Damper for a personal watercraft

A damper which can be easily fitted to the engine E of a personal watercraft W having little space, said damper damping the torsional vibration caused by the torsional stress acting on the crankshaft 1 of the engine E mounted in a personal watercraft W. A flange 2, which is a component of the coupling D that couples the crankshaft 1 and the propeller shaft 11 of a propulsion pump P, is fixed to the end of the crankshaft 1 which is adjacent to the pump P. The damper includes a damping mass fixed via an elastomeric member 4 to the periphery of a flange 2 in such a manner that the mass 5 can elastically move with respect to the crankshaft 1 in the direction of rotation.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates to dampers for an engine and a propulsion 
pump which are mounted on a personal watercraft. 
High torsional stress caused by torque acts on a crankshaft of an engine, 
and in particular of an automobile engine, which needs to rotate at a high 
speed. In order to resist the torsional stress, the crankshaft is designed 
to have an allowable torsional stress which is higher than the torsional 
stress acting on the crankshaft. In order to make the allowable torsional 
stress of the crankshaft higher, it is necessary to increase the sectional 
area of the crankshaft. This increases the engine weight. However, such 
increase in weight is contrary to the effort in recent years to lighten 
the weight of vehicles in order to reduce the fuel consumption and save 
resources. 
When the torsional stress acting on the crankshaft exceeds the allowable 
torsional stress of the crankshaft, the durability of the crankshaft 
decreases, and the vibration and noise from the engine due to the 
torsional stress increases. In the case of an engine in a four-wheeled 
vehicle, it is known that for reducing the torsional vibration of the 
crankshaft a damping mass (weight) is fixed through an elastic member to 
the pulley which drives a cooling fan or a water pump, and is fixed to the 
front end of the crankshaft. 
In the case of a motorcycle engine, as shown in FIG. 9 of the accompanying 
drawings, it is known that a damping mass 105 may be fixed to the 
longitudinal outer side portion of a magneto rotor 114 which is fixed to a 
front end 101a of the crankshaft for reducing the torsional vibration 
thereof (Japanese Patent Publication H.8-23304). 
For personal watercraft, various measures have been taken to damp the 
torsional vibration caused by the engine rotation. For example, Japanese 
Utility Model Publication S.53-25062 discloses elastomeric (i.e. synthetic 
or natural polymers which are elastic, e.g. rubber buffers) members 
arranged in a coupling, interconnecting a crankshaft of an engine and a 
propeller shaft of a propulsion pump, in order to reduce the transmission 
of the torsional vibration of the engine to the pump. 
This coupling, as shown in FIG. 10 of the accompanying drawings, includes a 
pair of flanges 102,112 each fixed to the adjacent ends of the crankshaft 
101 and propeller shaft 111. The flanges include axially extending claws 
102b and 112b. The claws 102b of flange 102 and the claws 112b of flange 
112 protrude axially toward the each other, and overlap each other when 
rotating (overlap each other in an axial direction in FIG. 10). The 
elastomeric member 113 is interposed between adjacent claws 102b and 112b. 
In order to further damp the torsional vibration from the engine of a 
personal watercraft, it might be possible to provide a damper, as in the 
case with the foregoing motorcycle, by providing additional space at the 
front out side portion of the crankshaft. For a personal watercraft, 
however, it is necessary to arrange the engine and accessories (auxiliary 
parts) very compactly in the small inner space of a body. More 
specifically, there is a need to arrange most of the engine, the fuel 
tank, the battery, the muffler and the other parts in the space defined 
between the hull and the deck. Furthermore, walls of the bulkhead and the 
pump chamber are arranged with little room or clearance rearward of the 
engine. Accordingly, under these circumstances, there is no available 
space in which the damper can be fitted in the manner that the motorcycle 
damper is fitted. 
Furthermore the dampers of personal watercraft are used in an environment 
of fresh or saltwater except when the damper is fitted in substantially 
closed casings, like a crankcase. It is necessary for the damper used in 
such an environment to be directly visible when daily checks are made, and 
to be easily replaced. 
Many of the engines mounted on personal watercraft are also supplied with 
fuel through a carburetor. The engine vibration, and in particular the 
torsional vibration in the normal revolution speed range of the engine 
makes it difficult to optimally set the fuel ratio of the carburetor for 
the revolution speed range of the engine, and to maintain this ratio. 
Technical problems special to personal watercraft also include the 
torsional vibration at the propeller shaft of the propulsion pump. This 
torsional vibration causes cavitation on the surface of the pump impeller 
blades which reduces the propulsive force. It is therefore desirable to 
damp the torsional vibration of the propeller shaft as much as possible. 
SUMMARY OF THE INVENTION 
In view of the foregoing, it is a first object of the present invention to 
provide a torsional vibration damper which can be easily fitted to an 
engine mounted in a personal watercraft, which has little space or room. 
It is a second object of the invention to provide a torsional vibration 
damper which can be easily fitted to a propulsion pump mounted in a 
personal watercraft, which has little space or room. 
According to a first aspect of this invention, there is provided a damper 
for damping the torsional vibration caused by the torsional stress acting 
on a crankshaft of a propulsion engine mounted in a personal watercraft, 
characterized in that a damping mass is fixed to the periphery of a flange 
by an elastomeric (i.e. synthetic or natural polymers, e.g. rubber, which 
are elastic) member in such a manner that the mass can elastically move 
with respect to the crankshaft in the direction of rotation, said flange, 
which is a component of a coupling that couples the crankshaft and a 
propeller shaft of a propulsion pump, being fixed to the rear end of the 
crankshaft. 
By providing such a damper, it is possible to effectively use for the 
damper the space around the periphery of the flange of the conventionally 
fitted coupling. This damping mass, which is fixed through the elastomeric 
member to a flange which is fixed to one end of the crankshaft, belongs to 
the same vibration system which the crankshaft belongs to. In relation to 
the torsional force, the mass is out of phase with the torsional stress 
acting on the crankshaft. As a result, the torsional forces cancel each 
other. Thus, the torsional vibration and the noise of the crankshaft is 
reduced. The damper is fixed to the coupling which is visible and 
therefore the damper can easily be inspected and easily attached and 
removed in comparison with a conventional damper. 
The elastomeric member may be fixed to the periphery of the flange of the 
coupling via a sleeve. This makes it possible to separately produce the 
flange of the coupling and other peripheral parts (damper unit) which 
includes the sleeve, the elastomeric member and the damping mass, and then 
assemble them. It is also possible to change the damping mass capacity by 
replacing the sleeve, the elastomeric member and the damping mass together 
as a unit. It is therefore easy to change the mass, simply and quickly. 
It is preferable that the watercraft's engine, which includes a generator 
with a magneto rotor fixed to a front end of the crankshaft, should also 
include a second damper. The second damper includes a damping mass fixed 
through an elastomeric member to the periphery of the rotor in such a 
manner that this mass can elastically move with respect to the crankshaft. 
The second damper is positioned in the small space around the periphery of 
the magneto rotor. It is therefore possible to fit the damper compactly in 
the engine casing without elongating the engine longitudinally. The 
damping masses of the dampers at both ends of the crankshaft may have 
different natural frequencies. This makes it possible to obtain a damping 
effect in a wider frequency range where the damping effects of the dampers 
are cumulative. 
According to a second aspect of this invention, there is provided a damper 
for damping the torsional vibration caused by the torsional stress acting 
on a crankshaft of a propulsion engine mounted on a personal watercraft, 
characterized in that a damping mass is fixed to the periphery of a 
fitting member by an elastomeric member in such a manner that the mass can 
elastically move with respect to the crankshaft in the direction of 
rotation, said fitting member being bolted to a side surface of a flange, 
which is fixed to the rear end of the crankshaft; the flange being a 
component of a coupling which couples the crankshaft and a propeller shaft 
of the propulsion pump. This damper is easy to mount and remove by 
tightening and loosening the bolts, respectively. Therefore, it is easy to 
replace the damping mass with another damping mass having a different 
weight or with a new damping mass. 
The fitting member may take the form of a ring, which is shaped like a "T" 
in axial section, and the damping mass may be positioned around the 
periphery of the fitting member. This structure is simple because of the 
damping mass being positioned around the periphery of the flange. 
According to a third aspect of this invention, there is provided a damper 
for a personal watercraft including an engine for propulsion and a 
propulsion pump which are mounted thereon, the engine including a 
crankshaft coupled to a propeller shaft of the pump through a coupling, 
which includes first and second flanges, each of the flanges including 
claws protruding axially toward the other flange, the claws of the first 
flange alternating with the claws of the second flange in the direction of 
rotation, the coupling further including an elastomeric buffer interposed 
between the flanges for transmitting power from the engine to the pump, 
the elastomeric buffer including a circular body portion and first and 
second alternating protrusions formed integrally on the periphery of the 
body portion, each of the first protrusions being elliptic and including a 
narrow root in radial section, each of the second protrusions taking the 
form of a strip in radial section, each of the first protrusions being 
positioned on the leading side of one of the claws of the first flange, 
each of the second protrusions being positioned on the trailing side of 
one of the claws of the first flange, characterized in that a damping mass 
is fixed, via an elastic member, to the periphery of the first flange in 
such a manner that the mass can elastically move with respect to the 
crankshaft in the direction of rotation. 
Using this damper, it is possible to place the damper in the space around 
the periphery of the first flange, which is fixed to the crankshaft of the 
engine, of the coupling for transmitting motive power from the engine to 
the pump. This damping mass, which is fixed via the elastomeric member to 
the first flange which in turn is fixed to one end of the crankshaft, 
belongs to the vibration system as the crankshaft. The damping mass 
therefore reduces the torsional vibration and the noise of the crankshaft. 
Furthermore, the elastomeric buffer in the coupling absorbs torsional 
vibration transmitted from the engine to the propeller shaft of the pump 
which even more effectively damps the torsional vibration generated at the 
propeller shaft. 
If the engine is of the type where the carburetor is fixed to the engine 
through the manifold, it is easy to set the carburetor because the engine 
vibration is damped. 
According to a fourth aspect of this invention, there is provided a damper 
for damping the torsional vibration caused by the torsional stress acting 
on a propeller shaft of a propulsion pump mounted on a personal 
watercraft, characterized in that a damping mass is fixed via an 
elastomeric member to the periphery of a flange in such a manner that the 
mass can elastically move with respect to the propeller shaft in the 
direction of rotation, the flange being fixed to the input end of the 
propeller shaft which is connected to the engine of the personal 
watercraft, said flange being a component of a coupling which couples the 
propeller shaft and the crankshaft of the engine. 
This damper, which is simple in structure, can damp the torsional vibration 
of the propeller shaft and the torsional vibration of the impeller 
attached to the propeller shaft of the pump. This prevents cavitation on 
the surface of the impeller blades. It is also possible to prevent the 
outer ends of the blades from contacting the inner surface of the pump 
casing due to the torsional vibration. Accordingly, the clearance between 
the casing and the blades can be small. Consequently, the propulsive 
efficiency of the pump is high. 
The elastomeric member may be fixed via a sleeve to the periphery of the 
second flange of the coupling. This makes it possible to separately 
produce the flange of the coupling and other peripheral parts (damper 
unit) which includes the sleeve, the elastomeric member and the damping 
mass, and then assemble them. It is also possible to change the damping 
mass capacity by replacing the sleeve, the elastomeric member and the 
damping mass together as a unit. It is therefore easy, simple and quick to 
change the mass. 
According to a fifth aspect of this invention, there is provided a damper 
for damping the torsional vibration caused by the torsional stress acting 
on a propeller shaft of a propulsion pump mounted on a personal 
watercraft, characterized in that a damping mass is fixed via an 
elastomeric member to the periphery of a fitting member in such a manner 
that the mass can elastically move with respect to the propeller shaft in 
the direction of rotation, said fitting member being bolted to a side 
surface of a flange which is fixed to the end of the propeller shaft which 
is connected to the engine of the personal watercraft, the flange being a 
component of a coupling which couples the propeller shaft and the 
crankshaft of the engine. 
This damper is easy to mount and remove by tightening and loosening the 
bolts, respectively. Therefore, it is easy to replace the damping mass 
with a damping mass having a different capacity or with a new damping 
mass. 
The fitting member may be in the form of a ring, which is shaped like a "T 
" in axial section, and the damping mass may be positioned around the 
periphery of the fitting member. This structure is simple because of the 
damping mass being positioned around the periphery of the flange.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Dampers for a personal watercraft according to one embodiment of the 
present invention will be described below with reference to the drawings. 
FIG. 4 shows a personal watercraft W, to which the invention is applied. 
The personal watercraft W has a body which includes a hull B and a deck C 
covering the hull top. In FIG. 4, B shows a hull , J shows a bulkheads, M 
shows muffler, T shows a fuel tank and N shows a battery. 
An engine E is mounted in the space between the hull B and deck C, and is 
arranged midship. A crankshaft 1 of the engine extends longitudinally 
along the hull B. A propulsion pump P for propelling the personal 
watercraft W is mounted in a rear portion of the hull B. A propeller shaft 
11 of the pump P extends longitudinally along the hull B. 
The adjacent ends of the crankshaft 1 and propeller shaft 11 are coupled 
together via a coupling D so that the output from the engine E is 
transmitted to the pump P. The coupling D has a flange 2 fixed to the rear 
end 1R (see FIG. 4, FIG. 2) of the crankshaft 1 and a flange 12 fixed to 
the input end 11F (see FIG. 4 , FIG. 5) of the propeller shaft 11. 
As shown in FIGS. 5(a) and 5(b), the flanges 2 and 12 include claws (pawls) 
2b and 12b, respectively, formed integrally on the flanges. 
The claws 2b and 12b protrude axially toward the opposite surfaces 12a and 
2a of the flanges 12 and 2, respectively, in such a manner that the claws 
2b overlap with the claws 12b in the direction of rotation (overlap each 
other in an axial direction looking from the side view). 
As shown in FIG. 5(a), the flanges 2 and 12 are interconnected by a 
circular elastomeric buffer 13 interposed between them. As shown in FIG. 
5(b), the buffer 13 includes a body portion 13a generally in the form of a 
disk and a number of protrusions 13b and 13c formed integrally on the 
periphery of the body portion 13a. The protrusions 13b alternate with the 
protrusions 13c in the direction of rotation. Each protrusion 13b is 
generally elliptic in radial section and includes a narrow root. Each 
protrusion 13c has the form of a strip in radial section. 
As shown in FIG. 5(b), the coupling rotates in the direction R. Each 
elliptic protrusion 13b of the elastomeric buffer 13 is interposed between 
the leading side 2c of one of the claws 2b on the flange 2 fixed to the 
crankshaft 1 and the trailing side 12c of one of the claws 12b on the 
flange 12 fixed to the propeller shaft 11. Likewise, each strip-like 
protrusion 13c is interposed between the trailing side 2d of the claw 2b 
and the leading side 12d of the claw 12b. The structure of the buffer 13 
is not limited to that shown in FIGS. 5(a) and 5(b), but may have other 
structures known to those skilled in the art. 
As also shown in FIG. 2, the flange 2 is fixed to the rear end of the 
crankshaft 1. As shown partially on an enlarged scale in FIGS. 1(a) and 
1(b), a metal sleeve 3 in the shape of a ring is fixed to the peripheral 
surface 2e of the flange 2 by press fitting, welding or bonding with an 
adhesive or the like so as to rotate with flange 2. 
As already stated with reference to FIG. 5(a), the flange 12 is fixed to 
the input end of the pump propeller shaft 11 so as to rotate with the 
shaft 11. A metal sleeve 3 in the form of a ring, which is similar to that 
shown in FIGS. 1(a) and 1(b), is fixed to the peripheral surface 12e of 
the flange 12 by press fitting, welding or bonding with an adhesive or the 
like so as to rotate with the flange 12. 
As shown in FIGS. 1(a), 1(b) and 5(a), a damper unit, wherein an annular 
damping mass or weight 5 is fixed to the peripheral surface of the sleeve 
3 through the intermediary of a ring-shaped elastomeric member 4, is 
formed to be one body. The damper unit is fixed to the peripheral surface 
of the associated flange 2 or 12, as stated above. More specifically, in 
this embodiment the damper unit consist of the sleeve 3, the elastomeric 
member 4, and damping mass 5. The elastomeric member 4 is made of NBR 
(nitrile butadiene rubber) but may be other elastomeric material made from 
natural or synthetic polymers which are elastic. Elastomeric member 4 is 
fixed to the peripheral surface of the sleeve 3 with an adhesive, or by 
baking (vulcanized bonding) so as to rotate with the sleeve 3. The annular 
damping mass 5 is made of metal, and fixed to the peripheral surface 4a of 
the elastomeric member 4 so as to rotate with elastomeric member 4 and 
move elastically with respect to the sleeve 3 within the range of 
elasticity of the elastomeric member 4 in the direction R (FIG. 1(b)) or 
in the opposite direction. 
The operation of the damper of the personal watercraft will be described 
below. When torsional stress causes torsional vibration of the crankshaft 
1 or the propeller shaft 11, the damping mass 5 vibrates independently of 
the crankshaft 1 or propeller shaft 11. Accordingly, by properly selecting 
a natural frequency of the damper in accordance with the natural frequency 
of the crankshaft 1 or propeller shaft 11, it is possible to restrain the 
vibration of the crankshaft 1 or propeller shaft 11 and to prevent noise. 
As shown in FIG. 3(a), the result of an experiment of the inventors, when a 
torsional stress acting on the crankshaft 1, without a damper as described 
above, the twist angle of the crankshaft 1 in the rotation created by the 
torsional vibration peaked at particular rotation frequencies. 
Another experiment was carried out under the conditions stated above, but 
with a damper constructed as described above, and which was set so that 
its characteristics were effective in a certain frequency band. As shown 
in FIG. 3(b), the result of this experiment shows that the maximum twist 
angle of the crankshaft 1 in this case was 50% less than that shown in 
FIG. 3(a). 
A further experiment was carried out under the same conditions with a 
damper constructed as described above, and which was set so that its 
characteristics were effective in another frequency band. As shown in FIG. 
3(c), the result of this experiment demonstrates that the maximum twist 
angle of the crankshaft 1 in this case was about 60% of that shown in FIG. 
3(a). It is therefore possible to provide a damper which fits the 
characteristics of an engine by changing the damper characteristics by 
varying the modulus of elasticity and/or the damping factor of the 
elastomeric member 4 and/or the weight of the annular damping mass 5. 
The torsional vibration of the pump propeller shaft 11, as well as the 
engine crankshaft 1, can be damped when the damper is on the flange 12 of 
the propeller as stated above. As a result, even when the propeller shaft 
11 rotates at high revolution speed which is the normal revolution speed 
range for personal watercraft, the vibration caused by the torsional 
vibration of the blades F of the propulsion pump P, which is fitted to the 
rear end of the propeller shaft 11, is damped in comparison with that of 
the prior art. 
Consequently, cavitation on the surface of the blades F of the pump P is 
decreased. It is therefore possible to prevent reduction of the propulsive 
force and shortening the life of the blades F because of cavitation. It is 
also possible to prevent the pump blades F from contacting the pump casing 
due to torsional vibration, even though there is only a small gap between 
the casing and the blades because of pump efficiency. 
In addition to the dampers at the crankshaft 1 and propeller shaft 11, the 
coupling means with the elastomeric buffer 13 may be interposed between 
the flanges 2 and 12 as shown in FIG. 5(a). This reduces the vibration at 
the connection between the engine E and pump P. As a result, the vibration 
of the whole drive system of the personal watercraft can be reduced more 
effectively. When the torsional vibration caused by the torsional stress 
decreases, the vibration and noise of the personal watercraft decrease. 
This gives a feeling of high smoothness when planing. Moreover, the 
torsional rigidity of the crankshaft 1 and propeller shaft 11 is lower. As 
a result, the crankshafts 1 and propeller shaft 11 may be lighter in 
weight, improving the planing performance and fuel consumption. 
As shown in FIGS. 2 and 6, the engine E is fitted with a generator G at the 
front end (left end in FIG. 2) IF of the crankshaft 1 in the engine 
casing. The generator G includes a generally cylindrical magneto rotor 14, 
which is fixed to the front end of the crankshaft 1 so as to rotate with 
the crankshaft 1. As stated already, a damper is fitted to the rear end of 
the crankshaft 1. In place of or in addition to this damper, a second 
damper may be fitted on the periphery of the magneto rotor 14 of the 
generator G. The second damper includes an annular damping mass 5 fixed by 
sleeve 3 and an elastomeric member 4 to the periphery of the rotor 14. 
In particular, if the dampers (one of the dampers shown in FIG. 5(a) and 
the second damper) are thus fitted at both ends of the crankshaft 1, and 
each damper has a different natural frequency, it is possible to increase 
the frequency range where the damping effect is obtained. This makes it 
possible to obtain a damping effect in a fairly wide speed range of the 
engine. 
The engine E includes a carburetor (not shown) as a fuel supply means, 
which is bolted to the manifold 7 which in turn is bolted to the crankcase 
H (FIG. 2). Accordingly, engine vibration is transmitted directly to the 
carburetor. The reduction in engine vibration by the dampers suppresses 
the spillage of fuel into the suction passage of the carburetor. 
Therefore, it is very easy to set the carburetor, and this setting may be 
maintained even during planing of the personal watercraft. 
As already stated, the annular damping mass 5 surrounds the elastomeric 
member 4 adhered or fixed to the periphery of sleeve 3. Such a damper can 
be fitted (mounted) easily and simply by press fitting, using an adhesive, 
or by mechanical engagement or the like, to the flange of the coupling 
between the engine and the propulsion pump of an existing personal 
watercraft. 
If necessary, it is possible to replace the damper with a new one by 
removing the sleeve 3 together with the elastomeric member 4 and the 
damping mass 5 from the coupling flange. Of course, the whole flange (or 
the whole rotor of the generator) may be replaced. 
As already stated, the damper unit which consist of the sleeve 3 , the 
elastomeric member 4 and damping mass 5 may be produced as a unit ,then 
the damper unit is fixed to the periphery of the flanges 2 ,12 of the 
coupling. Of course, after the sleeve 3 is fixed to the flange 2 or 12, 
the elastomeric member 4 and damping mass 5 may be fixed to the sleeve 3. 
The elastomeric member 4 may, without using the sleeve 3, be directly fixed 
to the flange 2 or 12 or the magneto rotor 14 with an adhesive or the 
like. In such case, the number of parts can be reduced. 
A damper unit which consists of a sleeve, an elastomeric member and an 
annular damping mass may be connected by splines to the periphery of the 
flange. In this case, the damper may be kept from moving axially with 
respect to the flange by a circlip , snap ring or the like. The inner 
surface of the sleeve 3 might also be serrated for connection by press 
fitting to the flange 2 or 12. The sleeve 3 might also be fixed to the 
flange 2 or 12 by screws. 
FIGS. 7 and 8 show another embodiment of the invention. With reference to 
FIG. 7, a fitting member 3 is provided which is shaped like a ring in 
front view and a "T" shape in axial section on its axis. The fitting 
member 3 has a flange portion 3e which is bolted to a side face of a 
flange 2 or 12. The outer portion 3f is attached to damping mass 5 via 
elastomeric member 4. In this case, the fitting member 3, elastomeric 
member 4 and annular damping mass 5 are assembled as a damper unit before 
fitting to the flange 2 or 12, then this damper unit is bolted to the 
flange 2 or 12. The damping mass 5 can elastically move with respect to 
the flange 2 of the crankshaft 1 or flange 12 of the propeller shaft 11 in 
the direction of rotation. The damper unit is easier to fit and remove.