Patent Publication Number: US-6988919-B2

Title: Oil cooler and small watercraft

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an oil cooler configured to cool oil that circulates within an engine, and a small watercraft comprising the oil cooler. 
   2. Description of the Related Art 
   In recent years, jet-propulsion personal watercraft, which are one type of small watercraft, have been widely used in leisure, sport, rescue activities, and the like. The personal watercraft is equipped with an engine within a space surrounded by a hull and a deck. The personal watercraft is configured to have a water jet pump that pressurizes and accelerates water sucked from a water intake generally provided on a hull bottom surface and ejects it rearward from an outlet port. As the resulting reaction, the personal watercraft is propelled forward. 
   Oil circulates within the engine mounted in the personal watercraft to lubricate and cool various components within the engine. In order for the oil to fully function, the oil is required to have a proper temperature. However, the oil that has circulated within the engine has a relatively high temperature, and therefore, an oil cooler is used to cool the oil (see Japanese Issued Patent No. 3276593,  FIG. 2 ). In some cases, conventional oil coolers are positioned distant from the engine (mainly comprised of a crankcase, a cylinder block, and a cylinder head), for example, in the vicinity of an oil tank provided to be independent of the engine. 
   A number of pipes, for example, a pipe that draws the oil into the oil cooler, a pipe that draws the oil out of the oil cooler, a pipe that draws a coolant to the oil cooler, and a pipe that discharges the coolant from the oil cooler, are connected to the oil cooler. For maintenance of the oil cooler, it is necessary to attach and detach these pipes to and from the oil cooler, which is burdensome. In addition, these pipes may have a complex piping configuration. Since the piping configuration is complex and the oil cooler is positioned distant from the engine, the lengths of the pipes are extended, which makes it difficult to achieve a lightweight small watercraft. 
   Another prior oil cooler is made of steel plate and mounts directly on an outer wall face of a crankcase. However, such an oil cooler has difficulty in removing unwanted substances from a coolant passage, and hence is not suitable for use in an engine having an open-looped cooling system in which water is drawn from outside for use as cooling water. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above described conditions, and an object of the present invention is to provide an oil cooler that has a simple piping configuration, is easy to maintain, and achieves a lightweight small watercraft, as well as a small watercraft comprising the oil cooler. 
   According to the present invention, there is provided an oil cooler of an engine for small watercraft, comprising a mounting portion configured to mount the oil cooler on an outer wall face of the engine, an oil passage configured to allow oil to flow therethrough and lead to outside of the oil cooler at the mounting portion, and a coolant passage through which coolant for cooling the oil flows, wherein the oil cooler is capable of being disassembled such that an inside of at least the coolant passage is exposed. 
   In accordance with the above construction, by directly mounting the oil cooler on the wall face of the engine, an oil gallery formed within the engine can communicate with the oil passage of the oil cooler without a tube or the like. Therefore, the number of pipes around the oil cooler can be reduced and piping configuration is simplified. In addition, since the oil cooler is disassembled to allow an inside of at least the coolant passage to be exposed, maintenance of the oil cooler, for example, removal of unwanted substances from the coolant passage can be carried out. 
   The oil cooler may include a passage forming plate provided with grooves on one face thereof and an opposite face thereof, and first and second cover members configured to cover the grooves, respectively, the oil passage may be formed by covering the groove formed on the one face of the passage forming plate with the first cover member, the coolant passage may be formed by covering the groove formed on the opposite face of the passage forming plate with the second cover member, and the second cover member may be at least partially removably attachable to allow the inside of the coolant passage to be exposed. 
   In accordance with the above construction, by removing the first or second cover member from the passage forming plate, an inside of the oil passage or the coolant passage is easily exposed. Alternatively, the cover member may be removably attachable only at a portion of the coolant passage that tends to be clogged with unwanted substances. As another alternative, the cover member may be formed integrally with the passage forming plates at portions of the oil passage and the coolant passage that need not be exposed. 
   The first cover member may be provided with a sensor-attaching portion configured to attach a hydraulic-pressure sensor and/or an oil-temperature sensor. The sensor-attaching portion allows the hydraulic-pressure sensor and the oil-temperature sensor to be easily attached, and hence maintenance of these sensors can be easily carried out. 
   The oil cooler may further comprise an oil filter attaching and detaching portion configured to removably attach an oil filter of the engine on the first cover member, wherein an oil hole may be formed in the first cover member in the vicinity of the oil filter attaching and detaching portion to allow the oil filter and the oil passage to communicate with each other with the oil filter attached on the first cover member. With this configuration, the engine provided with the oil cooler becomes compact. 
   An oil-receiving portion may be provided on the first cover member in the vicinity of the oil filter attaching and detaching portion and below the attached oil filter. Since the oil-receiving portion can receive the oil that leaks out when the oil filter is removed for a built-in filter element to be changed, the element is changed efficiently. The oil-receiving portion may be formed integrally with or independently of the first cover member. 
   The oil-receiving portion may be plate shaped and may be configured to extend from the first cover member along a center axis of the oil filter. 
   The cooler may further comprise an adapter configured to allow the oil passage of the oil cooler to communicate with another oil cooler, wherein the adapter may be provided between the oil filter and the first cover member. The adapter may be removably attached on the first cover member by means of a tubular mounting bolt of the oil cooler. In accordance with the above configuration, another oil cooler may be connected to the oil cooler of the present invention through the adapter as necessary in order to gain desired oil cooling capability in the engine. Thus, by increasing the number of oil coolers to increase cooling capacity, the design of the oil cooler itself need not be changed in order to address changes in cooling requirements of the engine. 
   The oil cooler of an engine for small watercraft, comprising a mounting portion configured to mount the oil cooler on an outer wall face of the engine, an oil passage configured to allow oil to flow therethrough and lead to outside of the oil cooler at the mounting portion, and a coolant passage through which coolant for cooling the oil flows, wherein the oil cooler is capable of being disassembled such that an inside of at least the coolant passage is exposed, may further comprise a plurality of passage forming plates each provided with a groove on at least one face thereof, wherein the passage forming plates may be removably disposed to have a layered structure, and the oil passage and the coolant passage may be each formed by the groove between the passage forming plates. 
   In accordance with the construction, by disassembling the oil cooler into the passage forming plates, the oil passage and the coolant passage are exposed, and hence are easy to maintain. By changing the number of the passage forming plates to be stacked, the cooling capability of the oil can be changed. 
   According to an aspect of the above-discussed oil cooler, the passage forming plates may be comprised of an oil passage forming plate forming the oil passage and a coolant passage forming plate forming the coolant passage which are alternately disposed to have a layered structure. In such a configuration, the cooling capability of the oil cooler can be easily changed by disposing stacked pairs of plates, each pair having an oil passage forming plate and coolant passage forming plate, in a suitable number. 
   In the oil cooler of an engine for small watercraft, comprising a mounting portion configured to mount the oil cooler on an outer wall face of the engine, an oil passage configured to allow oil to flow therethrough and lead to outside of the oil cooler at the mounting portion, and a coolant passage through which coolant for cooling the oil flows, wherein the oil cooler is capable of being disassembled such that an inside of at least the coolant passage is exposed, an inside of at least part of the coolant passage may be exposed at the mounting portion. In other words, with the oil cooler mounted on the outer wall face of the engine, at least part of the coolant passage may be formed by the outer wall face. This makes it possible to cool the wall portion of the crankcase as well as the oil. Also, when the oil gallery is formed in the wall portion, the oil following through the oil gallery can be cooled. 
   The oil cooler may further comprise a passage forming plate provided with a groove on at least one face thereof, and a first cover member configured to cover the groove formed on the one face of the passage forming plate, wherein the passage forming plate may be removably mounted on the outer wall face of the engine with an opposite face thereof in contact with the outer wall face of the engine, the oil passage may be formed by covering the groove formed on the one face with the first cover member, and the coolant passage may be formed between the opposite face of the passage forming plate and the outer wall face of the engine. 
   In such a configuration, since the opposite face of the passage forming plate on the coolant passage side is in contact with the outer wall face of the engine, the outer wall face of the engine is cooled, and when the oil gallery is formed in the wall portion of the engine, the oil flowing within the oil gallery is cooled. Further, since a cover member configured to cover the opposite face of the passage forming plate on the coolant passage side need not be provided, the number of components can be reduced and hence lightweight watercraft can be achieved. 
   The passage forming plate may be provided with a groove on the opposite face thereof, and the groove formed on the opposite face may be covered with the outer wall face of the engine. Such a structure increases the flow-cross-sectional area of the coolant. 
   A groove may be formed on the outer wall face of the crankcase that partially forms the coolant passage. In such a configuration, since the flow-cross-sectional area of the coolant passage and a contact area of the coolant with the outer wall face of the engine can be increased, the cooling capability of the oil cooler can be improved. 
   The oil cooler may further comprise an oil filter attaching and detaching portion configured to removably attach the oil filter of the engine on the first cover member, wherein an oil hole may be formed in the first cover member in the vicinity of the oil filter attaching and detaching portion to allow the oil filter and the oil passage to communicate with each other with the oil filter attached on the first cover member. Also, the oil cooler may further comprise an adapter configured to allow the oil passage of the oil cooler to communicate with another oil cooler, wherein the adapter may be provided between the oil filter and the first cover member on the oil passage side. 
   In the engine, air-intake pipes and exhaust pipes extending from a cylinder head of the engine are arranged in various configurations. In the case of personal watercraft, typically, the pipes extend from the cylinder head to the position lateral of a crankcase of the engine. In such piping configuration, there is an unused space between the air-intake pipe or the exhaust pipe and the outer wall face of the crankcase. 
   Accordingly, a small watercraft of the present invention comprises an engine configured to drive a propulsion mechanism, an air-intake pipe and an exhaust pipe extending from a cylinder head of the engine, and an oil cooler configured to cool oil that circulates with the engine, wherein the air-intake pipe or the exhaust pipe extends from the cylinder head to a lateral side of a crankcase of the engine to have a space between the air-intake pipe and an outer wall face of the crankcase or between the exhaust pipe and the outer wall face of the crankcase, and the oil cooler is mounted on the outer wall face within the space. 
   In accordance with the above construction, the unused space can be utilized for the oil cooler to be placed close to the crankcase. As a result, piping configuration is simplified and compact small watercraft is achieved. 
   In the small watercraft, an oil gallery may be formed within a wall portion of the crankcase of the engine to allow the oil to flow therethrough, the oil cooler may include an oil passage through which the oil flows and a coolant passage through which coolant for cooling the oil flows, the oil cooler may be mounted on the wall face of the crankcase such that the oil passage communicates with the oil gallery, and the oil cooler may be capable of being disassembled such that an inside of at least the coolant passage is exposed. In accordance with the above construction, maintenance of the oil cooler, for example, removal of unwanted substances from the coolant passage can be carried out, in addition to the above described simplified piping configuration. 
   The engine may employ an open-looped cooling system. Specifically, the engine mounted in the small watercraft is commonly configured to take in water from outside for use as coolant (cooling water). In the open-looped cooling system, the cooling water taken in from outside sometimes contains substances such as water borne plants. Since the oil cooler can be disassembled into the coolant passages as described above, the substances within the coolant passage can be easily removed. 
   The small watercraft may be a personal watercraft comprising a water jet pump driven by the engine. The small watercraft includes a jet-propulsion personal watercraft equipped with the water jet pump as a propulsion mechanism. The personal watercraft has a limited inner space, and engine components and the oil cooler are generally difficult to maintain. By applying the present invention to the personal watercraft, the piping configuration is significantly simplified and maintenance becomes much easier. 
   The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a personal watercraft according to an embodiment of the present invention; 
       FIG. 2  is a plan view of the personal watercraft in  FIG. 1 ; 
       FIG. 3  is a side view of the engine mounted in the personal watercraft in  FIG. 1 ; 
       FIG. 4  is a front view of the engine in  FIG. 3 ; 
       FIG. 5  is a partially enlarged view of the engine in  FIG. 3 , and a partial cross-sectional view showing an oil cooler and an oil filter mounted on a mounting face of a crankcase; 
       FIG. 6A  is a rear view of a cover plate constituting a rear-face of the oil cooler in  FIG. 5 ; 
       FIG. 6B  is a rear view of a passage forming plate, with the cover plate in  FIG. 6A  removed from the oil cooler in  FIG. 5  to expose an inner face of a cooling water passage; 
       FIG. 7A  is a front view of a cover plate constituting a front face of the oil cooler in  FIG. 5 ; 
       FIG. 7B  is a front view of a passage forming plate, with the cover plate in  FIG. 7A  removed from the oil cooler in  FIG. 5  to expose an inner face of an oil passage; 
       FIG. 8  is a side view of the engine comprising an oil cooler according to another embodiment of the present invention; 
       FIG. 9  is a view of an external appearance of the oil cooler in  FIG. 8 ; 
       FIG. 10  is a cross-sectional view of the oil cooler taken along line X—X in  FIG. 8 ; 
       FIG. 11  is a cross-sectional view of the oil cooler taken along line XI—XI in  FIG. 8 ; 
       FIG. 12  is a partial cross-sectional view of a two-layered oil cooler according to another embodiment of the present invention; 
       FIG. 13A  is a front view of a rear-face cover plate forming the oil cooler in  FIG. 12 ; 
       FIG. 13B  is a cross-sectional view of the rear-face cover plate taken along line XIIIb—XIIIb in  FIG. 13A ; 
       FIG. 13C  is a rear view of the rear-face cover plate in  FIG. 13A ; 
       FIG. 14A  is a front view of a front-face cover plate forming the oil cooler in  FIG. 12 ; 
       FIG. 14B  is a cross-sectional view of the front-face cover plate taken along line XIVb—XIVb in  FIG. 14A ; 
       FIG. 14C  is a rear view of the front-face cover plate in  FIG. 14A ; 
       FIG. 15A  is a front view of an oil passage forming plate forming the oil cooler in  FIG. 12 ; 
       FIG. 15B  is a cross-sectional view taken along line XVb—XVb of the oil passage forming plate in  FIG. 15A ; 
       FIG. 15C  is a rear view of the oil passage forming plate in  FIG. 15A ; 
       FIG. 16A  is a front view of a cooling water passage forming plate forming the oil cooler in  FIG. 12 ; 
       FIG. 16B  is a cross-sectional view of the cooling water passage forming plate taken along line XVIb—XVIb in  FIG. 16A ; 
       FIG. 16C  is a rear view of the cooling water passage forming plate in  FIG. 16A ; 
       FIG. 17  is a schematic view of an oil passage within the oil cooler in  FIG. 16A ; 
       FIG. 18  is a schematic view of a cooling water passage within the oil cooler in  FIG. 12 ; 
       FIG. 19  is a partial cross-sectional view of a three-layered oil cooler obtained by altering part of a configuration of the oil cooler in  FIG. 12 ; 
       FIG. 20  is a side view of the engine comprising oil coolers (first and second oil coolers) according to another embodiment of the present invention; 
       FIG. 21A  is a schematic view of an external configuration of an adapter in  FIG. 20 ; 
       FIG. 21B  is a cross-sectional view of the adapter taken along line XXIb—XXIb in  FIG. 21A ; 
       FIG. 22  is an exploded view showing the first oil cooler and the adapter in  FIG. 20 ; and 
       FIG. 23  is a schematic view showing flow of oil within the first oil cooler and a second oil cooler in  FIG. 20 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of small watercraft of the present invention will be described with reference to the accompanying drawings. Here, a personal watercraft will be described. The personal watercraft in  FIG. 1  is a straddle-type personal watercraft provided with a seat  7  straddled by a rider. A body  1  of the watercraft comprises a hull  2  and a deck  3  covering the hull  2  from above. A line at which the hull  2  and the deck  3  are connected over the entire perimeter thereof is called a gunnel line  4 . In  FIG. 1 , reference numeral  5  denotes a representative waterline on the personal watercraft. 
   As shown in  FIG. 2 , an opening  6 , which has a substantially rectangular shape as seen from above, is formed at a substantially center section of the deck  3  in the upper portion of the body  1  such that its longitudinal direction corresponds with the longitudinal direction of the body  1 . The seat  7  straddled by the rider is removably mounted over the opening  6 . 
   An engine room  8  is provided in a space defined by the hull  2  and the deck  3 , below the opening  6 . An engine E for driving the personal watercraft is mounted within the engine room  8 . The engine room  8  has a convex-shaped transverse cross-section and is configured such that its upper portion is smaller than its lower portion. In this embodiment, the engine E is an in-line four-cylinder four-cycle engine. As shown in  FIG. 1 , the engine E is mounted such that a crankshaft  9  extends along the longitudinal direction of the body  1 . 
   An output end of the crankshaft  9  is rotatably coupled integrally with a pump shaft  11  of a water jet pump P provided on the rear side of the body  1  through a propeller shaft  10 . An impeller  12  is attached on the pump shaft  11  of the water jet pump P. Fairing vanes  13  are provided behind the impeller  12 . The impeller  12  is covered with a pump casing  14  on the outer periphery thereof. 
   A water intake  15  is provided on the bottom of the body  1 . The water intake  15  is connected to the pump casing  14  through a water passage. The pump casing  14  is connected to a pump nozzle  16  provided on the rear side of the body  1 . The pump nozzle  16  has a cross-sectional area that gradually reduces rearward, and an outlet port  17  is formed on the rear end of the pump nozzle  16 . 
   Water outside the watercraft is sucked from the water intake  15  and fed to the water jet pump P. The water jet pump P pressurizes and accelerates the water and the fairing vanes  13  guide water flow behind the impeller  12 . The water is ejected through the pump nozzle  16  and out the outlet port  17 , and, as the resulting reaction, the watercraft obtains a propulsion force. 
   The engine E of this embodiment employs an open-looped cooling system. Specifically, as shown in  FIG. 1 , the pump casing  14  is provided with a water drawing hole  18 , and the water pressurized by the water jet pump P is partially drawn into the watercraft through the water drawing hole  18 , and is used as cooling water to cool the engine E or the like. 
   A bar-type steering handle  19  is provided at a front portion of the deck  3 . The steering handle  19  is connected to a steering nozzle  20  provided behind the pump nozzle  16  through a cable  21  (in  FIG. 2 ). When the rider rotates the handle  19  clockwise or counterclockwise, the steering nozzle  20  is swung toward the opposite direction so that the ejection direction of the water being ejected through the pump nozzle  16  can be changed, and the watercraft can be correspondingly turned to any desired direction while the water jet pump P is generating the propulsion force. 
   As shown in  FIG. 1 , a bowl-shaped reverse deflector  22  is provided on the rear side of the body  1  and on the steering nozzle  20  such that it can vertically swing around a horizontally mounted swinging shaft  23 . The deflector  22  is swung downward to a lower position around the swinging shaft  23  to deflect the ejected water from the steering nozzle  20  forward, and as the resulting reaction, the personal watercraft moves rearward. 
   Embodiment 1 
   A first Embodiment of the present invention will be described with reference to  FIGS. 3 to 7B . As shown in a side cross-sectional view in  FIG. 3 , the engine E mainly comprises a cylinder head  31  covered with a cylinder head cover  30  from above, a cylinder block  32  located below the cylinder head  31 , and a crankcase  33  located below the cylinder block  32 . Four air-intake ports  34  are provided on one side portion of the cylinder head  31  to be spaced equally apart from one other in the longitudinal direction of the engine E. The air-intake ports  34  open toward a lateral side of the engine E. One end portions  35 A of air-intake pipes  35  are respectively connected to the air-intake ports  34 . 
   As shown in  FIG. 4 , each of the air-intake pipes  35  extends from the corresponding air-intake port  34  toward outer side of the engine E. Then, the air-intake pipe  35  is curved downwardly at a position thereof and then extends to a position lateral of the crankcase  33 . As shown in  FIG. 3 , opposite end portion  35 B of the air-intake pipes  35  are arranged to be closer to one another at a position slightly behind the center in the longitudinal direction of the engine E. An air-intake chamber  36  having an inner space of a predetermined volume is provided on one side of the crankcase  33 . The air-intake pipes  35  are connected to an upper portion of the air-intake chamber  36  such that the opposite end portions  35 B protrude into the air-intake chamber  36 . The air-intake chamber  36  communicates with an air cleaner with a throttle body (not shown) provided between the air-intake chamber  36  and the air cleaner (not shown). As shown in  FIG. 4 , a space  37  is formed between the air-intake pipe  35  and an outer peripheral wall of the crankcase  33 . 
   As shown in  FIG. 4 , four exhaust ports  40  are provided on an opposite side portion of the cylinder head  31 . The exhaust ports  40  are arranged to be spaced equally apart from one another in the longitudinal direction of the engine E. The exhaust ports  40  open to the lateral side of the engine E. One end portion  41 A of each of the exhaust pipes  41  is connected to a corresponding exhaust port  40 . Each of the exhaust pipes  41  extends from the corresponding exhaust port  40  toward outer side of the engine E. Then, the exhaust pipe  41  is curved downwardly at a position thereof and extends to a position lateral of the crankcase  33 . In addition, opposite end portions (not shown) of the exhaust pipes  41  extend rearward of the engine E from the position lateral of the crankcase  33  and are merged into one pipe connected to a muffler (not shown). In this structure, a space  42  is formed between the exhaust pipe  41  and the outer wall face of the crankcase  33 . 
   As shown in  FIG. 4 , an engine-side mounting face  43  of an oil cooler  50  is formed on the outer wall portion of the crankcase  33  on an air-intake system side. The oil cooler  50  is mounted on the engine-side mounting face  43 . An oil filter  51  is attached on the oil cooler  50 . As shown in  FIG. 3 , when the engine E is seen from the air-intake system side, the oil cooler  50  placed in the space  37  is configured such that the oil filter  51  is entirely exposed. In  FIG. 4 , an oil tank  52  having a predetermined volume is provided under the crankcase  33 . An oil passage  53  is formed on a wall portion of the crankcase  33  on the air-intake system side to lead the oil from the oil tank  52  to the engine-side mounting face  43  of the oil cooler  50 . 
   An oil gallery  54  is formed within the wall portion of the engine E to communicate with various components within the engine E to deliver the oil to the various components. One end of the oil gallery  54  is located in the vicinity of the engine-side mounting face  43  formed on the outer wall portion of the crankcase  33  to communicate with an inside of the oil filter  51 . The engine-side mounting face  43  is formed such that its normal direction is inclined slightly upward from a horizontal direction. 
   As defined hereinafter, the X-axis shown in  FIG. 3  is parallel to the longitudinal direction of the engine E and its positive side is directed to forward of the engine E. And, Y-axis shown in  FIG. 3  is parallel to the normal direction of the engine-side mounting face  43  and its positive side is directed away toward the right side of the engine E (see  FIG. 4 ). Further, Z-axis is perpendicular to the X-axis and the Y-axis, and its positive side is directed upward. The X-axis, the Y-axis, and the Z-axis are identical to the X-axis, the Y-axis, and the Z-axis described below and shown in Figures. For the sake of simplicity, the positive side of the X-axis is a forward side of the watercraft and its negative side is an aft side of the watercraft. And, a positive side of the Y-axis is a front-face side of the oil cooler  50 , and its negative side is a rear-face side of the oil cooler  50 . Further, a positive side of the Z-axis is an upper side and its negative side is a lower side. 
   As shown in a partially cross-sectional view in  FIG. 5 , the oil cooler  50  comprises a substantially plate shaped passage forming plate  55  formed by casting using metal such as aluminum, a front-face cover plate (first cover member)  56  that covers a front face (one face) of the passage forming plate  55 , and a rear-face cover plate (second cover member)  57  that covers a rear-face (an opposite face) of the passage forming plate  55 . An oil passage groove  55 A is formed on the front face of the passage forming plate  55  by casting, and a cooling water passage groove  55 B is formed on the rear-face of the passage forming plate  55 . 
   The front-face cover plate  56  and the rear-face cover plate  57  are connected to each other with the passage forming plate  55  disposed between them. A seal member  58  made of synthetic resin is provided between the plates  56  and  55  and between the plates  57  and  55 . The plates  55 ,  56 , and  57  are fixed by means of a screw means  59 . By connecting these plates  55 ,  56 , and  57 , an oil passage  60  is formed by the oil passage groove  55 A and the front-face cover plate  56 , and a cooling water passage  61  is formed by the cooling water passage groove  55 B and the rear-face cover plate  57 . An oil-cooler side mounting face  62  is provided on the rear face of the rear-face cover plate  57 . The oil-cooler side mounting face  62  is a contact face with which the oil cooler  50  is mounted on the engine E. 
   The passage forming plate  55 , the front-face cover plate  56 , and the rear-face cover plate  57  are provided with holes  63  to  65  each having a relatively-large diameter, respectively. The holes  63  to  65  are configured so that their center axes conform to one another when the plates  55 ,  56 , and  57  are fixed by means of the screw means  59 , and a tubular mounting bolt  66  having a penetrating hole along a center axis thereof is inserted through the holes  63  to  65 . The axial length of the mounting bolt  66  is larger than the thickness of the oil cooler  50 . A male screw portion (portion configured to attach and remove the oil filter  51 )  66 A is formed in an end portion on a front-face side of the mounting bolt  66  to protrude from the front face of the oil cooler  51 . A male screw portion (portion configured to attach and remove the oil filter  51 )  66 B is formed in an end portion on a rear-face side of the mounting bolt  66  to protrude from the rear face of the oil cooler  51 . 
   A female screw portion  67  having a relatively large diameter is formed on the engine-side mounting face  43  of the crankcase  33 . The female screw portion  67  communicates with the oil gallery  54 . The oil cooler  50  is directly mounted on the outer wall face of the engine E in such a manner that the passage forming plate  55 , the front-face cover plate  56 , and the rear-face cover plate  57  are fixed by the screw means  59 , the oil-cooler side mounting face  62  of the rear-face cover plate  57  and the engine-side mounting face  43  of the crankcase  33  are in contact with each other, and the male screw portion  66 B of the mounting bolt  66  inserted through the holes  63  to  65  is screwed to the female screw portion  67  of the engine-side mounting face  43 . Also, by mounting the oil cooler  50  on the engine-side mounting face  43  of the crankcase  33 , the penetrating hole of the tubular mounting bolt  66  communicates with the oil gallery  54 . 
   The oil filter  51  is provided on the front face of the oil cooler  50 . The oil filter  51  is tubular with a bottom and opens at one end thereof. The oil filter  51  contains a filter element (not shown). A female screw portion  68  is provided substantially at a center of an opening of the oil filter  51 . The oil filter  51  is directly attached on the front-face cover plate  56  of the oil cooler  50  by screwing the female screw portion  68  to the male screw portion  66 A of the mounting bolt  66 . Under this condition, the inner space of the oil filter  51  communicates with the oil gallery  54  through the penetrating hole of the mounting bolt  66 . 
   As shown in  FIG. 6B , a tubular joint  70  and a tubular joint  71  are attached on a forward end portion of the passage forming plate  55 . The cooling water flows into the oil cooler  50  through the joint  70  and flows out of the oil cooler  50  through the joint  71 . Tubes  72  are connected to the joints  70  and  71 , respectively (see  FIG. 3 ) and the cooling water drawn through the water drawing hole  18  formed in the pump casing  14  in  FIG. 1  flows through the joints  70  and  71 . 
   As described above, the cooling water passage groove  55 B is formed on the rear-face of the passage forming plate  55  in  FIG. 6B . The cooling water passage groove  55 B extends from an attaching portion of the joint  70  to an attaching portion of the joint  71 , and its passage is sinuously shaped. Specifically, the cooling water passage groove  55 B extends from a forward end portion of the plate  55  toward an aft end portion thereof and is bent at a position to return toward the forward end portion, which is repeated. Finally, the cooling water passage groove  55 B reaches the joint  71 . A fin  73  is provided within and along the passage of the cooling water passage groove  55 B. 
   In the cooling water passage groove  55 B so configured, the cooling water flows thereinto though the joint  70  (see arrow Y 1 ), flows along the sinuously-shaped groove  55 B (see arrows Y 2  and Y 3 ), and is finally delivered outside through the joint  71  (see arrow Y 4 ). By removing the rear-face cover plate  57  in  FIG. 6A  from the passage forming plate  55 , the cooling water passage groove  55 B is exposed, and hence, an inner face  61 A of the cooling water passage  61  is entirely exposed. 
   As shown in  FIG. 6B , a plurality of oil holes  74  are formed to penetrate the passage forming plate  55  in the vicinity of the hole  63  formed in the passage forming plate  55 . As shown in  FIG. 6A , a plurality of oil holes  76  are formed to penetrate the rear-face cover plate  57  in the vicinity of the hole  65  formed in the rear-face cover plate  57 . By attaching the passage forming plate  55  on the rear-face cover plate  57 , the oil holes  74  and  76  are combined to be formed into an oil route. The oil route opens into the oil-cooler side mounting face  62  (see  FIG. 5 ). By mounting the oil cooler  50  on the engine-side mounting face  43 , the oil route communicates with the passage  53  formed in the wall portion of the crankcase  33  to allow the oil flowing through the passage  53  to be drawn to the oil passage  60  (see  FIG. 5 ). As shown in  FIG. 6B , on the rear-face of the passage forming plate  55 , a seal member  58  is provided on each of a peripheral portion of the plate  55 , a peripheral portion of the hole  63 , and a peripheral portion of the oil hole  74 , to inhibit leakage of the oil and the cooling water to outside. 
   As shown in  FIG. 7B , the oil passage groove  55 A is formed on the passage forming plate  55 . The oil passage groove  55 A starts from the oil holes  74  formed in the passage forming plate  55  and extends to a terminal point  55 B located in the vicinity of the oil holes  74  on the front face of the passage forming plate  55 . The oil passage groove  55 A is sinuously shaped. More specifically, the oil passage groove  55 A extends from the oil holes  74 , is bent at an aft end portion of the plate  55 , and then extends toward a forward end portion of the pate  55 . This pattern is repeated one or more times, until the oil passage groove  55 A reaches the terminal point  55 B in the vicinity of the oil holes  74 . 
   In the oil passage groove  55 A so configured, the oil flows thereinto through the oil holes  74  (see arrows Y 11 ), flows along the sinuous oil passage groove  55 A (see arrows Y 12  to Y 14 ), and flows to the terminal point  55 B in the vicinity of the oil holes  74 . By removing the front-face cover plate  56  in  FIG. 7A  from the passage forming plate  55 , the oil passage groove  55 A is exposed, and hence, an inner face  60 A of the oil passage  60  is entirely exposed. 
   As shown in  FIG. 7A  and  FIG. 5 , an oil hole  75  is formed to penetrate the front-face cover plate  56  at a position of the front-face cover plate  56  corresponding to the terminal point  55 B of the oil passage groove  55 A. The oil hole  75  communicates with the inner space of the oil filter  51  directly attached on the front face of the oil cooler  50  (see  FIG. 5 ) to lead the oil flowing within the oil passage  60  to the oil filter  51 . As shown in  FIG. 7B , on the front face of the passage forming plate  55 , the seal member  58  is provided on each of the peripheral portion of the plate  55  and the peripheral portion of the hole  63  to inhibit leakage of the oil from its proper route to outside. 
   Furthermore, a plurality of sensor attaching holes  56 A are formed to penetrate the front-face cover plate  56  and to communicate with the oil passage  60 . Within the sensor attaching holes  56 A, various types of sensors are attached. With the oil cooler  50  mounted on the engine E, the sensor attaching holes  56 A open toward the outer lateral side of the engine E. As shown in  FIG. 4 , a hydraulic-pressure sensor  77  and an oil-temperature sensor  78  are attached within the sensor attaching holes  56 A. The sensor attaching holes  56 A allow the sensors  77  and  78  to be easily attached and detached. 
   In the personal watercraft comprising the oil cooler  50 , the cooling water taken in from outside through the water drawing hole  18  (see  FIG. 1 ) formed in the pump casing  14  is delivered through the tube  72  and flows into the oil cooler  50  through the joint  70  as shown in  FIG. 6B . The cooling water flows along the cooling water passage  61  within the oil cooler  50  and is discharged outside the oil cooler  50  through the joint  71 . 
   The cooling water discharged from the oil cooler  50  is delivered to a water jacket (not shown) formed in the cylinder block  32  for use as the cooling water to cool the cylinder block  32 . Since the cooling water discharged from the oil cooler  50  is slightly higher in temperature than the cooling water before flowing into the oil cooler  50 , the cylinder block  32  is inhibited from being cooled excessively, thereby inhibiting dilution of the oil. 
   As shown in  FIG. 4 , the oil temporarily stored within the oil tank  52  flows within the passage  53  formed in the wall portion of the crankcase  33  and reaches the engine-side mounting face  43  (see arrow Y 10 ). Then, as shown in  FIG. 7B , the oil flows into the oil passage  60  formed by the passage forming plate  55  (see arrow Y 11 ) through the oil holes  76  of the rear-face cover plate  57  (see  FIG. 6A ) and the oil holes  74  of the passage forming plate  55  (see  FIG. 6B ). Then, the oil flows along the oil passage  60  within the oil cooler  50  and is delivered to the oil filter  51  through the oil hole  75  of the front-face cover plate  56  (see arrows Y 15  and Y 16 ). The oil flowing into the oil filter  51  is filtered inside thereof. Then, as shown in  FIG. 5 , the oil flows within the mounting bolt  66  (see arrow Y 17 ) and flows through the oil gallery  54  (see arrow Y 18 ) formed in the wall portion of the crankcase  33  to be delivered to the components within the engine E. 
   As described above, since low-temperature cooling water flows within the cooling water passage  61  on the rear-face side of the passage forming plate  55  and high-temperature oil flows within the oil passage  60  on the front-face side of the passage forming plate  55 , the oil is cooled by heat exchange with the cooling water. In addition, since the oil passage  60  is sinuously shaped, a relatively long cooling time is ensured, and hence the oil cooler  50  has a high cooling capability. Further, the fin  73  provided within the cooling water passage  61  allows the oil to be cooled efficiently. 
   The hydraulic-pressure sensor  77  and the oil-temperature sensor  78  attached within the sensor mounting holes  56 A penetrating the front-face cover plate  56  are in contact with the oil flowing within the oil passage  60 . Therefore, the hydraulic-pressure sensor  77  detects information relating to the pressure of the oil, and the oil-temperature sensor  78  detects information relating to the temperature of the oil. 
   In the oil cooler  50  configured as described above, by removing the screw means  59  and the mounting bolt  66 , the oil cooler  50  is disassembled into the passage forming plate  55 , the front-face cover plate  56 , and the rear-face cover plate  57 . As a result, since the inner face  60 A of the oil passage  60  and the inner face  61 A of the cooling water passage  61  are exposed, the interior of the oil cooler  50  is easily cleaned. In the personal watercraft comprising the oil cooler  50 , since the oil cooler  50  is directly mounted on the outer wall face of the crankcase  33 , externally attached pipes to lead the oil flowing within the oil gallery  54  to the oil passage  60  of the oil cooler  50  become unnecessary. As a result, piping configuration around the engine E is simplified, lightweight personal watercraft is achieved, and manufacturing cost is reduced. 
   While in this embodiment, the oil cooler  50  is placed within the space  37  between the engine E and the air-intake pipe  35 , the oil cooler  50  may be placed within the space  42  between the engine E and the exhaust pipes  41 . In this case, the engine-side mounting face  43  of the oil cooler  50  is formed on a side face of the crankcase on the exhaust pipe side. 
   Embodiment 2 
   An oil cooler having another configuration will be described with reference to  FIGS. 8 to 11 . In  FIGS. 8 to 11 , the same reference numerals as those in  FIGS. 1 to 7  denote the same or corresponding parts. The oil cooler of this embodiment is applicable to the personal watercraft described with reference to  FIGS. 1 and 2 . As shown in a side view of the engine E in  FIG. 8 , an oil cooler  80  is provided within the space  37  between the air-intake pipe  35  and the crankcase  33 , as in the oil cooler  50  of the first embodiment. 
   As shown in a front view of the oil cooler  80  in  FIG. 9 , the oil cooler  80  is substantially rectangular. The oil cooler  80  is provided with a joint  81  and a joint  82  at a forward end portion thereof. The cooling water flows into the oil cooler  80  through the joint  81  and flows out of the oil cooler  80  through the joint  82 . The ends of the tubes  72  are connected to the joints  81  and  82 , respectively. The oil filter  51  is attached on a front face of the oil cooler  80 . As shown in  FIG. 8 , the oil cooler  80 , which is placed within the space  37  between the crankcase  33  and the air-intake pipe  35 , is configured such that the oil filter  51  is substantially entirely exposed when the engine E is seen from the air-intake system side. 
   As shown in  FIGS. 10 and 11 , the rear-face over plate  57  of the oil cooler  50  of the first embodiment is not provided on the oil cooler  80  of this embodiment, and a rear-face of a passage forming plate  85  is directly attached on the outer wall face of the crankcase  33 . 
   As shown in  FIGS. 10 and 11 , the oil cooler  80  comprises a substantially plate-shaped passage forming plate  85  formed by casting using metal such as aluminum. An oil passage groove  85 A is formed on a front face of the passage forming plate  85  and a first cooling water passage groove  85 B is formed on a rear-face of the passage forming plate  85 . The oil passage groove  85 A and the first cooling water passage groove  85 B have structures similar to those of the oil passage groove  55 A and the cooling water passage groove  55 B of the first embodiment. The front face of the passage forming plate  85  is covered with a front-face cover plate  86  and an oil passage  87  is formed by the front-face cover plate  86  and the oil passage groove  85 A. 
   As shown in  FIGS. 9 and 10 , the front-face cover plate  86  is provided with an oil-receiving portion  88 . As shown in  FIG. 9 , the oil-receiving portion  88  is provided below the oil filter  51 . The oil-receiving portion  88  is formed by circular-arc shaped plate member to surround a lower portion of the oil filter  51 . The oil-receiving portion  88  extends from the front face of the front-face cover plate  86  along a center axis (toward the front face) of the oil filter  51 . The oil-receiving portion  88  serves to receive oil that leaks out when the oil filter  51  is removed to allow the filter element to be changed. 
   The oil-receiving portion  88 , which is formed on the front-face cover plate  86  forming the oil passage  87 , serves as a heat-release fin to release more heat of the oil flowing within the oil passage  87  to outside. The oil-receiving portion  88  may be formed independently of the front-face cover plate  86  and thereafter may be attached on the front-face cover plate  86 . Alternatively, the oil-receiving portion  88  may be cast integrally with the front-face cover plate  86 . In this case, the number of components is reduced and manufacturing steps is reduced. 
   As shown in  FIG. 10 , a rear-face of the passage forming plate  85  forms an oil cooler-side mounting face  90  of the oil cooler  80 . An engine-side mounting face  91  of the oil cooler  80  is formed on the outer wall face of the crankcase  33  in the vicinity of the oil gallery  54 . The front-face cover plate  86  is attached on the front face of the passage forming plate  85 , and under this condition, the passage forming plate  85  is mounted on the wall portion of the crankcase  33  by means of the mounting bolt  66  and a screw means  92  to allow the oil cooler-side mounting face  90  to be connected to the engine-side mounting face  91  of the crankcase  33 . As in the first embodiment, the oil filter  51  is attached on a front-face end portion of the mounting bolt  66 . An inner space of the oil filter  51  communicates with an oil passage  87 . 
   By mounting the oil cooler  80  on the engine-side mounting face  91 , the oil passage  87  communicates with the passage  53  formed in a wall portion of the crankcase  33 , and further communicates with the oil tank  52  through the passage  53  (see  FIG. 4 ). The oil filter  51  communicates with the oil gallery  54  through the inside of the mounting bolt  66 . 
   A second cooling water passage grove  91 B is formed on the engine-side mounting face  91 . The second cooling water passage groove  91 B is substantially symmetric with respect to the first cooling water passage groove  85 B formed on the rear-face of the passage forming plate  85 . By mounting the oil cooler  80  on the engine-side mounting face  91 , the first cooling water passage groove  85 B and the second cooling water passage groove  91 B form a cooling water passage  93 . The cooling water passage  93  is sinuously shaped in the vicinity of the oil gallery  54 , as in the cooling water passage  61  (see  FIG. 8 ) described in the first embodiment, and communicates with the tubes  72  through the joints  81  and  82 . 
   In the oil cooler  80  configured as described above, since it is not necessary to provide the cover plate on the rear-face side (cooling water passage side) of the passage forming plate  85 , small-sized and lightweight oil cooler  80  is achieved. In addition, since the cooling water passage  93  is formed by the passage forming plate  85  and the crankcase  33 , the crankcase  33  can be cooled. In particular, the oil flowing within the oil gallery  54  located in the vicinity of the cooling water passage  93  can be cooled. 
   As in the oil cooler  50  described in the first embodiment, the oil cooler  80  is dissembled into the passage forming plate  85  and the front-face cover plate  86  and an inner face of the cooling water passage  93  and an inner face of the oil passage  87  are exposed, by removing the mounting bolt  66  and the screw means  92 . 
   Embodiment 3 
   An oil cooler having another configuration will be described with reference to  FIGS. 12 to 19 . The oil cooler of this embodiment is applicable to the personal watercraft described with reference to  FIGS. 1 and 2 . 
   Referring to  FIG. 12 , an oil cooler  100  of this embodiment comprises a number of passage forming plates formed by casting using metal such as aluminum between the rear-face cover plate  101  and the front-face cover plate  102 . The oil cooler  100  of this embodiment has a two-layered structure comprising two stacked pairs of plates, each pair having an oil passage forming plate  103  and a cooling water passage forming plate  104 . The passage forming plate  103  forms the oil passage  105  and the cooling water passage forming plate  104  forms the cooling water passage  106 . 
   Referring to  FIGS. 13A to 13C , the rear-face cover plate  101  has a predetermined thickness. A cooling water passage groove  111  is formed to extend sinuously on a front face of the rear-face cover plate  101  and concave portions  112  are formed on a rear face thereof by partially thinning the plate  101  for the purpose of light weight. A hole  113  is formed to penetrate the rear-face cover plate  101  in a thickness direction. The hole  113  forms a bolt hole  150  through which the mounting bolt  66  (see  FIG. 12 ) is inserted. An oil cooler-side mounting face  114  of the rear-face cover plate  101  is located in the vicinity of the hole  113 . An oil inflow hole  115  is formed on the oil-cooler side mounting face  114  to penetrate the rear-face cover plate  101  in the thickness direction thereof. The oil inflow hole  115  forms an oil inflow passage  151  of the oil cooler  100 . 
   As shown in  FIGS. 14A to 14C , the front-face cover plate  102  is substantially equal in thickness to the rear-face cover plate  101 . A rear face of the front-face cover plate  102  is flat and concave portions  121  are formed on a front face of the front-face cover plate  102  by partially thinning the plate  102  for the purpose of light weight. A large-diameter hole  122  is formed to penetrate the front-face cover plate  102  in the thickness direction. The hole  122  forms a bolt hole  150 . An oil outflow hole  123  is formed to penetrate the front-face cover plate  102  in the thickness direction in the vicinity of the hole  122 . The oil outflow hole  123  forms an oil outflow passage  152  of the oil cooler  100 . Further, a cooling water inflow hole  124  and a cooling water outflow hole  125  are formed to penetrate the front-face cover plate  102  in the thickness direction. The cooling water inflow hole  124  forms a cooling water inflow passage  153  of the oil cooler  100  and the cooling water outflow hole  125  forms a cooling water outflow passage  154  of the oil cooler  100 . The cooling water inflow hole  124  and the cooling water outflow hole  125  have threaded inner peripheral faces with which joints configured to connect hoses (not shown) are threadedly engaged. 
   As shown in  FIGS. 15A to 15C , the oil passage forming plate  103  has a predetermined thickness smaller than that of the rear-face cover plate  101 . An oil groove  131  is formed on a front face of the oil passage forming plate  103  to extend sinuously, and a rear face of the plate  103  is flat. A large-diameter hole  132  is formed to penetrate the oil passage plate  103  in the thickness direction. The hole  132  forms the bolt hole  150 . An oil inflow hole  133  and an oil outflow hole  134  are formed to penetrate the oil passage forming plate  103  in the thickness direction thereof in the vicinity of the hole  132 . The oil inflow hole  133  forms the oil inflow passage  151  and the oil outflow hole  134  forms an oil outflow passage  152 . Further, a cooling water inflow hole  135  and a cooling water outflow hole  136  are formed to penetrate the oil passage forming plate  103  in the thickness direction. The cooling water inflow hole  135  forms a cooling water inflow passage  153  and the cooling water outflow hole  136  forms a cooling water outflow passage  154 . 
   As shown in  FIGS. 16A to 16C , the cooling water passage forming plate  104  is substantially equal in thickness to the oil passage forming plate  103 . A cooling water passage groove  141  is formed on a front face of the cooling water passage forming plate  104  to extend sinuously, and a rear face of the cooling water passage forming plate  104  is flat. A large-diameter hole  142  is formed to penetrate the cooling water passage forming plate  104  in the thickness direction. The hole  142  forms the bolt hole  150 . An oil inflow hole  143  and an oil outflow hole  144  are formed to penetrate the cooling water passage forming plate  104  in the thickness direction in the vicinity of the hole  142 . The oil inflow hole  143  forms the oil inflow passage  151  and the oil outflow hole  144  forms the oil outflow passage  152 . Further, a cooling water inflow hole  145  and a cooling water outflow hole  146  are formed to penetrate the cooling water passage forming plate  104  in the thickness direction thereof. The cooling water inflow hole  145  forms the cooling water inflow passage  153  and the cooling water outflow hole  146  forms the cooling water outflow passage  154 . 
   The oil cooler  100  is configured such that the oil passage forming plate  103  and the cooling water passage forming plate  104  are alternately disposed between the rear-face cover plate  101  and the front-face cover plate  102  to allow passages to be formed between the plates. In the structure of this embodiment, two stacked pairs of oil passage forming plate  103  and cooling water passage forming plate  104  are provided. 
   As shown in  FIG. 12 , the first oil passage forming plate  103  is placed in contact with the front face of the rear-face cover plate  101  and a cooling water passage  106  is formed between the cooling water passage groove  111  formed on the rear-face cover plate  101  and the rear face of the first oil passage forming plate  103 . The first cooling water passage forming plate  104  is placed in contact with the front face of the first oil passage forming plate  103 , and an oil passage  105  is formed between the oil passage groove  131  formed on the first oil passage forming plate  103  and the rear face of the first cooling water passage forming plate  104 . The second oil passage forming plate  103  is placed in contact with the front face of the first cooling water passage forming plate  104 , and a cooling water passage  106  is formed between the cooling water passage groove  141  formed on the first cooling water passage forming plate  104  and the rear face of the second oil passage forming plate  103 . The second cooling water passage forming plate  104  is placed in contact with the front face of the second oil passage forming plate  103 , and an oil passage  105  is formed between the oil groove  131  formed on the second oil passage forming plate  103  and the rear face of the second cooling water passage forming plate  104 . Further, the front-face cover plate  102  is placed in contact with the front face of the second cooling water passage forming plate  104 , and the cooling water passage  106  is formed between the cooling water passage groove  141  formed in the second cooling water passage forming plate  104  and the rear face of the front-face cover plate  102 . 
   As shown in  FIG. 12 , bolt hole  150  is formed by connecting the plates  101 ,  102 ,  103 , and  104  to one another such that center axes of the holes  113 ,  122 ,  132 , and  142  of these plates conform to one another. These plates  101 ,  102 ,  103 , and  104  are fixed by means of a screw means  59 . The oil cooler  100  is directly mounted on the outer wall face of the crankcase  33  in such a manner that the oil cooler-side mounting face  114  is brought into contact with the engine-side mounting face  43  by inserting the tubular mounting bolt  66  into the bolt hole  150  and by screwing the male screw portion  66 A of the mounting bolt  66  to the female screw portion  67  on the engine E side. As a result, the penetrating hole of the mounting bolt  66  communicates with the oil gallery  54  formed in the wall portion of the engine E. Also, a screw means  160  is used to fix the oil cooler  100  on the engine-side mounting face  43 . As in the oil coolers  50  and  80  described in the first and second embodiments, the oil filter  51  is screwed to the male screw portion  66 B at the end portion of the front face of the mounting bolt  66 . 
   As shown in  FIG. 17 , the oil inflow holes  115 ,  133 , and  143  of the plates  101 ,  103 , and  104  (typically not including the front-face cover plate  102 ) communicate with one another to be formed into the oil inflow passage  151 , and the oil outflow holes  123 ,  134 , and  144  of the plates  102 ,  103 , and  104  other than the rear-face cover plate  101  communicate with one another to be formed into the oil outflow passage  152 . The oil inflow passage  151  communicates with the oil passages  105  formed by the first and second oil passage forming plates  103  and also communicates with a passage  53  (see  FIG. 12 ) formed in the wall portion of the engine E to lead to the oil tank  52  (see  FIG. 4 ). The oil outflow passage  152  communicates with the oil passages  105  and an inner space of the oil filter  51 . 
   In the rear-face cover plate  101 , the oil outflow hole  115  is sealed on its periphery to seal between the periphery and the cooling water passage  106 . In the cooling water passage forming plate  104 , the oil inflow hole  143  and the oil outflow hole  144  are sealed on their peripheries to seal between the peripheries and the cooling water passage  106 . 
   The oil flows from the oil tank  52  into the oil cooler  100  through the passage  53 . As shown in  FIG. 17 , the oil flows through the oil inflow passage  151  and is divided at a position to flow into the oil passages  105  formed by the first and second oil passage forming plates  103 . The oil flowing along each of the oil passages  105  is cooled by the cooling water flowing through the cooling water passage  106  as described later with reference to  FIG. 18 . Then, the oil within the oil passage  105  flows into the oil outflow passage  152  to be merged therein and flows into the inner space of the oil filter  51 . 
   As shown in  FIG. 18 , the cooling water inflow holes  124 ,  135 , and  145  of the plates  102 ,  103 , and  104  other than the rear-face cover plate  101  communicate with one another to be formed into the cooling water inflow passage  153  and the cooling water outflow holes  125 ,  136 , and  146  communicate with one another to be formed into a cooling water outflow passage  154 . The cooling water inflow passage  153  and the cooling water outflow passage  154  communicate with the cooling water passages  106  formed by the first and second cooling water passage forming plates  104 . In the oil passage forming plate  103 , the cooling water inflow hole  135  and the cooling water outflow hole  136  are sealed on their peripheries to seal between the peripheries and the oil passage  105 . 
   The cooling water flows into the oil cooler  100  through the cooling water inflow hole  124  of the front-face cover plate  102 . The cooling water flows through the cooling water inflow passage  153  and is divided at a position to flow into the cooling water passages  106  formed by the first and second cooling water passage forming plates  104  and the rear-face cover plate  101 . The cooling water flows along the cooling water passages  106  while cooling the oil flowing through the oil passage  105  as described above with reference to  FIG. 17 . Then, the cooling water within the cooling water passage  106  flows into the cooling water outflow passage  154  to be merged therein and flows outside through the cooling water outflow hole  125  of the front-face cover plate  102 . 
   The oil cooler  100  of this embodiment can be disassembled into the plates  101 ,  102 ,  103 , and  104  by removing the screw means  59 ,  160 , and the mounting bolt  66 . As a result, the oil passages  105  and the cooling water passages  106  are easily exposed, and hence, are easy to maintain. 
   Further, a heat exchange area of the oil cooler can be changed freely by adjusting the number of the oil passage forming plates  103  and the cooling water passage forming plates  104 . Therefore, the cooling capability can be set flexibly to be adapted to the engine E to be used. The oil cooler  100  has a two-layered structure comprising two stacked pairs of plates, each pair including an oil passage forming plate  103  and a cooling water passage forming plate  104 , but this structure is only illustrative. For example, three stacked pairs may be adopted as in an oil cooler  170  in  FIG. 19 . Such a structure increases the heat exchange area of the oil cooler, and hence improves the cooling capability, in contrast to the oil cooler  100  having the two-layered structure. 
   Embodiment 4 
   The oil coolers  50 ,  80 ,  100  and  170  described in the first to third embodiments may be each connected to another oil cooler through an adaptor. In this embodiment, as shown in  FIGS. 20 to 23 , assume that the oil cooler  80  of the second embodiment (hereinafter referred to as “first oil cooler  80 ”) is connected to another oil cooler through the adapter. In  FIGS. 20 to 23 , the same reference numerals as those in  FIGS. 1 to 19  denote the same or corresponding parts. The oil cooler of this embodiment is applicable to the personal watercraft described with reference to  FIGS. 1 and 2 . 
   As shown in  FIG. 20 , a second oil cooler  180  independent of the first oil cooler  80  is placed behind the engine E and is connected to the first oil cooler  80  disposed in the space  37  through an adapter  181  and tubes  182  and  183 . 
   As shown in  FIGS. 21A and 21B , the adapter  181  is cylindrical to have a length along center axis that is shorter than its width. The adapter  181  has a center hole  184  extending along the center axis, and first and second spaces  185  and  186 . The first space  185  communicates with an oil passage  87  of the first oil cooler  80  (see  FIG. 10 ) through a hole  185 A formed in one end face of the adapter  181 . The second space  186  communicates with an inner space of the oil filter  51  (see  FIG. 10 ) through a plurality of holes  186 A formed in an opposite end face of the adapter  181 . 
   Tubular joints  187  and  188  protrude at an outer peripheral portion of the adapter  181 . The joint  187  communicates with the first space  185  and is connected to tube  182 , which draws the oil from the adapter  181  to the second oil cooler  180 . The joint  188  communicates with the second space  186  and is connected to tube  183 , which draws the oil from the second oil cooler  180  to the adapter  181 . 
     FIG. 22  shows an exploded view of the first oil cooler  80  and the adapter  181 . The passage forming plate  85 , the front-face cover plate  86 , and the adapter  181  are arranged in successive order, and the rear-face of the passage forming plate  85  is opposed to the engine-side mounting face  91  of the crankcase  33 . A tubular mounting bolt  189  having a penetrating hole along a center axis thereof is inserted through the center hole  184  of the adapter  181 , the hole  64  of the front-face cover plate  86 , and the hole  63  of the passage forming plate  85  and a male screw portion  189 A formed on an end portion of the mounting bolt  189  is screwed to the female screw portion  67  formed on the engine-side mounting face  91 . The front-face cover plate  86  and the passage forming plate  85  are fixed to the engine-side mounting face  91  by means of the screw means  92 . 
   Further, the female screw portion  68  of the oil filter  51  is screwed to a male screw portion (portion configured to attach and detach the oil filter  51 )  189 B formed on an opposite end portion of the mounting bolt  189 . Thereby, the first oil cooler  80 , the adapter  181 , and the oil filter  51  are mounted on the outer wall face of the crankcase  33  at the engine-side mounting face  91 . In the oil cooler  80  described above, the mounting bolt  189  has a portion with which the adapter  181  is removably attached on the oil cooler  80 . The second oil cooler  180  (see  FIG. 20 ) is typically connected to the first oil cooler  80  through the adapter  181  and the tubes  182  and  183 . It will be appreciated that the second oil cooler  180  may have a sinuous passage, multiple plate structure similar to the first oil cooler  80 , or may be of a different structure suitable for cooling the oil passing therethrough. The mounting bolt  189  of this embodiment is slightly longer than the mounting bolt  66  described in the second embodiment, and the other structure may be substantially the same. The male screw portions  189 A and  189 B of the mounting bolt  189  have structures similar to those of the male screw portions  66 A and  66 B of the mounting bolt  66 . 
   How the oil follows within the first oil cooler  80  and the second oil cooler  180  will be described with reference to the drawings. As shown in  FIG. 23 , the oil flows into the first oil cooler  80  through the passage  53  formed in the wall portion of the crankcase  33  ( FIG. 10 ). While the oil is flowing through the oil passage  87  within the first oil cooler  80  (see arrow Y 20 ), the oil is cooled. The oil that has reached a terminal point of the oil passage  87  (see arrow Y 21 ) flows through a penetrating hole  86 A formed in the front-face cover plate  86  at a position corresponding to the terminal point (see arrow Y 22 ) and into the first space  185  through the hole  185 A of the adapter  181  (see arrow Y 23 ). Then, the oil is delivered to the second oil cooler  180  through the joint  187  and the tube  182  (see arrow Y 24 ). 
   The oil delivered to the second oil cooler  180  is cooled within the second oil cooler  180  and is returned through the tube  183  (see arrow Y 25 ). The oil flows into the second space  186  of the adapter  181  through the joint  188  (see arrow Y 26 ). The oil is delivered to the oil filter  51  through the hole  186 A of the adapter  181  (see arrow Y 27 ). The oil flowing within the oil filter  51  is filtered by the filter element (not shown) located inside. Thereafter, the oil flows through the inside of the mounting bolt  189  (see arrow Y 28 ) and is delivered to the oil gallery  54  formed in the wall portion of the crankcase  33  (see arrow Y 29 ). 
   As should be appreciated from the foregoing, since the first oil cooler  80  configured as described above is connected to the second oil cooler  180  through the adapter  181 , this configuration may provide for proper cooling even where Engine E is a large-sized engine mounted in the personal watercraft. 
   As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.