Abstract:
The use of an oil cooler in an internal combustion engine for a personal watercraft is controlled, whereby excessive cooling of the engine during warmup with cold water operation is avoided, and dilution of oil is substantially prevented. The internal combustion engine includes a bypass oil path connecting respective oil paths upstream and downstream of the oil cooler. The bypass oil path permits oil to be selectively detoured around the oil cooler. An oil thermostat is provided in the upstream oil path for selectively opening either a path to the oil cooler or the bypass oil path, and the oil thermostat is operable to switch the flow of oil therebetween. The oil thermostat opens the bypass oil path when the temperature of the oil is below a predetermined temperature, and directs oil to the oil cooler when the temperature of the oil is equal to or exceeds a predetermined temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention claims priority under 35 USC 119 based on Japanese patent application No. 2004-284547, filed on Sep. 29, 2004. The subject matter of this priority document is incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an internal combustion engine, mounted on a personal watercraft which is designed for operation in water. More particularly, the invention relates to an internal combustion engine having an improved lubricating oil cooling structure. 
   2. Description of the Background Art 
   The personal watercraft, or small-sized planing boat, is constructed such that an internal combustion engine, for driving a jet propulsion pump, is mounted in a boat body and enclosed by a hull and a deck. A driver and up to two other crew members ride on the deck, so that an internal cabin space, constituted by the space between the hull and the deck, is narrow. The internal combustion engine is stored in a substantially closed and sealed state within the narrow space between the hull and the deck. 
   As a consequence, a compact internal combustion engine is required. In order to minimize the height of the internal combustion engine, a dry sump engine has been used, having no significant oil reservoir at the bottom of the engine, but storing the oil in a separate oil tank instead. Such an internal combustion engine is disclosed, for example, in Japanese published patent document No. 2003-35201. 
   A water-cooled oil cooler is provided for an oil path according to the dry sump lubrication system of JP-A No. 2003-35201, whereby the rise of the temperature of lubricating oil is inhibited. In the disclosed small-sized personal watercraft, cooling water taken from the side of positive pressure of a jet propulsion pump is used for cooling an internal combustion engine and the oil cooler also utilizes the cooling water. 
   In this cooling system, since cooling water is not circulated between a radiator and the internal combustion engine, and since new cooling water is constantly supplied, the cooling power of the water is high. However, in very cold water conditions, supercooling of the internal combustion engine may occur. During supercooling, fuel in a combustion chamber invades a crankcase from between a cylinder and a piston, is mixed with lubrication oil within the crankcase, so-called dilution occurs. As a result, deterioration of the lubrication oil is accelerated, having a negative effect upon the life of oil. 
   The invention is made in view of the above described problem. The object of the invention is to provide an internal combustion engine for a personal watercraft in which the use of an oil cooler is controlled, so that supercooling is avoided in very cold water conditions and the dilution of lubricating oil can be prevented. 
   SUMMARY OF THE INVENTION 
   To achieve the object, a first aspect of the invention relates to an internal combustion engine for a personal watercraft provided with an oil cooler for cooling lubricating oil. The personal watercraft comprises an internal combustion engine for driving a jet propulsion pump. The internal combustion engine is mounted in a hull so as to be encircled from below by the bottom of the hull and from above by a deck. A rider rides on the deck. The invention is characterized in that a bypass oil path connects an oil path on the upstream side of the oil cooler and an oil path on the downstream side. The bypass oil path detours around the oil cooler. An oil thermostat is provided to the upstream oil path for selectively opening a pathway to one of the oil cooler and the bypass oil path, and switching the flow of lubricating oil therebetween. The oil thermostat opens the bypass oil path when the temperature of lubricating oil is below predetermined temperature, and opens pathway to the oil cooler when the temperature of the lubricating oil is equal to or exceeds the predetermined temperature. 
   According to the first aspect of the invention, based on the temperature of the lubricating oil when it reaches the oil thermostat provided in the upstream oil path, the thermostat regulates the oil so that it bypasses the oil cooler when the temperature of the lubricating oil is below the predetermined temperature. The oil which bypasses the oil cooler is not cooled, whereby warm-up of the engine is accelerated. Thus, in cold water operation, supercooling is prevented. Moreover, even if fuel in a combustion chamber invades the crankcase and is mixed with the oil, evaporation is accelerated as the temperature of the oil rises, and the occurrence of dilution is prevented. 
   A second aspect of the invention relates to the internal combustion engine for a personal watercraft of the first aspect, and is further characterized in that a low-pressure oil switch is provided in the bypass oil path, and a high-pressure oil switch is provided in the oil path downstream of the oil cooler. 
   According to second aspect of the invention, the high-pressure oil switch is provided in the downstream oil path. As a result, an abnormal rise of oil pressure, caused for example, by the clogging of the downstream oil path, can be detected by the high-pressure oil switch. 
   As the downstream side of the bypass oil path communicates with the oil path upstream of the oil cooler not only when the oil thermostat opens the bypass oil path but also when the oil thermostat opens the side of the oil cooler and closes the bypass oil path, the bypass oil path is constantly filled with lubricating oil. An abnormal drop in oil pressure can be constantly and stably detected by providing the low-pressure oil switch in the downstream side of the bypass oil path. 
   Modes for carrying out the present invention are explained below by reference to a selected illustrative embodiment of the present invention, shown in the attached drawings. For a more complete understanding of the present invention, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following description and in the drawings, like numbers refer to like parts throughout the several views, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevational view of a personal watercraft, having an internal combustion engine according to a selected illustrative embodiment of the present invention mounted therein below a seat. 
       FIG. 2  is a top plan view of the personal watercraft of  FIG. 1  showing the internal combustion engine mounted along the longitudinal centerline of the personal watercraft. 
       FIG. 3  is a sectional view of the personal watercraft taken along line III—III in  FIG. 1  showing the engine compactly mounted between a deck on an upper side and a hull on a lower side. 
       FIG. 4  is a front elevational and partially sectional view of the boat body and the internal combustion engine of  FIG. 1  showing the internal combustion engine inclined to a right side of the boat body. 
       FIG. 5  is an isolated perspective view of the internal combustion engine of  FIG. 1  showing a surge tank and intercooler mounted on a left side thereof, a turbocharger mounted on a right side thereof, and connecting pipes therebetween. 
       FIG. 6  is a front elevational and partially sectional view of the internal combustion engine of  FIG. 1  showing a dividing, or split, plane between the cylinder block and the crankcase oriented at an acute angle with respect to a horizontal plane. 
       FIG. 7  is a side sectional view of the internal combustion engine of  FIG. 1  showing a crankshaft supported on a plurality of ribs formed on an interior surface of the cylinder block, and showing an oil cooling system mounted on a front face of the engine. 
       FIG. 8  is a right side elevational view of the internal combustion engine of  FIG. 1  with a part being cut-away showing an oil cooler mounted within the oil a thermostat positioned upstream of an oil cooler, and an oil cooler bypass path bypassing the oil cooler which permits the thermostat to redirect oil around the oil cooler under certain conditions. 
       FIG. 9  is a bottom sectional view of a cylinder block of the internal combustion engine of  FIG. 1  showing the configuration of the ribs formed on an interior surface of the cylinder block, and showing balance shafts extending longitudinally along the left and right sides of a front portion of the cylinder block. 
       FIG. 10  is a bottom view of a crankcase of the internal combustion engine of  FIG. 1  showing a longitudinally elongate rectangular opening formed in the bottom surface of the crankcase, and showing the aligning surface comprised of a circumferential edge of the opening, upon which the oil pan is fixed from below. 
       FIG. 11  is a bottom view of an oil pan of the internal combustion engine of  FIG. 1  showing an oil recovery path opening to one end, and showing fixing holes at spaced intervals about the periphery of the oil pan. 
       FIG. 12  is a top plan view of the oil pan of  FIG. 11  showing a cavity formed on three sides by a three-sided wall structure and on a fourth side by an oil strainer, and showing the oil recovery path opening into the cavity. 
       FIG. 13  is a side elevational view of the oil strainer of  FIG. 12 , showing a screen supported by a frame, and showing protrusion of the cover part of the oil strainer offset toward a lower side of the oil strainer. 
       FIG. 14  is a top plan view of the oil strainer of  FIG. 12 , showing the pyramidal protrusion of the cover part, and showing an opening formed in a lower face of the protrusion. 
       FIG. 15  is a sectional view of the oil strainer taken along line XV—XV in  FIG. 13 , showing the opening formed in a lower face of the cover part. 
       FIG. 16  is a diagram showing a circulation path of lubricating oil within the internal combustion engine of  FIG. 1 . 
       FIG. 17  is a diagram showing a circulation path of cooling water within the internal combustion engine of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   A selected illustrative embodiment of the invention will now be described in some detail, with reference to  FIGS. 1 through 17 . It should be understood that only structures considered necessary for clarifying the present invention are described herein. Other conventional structures, and those of ancillary and auxiliary components of the system, are assumed to be known and understood by those skilled in the art. Further, in the description provided herein, the right and left orientation is determined with reference forward advancing direction of the watercraft body. 
   A side plan view of a personal watercraft  1 , according to the present invention, is illustrated in  FIG. 1 . The personal watercraft  1  has an internal combustion engine  20  mounted therein in accordance with a selected illustrative embodiment hereof.  FIG. 2  illustrates a top plan view of the personal watercraft  1  of  FIG. 1 , and  FIG. 3  illustrates a sectional view of the personal watercraft  1  of  FIG. 1 . 
   The personal watercraft  1  is a small saddle-ride type planing boat, of a type which is sometimes referred to as a jet-ski. The watercraft  1  is made such that an inner space is defined between a hull  3  (lower boat bottom section) and an upper deck  4 . The hull  3  and the deck  4  constitute the primary components of a boat body  2 . An internal combustion engine  20  is stored in the inner space within the boat body  2 . The personal watercraft  1  is sized such that one to three crew members may straddle a central seat  5  provided on the deck  4  of the boat body  2 , and a handlebar  6  at the front part of the seat  5  is operated to steer the boat. 
   A propulsion means for the personal watercraft  1  is a jet propulsion pump  10  driven by the internal combustion engine  20 . The jet propulsion pump  10  is arranged at the rear part of the hull  3 . The jet propulsion pump  10  is an axial pump having a structure in which an impeller  11  is installed in a flow passage extending from a water inlet  12 , opened at the underside of the boat, to a nozzle  13 , arranged to form an outlet port opened at the rear end of the boat body (refer to  FIG. 17 ). A shaft  15  of the impeller  11  is connected to a crankshaft  21  of the internal combustion engine  20  through a coupler  56 . 
   Accordingly, when the impeller  11  is rotationally driven by the internal combustion engine  20  through the shaft  15 , water which has been drawn in at the water inlet  12  is forced outwardly through nozzle  13  at the outlet port. As a result, the boat body  2  is propelled forwardly under its reacting action, and then, at appropriate speeds, the personal watercraft  1  planes on the water. 
   Propulsion force generated by the jet propulsion pump  10  is controlled through operation of a throttle lever  7  mounted to the handlebar  6 . The nozzle  13  is rotatably operated through an operating wire corresponding to a steering operation of the operating handle  6 , and an advancing direction of the vehicle is changed by pivotally moving the outlet port of the nozzle  13 . The internal combustion engine  20  is arranged below the seat  5  substantially at a central part of the boat body  2 . The front part of the boat body  2  has a storage chamber  8 , and a fuel tank  9  is installed in the boat body between the storage chamber  8  and the internal combustion engine  20 . 
   In the depicted embodiment, the internal combustion engine  20  is an in-line four-cylinder double overhead cam (DOHC) type internal combustion engine operating on a 4-stroke cycle, where the crankshaft  21  is oriented in a forward-to-rearward (longitudinal) direction of the boat body  2 . The main body of the internal combustion engine  20  is made such that a cylinder block  22  and a crankcase  23  are vertically stacked, and are connected to each other along a split, or dividing, plane  24  ( FIG. 6 ) in such a way that the crankshaft  21  is rotatably supported along the split plane  24 . Moreover, the cylinder head  25  overlies the cylinder block  22 , and the cylinder head cover  26  is applied to the upper surface of the cylinder head  25 . In addition, an oil pan  27  is fixed below the crankcase  23  to the underside thereof. 
   A pair of right-side mounting brackets  22   a ,  22   a  protrude at the front and rear lower ends of the right side of the cylinder block  22  so as to slant upwardly (refer to  FIGS. 6 and 9 ). Similarly, a pair of front and rear left-side mounting brackets  23   a ,  23   a  protrude from the left side of the crankcase  23 , in parallel with the split plane  24  (refer to  FIGS. 6 and 10 ). 
   Accordingly, the right-side mounting bracket  22   a  and the left-side mounting bracket  23   a , arranged respectively at the right and left sides of the internal combustion engine  20 , protrude at an obtuse angle relative to each other. As shown in  FIG. 4 , each of the mounting brackets  22   a ,  23   a  is fixed to mounts  28 L,  28 R formed on the interior surface of the hull  3 . The mounts  28 L,  28 R are arranged at the same horizontal height and at the right and left sides of the hull  3  through rubber anti-vibration members  29 ,  29 , so as to supportively receive the internal combustion engine  20  thereon. 
   Accordingly, the split plane  24  between the cylinder block  22  and the crankcase  23  is in parallel with the protruding direction of the left side mounting bracket  23   a . As a result, the split plane  24  has an angle increased leftward in respect to a horizontal line H and is generally inclined (refer to  FIGS. 4 and 6 ). 
   The internal combustion engine  20  is formed such that a cylinder  22   b  of the cylinder block  22  extends in a direction perpendicular to the split plane  24 , and a cylinder head  25  and a cylinder head cover  26  are arranged in direction of extension. At the same time, the oil pan  27  is also fixed to the underside of the crankcase  23  in a direction perpendicular to the split plane  24 , so that the internal combustion engine  20  is inclined toward the right side as shown in  FIG. 4  (and  FIG. 6 ) and mounted on the boat body  2 . 
   As shown in  FIG. 6 , a piston  30  reciprocates within the rightward-inclined cylinder  22   b , whereby the crankshaft  21  is rotated through a connecting rod  31 . The cylinder head  25  resides on an upper side of the cylinder  22   b , and is made such that a combustion chamber  32  is formed in opposition against the top surface of the piston  30 . The combustion chamber  32  has openings, and an intake port  33 I and an exhaust port  33 E extend from these openings in a lateral direction. 
   Camshafts  35 I,  35 E respectively actuate an intake valve  34 I for opening or closing an opening of the intake port  33 I, and an exhaust valve  34 E for opening or closing of the exhaust port  33 E. The camshafts  35 I,  35 E are arranged at an aligning surface that is formed on an upper surface of the cylinder head, such that the camshafts are positioned between the cylinder head  25  and the cylinder head cover  26 . 
   A surging tank  40 , communicating with the intake port  33 I and an intercooler  41 , is connected to and arranged on the left side of the main body of the internal combustion engine  20 . An exhaust manifold  42 , communicating with the exhaust port  33 E, is connected to and arranged on the right side of the engine  20  (refer to  FIGS. 4 and 5 ). 
   As shown in  FIG. 5 , a turbocharger  43  is arranged at a rear part of the internal combustion engine  20 . The turbocharger  43  is constructed such that an exhaust outlet of the exhaust manifold  42  is connected to an intake port of its turbine segment  43 T, and further, a connecting pipe  44  from the intercooler  41  is connected to an outlet extending from the compressor part  43 C of the turbocharger  43 . 
   A cooling water feeding hose  45  permits feeding of cooling water from a positive pressure side of the jet propulsion pump  10  and is branched downstream of the pump  10 . A first branch thereof forms a cooling water hose  41   a , which extends between the feeding hose  45  and the intercooler  41 . A cooling water drain hose  41   b  extends from the other (downstream) side of the intercooler  41 , and is connected to the turbocharger  43  (refer to  FIG. 17 ). 
   Another cooling water hose  46 , formed of the second branch of the cooling water feeding hose  45 , extends toward an oil cooler  100  located at the front side of the internal combustion engine  20 , to be described later (refer to  FIG. 17 ). Further, as shown in  FIGS. 1 and 2  and referring to  FIG. 17 , the exhaust gas, used to rotate the turbine wheel at the turbine segment  43 T of the turbocharger  43 , passes in sequence through an exhaust pipe  47   a , an anti-back flow chamber  47   b  (a chamber for preventing back-flow of water to prevent water from entering into the turbocharger or the like at the time of turnover), a water muffler  47   c  and piping  47   d , reaches the water chamber  47   e , which is in communication with water, and is then discharged into the water. 
   As described above, although the crankshaft  21  is rotatably pivoted by means of bearings positioned at each end of the split plane  24  between the cylinder block  22  and the crankcase  23 , two balance shafts  36 L,  36 R, which eliminate secondary vibration, are rotatably pivoted at bearings at the right and left sides of the crankshaft  21 . 
   A total number of five crank journals  21   j  are provided within the cylinder block  22 . Specifically, a crank journal  21   j  is positioned between each of the respective four pairs of crank webs  21   w  corresponding to four cylinders of the crankshaft  21 , providing three such crank journals  21   j . In addition, the two front and rear crank journals  21   j  are provided corresponding to the front and rear faces of the cylinder block  22 . The five crank journals are held and rotatably pivoted through metal bearings at semi-arcuate landings formed at five ribs  22   r ,  23   r  forming vertical walls in a forward-to-rearward direction. Ribs  22   r ,  23   r  are formed at each of both upper and lower sides of the cylinder block  22  and the crankcase  23  (refer to  FIGS. 7 and 9 ). The central rib of the five ribs  22   r  will be referred to as central rib  22   rc.    
   As shown in the bottom view of the cylinder block  22  in  FIG. 9 , the four non-central ribs  22   r , of the five ribs  22   r  for supporting the crankshaft  21  at its bearings, extend generally within a plane between both right and left ends, without being curved. However, the left and right ends of the central rib  22   rc  are curved so as to be biased, or displaced, forward of the bearings (left side in  FIG. 9 ) that pivotally support the crankshaft  21 . 
   The right and left forward-displaced portions of the central rib  22   rc  are provided with rear side bearings for the balance shafts  36 L,  36 R. The front side bearings for the balance shafts  36 L,  36 R are arranged at the right and left portions of the rib  22   r  that forms the forward-most outer wall. That is, the balance shafts  36 L,  36 R are arranged in parallel at the right and left portions of the crankshaft  21 , and are rotatable at their front and rear portions through metal bearings, for example, at the bearing of the forward-most rib  22   r  and the bearing of the central rib  22   rc . As a result, the balance shafts  36 L,  36 R are longitudinally arranged so as to be offset toward the front side of the cylinder block  22 . 
   The balance shafts  36 L,  36 R are divided by the central rib  22   rc  such that balance weights  36 Lw,  36 Rw are positioned on the balance shafts  36 L,  36 R between the central rib  22   rc  and its front adjoining rib  22   r . In addition, there are provided balance weights  36 Lw,  36 Rw cantilevered at the rear end portion of the balance shafts  36 L,  36 R, positioned rearward of the central rib  22   rc.    
   As seen in horizontal section, the cylinder block  22  is formed having a lateral width in the front portion thereof, where balance shafts  36 L,  36 R are arranged, that is large, and its lateral width in the rear portion thereof, where balance shafts  36 L,  36 R are not arranged, is relatively narrow. Since the balance shafts  36 L,  36 R have their rear portions supported at the bearings displaced forward of the central rib  22   rc , the rear portions of the balance shafts  36 L,  36 R are positioned as far forward as possible. Correspondingly, the proportion of the horizontal section that is of a narrow lateral width, that is, the rear side portion of the cylinder block  22 , is kept large so that the overall size of the main body of the internal combustion engine  20  is compact. 
   In addition, since the rear part balance weights  36 Lw,  36 Rw are not supported at both ends, but instead are supported in a cantilever form, the entire length of the respective balance shafts  36 L,  36 R is made short, and bearings are not required at the rear ends thereof. Correspondingly, the narrow lateral width at the rear portion of the cylinder block  22  is assured to be large, further enhancing the effect of forming the overall a size of the main body of the internal combustion engine  20  in a compact manner. 
   Further, the crankcase  23 , connected to the split plane  24  of the cylinder block  22 , also has five ribs  23   r  corresponding to five ribs  22   r  of the cylinder block  22  (refer to  FIG. 7 ). The central rib  23   rc  is displaced forward at its left and right ends. As a result, it is possible to assure a large narrow lateral width portion at the rear part of the main body of the internal combustion engine  20 , and auxiliary machines are arranged within the acquired lateral vacant space at the rear side of the internal combustion engine  20 , permitting the overall size of the internal combustion engine  20  to be made even more compact. 
   As shown in  FIGS. 7 and 9 , a drive gear  21   g  is formed at the outer circumference of the crank web  21   w  of the crankshaft  21  rotating along the inner surfaces of the ribs  22   r ,  23   r  which form the forward-most outer walls of the cylinder block  22  and the crankcase  23 . In turn, the balance shafts  36 L,  36 R are also formed with driven gears  36 Lg,  36 Rg along the inner surfaces of the ribs  22   r ,  23   r  which form the forward-most outer walls. 
   The driven gear  36 Lg of the left balance shaft  36 L and the drive gear  21   g  at the outer circumference of a crank web  21   w  of the crankshaft  21  are directly engaged to each other. In turn, as shown in  FIG. 6 , an intermediate shaft  37  is supported at the rib  22   r  of the cylinder block  22  at a diagonally left upper part of the driven gear  36 Rg of the right balance shaft  36 R. An intermediate gear  37   g  rotatably pivots on the intermediate shaft  37 , and is engaged with the driven gear  36 Rg of the right balance shaft  36 R, and further is also concurrently engaged with the drive gear  21   g  at the outer circumference of the crank web  21   w  of the crankshaft  21 . 
   Accordingly, the right and left balance shafts  36 L,  36 R are rotated in opposite directions through rotation of the crankshaft  21 , and are rotated at twice rotating speed of the crankshaft  21  so as to dampen or eliminate its secondary vibration. 
   A gear mechanism comprised of the drive gear  21   g  for transmitting a rotation of the crankshaft  21  to the right and left balance shafts  36 L,  36 R, intermediate gear  37   g , driven gears  36 Lg,  36 Rg is arranged inside the cylinder block  22  and the crankcase  23  along the inner surfaces of the ribs  22   r ,  23   r  forming the forward-most outer walls and is placed at the position where it is overlapped at the same rearward positions as those of the mounting brackets  22   a ,  23   a  of the cylinder block  22  and the crankcase  23  as seen from its side elevational view. 
   Accordingly, a rigidity around the gear mechanism for use in transmitting a rotating power force at the cylinder block  22  and the crankcase  23  and at the bearing portions of the balance shafts  36 L,  36 R can be assured in a sufficient high value without adding any special structure. 
   Since the cylinder block  22  of the crankshaft  21  and the crank web  21   w  inside the crankcase  23  are provided with a drive gear  21   g , the crankshaft  21  itself can be made shorter, and the entire length of the internal combustion engine  20  can be correspondingly shorter, as compared with those of the prior art structure where the drive gear is provided independently. 
   The portion of the crankshaft  21  that protrudes out of the ribs  22   r ,  23   r  which form the front outer walls of the cylinder block  22  and the crankcase  23  is provided with a driven gear  51  for a starter. The driven gear  51  is connected to the crankshaft  21  through a one-way clutch  50  as shown in  FIG. 9 , and is positioned along the outer surfaces of the ribs  22   r ,  23   r . At the same time an outer rotor  54   r  of an AC generator  54  is fixed at a more forward location than the driven gear  51  for a starter (refer to  FIG. 7 ). 
   The driven gear  51  for a starter itself can be made smaller than an arrangement in which the driven gear  51  for a starter, applied through the one-way clutch  50 , is arranged side by side to the drive gear not integral with the crank web, as found in the prior art, but instead is arranged independently so as to avoid an interference from each other. 
   As indicated by a two-dot chain line in  FIG. 6 , a small diameter gear  52   a , rotatably supported by a reduction gear shaft  52 , is engaged with the driven gear  51  for a starter. A large diameter gear  52   b , integral with the small diameter gear  52   a , is engaged with the drive gear  53   a  fitted to a driving shaft of the starter motor  53 , positioned above the left balance shaft  36 L. 
   In turn, the rear part of the crankshaft  21  is pivotally supported on the bearings  55  on the rear walls of the cylinder block  22  and the crankcase  23 , and protrudes rearward, as shown in  FIG. 7 . The rear end of the crankshaft  21  is connected to the shaft  15  connected to the impeller  11  of the jet propulsion pump  10  through a coupler  56 . 
   Referring to  FIG. 7 , this figure shows that a cam chain chamber  57  is formed between the rear-most ribs  22   r ,  23   r  and the rear walls of the cylinder block  22  and the crankcase  23 . A drive sprocket  58  is fitted to the crankshaft  21  within the cam chain chamber  57 , and a cam chain  60  encircles both the drive sprocket  58  and the driven sprockets  59 ,  59  which are fitted to the rear ends of the upper camshafts  351 ,  35 E. 
   As seen in a bottom view of the crankcase ( FIG. 10 ), the lower surface of the crankcase  23  has a longitudinally elongate rectangular opening formed thereon. A circumferential edge of the opening is formed with an aligning surface  23   b  upon which an oil pan  27  is fixed from below, in compliance with this aligning surface  23   b.    
   The rectangular aligning surface  23   b  is formed with a plurality of threaded holes  23   p  provided at spaced intervals about the aligning surface  23   b . As shown in  FIGS. 11  and  12 , a bolt  61  is passed through each of a corresponding fixing hole  27   p  formed at a rectangular circumferential edge aligning surface  27   b  of the oil pan  27 , and threadably inserted into a threaded hole  23   p  whereby the oil pan  27  is fixed to the crankcase  23 . 
   Referring to  FIG. 10 , a main oil passage  23 C extends longitudinally along the lower surface of the crankcase  23 , and opens at the front wall of the crankcase  23 . Bolt holes  23   d  are formed on the right and left sides of each rib  23   r  so as to be laterally opposed across oil passages  23 C. A fastening bolt  38  is passed through each bolt hole  23   d , and is threadably inserted into the cylinder block  22  to fasten the crankcase  23  to the cylinder block  22 , whereby they are coupled together (refer to  FIG. 6 ). 
   Further, oil passages  23 L,  23 R for the right and left balancers, used to supply oil to the bearings of the right and left balance shafts  36 L,  36 R, are arranged along the right and left sides of the main oil passage  23 C so as to be in parallel with the main oil passage  23 C. The oil passages  23 L,  23 R for the right and left balancers are open at the front wall of the crankcase  23  (refer to  FIG. 6 ). 
   In addition, within the periphery of the rectangular aligning surface  23   b  of the crankcase  23 , and at its rear half part, an elongate, longitudinally extending, rectangular box-shaped (parallelepiped) frame wall  70 , having four sides is formed. An inside part of the frame wall  70  has an upper surface  71  (corresponding to the bottom of the crankcase), and the lower side is open (refer to  FIG. 10 ). The lower end surface of the frame wall  70  is set at the same height as, that is, lies flush with, that of the aligning surface  23   b  with the oil pan  27 . 
   In turn, as shown in  FIGS. 11 and 12 , the oil pan  27  is provided with a frame wall  27  on an upper surface thereof. The frame wall  72  is composed of three side walls, i.e. a front wall, a rear wall and a left wall, and a fourth (right) wall thereof is absent. The right side wall of the frame wall  70  of the crankcase  23  is vertically installed downward from the bottom surface of the crankcase to a location within the oil pan  27 . An oil recovering passage  73 , having a circular opening and extending straight forward from the front wall of the frame wall  72 , is opened at the front wall of the oil pan  27  (refer to  FIG. 6 ) and communicates with an oil pump  90  to be described later. 
   As shown in  FIG. 12 , inner edges of three sides of the frame wall  72  which bound the absent right wall, that is, the front wall, rear wall and bottom wall, are formed with grooves  72   a . A long rectangular oil strainer  74  is fitted within the grooves  72   a  in a substantially vertical posture. 
   As shown in  FIGS. 13 to 15 , the oil strainer  74  is made such that the circumferential edge of a band-like long oil screen  75  is held at its right and left portions by a stopper frame  76  and a screen cover  77 , and the holding part is enclosed by a rubber member  78 . 
   The stopper frame  76  includes a flat rectangular frame, closed in shape, and cross members  76   b . In particular, the stopper frame  76  has a shape in which three cross-member  76   b  extend between the long opposed sides of the flat rectangular frame  76   a  to form large four openings. The screen cover  77  comprises a frame part  77   a  surrounding a cover  77   b . The cover  77   b  protrudes outward in pyramid-shape, the apex of the pyramid being displaced to one side, adjacent to a frame part  77   a . Frame part  77   a  corresponds to the frame  76   a  of the stopper frame  76 , and a rectangular shape is cut out of lower portion of the cover  77   b  to form an opening  77   c.    
   The frame  77   a  of the screen cover  77  holds the circumferential edge of the oil screen  75  between itself and the frame  76   a  of the stopper frame  76 , goes around the back part of the frame  76   a , and fastens it to apply tension to the oil screen  75 . 
   The aforesaid oil strainer  74  is fitted by means of the rubber member  78  to the grooves  72   a  of three sides adjacent the absent right wall of the frame wall  72  in the oil pan  27 . When in place, the cover part  77   b  of the screen cover  77  protrudes to the right side (refer to  FIG. 12  and the oil strainer  74  is indicated by a two-dot chain line), and the opening  77   c  opens downward. 
   When the oil pan  27  is fixed to the crankcase  23  while the oil strainer  74  is fitted to the groove  72   a , the frame wall  70  of the crankcase  23  and the frame wall  72  of the oil pan  27  are abutted to each other at their end surfaces, the upper end rubber member  78  of the oil strainer  74  is abutted against the right wall of the frame wall  70 , a space in the oil pan  27  is partitioned by the frame walls  70 ,  72 , upper surface  71 , oil pan bottom surface and oil screen  75  to form a rectangular parallelepiped cavity  79 . The cavity  79  communicates with the oil recovering passage  73  through an opening at the front wall of the frame wall  72 . 
   As described above, since the internal combustion engine  20  is mounted on the boat body  2  so as to be inclined rightwardly, the rectangular parallelepiped cavity  79  defined in the oil pan  27  is set such that the oil screen  75  of the oil strainer  74  occupies the right opening, which is placed at a lower position of the cavity  79 . That is, oil accumulated in the oil pan  27  is gathered eccentrically at the right side to enable the oil strainer  74 , defining the right opening of the cavity  76 , to be constantly submerged in the oil. 
   Oil accumulated in the oil pan  27  is drawn in an opening  77   c  of the screen cover  77  of the oil strainer  74 , passes through the oil screen  75  and flows into the cavity  79 . At this time, a minimal amount of air is drawn in because the oil strainer  74  is constantly submerged in the oil. 
   Since the oil strainer  74  occupies the cavity  79  in a substantially vertical orientation, the lateral width of the oil pan  27  can be reduced than compared to case in which the oil pan is installed horizontally as shown in the prior art. Thus, it becomes easy to align the oil strainer  74  to fit with the right or left inclination from the center of the bottom of the personal watercraft  1 , and the internal combustion engine  20  can be mounted at a slightly lower position. 
   In addition, although it is necessary to have a space including a certain degree of margin in its vertical orientation when the oil pan is installed using the prior art horizontal orientation, installation under a substantial vertical orientation, as in the case of the present oil strainer  74 , enables a sufficient space to be assured at the lateral sides of the oil strainer  74  even if the vertical width of the oil pan is small, enables a vertical width of the oil pan  27  itself to be reduced, enables an entire height of the internal combustion engine  20  to be shortened, and further facilitates mounting the engine onto the boat bottom part of the personal watercraft  1 . 
   Since the cavity  79 , defined by the oil strainer  74 , is constituted by the frame wall  70  formed at the crankcase  23 , the upper surface  71 , the frame wall  72  formed at the oil pan  27  and the oil pan bottom surface, no special or exclusive parts are required, and the number of component parts can be reduced. Additionally, the oil strainer  74  is also constructed to be held between the crankcase  23  and the oil pan  27  providing superior assembly characteristics. 
   Front surfaces of the aforesaid cylinder block  22 , crankcase  23  and oil pan  27  are formed with aligning surfaces  22   f ,  23   f  and  27   f  forming a common plane (refer to  FIG. 6 ). A tank main body  81  of the oil tank  80  is connected to the aligning surfaces  22   f ,  23   f  and  27   f . Further, the oil tank  80  is constituted of the tank main body  81  and the tank cover  88 , which covers the front surface of the tank main body  81 . 
   As shown in  FIGS. 4 and 7 , the tank main body  81  has an aligning surface  81   r  connected to the aligning surfaces  22   f ,  23   f  and  27   f  formed at the front surfaces of the cylinder block  22 , crankcase  23  and the oil pan  27 . The tank main body  81  also has an aligning surface  81   f  for connection with the tank cover  88 , the aligning surfaces  81   r ,  81   f  being in parallel with each other. An ACG cover part  82 , protruding forward from the aligning surface  81   r  to cover the AC generator  54  or reduction gears  52   a ,  52   b , is provided. An entire longitudinal oil storing part  83  is formed over above and right and left sides of the ACG cover  82 , and a water-cooled type oil cooler housing  85  is formed to protrude above the crankshaft  21  at the right side of the oil storing part  83 . 
   Further,  FIG. 4  is a front plan view that shows the tank main body  81  fixed to the front surfaces of the cylinder block  22 , crankcase  23  and oil pan  27 . The upper space of the oil storing part  83  is provided with a breather chamber  84 . 
   As shown in  FIG. 7 , an outer rotor  54   r  of the AC generator  54  is fixed to the outer tip end of the crankshaft  21 , together with the coupling  62   a , by a bolt  63 . The coupling  62   a  is connected to a coupling  62   b  at the rear end of a pump shaft  95  of the oil pump  90 , to be described later. 
   A coupling cover part  82   a  covering the couplings  62   a ,  62   b  protrudes rearward at the central part of the ACG cover  82 . An inner stator  54   s  of the AC generator  54  is supported by being fixed to the coupling cover part  82   a.    
   An oil pump  90  is provided at a front part of the ACG cover part  82  covering the AC generator  54  from the front side. The oil pump  90  includes a first case  92  connected to a front part to the tank main body  81 , and a second case  93  connected to a front part, and fixed to, the tank main body  81  by a bolt  94  together with the first case  92 . The pump shaft  95 , coaxial with the crankshaft  21 , passes through both of the front and rear first and second cases  92 ,  93 , and together with the crankshaft  21  passes through the ACG cover part  82 . The coupling  62   b  is fixed at its rear end by a bolt  95   a  from a rear side. 
   An inner rotor is fitted to a shaft part in the first case  92  of the pump shaft  95 . A scavenging pump  90 S is provided. An inner rotor is fitted to a shaft part in the second case  93 , and a feed pump  90 F is provided. Accordingly, rotation of the crankshaft  21  is transmitted to a rotation of the pump shaft  95  through couplings  62   a ,  62   b  so as to drive the scavenging pump  90 S and the feed pump  90 F. 
   Referring to  FIGS. 4 and 7 , an oil recovering passage  86  that communicates with the oil recovering passage  73  of the oil pan  27  is formed at the lower part of the tank main body  81 . The oil recovering passage  86  is partially formed at the rear surface of the first case  92 , extends upward and reaches to the scavenging pump  90 S. 
   Accordingly, lubricating oil accumulated at the oil pan  27  passes through the oil strainer  74  under driving operation of the scavenging pump  90 S and is drawn in at the front part of the oil recovering passage  73 , passes through the oil recovering passage  86  and reaches to the upper scavenging pump  90 S. 
   Referring to  FIG. 7 , a common recovering oil discharging passage  87  is formed above the scavenging pump  90 S near the rear surface of the first case  92  and the front surface of the tank main body  81 . The upper end of the recovering oil discharging passage  87  opens to the oil storing part  83  of the oil tank  80 . Accordingly, the recovering oil discharged under a driving of the scavenging pump  90 S passes through the recovering oil discharging passage  87  and is recovered at the oil storing part  83  of the oil tank  80 . 
   In addition, as shown in  FIG. 7 , the supplying oil suction passage  96  is formed below the feed pump  90 F between the front surface of the first case  92  and the rear surface of the second case  93 , and at the same time, the supplying oil discharging passage  98  is formed above the feed pump  90 F. The lower end of the supplying oil suction passage  96  opens at a height near the bottom surface of the oil storing part  83 , and its upper end communicates with the suction port of the feed pump  90 F. A screen oil filter  97  is installed at the midway part of the supplying oil suction passage  96 . 
   The supply oil discharging passage  98  extends upward from the discharging port of the feed pump  90 F. Thereafter, it is bent rearward and is connected to a lateral hole  98   a  formed at the tank main body  81 . The lateral hole  98   a  communicates with a vertical hole  98   b  formed at the same tank main body  81 , the upper end of the vertical hole  98   b  opens in an annular shape at the fixing surface of the oil filter  110 , to be described later, and communicates with an oil inlet  111  of the oil filter  110  (refer to  FIG. 8 ). 
   Accordingly, when the feed pump  90 F is driven, the lubricating oil is drawn up through the supply oil suction passage  96  from the lower part of the oil storing part  83  of the oil tank  80 , discharged to the supply oil discharging passage  98 , forcedly fed upward at the lateral hole  98   a  and the vertical hole  98   b  formed at the tank main body  81 , and then reaches the oil filter  110 . 
   Further, a relief valve  99  is installed at the midway part of the supply oil discharging passage  98  between it and the oil storing part  83 , and when a discharging pressure of the supply oil is too high, surplus oil is returned back to the oil storing part  83 . 
   As shown in  FIGS. 4 and 8 , the water-cooling type oil cooler  100  is provided within the oil cooler housing  85 , and protrudes longitudinally from the front surface of the tank main body  81 . The oil cooler  100  is longer than it is wide, and comprises a plurality of heat exchanging plates  100   a  through which oil flows. An upstream side pipe  100   b  communicates with the upper part in the plates  100   a , and a downstream side pipe  100   c  communicates with the lower part in the plates  100   a , and each of the upstream side pipe  100   b  and the downstream side pipe  100   c  is connected to a respective upper hole and lower hole formed at the tank main body  81 . The oil cooler  100  is fixed to the tank main body  81 . 
   The oil cooler  100  is covered on its front side with a part of the tank cover  88  as shown in  FIG. 8 , so as to cause cooling water to flow in or flow out of the oil cooler housing  85 , and within it, whereby the oil in the oil cooler  100  is cooled. 
   As shown in  FIG. 8 , the upper hole in the tank main body  81 , to which the upstream side pipe  100   b  of the oil cooler  100  is connected, communicates with one outlet of an oil thermostat  105  provided with a changing-over valve  105   a  at the rear part of the upstream side pipe  100   b . The lower hole, to which the downstream side pipe  100   c  of the oil cooler  100  is connected, communicates with an substantially vertical oil passage  107  extending downward of the downstream side oil passage of the oil cooler  100 . Another outlet of the oil thermostat  105  bypasses the oil cooler  100 , and communicates with a bypass oil passage  106 , which is connected to the substantially vertical oil passage  107 . 
   In addition, as shown in  FIG. 8 , the inlet of the oil thermostat  105  communicates with the oil outlet  112  of the oil filter  110 . The oil outlet  112  is fixed to the upper part of the oil thermostat  105  by means of the upstream side oil passage  113  of the oil cooler  100 . The oil filter  110  is operated such that the oil, forcedly fed by the feed pump  90 F as described above, flows into the oil inlet  111 , and the filtered oil flows out of the oil outlet  112 . 
   When the lubricating oil is equal to or more than a predetermined temperature, the oil thermostat  105  opens the side of the oil cooler  100 , and closes the bypass oil path  106 , respectively, by means of the motion of the changing-over valve  105   a . Moreover, when the lubricating oil temperature is lower than the predetermined temperature, the oil thermostat  105  opens the bypass oil passage  106 , and closes the side of the oil cooler  100 . 
   A low-pressure oil switch  115  is fixed to the bypass oil passage  106  so as to detect an abnormal reduction of hydraulic pressure, and a high-pressure oil switch  116  is fixed to the substantially vertical oil passage  107  downstream side of both the oil cooler  100  and the bypass oil passage  106 , so as to detect an abnormal increasing of hydraulic pressure. 
   As shown in  FIG. 8 , the low-pressure oil switch  115  is fixed to the bypass oil passage  106  so as to protrude in a rightward direction, and in turn, the high-pressure oil switch  116  is fixed to the substantially vertical oil passage  107  so as to protrude in a forward direction, using the space below the oil cooler  100 . 
   As indicated by a dotted line in  FIG. 4 , the substantially vertical oil passage  107  is bent at the lower part of the tank main body  81  in a leftward direction and communicates with the oil lateral passage  108 . The oil lateral passage  108  has three branched passages directed rearward. The central part of the oil lateral passage  108  is provided with a main gallery supplying passage  109   c  that supplies oil to the main gallery  23 C of the internal combustion engine  20 . The respective left and right ends of the oil lateral passage  108  are provided with a left balancer supplying passage  109   l  and a right balancer supplying passage  109   r  for supplying oil to the bearings for each of the right and left balance shafts  36 L,  36 R (refer to  FIG. 10 ). 
   As shown in  FIGS. 7 and 16 , the main gallery supplying passage  109   c  is connected to the main oil passage  23 C of the crankcase  23  and oil is distributed from the main oil passage  23 C to each of the bearings of the crankshaft  21  and supplied to the passage in the rib  23   r.    
   The left balancer supplying passage  109   l  and the right balancer supplying passage  109   r  are connected to each of the left balancer oil passage  23 L and the right balancer oil passage  23 R, respectively (refer to  FIG. 10 ), whereby oil is supplied to the bearings of the right and left balance shafts  36 L,  36 R. 
   Further, oil is supplied from the main oil passage  23 C to the bearings of the upper camshafts  351 ,  35 E and at the same time oil is also supplied to the turbocharger  43  so as to form circulation paths each returning to the oil pan  27 . 
   In  FIG. 16 , a circulation path diagram for lubricating oil described above is illustrated, and its entire flow will now be described. Lubricating oil, accumulated at the oil pan  27 , is drawn by means of a driving operation of the scavenging pump  90 S, filtered through the oil strainer  74 , passes through the oil recovering passages  73 ,  86  and is drawn into the scavenging pump  90 S. Lubricating oil discharged out of the scavenging pump  90 S is recovered into the oil tank  80 . 
   Lubricating oil recovered into the oil tank  80  is drawn by means of a driving operation of the feed pump  90 F, passes through the screen oil filter  97 , and is drawn into the feed pump  90 F. Lubricating oil discharged out of the feed pump  90 F passes through the lateral hole  98   a  and the vertical hole  98   b , passes through a medial relief valve  99 , flows into the oil filter  110  where it is filtered, and then reaches the oil thermostat  105 . 
   When the lubricating oil reaches a temperature equal to or higher than a predetermined temperature, the changing-over valve  105   a  opens a pathway to the oil cooler  100 , permitting the lubricating oil to flow to the oil cooler  100  and to be cooled, while closing off access to a bypass oil path  106 . Cooled lubricant is discharged to substantially vertical oil passage  107 . Alternatively, if the lubricating oil reaches the thermostat  105  at a temperature below the predetermined temperature, the changing-over valve  105   a  closes the pathway to the oil cooler, and opens the bypass oil passage  106 , thereby permitting the lubricating oil to flow through the bypass oil passage  106 , avoiding the cooling action of the oil cooler  100 , and flowing downstream from the bypass oil passage to the substantially vertical oil passage  107 . In addition, a low-pressure oil switch  115  is fixed to the bypass oil passage  106 , and the high-pressure oil switch  116  is fixed to the substantially vertical oil passage  107 . 
   Lubricating oil that has flowed down the substantially vertical oil passage  107  is branched at the lower end thereof within oil lateral passage  108  into three branch passages, whereby lubricating oil flows at the lower part of the crankcase  23  in a rearward direction. Lubricating oil branched at the right and left balancer supplying passages  109   l ,  109   r  passes through each of the right and left balancer oil passages  23 L,  23 R and is supplied to the bearings of the right and left balance shafts  36 L,  36 R. 
   Lubricating oil branched at the central main gallery supplying passage  109   c  is further branched while passing through the main oil passage  23 C and is supplied to each of the bearings of the crankshaft  21 . Further, lubricating oil supplied to each of the bearings of the crankshaft  21  passes through the oil passage formed in the crankshaft  21  and is supplied to a connecting part with a large end of the connecting rod  31 . 
   In addition, a camshaft oil supplying passage  120  is formed to extend from the main oil passage  23 C in an upward direction. Lubricating oil that has ascended the camshaft oil supplying passage  120  flows in each of the in-shaft oil passages of the right and left camshafts  351 ,  35 E, and supplies the in-shaft oil passages to each of the bearings and each of the cam surfaces. Lubricating oil that has lubricated the crankshaft  21 , right and left balance shafts  36 L,  36 R and right and left camshafts  351 ,  35 E and the like finally returns back to the oil pan  27 . 
   Further, the turbocharger oil supplying pipe  122  extends from the main oil passage  23 C to the turbocharger  43  through the oil filter  121 . A part of the lubricating oil that has flowed through the main oil passage  23 C passes through the turbocharger oil supplying pipe  121  and is supplies the turbocharger  43 . 
   Lubricating oil supplied to the turbocharger  43  is branched to provide a first branch for lubricating the bearings and a second branch for shutting off heat at the turbine and cooling it. The lubricating oil within the two branches is returned back to the oil pan  27  through the two oil discharging pipes  123 ,  124 . 
   Meanwhile, a cooling system for the internal combustion engine  20  of the present invention mounted on the personal watercraft  1  uses water on which the personal watercraft  1  floats.  FIG. 17  illustrates the circulation path for the cooling water which is described as follows. As presented above, cooling water is fed from the cooling water intake port  131  at the downstream positive pressure side of the impeller  11  of the jet propulsion pump  10  by means of the cooling water feeding hose  45 . Cooling water passing through one branched cooling water hose  46  of the cooling water feeding hose  45  is supplied to the oil cooler housing  85  of the oil cooler  100  placed at an upstream side of the jet propulsion pump  10 . Cooling water is directed in from the downstream side cooling water in-flow part  85   a  to cool the lubricating oil, thereafter, the cooling water flows out of the upper cooling water out-flow part  85   b , circulates at the water jacket of the cylinder block  22  of the internal combustion engine  20  to cool the internal combustion engine  20 , and is discharged out of the boat. 
   Cooling water passing through the other cooling water hose  41   a  branched from the cooling water feeding hose  45  flows into the intercooler  41  to cool intake gas, and then flows to the turbocharger  25  to cool the turbocharger  25 . Thereafter, the cooling water reaches the exhaust pipe  47   a  to cool the exhaust pipe  47   a  and at the same time the exhaust gas is taken into the cooling water, then the cooling water passes through the anti-backflow chamber  47   b , water muffler  47   c  and pipe  47   d  in sequence and reaches the water chamber  47   e  communicating with the water, and then the cooling water is discharged into the water. 
   The oil thermostat  105  in the aforesaid lubricating system opens the oil path through the oil cooler  100  when the lubricating oil shows a temperature equal to or more than the predetermined temperature, so as to cool the lubricating oil, thereby cooling of the internal combustion engine  20  can be promoted. 
   In turn, when the lubricating oil shows a temperature lower than the predetermined temperature, the bypass oil passage  106  is opened directing the lubricating oil bypass the oil cooler  100  and not to be cooled. In this manner, idling operation is promoted and over-cooling at the time of a cooling operation is prevented in advance. 
   The personal watercraft  1  is operated such that cooling water fed from the positive pressure side of the jet propulsion pump  10  is used for cooling the internal combustion engine  20 , and the oil cooler  100  also utilizes this cooling water, so that it is easy for over-cooling to occur during a cooling operation, and passing the lubricating oil through the oil cooler causes it to reach an over-cooled state more easily. To avoid this situation, the lubricating oil is not passed through the oil cooler  100  under a control of the oil thermostat  105  at a temperature lower than the predetermined temperature, where the over-cooling is apt to occur, but instead bypasses the oil cooler  100  to avoid the over-cooling at the time of cooling operation. 
   Since over-cooling is avoided, even if fuel in the combustion chamber  32  enters into the crankcase  23  and is mixed with oil, evaporation of oil is promoted since the oil temperature is increased, and dilution is prevented, whereby oil deterioration is restricted. 
   Since both the bypass oil passage  106  and the discharge from the oil cooler communicate with the downstream side of the bypass oil passage  106 , the bypass oil passage  106  is always filled with lubricating oil. The bypass oil passage  106  is provided with the low-pressure oil switch  115 , whereby an abnormal reduction in hydraulic pressure is stably detected. 
   The substantially vertical oil passage  107  at the downstream side of the oil cooler  100  is provided with the high-pressure oil switch  116  to enable detection of an abnormal increasing of hydraulic pressure caused by clogging at the oil passage to be lubricated such as each of the downstream side bearings or the like. When the abnormal state of hydraulic pressure is detected by one or both of the low-pressure oil switch  115  and the high-pressure oil switch  116 , countermeasures, including producing an alarm for bringing the condition to an operator&#39;s attention, are carried out. 
   The oil cooler  100  is made such that a size of the heat exchanging plates  100   a  is short and small as compared with that of the prior art. Moreover, the lower part of the oil cooler  100  is displaced upward and located at a higher position than the crankshaft  21 , and the oil cooler housing  85  itself is also located at a higher position than the crankshaft  21  at its lower part. Accordingly, as shown in  FIG. 8 , a space is formed below the oil cooler  100 , which protrudes from the tank main body  81 . Thus, some auxiliary units can be arranged below the oil cooler  100  to utilize the space, and the high-pressure oil switch  116  is arranged to protrude within this space about the internal combustion engine  20  of the present invention. 
   Since the high-pressure oil switch  116  is arranged to protrude just below a part of the tank cover  88  covering the oil cooler  100  from its front side, its upper part is covered by the tank cover  88  to prevent water from dropping from above onto the high-pressure oil switch  116 . 
     FIG. 17  illustrates the circulation path for the cooling water, wherein a relative height between the internal combustion engine  20  and the jet propulsion pump  10  is substantially illustrated in reference to its actual state. The crankshaft  21  and the rotating shaft of the impeller  11  are connected by the shaft  15  and they are also set substantially at the same height. 
   Referring to  FIG. 17 , as described above, the cooling water is taken through the cooling water intake port  131  at the downstream side positive pressure of the impeller  11  of the jet propulsion pump  10 , and flows through the cooling water feeding hose  45  and the cooling water hose  46 , and flows from the cooling water in-flow part  85   a  at the lower part of the oil cooler housing  85  to the oil cooler housing  85 . The cooling water in-flow part  85   a  of the oil cooler housing  85  is located at a higher position than that of the crankshaft  21 , and in turn, the cooling water intake port  131  at the positive pressure side of the jet propulsion pump  10  has a lower position than that of the crankshaft  21  kept at the same height position. The cooling water feeding hose  45  reaching the oil cooler housing  85 , and all the cooling passages of the cooling water hose  46 , are also located at a lower position than that of the cooling water in-flow part  85   a  at the lower part of the oil cooler housing  85 . 
   Accordingly, when the personal watercraft  1  is pulled up on land, water in the oil cooler housing  85 , covered by the tank cover  88 , flows out of the cooling water in-flow part  85   a , passes through the cooling water hose  46  and the cooling water feeding hose  45 , flows out of the cooling water intake port  131  at the positive pressure side of the jet propulsion pump  10 , and is naturally discharged. 
   While a working example of the present invention has been described above, the present invention is not limited to the working example described above, but various design alterations may be carried out without departing from the present invention as set forth in the claims.