Cooling apparatus for outboard motor

A cooling apparatus for an outboard motor which is provided with a water-cooled engine in a vertical alignment in which a crank shaft is vertically disposed, the engine being composed of a cylinder block, a cylinder head and an exhaust manifold into which water jackets are formed respectively and the water jackets are supplied with cooling water from a water pump disposed below the engine in a state mounted to a hull, the cooling apparatus comprising a cylinder cooling-water passage for supplying cooling water from the water pump to the water jackets of the cylinder block and said cylinder head, an exhaust cooling-water passage for supplying cooling water from the water pump to the water jacket of the exhaust manifold, the cylinder cooling-water passage and the exhaust cooling-water passage being independently disposed from each other and being joined together at downstream portions thereof, a thermostat provided for the water jacket of the cylinder block and a sensor for detecting a temperature of a cylinder surface provided for the water jacket of the cylinder block at a portion between the water jacket thereof and the thermostat.

BACKGROUND OF THE INVENTION 
The present invention relates to a cooling apparatus for a water-cooled 
type outboard motor. 
An engine unit of an outboard motor is mounted vertically in such a manner 
that a crank shaft of the engine unit is disposed vertically in a mounted 
state. The engine unit is formed by assembling a crank case, a cylinder 
block and a cylinder head. Moreover, an exhaust manifold for gathering 
flows of exhaust gases discharged from respective cylinders to cause the 
exhaust gas to flow downwards is provided on one of the side surfaces of 
the cylinder block. 
The engine unit of the structure mentioned above is provided with a cooling 
apparatus structured to supply sea water, river water or lake water pumped 
up by a water pump to a water jacket provided for the inside portions of 
the cylinder block, the cylinder head and the exhaust manifold, 
respectively. 
The conventional cooling apparatus for an outboard motor has a structure 
that water jackets respectively provided for the cylinder block, the 
cylinder head and the exhaust manifold are allowed to communicate with one 
another to cause cooling water to flow in these elements. The water jacket 
of the cylinder block or the cylinder head is provided with a thermostat 
to adjust the flow of cooling water to be an adequate quantity to be 
adaptable to the water temperature to prevent the cylinder from being 
excessively cooled. 
The thermostat is disposed above the uppermost portion of the water jacket. 
The reason for this arrangement is to obviate a problem which may arise in 
that air is accumulated in a portion upstream from the thermostat after 
the thermostat has been opened if the thermostat is disposed lower than 
the uppermost portion of the water jacket to thereby prevent sufficient 
circulation of the cooling water around the uppermost cylinder. In this 
case, the water-cooled engine cannot satisfactorily be cooled down. 
Moreover, the cylinder block is provided with a 
cylinder-surface-temperature sensor for detecting the temperature of the 
wall of the cylinder and a pressure valve for controlling an oil pressure 
of the cooling water. In a conventional arrangement, a sensor for 
detecting a temperature of the cylinder surface and the pressure valve are 
disposed apart from the thermostat. 
However, the conventional cooling apparatus for an outboard motor, having 
the structure that the cooling water is communicated among the water 
jackets for the cylinder head, the cylinder block and the exhaust 
manifold, suffers from deterioration in the efficiency for cooling the 
portion around the cylinder because of introduction of hot cooling water 
into the water jackets for the cylinder block and the cylinder head. 
Moreover, the temperature of the cooling water cannot satisfactorily be 
controlled by the thermostat. 
Since the thermostat of the conventional cooling apparatus for an outboard 
motor is disposed above the uppermost portion of the water jacket, the 
thermostat excessively projects over the surface of the engine unit, which 
results in an enlargement of the size of the engine unit. 
Since the cylinder-surface-temperature sensor is disposed apart from the 
thermostat, the temperatures detected by the cylinder-surface-temperature 
sensor are not stable. In particular, the temperature of the cooling water 
cannot easily be controlled during a warming-up operation, thus causing a 
problem to arise in that an excessively long time is required to warm up 
the engine unit. In case a breakdown of the thermostat may occur, the 
cylinder-surface-temperature sensor cannot detect the breakdown. 
The structure, in which the pressure valve is disposed apart from the 
thermostat, cause pipes extending from the thermostat and the pressure 
valve to be disposed apart from one another. As a result, the structure of 
a water drainage passage becomes too complicated. Thus, the engine unit 
cannot easily be assembled and the maintenance of the same cannot smoothly 
be performed. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to substantially eliminate 
defects or drawbacks encountered in the prior art described above and to 
provide a cooling apparatus for an outboard motor capable of stabilizing 
the temperature of cooling water, which flows in water jackets of a 
cylinder block and a cylinder head to appropriate levels so as to improve 
the performance for cooling the cylinder portion and to enable a 
thermostat to easily control the temperature. 
Another object of the present invention is to provide a cooling apparatus 
for an outboard motor capable of preventing undesirable enlargement of the 
size of the engine which occurs attributable to provision of a thermostat 
and stabilizing the temperatures which are detected by the thermostat so 
as to easily and accurately control the temperature of the cooling water, 
to easily and accurately assemble the cooling apparatus and to smoothly 
perform the maintenance of the cooling apparatus. 
A further object of the present invention is to provide a cooling apparatus 
for an outboard motor capable of cooling exhaust passages following an 
exhaust manifold without excessively cooling (supercooling) the cylinder. 
A still further object of the present invention is to provide a cooling 
apparatus for an outboard motor capable of simplifying the structure of 
passages for the cooling water in the cylinder block, the cylinder head 
and the exhaust manifold to easily manufacture the cooling apparatus. 
These and other objects can be achieved according to the present invention 
by providing a cooling apparatus for an outboard motor which is provided 
with a water-cooled engine in a vertical alignment in which a crank shaft 
is vertically disposed, the engine being composed of a cylinder block, a 
cylinder head and an exhaust manifold into which water jackets are formed 
respectively and the water jackets are supplied with cooling water from a 
water pump disposed below the engine, wherein a cylinder cooling-water 
passage for supplying cooling water from the water pump to the water 
jackets of the cylinder block and the cylinder head and an exhaust 
cooling-water passage for supplying cooling water from the water pump to 
the water jacket of the exhaust manifold are independently disposed from 
each other, the cylinder cooling-water passage and the exhaust 
cooling-water passage are joined to each other at downstream portions 
thereof. 
In preferred embodiments, the cylinder cooling-water passage is provided 
with a thermostat at a position higher in level than uppermost portions of 
the water jackets of the cylinder block and the cylinder head and lower in 
level than a top end of a timing belt disposed on an upper surface of the 
engine, in the mounted state of an outboard motor to a hull, and a sensor 
for detecting a cylinder surface temperature and a pressure valve are 
disposed adjacent to the thermostat. 
The cylinder cooling-water passage and the exhaust cooling-water passage 
are joined so that the cooling-water flow after the joined portion thereof 
is introduced into a water drainage passage formed adjacent to an exhaust 
passage disposed vertically so as to penetrate an inside portion of an oil 
pan which is disposed below the engine in the mounted state. The cylinder 
cooling-water passage and the exhaust cooling-water passage are provided 
with passages downstream side of the water jackets, the downstream side 
passages being formed of drainage hoses, respectively, which are 
collectively disposed on one side surface of the engine. 
In a more specified aspect, there is provided a cooling apparatus for an 
outboard motor which is provided with a water-cooled engine in a vertical 
alignment in which a crank shaft is vertically disposed, the engine being 
composed of a cylinder block, a cylinder head and an exhaust manifold into 
which water jackets are formed respectively and the water jackets are 
supplied with cooling water from a water pump disposed below the engine in 
a state mounted to a hull, the cooling apparatus comprising: 
a cylinder cooling-water passage for supplying cooling water from the water 
pump to the water jackets of said cylinder block and said cylinder head; 
an exhaust cooling-water passage for supplying cooling water from the water 
pump to the water jacket of the exhaust manifold, the cylinder 
cooling-water passage and the exhaust cooling-water passage being 
independently disposed from each other and being joined to each other at 
downstream portions thereof; 
a thermostat for controlling a temperature of the cooling-water passing 
through the cylinder cooling-water passage provided for the water jacket 
of the cylinder block; and 
a sensor for detecting a temperature of a cylinder surface provided for the 
water jacket of the cylinder block at a portion between the water jacket 
thereof and the thermostat. 
According to the structure and characters of the present invention 
mentioned above, hot cooling water, which has cooled the exhaust manifold 
down, can be prevented from flowing into the water jackets of the cylinder 
block and the cylinder head. Therefore, the temperature of cooling water, 
which flows around the cylinder, can be stabilized to an appropriate 
level. Thus, the performance for cooling the cylinder portion can be 
improved and the temperature can be easily controlled by the thermostat. 
Furthermore, according to the specific location of the thermostat, an 
excessive projection thereof over the upper portion of the engine can be 
prevented so that an undesirable enlargement of the size of the engine is 
prevented. Since the cylinder-surface-temperature sensor is disposed 
adjacent to the thermostat, the temperatures, which are detected by the 
sensor, can be stabilized and thus the performance for controlling the 
temperature of cooling water can be improved. Even if the thermostat 
produces trouble, the trouble can immediately be detected by the 
cylinder-surface-temperature sensor. Moreover, since pressure valve is 
disposed adjacent to the thermostat, pipes extending from the thermostat 
and the pressure valve are disposed adjacently. As a result, the cooling 
apparatus can easily and accurately be assembled and the maintenance of 
the same can smoothly be performed. 
Still furthermore, according to the present invention, the exhaust passages 
are cooled down by cooling water which has cooled each water jacket. 
Therefore, the necessity of enlarging the quantity of cooling water to 
cool the exhaust passages can be eliminated, and hence, the water jackets 
of the cylinder block and the cylinder head are not cooled down 
excessively. Therefore, the exhaust passages can be cooled down without 
supercooling of the cylinder. 
The necessity of forming passages for returning the cooling water in the 
cylinder block and the cylinder head can be eliminated. Therefore, the 
structures of the passages for the cooling water in the cylinder block and 
the cylinder head can be simplified and thus the engine can easily be 
manufactured. Since the water drainage hoses are collectively disposed on 
the side surface of the engine, the hose layout can easily be performed. 
As a result, the cooling apparatus can easily and accurately be assembled 
and the maintenance of the same can smoothly be performed. 
Other objects, features and advantages of the present invention will be 
made clear from the following detailed description of the preferred 
embodiments described in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described hereunder with 
reference to the accompanying drawings. 
With reference to FIG. 1, an outboard motor 1 is mounted on a transom 3 of 
a hull 2 through a crank bracket 4 in such a manner that the outboard 
motor 1 is able to turn horizontally about a swivel shaft 5 vertically 
disposed in the rear portion of the crank bracket 4. 
An engine 6 mounted in the uppermost portion of the outboard motor 1 is, 
for example, an in-line, four-cylinder, water-cooled and four-stroke-cycle 
gasoline engine, the engine 6 being mounted vertically in such a manner 
that a crank shaft 7 of the engine 6 is positioned vertically. The engine 
6 is assembled in such a manner that a crank case 9, a cylinder block 10, 
a cylinder head 11, a head cover 12 and the like are disposed sequentially 
in the longitudinal direction. 
An oil pan 14 is secured to the lower portion of the engine 6 through an 
engine holder 13 formed into a thick plate-like shape. A drive-shaft 
housing 15 is secured to the lower portion of the oil pan 14, and a gear 
housing 16 is secured to the lower portion of the drive-shaft housing 15. 
The engine 6, the engine holder 13 and the oil pan 14 are covered with an 
engine cover 18 which can be sectioned vertically. The engine cover 18 has 
a lower cover 18a, secured to spread over the engine holder 13 and the oil 
pan 14, and an upper cover 18b detachably attached above the lower cover 
18a. When maintenance of the engine 6 or the like is performed, the upper 
cover 18b is removed. 
A rotational drive shaft 19 extending downwards is integrally connected to 
the lower end of the crank shaft 7 of the engine 6. The drive shaft 19 is 
allowed to penetrate the engine holder 13, the oil pan 14 and the 
drive-shaft housing 15 to reach the inside portion of the gear housing 16. 
On the other hand, a propeller shaft 20 extending longitudinally is 
rotatively supported in the gear housing 16, and a rotational propeller 21 
is integrally connected to the rear end of the propeller shaft 20. A bevel 
gear mechanism 22 disposed at the intersection between the drive shaft 19 
and the propeller shaft 20 transmits rotations of the drive shaft 19 to 
the propeller shaft 20 so that the propeller 21 is rotated. 
Further, a pair of Note that right and left mounting portions 23 and 24 are 
formed in the front ends of the engine holder 13 and the drive-shaft 
housing 15. The vertical mounting portions 23 and 24 are pivotally 
supported at the upper and lower ends of the swivel shaft 5. 
An air intake unit 26 is attached to the left side surface, as viewed, of 
the engine 6. A starter motor 27 for starting the engine 6 is disposed on 
the front surface of the engine 6. The upper end of the crank shaft 7 
projects upwards over the upper surface of the engine 6. A rotational 
flywheel 28 is integrally mounted at the upper end of the crank shaft 7, 
while a drive pulley 29 is disposed below the flywheel 28. On the other 
hand, a camshaft 30 is pivotally disposed in the cylinder head 11 to be in 
parallel to the crank shaft 7. An upper end of the camshaft 30 projects 
over the upper surface of the engine 6. A rotational driven pulley 31 is 
integrally mounted to the projecting portion as shown in FIG. 5. 
A timing belt 32 is arranged between the drive pulley 29 of the crank shaft 
7 and the driven pulley 31 of the camshaft 30. The timing belt 32 
transmits rotations of the crank shaft 7 to the camshaft 30 so that a 
valve gear unit, not shown, accommodated in the cylinder head 11 is 
operated. 
A flange-shaped ring gear 28a is disposed around the flywheel 28 so that a 
pinion gear 27a of the starter motor 27 projects upwards to be engaged 
with the ring gear 28a when the starter motor 27 has been rotated. As a 
result, the crank shaft 7 is rotated so that the engine 6 is started. 
A cover 33 made of synthetic resin or the like is disposed to cover the 
starter motor 27, the flywheel 28, the drive pulley 29, the driven pulley 
31, the timing belt 32 and the like from an upper side so that water drops 
are blocked by the cover 33 to protect the inside portion from water. 
A water pump 35 is disposed on the upper surface of the gear housing 16. 
The drive shaft 19 also serves as the drive shaft of the water pump 35. A 
water inlet 36 is formed in the side surface of the gear housing 16. A 
water-inlet passage 37 formed to extend upwards from the water inlet 36 is 
connected to the water pump 35. An upper end of a water supply pipe 38 
extending upwards from the water pump 35 is connected to the lower surface 
of the oil pan 14. 
FIG. 2 is a top view of the oil pan 14. FIG. 3 is a bottom view of the 
engine holder 13. As shown in FIG. 2, the major portion of the inside 
space of the oil pan 14 is occupied by an oil accumulating tank 40. For 
example, an exhaust passage 41, a water-supply passage 42 and a water 
drainage passage 43 are formed in the right-hand portion in the oil pan 
14. The passages 41, 42 and 43 extend downwards from the engine 6 to 
vertically penetrate the inside portion of the oil pan 14, the passages 
41, 42 and 43 being formed integrally with the oil pan 14 by forming 
vertical walls 14a, 14b and 14c in the oil pan 14. 
The water-supply passage 42 and the water drainage passage 43 are formed to 
surround the exhaust passage 41. For example, the water-supply passage 42 
is formed to have an L-like planer shape. The exhaust passage 41 is formed 
on the inside of the corner of the L-shape water-supply passage 42, while 
the water drainage passage 43 is formed in front of the exhaust passage 
41. 
A water-supply joint 44 is formed in the bottom of the water-supply passage 
42. An upper end of the water supply pipe 38 extending from the water pump 
35 is connected to the water-supply joint 44. Further, an oil drain port 
45 for discharging oil in the oil accumulating tank 40 is formed in the 
lower portion of the left-hand side surface of the oil pan 14. 
As shown in FIG. 3, the engine holder 13 has an exhaust opening 46, a water 
supply opening 47 and a water drain opening 48 corresponding to the 
exhaust passage 41, the water-supply passage 42 and the water drainage 
passage 43 of the oil pan 14. Further, drive-shaft insertion openings 49 
and 50, through which the drive shaft 19 is inserted, are formed at the 
front ends of the engine holder 13 and the oil pan 14. Moreover, a pair of 
right and left mounting members 51 are formed in the front portion of the 
engine holder 13 to permit the mounting portion 23 to be mounted to the 
mounting members 51. 
As shown in FIG. 6, four suction ports 53 are formed in the left side 
surface of the cylinder head 11 of the engine 6. Four exhaust ports 54 
shown in FIG. 4 are formed in the right side, as viewed, surface of the 
cylinder head 11. Four suction branches 26a of the air intake unit 26 are 
connected to the suction ports 53, while an exhaust manifold 55 is secured 
to the exhaust ports 54. 
An exhaust collector passage 56 is formed in the exhaust manifold 55. The 
exhaust ports 54 are collected to the exhaust collector passage 56. The 
lower end of the exhaust manifold 55 overlaps the right surface of the 
engine holder 13. The lowermost end of the exhaust collector passage 56 is 
warped toward the engine holder 13 and connected to the exhaust opening 46 
opened in the left surface of the engine holder 13. Thus, an exhaust 
passage is formed in the sequential order as the exhaust ports 54, the 
exhaust collector passage 56, the exhaust opening 46 and the exhaust 
passage 41. 
As shown in FIG. 1, an exhaust outlet passage 57 is formed in the 
drive-shaft housing 15 and the gear housing 16. The lower end of the 
exhaust passage 41 of the oil pan 14 is opened in the upper portion of the 
exhaust outlet passage 57. 
An end of the exhaust outlet passage 57 is allowed to communicate with the 
outside from the central portion of the propeller 21. Exhaust gas 
discharged from the exhaust passage 41 is introduced into the exhaust 
outlet passage 57 so as to be expanded, and then discharged from the 
central portion of the propeller 21 to the outside into water. 
An exhaust water jacket 58, through which cooling water flows, is formed 
around the exhaust collector passage 56. An inlet 59 is formed in the 
lowermost portion of the exhaust water jacket 58, while an outlet 60 is 
formed in the uppermost portion. As shown in FIG. 9, a pressure valve 61 
is disposed in the outlet 60. Further, a water testing opening 62 is 
formed in the lowermost portion of the exhaust water jacket 58. The water 
testing opening 62 and a release opening 63 formed in the trailing end of 
the oil pan 14 are connected to each other through a hose as shown in FIG. 
4. 
As shown in FIG. 7, four cylinders 64 are disposed in the cylinder block 
10. A block water jacket 65, through which cooling water flows, is formed 
so as to surround the cylinders 64. An inlet 66 is formed in the lowermost 
portion of the block water jacket 65, while an outlet 67 is formed in the 
uppermost portion of the block water jacket 65. The outlet 67 accommodates 
a thermostat 68. 
The inlets 59 and 66 of the exhaust water jacket 58 and the block water 
jacket 65 are, through a water supply opening 47 of the engine holder 13, 
connected to the water-supply passage 42 of the oil pan 14. Water drainage 
hoses 70 and 71 disposed to pass through the outside portion of the engine 
6 are connected to the outlets 60 and 67. The other ends of the water 
drainage hoses 70 and 71 are joined to a water drainage hose 73 by a 
T-shape joint 72. The water drainage hose 73 is connected to the water 
drain opening 48 of the engine holder 13 through an L-shape joint 74, and 
then connected to the water drainage passage 43 of the oil pan 14. 
Further, the water drainage hoses 70, 71 and 73, the T-shape joint 72 and 
the L-shape joint 74 are collectively disposed on the side surface (i.e. 
right surface) of the engine 6. 
Therefore, a cooling apparatus 75 as shown in FIG. 10 will be formed, in 
which the block water jacket 65 is connected to a head water jacket 76 
formed in the cylinder head 11 so that the cooling water is communicated 
between the block water jacket 65 and the head water jacket 76. Further, a 
cylinder-surface-temperature sensor 69 is disposed between the block water 
jacket 65 and the thermostat 68. 
Thus, a cylinder cooling-water passage A is formed which includes the block 
water jacket 65, the head water jacket 76, the thermostat 68 and the water 
drainage hose 71. On the other hand, an exhaust cooling-water passage B is 
formed which includes the exhaust water jacket 58, the pressure valve 61 
and the water drainage hose 70. 
The cylinder cooling-water passage A is a passage for supplying cooling 
water supplied from the water pump 35 to the block water jacket 65 and the 
head water jacket 76. On the other hand, the exhaust cooling-water passage 
B is a passage for supplying cooling water supplied from the water pump 35 
to the exhaust water jacket 58. The cooling-water passages A and B are 
formed individually from each other and arranged to have downstream 
portions which are joined together in a portion including the T-shape 
joint 72 and the water drainage hose 73. 
When the engine 6 has been started, the water pump 35 is operated 
attributable to rotations of the drive shaft 19 so that external water is, 
as cooling water, sucked through the water inlet 36 of the gear housing 16 
into the water pump 35 so as to be supplied to the water supply pipe 38 
under pressure. The cooling water flows through the water-supply passage 
42 of the oil pan 14 and the water supply opening 47 of the engine holder 
13 and is then branched into the cylinder cooling-water passage A and the 
exhaust cooling-water passage B. Thus, the exhaust water jacket 58, the 
block water jacket 65 and the head water jacket 76 are cooled down. 
The cooling water allowed to flow through the cylinder cooling-water 
passage A and the exhaust cooling-water passage B are joined at the 
T-shape joint 72, and then introduced into the water drain opening 48 of 
the engine holder 13 and the water drainage passage 43 of the oil pan 14, 
after which the cooling water is discharged to the outside through the 
exhaust outlet passage 57. Further, a pressure valve 77 is disposed 
between the water pump 35 and the engine holder 13, as shown in FIG. 10. 
The thermostat 68 is closed when the engine 6 is in a state of stoppage, 
and when the temperature of cooling water in the block water jacket 65 and 
the head water jacket 76 has been raised to a predetermined level after 
start of the engine 6, the thermostat 68 is opened to permit cooling water 
to pass through. As a result, time required for the engine 6 to be warmed 
up can be shortened and the supercooling of the cylinders 64 can be 
prevented during the operation. 
Since the pressure in the portion downstream from the water pump 35 is 
raised excessively during the period in which the thermostat 68 is opened, 
the pressure valve 77 is opened to relieve excess pressure (cooling water) 
to the outside so that the cooling water passage can be protected. 
The thermostat 68 is disposed in the right-hand portion of the upper 
surface of the cylinder block 10, as shown in FIG. 5. As shown in FIG. 7, 
the thermostat 68 is disposed at a position higher than the uppermost 
portions of the block water jacket 65 and the head water jacket. In 
addition, the position of the thermostat 68 is made to be lower than the 
upper end of the timing belt 32. The portion of the outlet 67 in which the 
thermostat 68 is included is covered with the cover 33. 
As shown in FIGS. 7 and 8, the cylinder-surface temperature sensor 69 is 
disposed adjacent to the thermostat 68. The pressure valve 61 of the 
exhaust water jacket 58 is also disposed adjacent to the thermostat 68. 
The cooling apparatus 75 of the outboard motor 1 has the structure 
mentioned above. Accordingly, since the independent cylinder cooling-water 
passage A and the exhaust cooling-water passage B are provided for the 
cooling apparatus 75 and the downstream portions of the cylinder 
cooling-water passage A and the exhaust cooling-water passage B are 
joined, introduction of hot cooling water, which has cooled the exhaust 
manifold down, into the block water jacket 65 and the head water jacket 76 
can be prevented as has been experienced with the conventional outboard 
engine. 
Therefore, the temperature of the cooling water which flows around the 
block water jacket 65 can be stabilized at adequate levels so that the 
performance for cooling the portion including the block water jacket 65 is 
improved. Moreover, the thermostat 68 is able to easily control the 
temperature. 
As described above, the cooling apparatus 75 is arranged in such a manner 
that the thermostat 68 provided for the cylinder cooling-water passage A 
is positioned higher than the uppermost portions of the block water jacket 
65 and the head water jacket and lower than the top end of the timing belt 
32. Therefore, excessive projection of the thermostat 68 over the upper 
portion of the engine 6 can be prevented. Thus, undesirable enlargement of 
the size of the engine 6 can be prevented. 
Since the cooling apparatus 75 is structured in such a manner that the 
cylinder-surface-temperature sensor 69 is disposed adjacent to the 
thermostat 68, the cylinder-surface-temperature sensor 69 is able to 
stably detect the temperature. As a result, the temperature of the cooling 
water can easily and accurately be controlled. If the thermostat 68 
produces trouble, the trouble can immediately be detected by the 
cylinder-surface-temperature sensor 69. 
Since the cooling apparatus 75 is structured in such a manner that the 
pressure valve 61 is also disposed adjacent to the thermostat 68, the 
water drainage hoses 70 and 71 extending from the thermostat 68 and the 
pressure valve 61 respectively are disposed adjacently. As a result, the 
cooling apparatus 75 including the water drainage hoses 70 and 71 can 
easily be assembled and the maintenance of the same can smoothly be 
performed. 
The cooling apparatus 75 is arranged in such a manner that the flow of the 
joined cylinder cooling-water passage A and the exhaust cooling-water 
passage B is introduced into the water drainage passage 43 adjacent to the 
exhaust passage 41 provided in the oil pan 14. Therefore, the cooling 
water, which has cooled the water jackets 58, 65 and 76 down, is able to 
cool the water drainage passage 43. Thus, the necessity of enlarging the 
quantity of cooling water to cool the water drainage passage 43 can be 
eliminated. 
Since the quantity of the cooling water is not enlarged, the block water 
jacket 65 and the head water jacket 76 are not cooled down excessively. 
That is, the water drainage passage 43 can be cooled down without arising 
the problem that the block water jacket 65 is cooled down excessively. 
The cooling apparatus 75 is structured in such a manner that the downstream 
passages of the block water jacket 65, the head water jacket 76 and the 
exhaust water jacket 58 are, in the cylinder cooling-water passage A and 
the exhaust cooling-water passage B, formed by the water drainage hoses 
70, 71 and 73 which are disposed on the outside of the engine 6. Moreover, 
since the water drainage hoses 70, 71 and 73 are collectively disposed on 
the side surface of the engine 6, the necessity of forming passages for 
returning cooling water in the cylinder block 10, the cylinder head 11 and 
the exhaust manifold 55 can be eliminated. 
As a result, the structure of the passages for cooling water in the 
cylinder block 10, the cylinder head 11 and the exhaust manifold 55 can be 
simplified and easily be manufactured. Since the operation for piping the 
water drainage hoses 70, 71 and 73 can easily be performed, the cooling 
apparatus 75 can significantly easily be assembled and the maintenance of 
the same can smoothly be performed. 
As described above, the cooling apparatus for an outboard motor according 
to the present invention is structured in such a manner that the cylinder 
cooling-water passage for supplying cooling water from the water pump to 
the water jackets in the cylinder block and the cylinder head and the 
exhaust cooling-water passage for supplying cooling water from the water 
pump to the water jacket in the exhaust manifold are formed independently. 
Moreover, the downstream portions of the cylinder cooling-water passage 
and the exhaust cooling-water passage are joined together. Therefore, the 
temperature of cooling water which flows around the cylinder can be 
stabilized to adequate levels so that the performance for cooling the 
portion including the cylinder is improved. In addition, the thermostat is 
enabled to easily control the temperature. 
It is to be noted that the present invention is not limited to the 
described embodiment and many other changes and modifications may be made 
without departing from the scopes of the appended claims.