Abstract:
An exhaust system for a small boat is constructed in such a manner that an exhaust pipe extends rearwardly from an engine provided in a hull of the boat. A water muffler is provided in the exhaust pipe, and exhaust gas is discharged through a water lock pipe extending upward from a water muffler. A water lock pipe has an inverted U-shape. A resonator chamber for attenuating resonance of the water lock pipe is provided in the water muffler. The resulting configuration provides an effective system for reducing exhaust noise emanating from the small boat.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2001-269463 filed on Sep. 5, 2001, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust system for an engine mounted on a small boat, for example, a boat propelled by jet water from a jet pump. 
     2. Description of Background Art 
     A jet propulsion boat is a vessel provided with a jet pump mounted at the rear portion of the hull. An engine drives the jet pump to propel the boat by drawing in water from the vessel bottom, and then splashing the water in a rearward direction. 
     The jet propulsion boat disclosed, for example, in Japanese Patent Laid-Open No. 212936/1998, entitled “EXHAUST SYSTEM FOR A SMALL PLANING BOAT”, is known. This jet propulsion boat comprises a water muffler disposed midway in the exhaust pipe. FIG. 3 of this patent has been reproduced herein as FIG.  8 . 
     FIG. 8 is a cross sectional view showing the water muffler of the above described related art. 
     Cooling water used for cooling exhaust gas flows from the insert port  101  of the water muffler  100  into the second chamber  103 , as shown by the arrow (a). Then, the cooling water flows from the second chamber  103  flows via the first chamber  102  into the third chamber  104 . 
     On the other hand, exhaust gas discharged from the engine flows from the internal tube  105  of the water muffler  100  into the first chamber  102 , as shown by the arrow (b). Exhaust gas from the first chamber  102  flows through a communication cylinder  106  to the second chamber  103 , and thereafter, flows through the communication cylinder  107  and into the third chamber  104 . 
     Lastly, the exhaust gas in the third chamber  104 , as well as the cooling water which has been trapped in the third chamber  104 , flow together out through the exhaust pipe  108 . 
     With this water muffler  100 , it is possible to lower exhaust noise to a certain level by flowing exhaust gas into the expansion chambers of the first to third chambers and expanding exhaust gas therein. However, it is difficult to sufficiently eliminate the noise merely by the expansion of gas. Thus, an exhaust system having a sound-deadening resonator disposed in the exhaust pipe has been proposed in order to further lower the noise. Exhaust noise can be lowered by resonating with the resonator. 
     However, in order to provide a resonator in the exhaust pipe, it is necessary to secure a storing space for storing the resonator in the hull. The interior of the hull of most small boats is limited in size. Thus, in addition to planning for the space required for storing the resonator, consideration must be given to laying out the variety of accessories that also must be mounted in the hull of the vessel. Finding storage space in the hull for all of the required components, in addition to a resonator, has proved to be difficult. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an exhaust system for a small boat that can be stored in a practical manner, which at the same time can achieve the desired reduction in noise. 
     In order to solve this problem, a first aspect of the present invention provides a small boat in which an exhaust pipe extends rearwardly from the engine provided in the hull. A water muffler is disposed in the exhaust pipe, and exhaust gas is discharged through the water lock pipe in the inverted U-shape extending upward from the water muffler. Further, a resonator chamber for attenuating the resonance of a water lock pipe is provided in the water muffler. 
     The resonator chamber here refers to a sound box for attenuating resonance of the water lock pipe by utilizing the principle of resonance. 
     Since the resonator chamber for attenuating resonance of the water lock pipe is provided in the water muffler, it is not necessary to secure a separate storage space for a resonator in the limited space in the hull. 
     Therefore, the layout of the various accessories required for a vessel can be determined relatively easily. 
     In addition, by forming the resonator chamber by using a water muffler, the number of the members for constituting the resonator can be reduced. Thus, the resonator can easily be provided, and its cost can be reduced. 
     In a second aspect of the present invention, an expansion chamber is provided in the water muffler. Further, the extension chamber and the resonator chamber are able to communicate with each other by a communication pipe, the communication pipe being disposed above the level of the cooling water in the water muffler. 
     The communication pipe which enables the expansion chamber and the resonator chamber to communicate is disposed above the water level in the water muffler. Therefore, since the communication pipe is not closed by cooling water, the expansion chamber and the resonator chamber can freely communicate with each other at all times. Since characteristics of the resonator chamber are maintained, the resonance of the water lock pipe can be attenuated. 
     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 boat provided with an exhaust structure according to the present invention; 
     FIG. 2 is a side view of the exhaust system for a small boat according to the present invention; 
     FIG. 3 is a perspective view showing a water muffler of the exhaust system for a small boat according to the present invention; 
     FIG. 4 is a cross sectional view showing the water muffler of the exhaust system for a small boat according to the present invention; 
     FIG. 5 is a cross sectional view taken along the line  5 — 5  in FIG. 4; 
     FIG. 6 is a cross sectional view taken along the line  6 — 6  in FIG. 4; 
     FIG. 7 is an explanatory drawing illustrating the operation of the water muffler constituting the exhaust system for a small boat according to the present invention; and 
     FIG. 8 is a cross sectional view showing a water muffler in the related art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Though the present invention relates to a small boat, a jet propulsion boat will be described as an example. 
     FIG. 1 is a side view of a small boat comprising an exhaust structure according to the present invention. 
     The jet propulsion boat  10  comprises 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  20  provided in the pump chamber  16 . An exhaust system  30  for a small boat is attached to the engine  15  on the air intake side and to the pump chamber  16  on the exhaust side, a steering handle  28  is mounted above the fuel tank  14 , and a seat  29  is mounted rearwardly of the steering handle  28 . 
     The jet pump  20  comprises a housing  21  extending rearwardly 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 splashed via the rear end opening of the housing  21  from the steering pipe (steering nozzle)  25  by driving the engine  15  and rotating the impeller  22 . 
     The steering pipe (steering nozzle)  25  is a member mounted at the rear end of the housing  21  so as to be capable of swinging in the lateral direction. The steering nozzle  25  is a steering nozzle for controlling the steering direction of the hull  11  by the swinging operation of the steering handle  28  in the lateral direction. 
     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 splashing the water through the rear end of the housing  21  from the steering nozzle  25 . 
     FIG. 2 is a side view of the exhaust system for a small boat according to the present invention. 
     The exhaust system  30  for a small boat is such that an exhaust pipe  31  is connected to an exhaust manifold (not shown) of the engine  15 . The end  32  of the exhaust pipe  31  is mounted along the top wall  17  (wall surface of the hull) of the pump chamber  16 , a resonator  40  disposed on the top wall  17  is connected to the end  32  of the exhaust pipe  31 , and the exhaust port  41  of the resonator  40  is faced toward the internal space  16   a  of the pump chamber  16 . 
     Accordingly, the exhaust port  33  of the exhaust pipe  31  can communicate with the internal space  16   a  of the pump chamber  16  via the exhaust port  41  of the resonator  40 . Exhaust gas and cooling water can be discharged from the exhaust port  41  of the resonator  40  smoothly by disposing the exhaust port  41  of the resonator  40  at the position facing toward the exhaust port  33  of the exhaust pipe  31 . 
     The exhaust pipe  31  comprises an exhaust pipe  34  connected to the exhaust manifold, an exhaust body  35  connected to the exit of the exhaust pipe  34 , a water muffler  36  connected to the exit side of the exhaust body  35 , a water lock pipe  37  connected to the water muffler  36 , and a tail pipe  38  connected to the exhaust port of the water lock 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 water muffler  36  is a member disposed on the right side of the pump chamber  16 . The water muffler  36  will be described in detail later referring to FIG. 3 to FIG.  7 . 
     By disposing a resonator  40  in the internal space  16   a  of the pump chamber  16 , the space originally left as a dead space therein may be used for mounting the resonator  40 . Therefore, it is not necessary to secure a space for mounting the resonator  40  in the hull  11 . The resonator  40  can be formed into a compact rectangular shape. This can be accomplished by forming the hollow portion communicating with the water lock pipe  37  in the meandering shape. 
     By providing such resonator  40 , the principle of resonance can be used for attenuating the resonance of the water lock pipe  37 . 
     The water lock 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 water lock pipe  37  on top, in the unlikely event that water has entered water lock pipe  37  from the tail pipe  38 , the entered water is prevented from flowing over the convex portion  37   a  of the water lock pipe  37 . Thereby water is prevented from entering into the engine  15  side. In other words, the water lock pipe  37  has a water locking capability. 
     FIG. 3 is a perspective view showing a water muffler of the exhaust system for a small boat according to the present invention. 
     The water muffler  36  comprises a first cylindrical body  51  having a front cap  51   a  formed integrally therewith, a second cylindrical body  52  having a front wall surface  52   a  formed integrally therewith for closing the rear end opening  51   b  of the first cylindrical body  51 , a rear wall surface  53  for closing the rear end opening  52   b  of the second cylindrical body  52 , and a third cylindrical body  54  having a front end opening  54   a  attached to the rear wall surface  53  and the second cylindrical body  52  and having a rear cap  54   b  integrally formed therewith. The first cylindrical body  51  and the front wall surface  52   a  define a first expansion chamber  55 , the second cylindrical body  52  and the rear wall surface  53  define a second expansion chamber  56 , and the rear wall surface  53  and the third cylindrical body  54  define a resonator chamber  57 . 
     In addition, the water muffler  36  is constructed in such a manner that the exit  35   a  of the exhaust body  35  is attached to the side wall  51   c  of the first cylindrical body  51 , the front wall surface  52   a  is provided with a front communication pipe  58  enabling the first expansion chamber  55  and the second expansion chamber  56  to communicate with each other, and the inlet port  37   b  side of the water lock pipe  37  is attached on the side wall  52   c  of the second cylindrical body  52 . The rear wall surface  53  is provided with a rear communication pipe (communication pipe)  59  enabling the second expansion chamber  56  and the resonator chamber  57  to communicate with each other, and the rear wall surface  53  is formed with a communication hole  53   a  enabling the second expansion chamber  56  and the resonator chamber  57  to communicate with each other. 
     The resonator chamber  57  is a sound box for attenuating resonant of the water lock pipe  37 . The characteristics of the resonator chamber  57  can be determined by the volume of the resonator chamber (sound box)  57 , the diameter of the front communication pipe  58 , and the length of the front communication pipe  58 . 
     Therefore, the volume of the resonator chamber (sound box)  57 , the diameter of the front communication pipe  58 , and the length of the front communication pipe  58  may be determined as appropriate so as to correspond with resonance of the water lock pipe  37 . 
     The front cap  51   a  of the first cylindrical body  51  and the rear cap  54   b  of the third cylindrical body  54  are formed with substantially cross shaped recesses (hereinafter referred to as “reinforcing rib”)  51   d,    54   d  respectively. Since formation of the reinforcing ribs  51   d,    54   d  can enhance the rigidity of the water muffler  36 , vibration of the front cap  51   a  and the rear cap  54   b  can be suppressed. 
     Therefore, a so-called flattering sound can be prevented from being generated, and thus, the noise of the jet propulsion boat can be reduced. 
     FIG. 4 is a cross sectional view showing a water muffler of the exhaust system for a small boat according to the present invention, showing a state in which a resonator chamber  57  for attenuating resonance of the water lock pipe  37  is provided in the water muffler  36 . 
     By providing a resonator chamber  57  in the water muffler  36 , it is not necessary to provide a resonator outside the exhaust pipe  31 , and thus to secure a storing space for a resonator in the limited space in the hull  11  (shown in FIG.  1 ). 
     Therefore, the space in the hull  11  can be sufficiently utilized as a space for mounting various accessories required for a vessel, and thus, the layout of the various accessories required for a vessel can be determined relatively easily. 
     In addition, by forming a resonator chamber  57  by utilizing the water muffler  36 , the number of the members for constituting the resonator may be reduced. 
     The second expansion chamber  56  and the resonator chamber  57  in the water muffler  36  can communicate with each other by means of a rear communication pipe  59 . The rear communication pipe  59  is disposed above the water level of the cooling water  60  in the water muffler  36 . 
     Therefore, since the rear communication pipe  59  is not closed by cooling water  60 , the characteristics of the resonator chamber  57  can be maintained. Thus, the attenuation of resonance of the water lock pipe  37  is ensured. 
     FIG. 5 is a cross sectional view taken along the line  5 — 5  in FIG.  4 . 
     By forming the inlet port  37   b  of the water lock pipe  37  into the tapered shape at the angle θ, the inlet port  37   b  can be disposed near the side wall  52   c  of the second cylindrical body  52 . Therefore, cooling water  60  trapped in the bottom  52   d  of the second expansion chamber  56  can be conducted to the inlet port  37   b  smoothly with exhaust gas and discharged out the hull  11  (shown in FIG. 1) effectively from the water lock pipe  37 . 
     The upper limit of the water level of cooling water  60  trapped in the water muffler  36 (the height of water surface  60   a ) is the maximum height H of the inlet port  37   b  of the water lock pipe  37 . 
     Even when the water level of cooling water  60  reaches the upper limit of the height H, since the upper space  58   a  of the front communication pipe  58  is disposed above cooling water  60 , the first expansion chamber  55  (shown in FIG. 4) and the second expansion chamber  56  are always able to communicate with each other. 
     Therefore, exhaust gas in the first expansion chamber  55  can flow through the front connection pipe  58  into the second expansion chamber  56 . 
     FIG. 6 is a cross sectional view taken along the line  6 — 6  in FIG. 4, showing a state in which the rear communication pipe  59  enable the second expansion chamber  56  and the resonator chamber  57  (shown in FIG. 4) in the water muffler  36  to communicate with each other. The rear communication pipe  59  is mounted at the upper position of the rear wall surface  53 , and a communication hole  53   a  is formed at the lower portion of the rear wall surface  53 . 
     By disposing the rear communication pipe  59  at the upper portion of the rear wall surface  53 , the rear communication pipe  59  can be disposed at the position higher than the upper limit of the water level H of cooling water  60 . Therefore, the rear communication pipe  59  is not closed by cooling water  60 , and thus attenuation of resonant of the water lock pipe  37  is ensured while utilizing the principle of resonance in the resonator chamber  57 . 
     Subsequently, the exhaust system  30  for a small boat, especially the operation of the water muffler  36 , is described referring to FIG.  7 . 
     FIG. 7 is an explanatory drawing illustrating the operation of the water muffler constituting the exhaust system for a small boat according to the present invention. 
     Exhaust gas discharged from the engine  15  (shown in FIG. 2) flow through the exhaust pipe  34  (shown in FIG. 2) connected to the exhaust manifold (not shown) to the exhaust body  35 , and then from the exhaust body  35  to the first expansion chamber  55  of the water muffler  36  as shown by the arrow ( 1 ). 
     On the other hand, a part of cooling water  60  used for cooling exhaust gas also flows through the exhaust body  35  into the first expansion chamber  55  of the water muffler  36 . 
     Exhaust gas from the first expansion chamber  55  flows through the front communication pipe  58  to the second expansion chamber  56  as shown by the arrow ( 2 ), and exhaust gas from the second expansion chamber  56  circulates in the second expansion chamber  56  as shown by the arrow ( 3 ). 
     Circulated exhaust gas flows from the inlet port  37   b  of the water lock pipe  37  into the water lock pipe  37  as shown by the arrow ( 4 ), and is discharged from water lock pipe  37  to the outside of the hull  11  (shown in FIG.  1 ). 
     Since the second expansion chamber  56  and the resonator chamber  57  are able to communicate with each other by means of the rear communication pipe  59 , attenuation of resonance of the water lock pipe  37  is ensured while utilizing the principle of resonance of the resonator chamber  57 . Thus, the sound-deadening effect of exhaust noise can be enhanced sufficiently. 
     On the other hand, cooling water  60  from the first expansion chamber  55  flows through the front communication pipe  58  into the second expansion chamber  56 , and cooling water  60  in the second expansion chamber  56  flows through the communication hole  53   a  into the resonator chamber  57 . 
     As a consequence, cooling water  60  is retained in the entire area of the bottom surface  36   a  of the water muffler  36 . The retained cooling water  60  flows with exhaust gas through the inlet port  37   b  of the water lock pipe  37  into the water lock pipe  37 , and then from the water lock pipe  37  to the outside of the hull  11 . 
     Though an example in which the resonator chamber  57  is provided in the water muffler  36  and the end  32  of the exhaust pipe  31  is provided with a resonator  40  has been described in the aforementioned embodiment, it is not limited thereto. For example, it is possible to eliminate the resonator  40 , and provide only the resonator chamber  57  in the water muffler  36 . 
     Further, though a jet propulsion boat that is propelled by a jet pump is taken as an example of small boats for description in the aforementioned embodiment, propelling means for a small boat is not limited thereto. 
     In addition, though an example in which the water muffler  36  comprises the first and the second expansion chambers  55 ,  56  has been described in the aforementioned embodiment, it is not limited thereto. It is possible to determine the number of the expansion chamber arbitrarily. The configuration of the water muffler  36  can be changed as appropriate. 
     With this construction the present invention exerts the following effects. 
     According to the first aspect of the present invention, since the resonator chamber for attenuating resonant of the water lock pipe is provided in the water muffler, it is not necessary to secure the storing space for the resonator in the limited space in the hull. Therefore, the space in the hull can be utilized primarily as a space for mounting various accessories required for a vessel. Thus the layout of such various accessories required for a vessel can be determined relatively easily. 
     Therefore, since the resonator can be provided relatively easily, exhaust noise can be reduced easily. 
     In addition, by forming the resonator chamber using the water muffler, the number of the members for constituting the resonator can be reduced. This enables a reduction in cost. 
     According to the second aspect of the present invention, the communication pipe enabling the expansion chamber and the resonator chamber to communicate with each other is disposed above the water level in the water muffler. Therefore, the communication pipe is not closed by cooling water, and thus, the expansion chamber and the resonator chamber are able to communicate continuously. 
     Therefore, since the characteristics of the resonator chamber can be maintained, the capability of resonator chamber to attenuate the resonance of the water lock pipe can be maintained as well. Therefore, reduction of exhaust noise of exhaust gas is ensured. 
     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.