Outboard motor

An outboard motor includes an engine holder, an engine which is disposed above the engine holder in a state of the outboard motor mounted to a hull and in which a crankshaft extends substantially perpendicularly, and an oil pan disposed below the engine holder. A drive shaft housing is disposed below the oil pan and is formed with an exhaust chamber and a water pump is driven by the drive shaft so as to pump up a cooling water. A cooling water passage is formed in the oil pan and the drive shaft housing to guide the cooling water pumped up by the water pump to the engine. A relief valve is disposed on the way of the cooling water passage so as to discharge the cooling water into the exhaust chamber.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an outboard motor having an improved
 structure of a cooling water passage and an associated structure.
 2. Discussion of the Background
 Engines of outboard motors include an engine, generally of water-cooling
 type, having a cylinder block in which a plurality of cylinders are
 arranged in a vertical direction. In a cylinder head connected to the
 cylinder block, intake ports and exhaust ports are formed for the
 respective cylinders. Exhaust gas discharged from the exhaust ports is
 collected into a common exhaust passage and then discharged out from the
 outboard motor.
 Since the exhaust passage is heated to a very high temperature by the
 exhaust gas, a cooling water passage is formed around the exhaust passage
 to cool the exhaust passage by means of cooling water. Such a cooling
 water passage is provided for members of the engine, such as cylinder
 block, an engine holder and an oil pan, through which the exhaust passage
 passes.
 However, in a conventional structure, since fastening bolts for connecting
 the engine holder and the oil pan are disposed only around the members
 mentioned above, a pressure acting on a juncture (mating) surface between
 the engine holder and the oil pan near the central portion of both these
 members adversely becomes lower than that of a peripheral portion of the
 juncture surface. As a result, in a case where the cooling water passage
 is formed nearer to the central portions of the engine holder and the oil
 pan, if the cooling water pressure becomes high, the cooling water leaks
 from the central portion of the juncture surface of both the members.
 It is possible to arrange the cooling water passage closer to the
 peripheral portions of the engine holder and the oil pan than to the
 exhaust passage. However, in such a case, the engine holder can not be
 formed with a gas collecting port of the exhaust gas and the oil pan can
 also not be formed with a water checking port, for example.
 In the meantime, in the outboard motor equipped with the water-cooled
 engine, respective parts or elements of the engine are cooled by sea
 water, lake water, river water or like which is pumped up by a water pump
 driven by a drive shaft, and after cooling, such water is discharged
 outward.
 At a time when the temperature of the cooling water is low just after the
 starting of the engine operation, for example, the flow of the cooling
 water is interrupted by a thermostat valve provided for the engine to
 thereby assist or promote a warm-up of the engine.
 However, during an engine warm-up operation, in spite of the fact that the
 flow of the cooling water is interrupted by the thermostat valve, the
 water pump is driven, so that the water pressure in a upstream side of a
 cooling water passage of the thermostat valve is increased. In order to
 obviate such defect and protect a seal portion or like of the cooling
 water passage, in the prior art, there is provided a water pressure valve,
 for example, as a relief valve, to the cooling water passage at a portion
 upstream side of the thermostat valve to thereby discharge a portion of
 the cooling water of which pressure is excessively increased outside the
 cooling water passage.
 Japanese Patent Laid-open Publication No. HEI 8-100658 discloses one
 example of such water pressure valve, which is disposed inside the engine
 and Japanese Patent Laid-open Publication No. HEI 10-339163 discloses a
 structure in which a box-shaped passage casing is mounted to a bottom
 surface of an oil pan disposed below the engine to guide the cooling water
 into this casing and a water pressure valve is mounted to this case.
 However, these prior art publications provide the following problems or
 defects.
 That is, in the case where the water pressure valve is disposed in the
 inside portion of the engine, a layout of other equipments or members in
 the engine will be damaged and, moreover, it will be necessary to locate a
 specific discharge passage for discharging the cooling water from the
 water pressure valve, so that the number of the parts will be increased
 and the entire structure of the engine will be made complicated, thus
 providing a problem.
 Furthermore, in the case where the passage casing is mounted to the bottom
 surface of the oil pan, the number of parts or elements will be increased
 and, moreover, since the bottom surface of the oil pan is made flat,
 lubrication oil will not be sufficiently discharged at a time of
 exchanging the oil.
 Additionally, in the outboard motor shown in the Japanese Patent Laid-open
 Publication No. HEI 10-339163, an inside portion of a drive shaft housing
 disposed below the oil pan is divided by a partition wall into an exhaust
 chamber and a space in which cooling equipments or like are arranged, and
 the cooling water from the water pressure valve is discharged into this
 space. However, the wall portion of the drive shaft housing on the side of
 the exhaust chamber is heated by the exhaust gas and its temperature is
 increased, so that there is caused a problem of adhesion of calcium, for
 example, contained in the sea water to the drive shaft housing wall, thus
 being inconvenient.
 SUMMARY OF THE INVENTION
 The present invention has been accomplished in view of the above
 circumstances, and it is a primary object of the present invention to
 provide an outboard motor capable of arranging a cooling water passage
 nearer to a central portion of an engine unit than to an exhaust passage
 by increasing a pressure acting on a juncture surface of the engine unit
 near the central portion thereof.
 Another object of the present invention is to provide an outboard motor
 having a cooling water passage of an improved structure making the entire
 structure of the outboard motor compact.
 A further object of the present invention is to provide an outboard motor
 having an improved cooling water passage structure capable of reducing an
 exhaust gas temperature and a temperature in an exhaust chamber and
 capable of improving a lubrication oil discharging performance.
 A still further object of the present invention is to provide an outboard
 motor having an improved structure for mounting a water pressure valve to
 the cooling water passage.
 These and other objects can be achieved according to the present invention
 by providing, in one aspect, an outboard motor comprising:
 an engine holder;
 an engine which is disposed above the engine holder in a state of the
 outboard motor to be mounted to a hull and in which a crankshaft extends
 substantially perpendicularly;
 an oil pan disposed below the engine holder;
 a drive shaft housing disposed below the oil pan;
 an exhaust passage formed in the engine, the engine holder and the oil pan
 for discharging an exhaust gas from the engine in the drive shaft housing;
 and
 a cooling water passage and a return-water passage disposed in a vicinity
 of the exhaust passage; and
 fixing means disposed in the engine holder for fixing the oil pan to the
 engine holder.
 In a preferred embodiment of this aspect, the engine holder and the oil pan
 are provided with vertical wall sections forming the exhaust passage, the
 cooling water passage and the return-water passage and forming with a
 juncture portion between the engine holder and the oil pan and the fixing
 means is provided for the vertical wall sections. The engine holder and
 the oil pan are provided with outer wall sections forming the exhaust
 passage, the cooling water passage and the return-water passage and
 forming the juncture portion between the engine holder and the oil pan,
 the outer wall sections are connected to the engine by fixing means at a
 plurality of locations.
 The engine holder and the oil pan are provided with vertical wall sections
 forming the exhaust passage, the cooling water passage and the
 return-water passage and forming a juncture portion between the engine
 holder and the oil pan, the vertical wall sections being connected by
 means of rib, and the fixing means is provided at an intersection between
 the vertical wall sections and the rib. The holder and the oil pan are
 provided with outer wall sections forming the exhaust passage, the cooling
 water passage and the return-water passage and forming the juncture
 portion between the engine holder and the oil pan, the outer wall sections
 are connected to the engine by fixing means at a plurality of locations.
 As explained above, according to this first aspect of the outboard motor of
 the present invention of the structures mentioned above, the engine is
 disposed above the engine holder, and the oil pan is disposed below the
 engine holder, the drive shaft housing is disposed below the oil pan, the
 exhaust gas from the engine is discharge into the drive shaft housing
 through the exhaust passage formed in the engine, the engine holder and
 the oil pan, the cooling water passage and the return-water passage are
 provided around the exhaust passage, and the connecting means for
 connecting the engine holder and the oil pan is provided in the engine
 holder. Therefore, the pressure on the juncture surface between the engine
 holder and the oil pan is increased and the sealing performance of the
 cooling water passage can be enhanced. Further, the vertical walls forming
 the exhaust passage, the cooling water passage and the return-water
 passage and forming a juncture between the engine holder and the oil pan
 are formed in the engine holder and the oil pan, and the vertical walls
 are provided with the connecting means. Therefore, the pressure on the
 center portion of the juncture surface between the engine holder and the
 oil pan is increased and the sealing performance can be enhanced.
 Furthermore, the vertical walls forming the exhaust passage, the cooling
 water passage and the return-water passage and forming a juncture between
 the engine holder and the oil pan are formed in the engine holder and the
 oil pan, and the connecting means is provided at an intersection between
 the vertical walls and the connecting rib which connects the vertical
 walls to each other. Therefore, the pressure on the center portion of the
 juncture surface between the engine holder and the oil pan is further
 increased and the sealing performance is enhanced.
 Still furthermore, the outer walls of the engine holder and the oil pan
 which form the exhaust passage, the cooling water passage and the
 returning-water passage, and form the juncture between the engine holder
 and the oil pan are connected to the engine by connecting means at a
 plurality of locations. Therefore, the pressure on the juncture surface
 between the engine holder and the oil pan is increased and the sealing
 performance can be improved.
 According to another aspect of the present invention, there is provided an
 outboard motor comprising:
 an engine holder;
 an engine which is disposed above the engine holder in a state wherein the
 outboard motor is mounted to a hull and in which the crankshaft extends
 substantially perpendicularly;
 an oil pan disposed below the engine holder;
 a drive shaft housing which is disposed below the oil pan and in which an
 exhaust chamber is formed and a drive shaft extends vertically;
 a water pump driven by the drive shaft so as to pump up cooling water;
 a cooling water passage formed in the oil pan and the drive shaft housing
 to guide the cooling water pumped up by the water pump to the engine; and
 a relief valve disposed in the the cooling water passage so as to discharge
 the cooling water into the exhaust chamber.
 In a preferred embodiment of this aspect, the oil pan is provided, at a
 bottom surface thereof, with a horizontal cooling water passage which
 extends in a horizontal direction and to which the cooling water is guided
 from the water pump, provided with a vertical cooling water passage
 extending towards the engine from an intermediate portion of the
 horizontal cooling water passage and provided with a relief port disposed
 at a portion upstream side of a connecting portion of the horizontal and
 vertical cooling water passages so as to be opened towards the exhaust
 chamber, the relief valve being mounted to the relief port. The bottom
 surface of the oil pan is vertically staged to provide a low level portion
 and a high level portion and an oil drain port is formed to a wall section
 of the oil pan at the low level portion side. The oil pan is formed, at
 the bottom surface thereof, with a boss for mounting the relief valve.
 According to this second aspect of the present invention of the structure
 mentioned above, the drive shaft housing is disposed below the oil pan and
 the exhaust chamber is formed in the drive shaft housing in which the
 drive shaft extends vertically, the water pump is driven so as to pump up
 the cooling water, the cooling water passage is formed in the oil pan and
 the drive shaft housing to guide the cooling water pumped up by the water
 pump to the engine, and the relief valve is disposed on the way of the
 cooling water passage so as to discharge the cooling water into the
 exhaust chamber. Accordingly, the exhaust gas is effectively cooled and,
 hence, effect of increasing of the temperature of the drive shaft housing
 wall can be effectively prevented.
 Furthermore, the oil pan is provided with the horizontal cooling passage
 and the vertical cooling water passage, and the relief port is disposed at
 a portion upstream side of a connecting portion of the horizontal and
 vertical cooling water passages so as to be opened towards the exhaust
 chamber. The relief valve is mounted to this relief port. According to
 such arrangement, the cooling water passage and the relief valve mounting
 structure can be made compact.
 The formation of the oil drain port to the wall section of the oil pan at
 the lower level bottom portion makes smooth the lubrication oil
 discharging. Since the oil pan is formed, at the bottom surface thereof,
 with a boss for mounting the relief valve, the structure can be further
 made compact and the assembling performance can be improved.
 The nature and further characteristic features of the present invention
 will be made more clear from the following descriptions made with
 reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Preferred embodiments of the present invention will be described hereunder
 with reference to the accompanying drawings.
 The first embodiment of the present invention will be explained with
 reference to FIGS. 1 to 6.
 As shown in FIG. 1, the outboard motor 1 is in a state mounted to a hull H
 and includes an engine holder 2 and an engine 3 disposed above the engine
 holder 2. The engine 3 is a vertical type engine in which a crankshaft 4
 is disposed substantially vertically.
 An oil pan 5 is disposed below the engine holder 2, a bracket 6 is mounted
 to the engine holder 2 for example, and the outboard motor 1 is mounted to
 a transom of a hull H through the bracket 6. Peripheries of the engine 3,
 the engine holder 2 and the oil pan 5 of the outboard motor 1 are covered
 with an engine cover 7. The engine cover 7 comprises a lower cover section
 7a covering the lower portion of the engine 3, the engine holder 2 and the
 oil pan 5 and an upper cover section 7b covering the upper portion of the
 engine 3.
 A drive shaft housing 8 is disposed at a lower portion of the oil pan 5. A
 drive shaft 9 is disposed substantially vertically in the engine holder 2,
 the oil pan 5 and the drive shaft housing 8, and an upper end of the drive
 shaft 9 is connected to a lower end of the crankshaft 4. The drive shaft 9
 extends downward in the drive shaft housing 8 for driving a propeller 13
 through a bevel gear 11 and a propeller shaft 12 housed in a gear case 10
 provided in a lower portion of the drive shaft housing 8.
 The engine 3 mounted to the outboard motor 1 is a water-cooled
 four-stroke-cycle three-cylinder engine, which comprises, in combination,
 a cylinder head 14, a cylinder block 15, a crankcase 16 and the like. This
 engine 3 is a water-cooled engine which takes seawater, lake water or
 river water into the engine 3 from an intake port 17 provided in the gear
 case 10 to use the water as cooling water.
 The cylinder block 15 is disposed rearward (right side) of the crankcase 16
 disposed at the leftmost position in FIG. 1. The cylinder head 14 is
 disposed rearward the cylinder block 15.
 With reference to FIGS. 2 to 6, the drive shaft housing 8 is provided at
 its bottom portion, with a water pump 18 connected to the intake port 17.
 The water pump 18 is driven by the drive shaft 9, and a water-feed pipe 19
 extending upward from the water pump 18 is disposed.
 The oil pan 5 is formed, at its bottom portion, with a horizontally
 extending lateral cooling water passage (horizontal cooling water passage)
 20. The horizontal cooling water passage 20 is provided at its upstream
 end with a cooling water inlet 20a, and a downstream end of the water-feed
 pipe 19 is connected to the cooling water inlet 20a. A vertical cooling
 water passage 21 surrounded by an outer wall 5a and a vertical wall 5b of
 the oil pan 5 extends upward from an intermediate portion of this cooling
 water passage 20. The outer wall 5a of the oil pan 5 is formed with a
 cooling water take-out port 22 for checking water.
 The engine holder 2 is formed with a cooling water passage 23 passing
 through the engine 3 in the vertical direction and surrounded by an outer
 wall 2a and a vertical wall 2b of the engine holder 2, and the cooling
 water passage 23 is connected to the downstream end of the vertical
 cooling water passage 21 of the oil pan 5. A downstream end of the cooling
 water passage 23 is connected to a cooling water jacket, not shown, formed
 in the engine 3.
 Cooling water which has cooled various portions of the engine 3 is
 introduced, as return water, into return-water passages 24 and 25 formed
 in the engine holder 2 and the oil pan 5 so as to be surrounded by the
 outer walls 2a and 5a and the vertical walls 2b and 5b of the engine
 holder 2 and the oil pan 5. The oil pan 5 is formed, at its bottom
 surface, with a return-water outlet 26 through which the return-water is
 discharged into the space formed in the drive shaft housing 8.
 On the other hand, the exhaust gas from each cylinder of the engine 3 is
 discharged into the space formed in the drive shaft housing 8 through
 exhaust passages 27 and 28 formed in the engine holder 2 and the oil pan
 5. The exhaust passages 27 and 28 pass through the engine holder 2 and the
 oil pan 5 in the vertical direction and are formed so as to be surrounded
 by the outer walls 2a and 5a, and the vertical walls 2b and 5b forming the
 cooling water passages 21 and 23 and the return-water passages 24 and 25.
 The exhaust passages 27 and 28 are disposed so as to be deviated towards
 one side from the center line connecting front and rear portions of the
 outboard motor 1, i.e., left side in the present embodiment. The outer
 wall 2a of the engine holder 2 is formed with an exhaust gas collecting
 port 29 connected to the exhaust passage 27.
 The vertical cooling water passage 21 of the oil pan 5, the cooling water
 passage 23 of the engine holder 2, and the return-water passages 24 and 25
 formed in the engine holder 2 and the oil pan 5 are disposed around the
 exhaust passages 27 and 28. The exhaust gas is cooled by the cooling water
 and the return-water.
 As shown in FIG. 6, for example, the cooling water passage 23 formed in the
 engine holder 2 is disposed so that the passage 23 is formed into a
 substantially L-shape extending from a front side of the exhaust passage
 27 to a portion close to the central portion of the engine holder 2 on the
 opposite side from the exhaust gas collecting port 29. The return-water
 passage 24 is formed in the engine holder 2 on the rear side of the
 exhaust passage 27.
 The outer wall 2a around the engine holder 2 and a lower surface of the
 vertical wall 2b forming the cooling water passage 23, the return-water
 passage 24 and the exhaust passage 27 formed in the outer wall 2a; as well
 as the outer wall 5a of the periphery of the oil pan 5 and an upper
 surface of the vertical wall 5b forming the vertical cooling water passage
 21 and the exhaust passage 28 formed in the outer wall 5a; are aligned
 with each other to form a juncture (mating) surface 30 tightly contacting
 to each other (see FIGS. 3 and 6).
 The engine holder 2 and the oil pan 5 are fastened together to the engine
 3, comprising the cylinder head 14, the cylinder block 15 and the
 crankcase 16, by means of a plurality of through bolts 31 as connecting
 means after the engine holder 2 and the oil pan 5 have been previously
 connected to the juncture surface 30 by plurality of other bolts 32.
 Gaskets are interposed between the engine 3 and the engine holder 2 at its
 juncture (mating) surface and between the engine holder 2 and the oil pan
 5 at the juncture surface 30 thereof.
 As shown in FIG. 4, for example, four through bolts 31 for fastening the
 engine holder 2 and the oil pan 5 to the engine 3 are disposed on each of
 the opposite sides of the outer wall 5a of the periphery of the oil pan 5,
 i.e., in total, eight through bolts 31 are disposed, and are inserted
 toward the engine holder 2 from the lower portion of the oil pan 5. The
 bolts 32 previously connecting the engine holder 2 and the oil pan 5 are
 disposed so that one of the bolts 32 is disposed at the front side of the
 periphery of the oil pan 5 for example, and two bolts 32 are disposed at
 the rear side thereof, and three, in total, bolts are disposed. The bolts
 32 are also inserted towards the engine holder 2 from the lower portion of
 the oil pan 5.
 A bolt hole 34 is formed at an intersecting portion between the vertical
 walls 2b and 5b forming the cooling water passages 21 and 23, the
 return-water passages 24 and 25 and the exhaust passages 27 and 28 in the
 engine holder 2 and the oil pan 5, and the rib 33 for connecting the
 vertical walls 2b and 5b. A bolt 35 as connecting means is inserted into
 the bolt hole 34 from the upper surface side of the engine holder 2
 towards the oil pan 5 disposed below and is fastened thereto (see FIGS. 2
 and 5). With this structure, substantially central portions of the engine
 holder 2 and the oil pan 5 as viewed from top are connected.
 The first embodiment of the present invention mentioned above will operate
 in the following manner.
 When the engine 3 is driven and the drive shaft 9 is rotated, the cooling
 water is pumped up by the water pump 18, and the cooling water is
 introduced from the cooling water inlet 20a through the water-feed pipe 19
 into the horizontal cooling water passage 20 and the vertical cooling
 water passage 21 formed in the oil pan 5. The cooling water is then
 introduced into the various portions of the engine 3 through the cooling
 water passage 23 formed in the engine holder 2. The cooling water which
 has cooled the various portion of the engine 3 is discharged to the space
 formed in the drive shaft housing 8 through the return-water passages 24
 and 25.
 On the other hand, the exhaust gas is discharged from each cylinder in
 accordance with the engine operation into the drive shaft housing 8
 through the exhaust passage 27 in the engine holder 2 and the exhaust
 passage 28 in the oil pan 5.
 In the present invention, the cooling water passages 21 and 23, and the
 return-water passages 24 and 25 formed in the engine holder 2 and the oil
 pan 5 are disposed around the exhaust passages 27 and 28, and the exhaust
 gas is cooled by the cooling water and the return-water. Therefore, a
 temperature in the drive shaft housing 8 is restrained from increasing,
 and it is possible to prevent calcium from adhering to the outer wall of
 the drive shaft housing 8.
 Among the outer walls 2a and 5a, and the vertical walls 2b and 5b, the
 outer walls 2a and 5a can provide sufficient sealing performance because
 the through bolts 31 disposed at the opposite sides of the outer wall 5a
 of the periphery of the oil pan 5 fasten the engine holder 2 and the oil
 pan 5 to the engine 3 so as to apply a pressure to the juncture surface 30
 of the peripheral portion of the engine holder 2 and the oil pan 5.
 On the other hand, the vertical walls 2b and 5b can provide sufficient
 sealing performance because the bolt 35 provided at the intersection of
 the vertical walls 2b, 5b and the rib 33 connecting the vertical walls 2b,
 5b connects the engine holder 2 and the oil pan 5 to each other, thereby
 applying a pressure to the juncture surface 30 of the central portions of
 the engine holder 2 and the oil pan 5.
 As a result, even if the cooling water pressure is increased, no cooling
 water leaks from the juncture surface 30 at the central portions of the
 engine holder 2 and the oil pan 5, so that the cooling efficiency of the
 engine 3 and the exhaust gas can be enhanced.
 Further, the cooling water passages 21 and 23 and the return-water passages
 24 and 25 can be disposed nearer to the central portions of the engine
 holder 2 and the oil pan 5 than to the exhaust passages 27 and 28, so that
 the engine holder 2 can be formed with the collecting port 29 of the
 exhaust gas and the oil pan 5 can be formed with cooling water outlet 22
 for checking the cooling water.
 A second embodiment of the outboard motor of the present invention will be
 described hereunder with reference to FIG. 1 and FIGS. 7-11, in which FIG.
 1 is used for a common view to the first and second embodiments and the
 reference numerals 1 to 19 in FIG. 1 are commonly used hereunder for the
 second embodiment as like in the first embodiment.
 FIGS. 7 to 9 are views similar to FIGS. 2 to 4, in which FIGS. 2 and 7 are
 views showing a central portion of the outboard motor in an enlarged scale
 and FIGS. 3-4 and 8-9 are views representing the oil pan.
 With reference to FIG. 7, the engine 3 of the outboard motor 1 is a
 water-cooled engine provided with the gear case 10 to which is formed the
 intake port 17 through which the cooling water such as sea water, lake
 water, river water or like is introduced into the engine to cool the same
 by the water pump 18 disposed at the bottom portion of the drive shaft
 housing 8 and driven by the drive shaft 9. The thus taken cooling water
 then rises upward through the water-feed pipe 19, as the cooling water
 passage, in the drive shaft housing 8 and then guided towards the engine 3
 through a cooling water passages 120, 121 and 122 formed in the engine
 holder 2 and the oil pan 5.
 With reference to FIGS. 1 and 8-9, as mentioned with reference to the first
 embodiment, the horizontal cooling water passage 120 is formed integrally
 to the bottom surface of the oil pan 5 so as to extend in the horizontal
 direction. The horizontal cooling water passage 120 extends in forward and
 backward direction at substantially the central portion of the bottom
 surface of the oil pan 5 and is provided, at its front end, with the
 cooling water intake port 120a opened towards the junction (mating)
 surface between the oil pan 5 and the drive shaft housing 8. The upper end
 portion of the feed-water pipe 19 is connected to the cooling water intake
 port 120a. Further, the rear end portion of the passage 120 is opened
 rearward of the oil pan 5 to form a flush port 120b which is closed by a
 plug 120c.
 The vertical cooling water passage 121 vertically extending from
 substantially the central portion of the horizontal cooling water passage
 120 towards the mating portion (junction surface) of the oil pan 5 and the
 engine holder 2 is formed integrally with the oil pan 5, and the upper end
 portion of the vertical cooling water passage 121 is connected to a
 cooling water passage 122 formed to the engine holder 2 as shown in FIG.
 1. A thermostat valve may be disposed on the downstream side of the
 cooling water passage 122 formed to the engine holder 2.
 A relief port 123 opened towards the space defined in the drive shaft
 housing 8 is formed, integrally with the oil pan 5, to the lower portion
 of the horizontal cooling water passage 120 on the side of the cooling
 water intake port 120a, i.e. on the upstream side of the connecting
 portion between the horizontal and vertical cooling water passages 120 and
 121, and for example, a water pressure valve 124 is mounted to this relief
 port 123 as a relief valve.
 With reference to FIG. 11, which is an enlarged sectional view of a portion
 near the relief port 123, the water pressure valve 124 is fixed, through a
 metallic plate 125, to a water pressure valve mounting boss 124a formed to
 the bottom surface of the oil pan 5, for example, integrally therewith.
 Further, it is designed that, as shown by arrows in FIG. 11, the flowing
 direction of the cooling water discharged through the water pressure valve
 124 is normal (perpendicular in FIG. 11) to the flow direction of the
 cooling water flowing through the horizontal cooling water passage 120.
 Incidentally, though not shown in detail, an exhaust passage 126 is formed
 in the engine holder 2 in the vicinity of the vertical cooling water
 passage 121 and the cooling water passage 122. The exhaust passage 126 is
 communicated, at its upper end, with an exhaust passage formed to the
 cylinder block or cylinder head and, at its lower end, is opened towards
 the space defined in the drive shaft housing 8, this space being therefore
 utilized as an exhaust chamber 127.
 Furthermore, with reference to FIGS. 8 to 10, the oil pan 5 is formed with
 an oil drain port 129 opened sideways at a lower front portion of the oil
 pan 5, and the oil drain port 129 is closed by a drain cap 129a. As shown
 in FIG. 10, the bottom portion of the oil pan 5 is formed with a vertical
 step D having an upper level portion and a lower level portion with the
 horizontal cooling water passage 120 being interposed. The lower level
 portion is positioned at substantially the same level of the lowest
 portion of the drain port 129 and the upper level portion is positioned at
 substantially the same level as the horizontal cooling water passage 120.
 The second embodiment of the structure mentioned above will attain the
 following operation and function.
 As mentioned with reference to the first embodiment, when the engine 3 is
 driven and the drive shaft 9 is rotated, the cooling water is pumped up by
 the water pump 18, and the cooling water is introduced from the cooling
 water inlet 120a through the water-feed pipe 19 into the horizontal
 cooling water passage 120 and the vertical cooling water passage 121
 formed in the oil pan 5. The cooling water is introduced into the various
 portions of the engine 3 through the cooling water passage 122 formed in
 the engine holder 2. The cooling water which has cooled the various
 portion of the engine 3 is discharged to the exhaust chamber 127 formed in
 the drive shaft housing 8 through the cooling water discharge passage 128.
 At the time when the cooling water temperature is low, for example, just
 after the engine operation starting, the thermostat valve disposed
 downstream side of the cooling water passage 122 formed to the engine
 holder 2 is operated so as to shut off the cooling water flow to thereby
 assist or promote the engine warm-up operation.
 Then, during the engine warm-up operation, the water pump 18 is driven in
 spite of the shut-off of the cooling water flow, and the pressure of the
 cooling water on the upstream side of the thermostat valve is increased.
 However, since the water pressure valve 124 as the relief valve is
 provided to the horizontal cooling water passage 120 at a portion upstream
 side of the thermostat valve, a portion of the cooling water of which
 pressure is excessively increased is discharged. At this time, the water
 pressure valve 124 is directed downward towards the lower portion of the
 horizontal cooling water passage 120, so that the cooling water is
 discharged into the exhaust chamber 127 formed in the drive shaft housing
 8.
 Furthermore, according to this embodiment, since the discharging direction
 of the water pressure valve 124 is set to the direction normal (i.e.
 perpendicular) to the cooling water flow direction in the horizontal
 cooling water passage 120, the cooling water collides against the water
 pressure valve 124 at its lower portion, so that the cooling water is
 splashed in all directions.
 The cooling water splashed into the exhaust chamber 127 is mixed with the
 exhaust gas from the exhaust passage 126 and cool the gas to thereby
 prevent the temperature of the wall section of the drive shaft housing 8
 from raising. Further, the cooling water splashed in the exhaust chamber
 127 directly adheres to the inner wall section of the drive shaft housing
 8, so that the temperature of the wall section of the drive shaft housing
 8 is further lowered. The problem of the adhesion of calcium component in
 the sea water, pumped up as cooling water, to the drive shaft housing wall
 will be solved by lowering the temperature of the wall section thereof.
 Furthermore, in the second embodiment, the horizontal cooling water passage
 120 and the vertical cooling water passage 121 are formed integrally with
 the oil pan 5 and the water pressure valve 124 is fixed, through a
 metallic plate 125, to the water pressure valve mounting boss 124a formed
 to the bottom surface of the oil pan 5. Accordingly, any sealing means is
 not required to be located for the cooling water passage which requires
 water-tight performance and, hence, the number of element or part can be
 reduced and the entire structure can be made compact and easily assembled,
 thus being advantageous.
 Still furthermore, the vertical step is formed to the bottom portion of the
 oil pan 5 and the oil drain port 129 is formed to the side wall of the
 lower bottom portion of the oil pan 5, so that the oil can be surely
 discharged at the lubrication oil exchanging time. Moreover, the oil drain
 port 129 will be effectively inspected and maintained by locating it at
 the front side of the oil pan 5.
 Further, in the described embodiment, the horizontal cooling water passage
 120 is formed to the oil pan 5 and the water pressure valve 124 is mounted
 to this horizontal cooling water passage 120. In an alternation, however,
 the horizontal cooling water passage may be formed to the drive shaft
 housing 8 and the water pressure valve 124 may be mounted to this drive
 shaft housing 8. Furthermore, the water pressure valve 124 may be disposed
 on the way of the water-feed pipe 19 disposed in the exhaust chamber 127.
 It is to be noted that the present invention is not limited to the
 described embodiments and many other changes and modifications may be made
 without departing from the scopes of the appended claims.