached on the mounting port  44 . The end  32  of the tail pipe  38  can be inserted into the packing  45  so that the exhaust port  33  of the tail pipe  38  (that is, the exhaust port of the exhaust pipe  31 ) faces toward the hollow portion  43  of the base  42 . 
     The resonator body  50  is a hollow pipe of rectangular in cross section extending in the meandering shape from the right rear corner  42   c  of the base  42 , which is brought into communication with the hollow portion  43  of the base  42 . 
     The resonator body  50  includes a first bent portion  51  bent from the right rear corner  42   c  of the base  42  counterclockwise by about 180°, a first extended portion  52  extending forward from the tip of the first bent portion  51  along the right side  42   d  of the base  42 , a second bent portion  53  bent from the tip of the first extended portion  52  clockwise by about 180°, and a second extended portion  54  extending rearward from the tip of the second bent portion  53  along the right side  52   a  of the first extended portion  52 . Also included are a third bent portion  55  bent from the tip of the second extended portion  54  clockwise by about 90°, and a third extended portion  56  extending from the tip of the third bent portion  55  along the rear side  51   a  of the first bent portion  51  and the rear side  42   e  of the base  42 . 
     The tip  56   a  of the third extended portion  56 , that is, the tip of the resonator body  50  is formed in the closed state. 
     By bending the resonator body  50  in the meandering state as described above, the length L 1  of the resonator  40  can be secured to a desired length while keeping the size of the resonator  40  to a minimum. Since the resonator  40  can be formed to have a desired length, the sound-deadening effect of the exhaust noise in from the long exhaust pipe can be sufficiently attenuated. 
     In addition, a first gap  61  and a second gap  62  are formed by bending the resonator  40  in the curved state. Therefore, by providing a first rib  63  (shown in FIG. 5) and a second rib  64  (shown in FIG. 5) respectively at the first gap  61  and the second gap  62 , the two wall surfaces constituting the first gap  61  are integrally connected, and likewise, the two wall surfaces constituting the second gap  62  are integrally connected. 
     Accordingly, the resonator  40  is formed generally into a substantially rectangular shape (flat plate shape). By forming the resonator  40  into the plate shape, the size of resonator  40  may be reduced to a minimum. Thus, the resonator  40  can be disposed in a relatively small storage space. 
     With the resonator  40  bent in the meandering shape, by bringing the hollow portion  50   a  (shown in FIG. 5) of the resonator body  50  into communication with the hollow portion  43  of the base  42 , the resonator body  50  can be brought into communication with the connecting pipe  37  through the tail pipe  38 . Accordingly, resonance from the connecting pipe  37  can be attenuated, thereby reducing the exhaust noise. 
     The plate shaped resonator  40  thus constructed includes a front mounting bracket  65  (shown in FIG. 2) on the front wall  40   a  thereof, and a rear mounting bracket  66  on the rear wall  40   b  thereof. 
     The resonator  40  can be mounted on the top wall  17  in the pump chamber  16  by attaching the front mounting bracket  65  on the front wall  18   a  of the pump chamber  16  with bolts  67 ,  67 , and attaching the rear mounting bracket  66  on the top wall  17  of the pump chamber  16  with bolts  67 ,  67 . 
     With most jet propulsion boats, due to mounting various accessories in the hull that is required for a vessel, there is very little extra space left in the hull. However, it is likely that there is a space left in the vicinity of the top wall  17  (wall surface) of the pump chamber  16 . 
     Therefore, as shown in FIG. 2, in order to make effective use the dead-space left near the top wall  17  of the pump chamber  16 , the resonator  40  is mounted onto the top wall  17 . 
     Furthermore, since the pump chamber  16  is located outside the hull  11 , by placing the resonator  40  along the top wall  17  of the pump chamber  16 , the resonator  40  can be mounted outside the hull  11 . By mounting the resonator  40  outside the hull  11 , it is not necessary to provide a storage space for storing the resonator  40  inside the hull  11 . 
     In this manner, by placing the resonator  40  along the top wall  17  in the pump chamber  16 , resonator  40  can easily be mounted. 
     In addition, by mounting the resonator  40  on the top wall  17  in the pump chamber  16 , the tail pipe  38  can be mounted on the top wall  17  in the pump chamber  16 . Therefore, the length of the connecting pipe  37  which communicates with the tail pipe  38  can be reduced as much as possible. Thus, the space for providing the tail pipe  38  can be secured in the hull  11  relatively easily. 
     FIG. 4 is an exploded perspective view showing a principal portion of the exhaust structure of the jet propulsion boat according to the present invention (first embodiment). 
     The exhaust structure  30  of the jet propulsion boat includes an opening  46  on the bottom wall  41  of the resonator  40 , and a valve body  70  mounted at the opening  46  via a supporting bracket  80 . The opening  46 , the valve body  70 , and the supporting bracket  80  will be described below. 
     The opening  46 , of the resonator  40  is an exhaust hole formed into the substantially rectangular shape, and is divided into the first opening  47   a  and the second opening  47   b  by laying a supporting beam  48  between the opposing front and rear sides  44   a ,  44   b  of the opening  46 . 
     The supporting beam  48  includes a guide portion  49  of V-shaped or substantially V-shaped cross section on the surface facing toward the hollow portion  43  (See FIG. 5) of the base  42 . By forming the guide portion  49  into the V-shape or the substantially V-shape, the guide portion  49  may be projected toward the exhaust port  33  of the tail pipe  38  in the tapered shape. 
     By forming the guide portion  49  on the supporting beam  48 , the lower side of the supporting beam  48  (on the side facing toward the pump chamber  16 ) is provided with a trough  48   a.    
     The valve body  70  is a rubber member formed into the substantially rectangular shape as a whole, and includes a mounting portion  71  capable of abutting against the supporting beam  48  at the substantially center thereof, a ridge  72  at the mounting portion  71 . Reinforcing ribs  72   a  are formed in the internal space of the ridge  72  at regular intervals, the ridge  72  being formed so as to be capable of engaging the trough  48   a  of the supporting beam  48 . A first and a second flap  74 ,  76  are formed respectively on both sides (left and right sides) of the mounting portion  71 . 
     The first flap  74  includes a reinforcing rib  75  along the peripheral edges  74   b - 74   d , and the second flap  76  includes a reinforcing rib  77  along the peripheral edges  76   b - 76   d.    
     The supporting bracket  80  includes a supporting portion  81  being capable of abutting against the mounting portion  71  of the valve body  70 , and a slanted portion  82  extending from the supporting portion  81  and slanting downward toward the rear. 
     As shown in FIG. 5, when mounting the valve body  70  on the bottom wall  41  of the resonator  40 , the mounting portion  71  of the valve body  70  is positioned on the supporting beam  48  by engaging the ridge  72  of the valve body  70  with the trough  48   a  of the supporting beam  48 . The supporting portion  81  of the supporting bracket  80  is abutted against the mounting portion  71 , and in this state, the rivets  85 ,  85  are knocked into the mounting holes  41   a ,  41   a  of bottom wall  41 , the mounting holes  78 ,  78  of the valve body  70 , and the mounting holes  83 ,  83  of the supporting bracket  80  and nuts  86 ,  86  to clamp the mounting portion  71  of the valve body  70  between the bottom wall  41  and the supporting bracket  80 . 
     The first and the second flaps  74 ,  76  provided on the valve body  70  are bent at the respective bending portions  74   a ,  76   a  by the weights of the respective flaps  74 ,  76  and suspended downwardly. The first flap  74  can be maintained in the slanted state (shown in FIG. 5) by supporting the first flap  74  by the slanted portion  82  of the supporting bracket  80 . 
     On the other hand, the second flap  76  is suspended vertically by being bent at the bending portion  76   a  as shown in FIG.  5 . 
     FIG. 5 is a cross sectional view taken along the line  5 — 5  in FIG. 3, showing a state in which a heat-shield plate  19  is attached on the back side of the top wall  17  of the pump chamber  16 . The resonator  40  is provided on the back side of the heat-shield plate  19 , and the end  32  of the tail pipe  38  is inserted into the mounting port  17   a  of the top wall  17  of the pump chamber  16  and into the mounting port  19   a  of the heat-shield plate  19 . The end  32  of the tail pipe  38  is fitted into the packing  45  so that the tail pipe  38  passes through the peripheral wall (upper wall  42   a  of the base  42 ) of the resonator  40  to dispose the exhaust port  33  of the tail pipe  38  in the base  42  (hollow portion  43 ) of the resonator  40 . The opening  46  is formed on the peripheral wall (bottom wall)  41  of the resonator  40  and faces upwardly toward the exhaust port  33  and downwardly toward the internal space  16   a  of the pump chamber  16 . The guide portion  49  is formed on the surface of the supporting beam  48  on the upstream side (that is, the surface facing toward the exhaust port  33  of the tail pipe  38 ) so as to project toward the exhaust port  33 . 
     As described above, since an the exhaust port  33  is disposed in the resonator  40  (hollow portion  43  of the base  42 ) by passing the tail pipe  38  of the exhaust pipe  31  through the peripheral wall of the resonator  40  and the opening  46  is formed on the bottom wall  41  of the resonator  40  facing toward the exhaust port  33 , exhaust gas discharged from the exhaust port  33  of the tail pipe  38  can be introduced to the opening  46  (first and second openings  47   a ,  47   b ) of the resonator  40  and discharged into the internal space  16   a  of the pump chamber  16  effectively. 
     In addition, by forming the guide portion  49  on the surface of the supporting beam  48  on the upstream side so as to project toward the exhaust port  33  of the tail pipe  38 , exhaust gas flowing out from the exhaust port  33  can be guided along the guide portion  49  smoothly to the first and the second openings  47   a ,  47   b.    
     The figure shows a state in which the opening  46  formed on the bottom wall  41  of the resonator  40  is divided into the first and the second openings  47   a ,  47   b  by the supporting beam  48 , and the valve body  70  is mounted on the supporting beam  48 . 
     Since the opening  41  of the resonator  40  is divided into the first and the second openings  47   a ,  47   b  by the supporting beam  48 . The first and the second openings  47   a ,  47   b  can be closed by the first and the second flaps  74 ,  76 . Since the opening  41  is divided in two smaller openings  47   a ,  47   b , the size of first and the second flaps  74 ,  76  can be made smaller also. 
     As such, the first and the second openings  47   a ,  47   b  can quickly be opened and closed by the first and the second flaps  74 ,  76 . Therefore, before water enters from the first and the second openings  47   a ,  47   b  into the resonator  40 , the first and the second openings  47   a ,  47   b  can be quickly closed by the first and the second flaps  74 ,  76 . 
     Subsequently, an example in which exhaust gas is discharged from the resonator  40  will be described referring to FIG.  5 . The first and the second flaps  74 ,  76  provided on the valve body  70  are bent downward at the respective bending portions  74   a ,  76   a  by the weights of the respective flaps  74 ,  76 . In this case, since the first flap  74  is supported by the slanted portion  82  of the supporting bracket  80 , the first flap  74  can be maintained in the slanted state. On the other hand, the second flap  76  is bent at the bending portion  76   a  by its own weight, and is suspended in a substantially vertical position. 
     Accordingly, the first and the second openings  47   a ,  47   b  provided on the bottom wall  41  of the resonator  40  may be opened. 
     Since the opening  46  of the resonator  40  faces upward toward the exhaust port  33  of the tail pipe  38 , exhaust gas discharged from the exhaust port  33  of the tail pipe  38  and cooling water discharged together with exhaust gas can be conducted downwardly to the opening  46  of the resonator  40  (that is, the first and the second openings  47   a ,  47   b ) effectively as shown by the arrow. 
     In addition, since the guide portion  49  of V-shaped or substantially V-shaped cross section is formed on the surface of the supporting beam  48  on the upstream side, exhaust gas flowing out from the exhaust port  33  of the tail pipe  38  and cooling water discharged together with exhaust gas can be guided along the guide portion  49  and conducted smoothly through the first and the second openings  47   a ,  47   b.    
     The purpose of supporting the first flap  74  by the slanted portion  82  of the supporting bracket  80  will be described referring to FIG.  7 . 
     FIG. 6 is a cross sectional view taken along the line  6 — 6  in FIG. 2, showing a state in which a jet pump  20  is provided at the center of the pump chamber  16 . A resonator  40  is attached on the top wall  17  in the pump chamber  16  while effectively utilizing the dead space. The reverse cable  28   a  and the pipe  28   b  are provided above the jet pump  20 , that is, on the left side of the jet pump  20 . A cable  28   c  is provided between the jet pump  20  and the resonator  40 , and a steering cable  28   d  is provided on the right side of the jet pump  20 . Seawater  87  is shown having entered to approximately the level of the upper surface of the jet pump  20 . 
     The reverse cable  28   a  is a cable for operating the reverse bucket  26  (See FIG.  2 ), and the pipe  28   b  is a pipe for taking cooling water. The cable  28   c  is a cable for trimming, and the steering cable  28   d  is a cable for operating the steering nozzle (See FIG.  2 ). 
     An example of seawater is prevented from entering from the opening  46  (the first and the second opening  47   a ,  47   b ) of the resonator  40  will now be described referring to FIG.  7 . 
     FIG. 7 illustrate a state in which the valve body prevents seawater from entering according to the first embodiment of the present invention. 
     In the unlikely event that the jet propulsion boat  10  overturned during operation, the first flap  74  moves from the opened position P 1  (position represented by a phantom line) to the closed position P 2  (position represented by a solid line) by its own weight, and the first flap  74  closes the first opening  47   a  of the resonator  40 . 
     Simultaneously, the second flap  76  moves from the opened position P 3  (position represented by a phantom line) to the closed position P 4  (position represented by a solid line) by its own weight, and the second flap  76  closes the second opening  47   b  of the resonator  40 . 
     Since the resonator is constructed so that the opening  46  is divided and the first and second openings  47   a ,  47   b  are closed individually by the first and the second flaps  74 ,  76 , the size of the first and the second flaps  74 ,  76  may be minimized. 
     By minimizing the first and the second flaps  74 ,  76 , they can be moved from the opened positions (P 1 , P 3 ) to the closed positions (P 2 , P 4 ) in a short time. Therefore, the first and the second openings  47   a ,  47   b  can be closed by the flaps  74 ,  76 , respectively, before seawater reaches the first and the second openings  47   a ,  47   b.    
     The purpose of supporting the first flap  74  in the slanted state by the slanted portion  82  of the supporting bracket  80  will now be described. 
     In the unlikely event that the jet propulsion boat  10  is overturned during operation, seawater  87  in the pump chamber  16  falls on the top wall  17  of the pump chamber  16 . In this case, since seawater  87  in the vicinity of the left wall  18   b  of the pump chamber  16  falls along the left wall  18   b  smoothly as shown by the arrow {circle around ( 1 )}, it would reach the first opening  47   a  of the resonator  40  relatively quickly. Therefore, it is necessary to quickly close the first opening  47   a  by the first flap  74  of the valve body  70 . 
     Therefore, when the jet propulsion boat  10  is in the normal operation, the first flap  74  is maintained in the slanted state by supporting it by the slanted portion  82  of the supporting bracket  80  as shown in FIG.  5 . As a consequence, in the unlikely event that the jet propulsion boat  10  is overturned, the first flap  74  can be moved quickly from the opened position P 1  to the closed position P 2 . Therefore, seawater  87  can be prevented from entering into the resonator  40  by closing the first opening  47   a  with the first flap  74 , before seawater  87  falls along the left wall  18   b  of the pump chamber  16  and reaches the first opening  47   a.    
     On the other hand, seawater  87  in the vicinity of the right wall  18   c  of the pump chamber  16  falls toward the second opening  47   b  as shown by the arrow {circle around ( 2 )}. Since the second opening  47   b  is located away from the right wall  18   c  and the cable  28   c  is laid in the vicinity of the second opening  47   b , the cable  28   c  blocks the dropping of seawater  87 . 
     Therefore, a relatively long time is necessary until seawater  87  reaches the second opening  47   b . Therefore, the second opening  47   b  does not need to be closed as quickly as the first opening  47   a.    
     Therefore, as shown in FIG. 5, the second flap  76  is suspended vertically when the jet propulsion boat  10  is in normal operation. As a consequence, the exhaust gas can be discharged effectively through the second opening  47   b.    
     When the jet propulsion boat  10  is overturned, since the second opening  47   b  is located away from the right wall  18   c  and seawater is blocked by the cable  28   c , the second flap  76  is moved from the opened position P 3  represented by a phantom line to the closed position P 4  represented by a solid line to close the second opening  47   b  with the second flap  76  before seawater  87  reaches the second opening  47   b . Thus, the entering of seawater  87  through the second opening  47   b  into the resonator  40  is prevented. 
     Referring now to FIG.  8  and FIG. 9, the second embodiment will be described. In the second embodiment, the same members as in the first embodiment are designated by the same reference numerals, and will not be described again. 
     FIG. 8 is a cross section of a principal portion of the exhaust structure for a jet propulsion boat according to the present invention (second embodiment). 
     The exhaust structure  90  for a jet propulsion boat differs from the first embodiment only in that the resonator  40  is mounted along the left wall  18   b  of the pump chamber  16 . The other structures are the same as the first embodiment. 
     In other words, FIG. 8 shows that the exhaust structure  90  for a jet propulsion boat is constructed such that the heat-shield plate  19  is mounted on the backside of the left wall  18   b  of the pump chamber  16 . The resonator  40  is provided on the backside of the heat-shield plate  19 . The end  32  of the tail pipe  38  is inserted into the mounting port  18   d  of the left wall  18   b  of the pump chamber  16  and the mounting port  19   a  of the heat-shield plate  19  and the end  32  of the tail pipe  38  is fitted into the packing  45  to face the exhaust port  33  of the tail pipe  38  toward the hollow portion  43  of the base  42 . The opening  46  of the bottom wall  41  of the resonator  40  is faced toward the inner space  16   a  of the pump chamber  16 . 
     The opening  46  is divided into the first opening  47   a  and the second opening  47   b  by the supporting beam  48 , as in the first embodiment. 
     Simultaneously, FIG. 8 shows a state in which the guide portion  49  is provided on the supporting beam  48  to face toward the exhaust port  33  of the tail pipe  38  and the ridge  72  of the valve body  92  is engaged with the trough  48   a  on the supporting beam  48  to position the mounting portion  71  of the valve body  92  with respect to the supporting beam  48 . The supporting portion  81  of the bracket  80  is abutted against the mounting portion  71 , and in this state, the rivets  85 ,  85  (only the one on the far side is shown in the figure) is knocked in, as in the first embodiment, to clamp the mounting portion  71  of the valve body  92  between the bottom wall  41  and the supporting bracket  80 . 
     The structure of the valve body  92  is the same as the valve body  70  in the first embodiment, except that the second flap  76  is removed from the valve body  70 . 
     The first flap  74  provided on the valve body  92  is bent downward at the bending portion  74   a  by being applied with its own weight. In this case, the first flap  74  is supported in the slanted state by supporting the first flap  74  by the slanted portion  82  of the supporting bracket  80 . 
     Consequently, the first opening  47   a  formed on the bottom wall  41  of the resonator  40  can be opened. On the other hand, since the second opening  47   b  is not provided with a flap, it is always in the opened state. 
     Therefore, exhaust gas discharged from the exhaust port  33  of the tail pipe  38  can be guided by the guide portion and conducted to the first and the second openings  47   a ,  47   b  as shown by the arrow. 
     Referring now to FIG. 9, an example of preventing entering of seawater from the opening  46  of the resonator  40  will be described. 
     FIG. 9 is an explanatory drawing showing a state in which entering of seawater is prevented by a valve body according to the second embodiment of the present invention. 
     In the unlikely event that the jet propulsion boat is overturned during travel, seawater  87  in the pump chamber  16  falls toward the top wall  17  of the pump chamber  16 . In this case, since seawater  87  in the vicinity of the left wall  18   b  of the pump chamber  16  falls along the bottom wall  41  of the resonator  40  as shown by the arrow {circle around ( 3 )}, it passes over the second opening  47   b  of the resonator  40 . Therefore, seawater  87  does not enter from the second opening  47   b , even though a flap is not provided at the second opening  47   b.    
     The second opening  47   b  is formed at the position that comes above the sea level when the boat is overturned. 
     On the other hand, seawater  87  in the center of the pump chamber  16  falls on the top wall  17  and flows toward the first opening  47   a  as shown by the arrow {circle around ( 4 )}. Therefore, the first flap  74  is provided at the first opening  47   a  so that the first flap  74  moves from the opened position P 5  shown by a phantom line to the closed position P 6  shown by a solid line, and closes the first opening  47   a  by the first flap  74  preventing seawater  87  from entering into the resonator  40 . 
     The exhaust structure  90  for a jet propulsion boat according to the second embodiment can provide the same effects as the first embodiment. 
     In other words, according to the second embodiment, the dead space left in the vicinity of the wall surface can be effectively utilized by placing the resonator  40  along the left wall  17  of the pump chamber  16  (See FIG.  8 ). In addition, it is not necessary to secure the storage space for storing the resonator  40  inside the hull  11 , because the resonator is mounted in the pump chamber  17  which is outside the hull  11 . 
     In this way, by placing the resonator  40  along the top wall  17  of the pump chamber  16 , the resonator  40  can be mounted with less trouble. 
     According to the second embodiment, since the opening  46  of the resonator  40  is faced toward the exhaust port  33  of the tail pipe  38 , exhaust gas discharged from the exhaust port  33  of the tail pipe  38  can be conducted effectively to the opening  46  (that is, the first and the second openings  47   a ,  47   b ) of the resonator  40 . 
     Further, according to the second embodiment, since the opening  46  is divided into the first and the second openings  47   a ,  47   b , the size of first flap  74  covering the first opening  47   a  can be minimized. 
     Since the first flap  74  can be moved from the opened position to the closed position in a short time, the first opening  47   a  can be closed by the first flap  74  before water enters the first opening  47   a.    
     Furthermore, according to the second embodiment, by providing a guide portion  49  of V-shaped or substantially V-shaped cross section on the surface of the supporting beam  48  on the upstream side, exhaust gas discharged from the exhaust port  33  of the tail pipe  38  and cooling water discharged with the exhaust gas can be guided along the guide portion  49  and conducted smoothly out through the first and the second openings  47   a ,  47   b.    
     Though the resonator  40  is provided on the top wall  17  of the pump chamber  16  in the first embodiment and the resonator  40  is provided on the left wall  17   b  of the pump chamber  16  in the second embodiment according to the description above, it is not limited thereto. It is possible to provide the resonator on other wall surfaces of the pump chamber  16 . It is also possible to provide the resonator  40  on the portion other than the wall surface in the pump chamber  16 . 
     Further, though the opening  46  on the bottom wall  41  of the resonator is rectangular in the embodiments described above, it is not limited thereto. It is also possible to form the opening  46  in other configurations such as circle. 
     In addition, while the example in which the resonator  40  is formed in the meandering state was described in the aforementioned embodiments, it is not limited thereto. It is possible to form the resonator linearly, and dispose it in the dead space in the pump chamber  16 . 
     Though the example in which the first flap  74  is supported by the slanted portion  82  of the supporting bracket  80  in the slanted state during normal operating conditions was described in the first and the second embodiments, the slanted state of the first flap  74  can be selected arbitrary. In addition, in the first embodiment, it is possible to eliminate the slanted portion  82  from the supporting bracket  80  and suspend the first flap  74  in the vertical direction. 
     Further, though the example in which the guide portion  49  is formed integrally with the supporting beam  48  was described in the aforementioned embodiments, it is not limited thereto. It is also possible to mount the separate guide portion  49  on the supporting beam  48 . 
     With the construction described above, the present invention provides the following effects. 
     A sound-deadening resonator is disposed while effectively utilizing the dead space in the pump chamber. Therefore, it is not necessary to take a long time considering how best to secure the storage space for the resonator in the vessel. 
     Therefore, the resonator can be mounted easily, and thus exhaust noise of the jet propulsion boat can be alleviated without trouble. 
     The exhaust port is disposed in the resonator, and the opening is formed on the peripheral wall of the resonator at the position facing toward the exhaust port. Therefore, exhaust gas discharged from the exhaust port and cooling water discharged with exhaust gas can be conducted effectively discharged out through the opening on the bottom of the resonator. 
     The opening of the resonator is divided into the first and the second openings, and the divided openings are individually closed by separate flaps. By providing separate flaps, the size of each flap can be minimized. Thus, the first and the second openings can be quickly closed by the respective flaps. 
     Therefore, the first and the second openings can be closed by the flaps before water enters into the resonator. 
     The guide portion of V-shaped or substantially V-shaped cross section is formed on the supporting beam so as to project toward the exhaust port. Thus exhaust gas and cooling water discharged with exhaust gas can be guided along the guide portion and discharged effectively out through the first and the second openings. 
     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.