Cavitation control for marine propulsion system

A jet drive cavitation control system briefly limits engine output power to prevent the onset of impeller cavitation when pressure upstream of the impeller indicates the likelihood of imminent impeller cavitation. The system uses a pressure sensor to sense water pressure, preferably immediately upstream of the impeller. The pressure sensor generates a signal that is transmitted to an electronic controller which controls the operation of the internal combustion engine that powers the jet drive. A threshold cavitation water pressure value is preselected at a point before the onset of impeller cavitation is likely. When the measured water pressure drops to or below the threshold cavitation water pressure value, the electronic controller immediately limits engine output to prevent impeller cavitation. Engine power output can be limited in any number of ways, for example, clipping spark plug ignition, retarding spark plug ignition, limiting throttle, limiting the amount of air supplied to the engine, limiting the amount of fuel supplied to the engine, adding water to the exhaust stream or modifying the configuration or operation of exhaust port valves, etc.

FIELD OF THE INVENTION 
The invention relates to cavitation control for marine propulsion systems. 
The invention is especially well-suited for minimizing impeller cavitation 
in marine jet drives. 
BACKGROUND OF THE INVENTION 
Marine jet drives are used in many marine applications, including 
propulsion for personal watercraft and jet boats. Jet drives for 
watercraft typically have an engine driven jet pump located within a duct 
in the hull of the watercraft. An inlet opening for the duct is positioned 
on the underside of the watercraft. The jet pump generally consists of an 
impeller and a stator located within the duct followed by a nozzle. A jet 
of water exits rearward of the watercraft to propel the watercraft. The 
impeller is driven by the engine to rotate within a wear ring. The 
rotating impeller provides thrust energy to the water flowing through the 
jet drive. The water then flows through the stator and the nozzle before 
exiting rearward through a generally tubular rudder than can be rotated to 
steer the watercraft. 
When accelerating at low speeds, water pressure in the duct immediately 
upstream of the impeller can drop significantly, thus contributing to 
impeller cavitation. Impeller cavitation is not normally a problem at 
medium or high watercraft speeds (even during acceleration) because water 
ram pressure in the duct against the impeller is significant. If 
cavitation occurs, the jet pump unloads the engine, which in turn causes 
the impeller to rotate at a higher rate and the cavitation worsens. If the 
impeller is fully cavitated and the engine is fully unloaded, the operator 
of the watercraft must normally slow the engine to idle to alleviate the 
cavitation. In extreme cases, impeller cavitation can cause damage to 
mechanical parts of the jet drive. Because watercraft are operated 
rigorously and under various operating conditions, it is difficult to 
predict the onset of impeller cavitation based merely on engine rpm and 
throttle position. This can also be true in other marine applications, 
e.g. jet boats. 
In order to eliminate the likelihood of impeller cavitation during 
acceleration at low speeds, marine jet drives are designed especially to 
minimize cavitation during acceleration at low speeds. For instance, the 
shape of the jet drive duct and the blade angle of the impellers are often 
selected to minimize impeller cavitation during low speed acceleration. 
However, such design configurations compromise jet drive performance at 
high speeds. 
The likelihood of impeller cavitation during low speed acceleration is 
higher with larger watercraft, and is also higher when more powerful 
engines are used. Impeller cavitation therefore restricts the use of jet 
drives in larger watercraft, and in watercraft having more powerful 
engines. 
SUMMARY OF THE INVENTION 
The invention is a cavitation control system that is especially well-suited 
for use on jet-propelled watercraft. The system uses a pressure sensor to 
monitor water pressure upstream of the impeller, preferably immediately 
upstream of the impeller. In accordance with the invention, engine output 
power is limited briefly to prevent impeller cavitation when the measured 
water pressure indicates that the onset of cavitation would otherwise be 
likely. 
The pressure sensor generates a water pressure signal that is preferably 
transmitted to an electronic controller which controls the operation of 
the internal combustion engine that powers the jet drive. The priority of 
the electronic control unit is to not limit engine output power unless the 
measured water pressure drops to or below a threshold cavitation water 
pressure value. The threshold water pressure value is preferably 
preselected at a pressure value slightly above the onset of impeller 
cavitation. Once the measured water pressure drops to or below the 
preselected water pressure value, the electronic controller immediately 
limits engine output to prevent impeller cavitation. Typically, engine 
output power need not be limited for more than approximately one-half 
second. Engine power output can be limited in any number of ways (for 
example, clipping spark plug ignition, retarding ignition timing advance, 
adding water to exhaust stream, modifying exhaust valve operation or 
configuration, limiting throttle, limiting the amount of air supplied to 
the engine, limiting the amount of fuel supplied to the engine), but 
clipping spark plug ignition is preferred. 
Inasmuch as damaged impellers normally cavitate at lower speeds than 
undamaged impellers, it may be desirable to include means to automatically 
modify the threshold cavitation water pressure value after the system 
detects that impeller cavitation has occurred previously. One way to 
identify impeller cavitation is to monitor impeller rpm during 
acceleration at low speeds (e.g. sharp rises in impeller rpm indicates 
cavitation), although other methods may be employed in accordance with the 
invention. 
One of the primary advantages of the invention is that the likelihood of 
impeller cavitation is detected accurately and shortly before the onset of 
actual impeller cavitation. Therefore, it is not necessary to limit engine 
output power for an excessively long period of time to prevent cavitation. 
This is possible because, in accordance with the preferred embodiment of 
the invention, water pressure is measured directly and immediately 
upstream of the rotating impeller, and the likelihood of imminent 
cavitation depends on the instantaneous water pressure at this location. 
The pressure sensor is preferably mounted to measure the pressure of water 
flowing through the wear ring in which the impeller rotates immediately 
upstream of the impeller. In addition, tests have shown that the most 
active pressure fluctuations during jet drive operation occur at the 
bottom of the wear ring. Therefore, placement of the pressure sensor 
through the bottom of the wear ring provides the greatest resolution for 
the pressure measurement. 
An impeller cavitation control system in accordance with the invention is 
practical and eliminates the need to compromise jet drive design to 
accommodate low speed acceleration. Jet drives can therefore be designed 
to better optimize high speed performance, while using a cavitation 
control system in accordance with the invention to eliminate impeller 
cavitation during acceleration at low speeds. Further, by implementing the 
invention, engines having higher power outputs can be used to power jet 
propelled watercraft without having to compromise system performance to 
account for impeller cavitation difficulties. 
Other features and advantages of the invention may be apparent to those 
skilled in the art upon inspecting the following drawings and description 
thereof.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows a personal watercraft 10. As previously mentioned, the 
invention has particular utility in small personal watercraft like the 
watercraft 10 depicted in FIG. 1, however, the application of the 
invention is not limited thereto. 
The personal watercraft 10 has a hull 12 and a deck 14, both preferably 
made of fiber reinforced plastic. A driver and/or passenger riding on the 
watercraft 10 straddles the seat 16. The driver steers the watercraft 10 
using a steering assembly 18 located forward of the seat 16. A throttle 
actuator 19 is normally mounted on the grip for the steering assembly 18. 
An engine compartment 20 is located between the hull 12 and the deck 14. A 
gasoline fueled internal combustion engine 22 is located within the engine 
compartment 20. The engine has an output shaft 23, FIG. 2, that is coupled 
via coupler 24 to a jet pump located rearward of the engine 22 generally 
in the vicinity of arrow 26. 
An electronic control unit 29 is provided within the engine compartment 20. 
The throttle actuator 19 actuates a throttle linkage, or communicates with 
the electronic control unit 29 as is known in the art, to adjust the 
engine throttle position in accordance with the position of the actuator 
19. The electronic control unit 29 controls the operation of the engine 
22. If the engine 22 is a carbureted engine, the electronic control unit 
29 controls the timing for the spark plug ignition system. If the engine 
22 is a fuel injected engine, the electronic controller not only controls 
timing for the spark plug ignition system, but also controls the timing 
and amount of fuel supplied to the engine. 
FIG. 2 shows a jet pump 26 implementing an impeller cavitation control 
system as in accordance with the invention. The pump 26 includes an intake 
housing 30 that is attached to the hull 12. The intake housing 30 has an 
inlet opening 32 that provides a path for sea water to flow into an intake 
duct 34 located within the intake housing 30. Sea water flows upward and 
rearward through the intake duct 34 to an impeller 38. The impeller 38 is 
rotatably driven by an impeller drive shaft 40. The impeller drive shaft 
40 passes through an impeller drive shaft opening 42 in the intake housing 
30, and is coupled to the engine Output 23 or crankshaft shaft via coupler 
24. As the impeller shaft 40 passes through the intake housing 30, the 
impeller shaft 40 is supported by a sealed bearing assembly 44. The 
preferred intake housing 30 as well as the preferred sealed bearing 
assembly 44 is described in detail in copending patent application Ser. 
No. 08/710,868, entitled "Intake Housing For Personal Watercraft", by 
James R. Jones, now U.S. Pat. No. 5,713,768, issued on Feb. 3, 1998, which 
is assigned to the assignee of the present application. 
External to the intake housing 30, coupling head 46 is threaded onto the 
impeller drive shaft 40. The impeller coupling head 46 is preferably 
driven by the coupler 24 through an elastomeric member 48, although other 
coupling techniques can be used in accordance with the invention. The 
preferred coupler 24, elastomeric member 48, and impeller coupling head 46 
are disclosed in detail in copending patent application Ser. No. 
08/735,325, entitled "Engine Drive Shaft Coupler For Personal Watercraft", 
by Jerry Hale, now U.S. Pat. No. 5,720,638, issued on Feb. 24, 1998, which 
is assigned to the assignee of the present application. 
The impeller 38 rotates within a wear ring 50 to accelerate sea water 
flowing through the jet pump 26. A stator 52 is located rearward of the 
impeller 38 and the wear ring 50. The stator 52 has several stationary 
vanes 54, preferably seven (7) vanes, to remove swirl from the accelerated 
sea water. After the sea water exits the stator 52, the water flows 
through a nozzle 56. As used herein, the term "jet drive duct" refers to 
the water flow passage defined by the combination of the intake duct 34, 
the wear ring 50, the stator 52, and the nozzle 56. The preferred 
construction of the stator 52 and the nozzle 56 is described in detail in 
copending U.S. patent application Ser. No. 08/710,869, entitled "Stator 
And Nozzle Assembly For Jet Propelled Personal Watercraft", by James R. 
Jones, now U.S. Pat. No. 5,713,769, issued on Feb. 3, 1998, which is 
assigned to the assignee of the present application. 
Sea water exiting the nozzle 56 is directed by rotating tubular rudder 58 
about a vertical axis to steer the personal watercraft 10. The reverse 
gate 28 is preferably mounted to the nozzle 56 along a horizontal axis. 
Alternatively, the reverse gate 28 can be mounted to a trimming gimbal 
along a horizontal axis. The preferred reverse gate mechanism is described 
in detail in copending patent application Ser. No. 08/783,440, entitled 
"Reverse Gate For Personal Watercraft", by James R. Jones, Peter P. 
Grinwald and Richard P. Christians, now U.S. Pat. No. 5,752,864, issued on 
May 19, 1998, which is assigned to the assignee of the present 
application. 
An inlet adapter plate 60 is connected to the intake housing 30 upstream of 
the intake duct 34 to adapt intake housing 30 to the hull 12 on the 
underside of the watercraft 10. A tine assembly 62 has a plurality of 
tines that extend rearward from the inlet adapter 60 to cover the inlet 
opening 32. A ride plate 64 is mounted to the inlet adapter 60 rearward of 
the inlet opening 32. The ride plate 64 covers the area rearward of the 
inlet opening 32 to the transom of the watercraft 10 so that the pump 
components are not exposed below the watercraft 10. The ride plate 64 is 
supported in part by a depending boss 66 on the nozzle 56. The preferred 
inlet adapter system, including the inlet adapter plate 60, the tine 
assembly 62, and the ride plate 64, are disclosed in detail in copending 
patent application Ser. No. 08/717,915, entitled "Inlet Adapter For A 
Personal Watercraft", by James R. Jones, now U.S. Pat. No. 5,700,160 
issued on Dec. 23, 1997, which is assigned to the assignee of the present 
application. 
The impeller 38 has a hub 68, and blades 70 which extend outward from the 
impeller hub 68. Preferably, the impeller 38 has three or four blades 70. 
The impeller blades 70 should be equally spaced and the impeller 38 should 
be balanced. The impeller hub 68 has an outer surface that diverges as the 
surface extends rearward. The impeller blades 70 angle rearward as the 
blades 70 extend partially around the hub 38. Each blade 70 typically 
extends more than one-quarter around the hub 38. An outer edge 72 of each 
impeller blade 70 is in close proximity to the inner surface of the wear 
ring 50. Both the impeller 38 and the wear ring 50 are preferably made of 
stainless steel. The preferred method of mounting the impeller 38 to the 
impeller shaft 40 is described in detail in copending patent application 
Ser. No. 08/719,621, entitled "Impeller Mounting System For A Personal 
Watercraft", by James R. Jones, now U.S. Pat. No. 5,759,074, issued on 
Jun. 2, 1998, which is assigned to the assignee of the present 
application. 
When the watercraft 10 is accelerating at low speeds, the pressure in the 
jet drive duct drops as the impeller 38 rotation speed increases to 
accelerate the watercraft 10. As the watercraft 10 speed increases, water 
ram pressure begins to counteract the pressure drop upstream of the 
impeller 38 caused by the accelerating impeller 38. Impeller cavitation is 
possible when the impeller 38 is rotating at high rates as the pressure 
drop in the duct immediately upstream of the impeller 38 peaks. 
Thereafter, impeller cavitation is unlikely. 
In accordance with the invention, a pressure sensor 74 measures the water 
pressure of water flowing through the jet drive duct (i.e. the intake duct 
34 and the wear ring 50) immediately upstream of the impeller 38. The 
pressure sensor 74 is preferably a mechanically actuated sensor including 
a diaphragm 74a. The bottom wall 76 of the wear ring 50 contains a 
pressure sensing access hole 78 therethrough. Various fittings or the like 
may be used to install the access hole 78, however, it is preferred that 
the access hole 78 be a cylindrical hole through wear ring 50 having a 
diameter of approximately 0.125 inches. The diaphragm 74a for the 
mechanical pressure sensor 74 is exposed to water flowing through the jet 
drive duct immediately upstream of the impeller 38 via the access hole 78. 
The pressure sensor 74 generates a water pressure signal in response to 
the measured water pressure. The water pressure signal is transmitted, 
line 80, to the electronic control unit 29. 
The electronic control unit 29 is programmed to immediately limit engine 
output power when the water pressure measured by the pressure sensor 74 
indicates that the onset of imminent impeller cavitation is probable 
unless engine power is limited. FIG. 3 schematically illustrates the 
operation of the cavitation control system 79 to limit engine output power 
and prevent impeller cavitation. The water pressure signal in line 80 from 
pressure sensor 74 inputs the electronic control unit 29 which is 
preferably programmed to clip ignition spark plug firing when the measured 
water pressure drops to or below a threshold cavitation water pressure, 
block 82. The electronic control unit 29 transmits control signals, line 
84, to the engine ignition coils which fire the engine spark plugs. 
Clipping ignition spark plug firing is the preferred way of limiting 
engine output power because it is important that engine output power be 
limited immediately upon detection that the water pressure has dropped to 
or below the threshold cavitation water pressure value. Typically, it is 
not necessary to clip spark plug firing for more than about one-half 
second to control water pressure upstream of the impeller 38 and prevent 
cavitation. 
Other methods of immediately limiting engine power output besides clipping 
ignition spark plug firing may be suitable or even more appropriate 
depending on the type of engine 22 used to power the watercraft 10. For 
instance, spark ignition coil be retarded in some engines to quickly limit 
engine output power. Also, the power output in some engines can be reduced 
by adding water into the exhaust stream, or by adjusting the timing of 
exhaust valves and/or configuration of exhaust ports. Further, less 
preferred methods of limiting engine output power such as limiting engine 
throttle position, limiting the amount of air supplied to the engine, or 
limiting the amount of fuel supplied to the engine may be suitable to 
immediately limit engine output power in some engines. 
The priority of the electronic control unit 29 is to operate the engine as 
normal without accommodating the cavitation control system 79, unless the 
water pressure measured by the pressure sensor 74 drops to or below the 
threshold cavitation water pressure value. Once the water pressure 
measured by the pressure sensor 74 drops to or below the threshold 
cavitation water pressure value, the electronic control unit 29 is 
triggered to immediately limit engine output power until the water 
pressure measured by the pressure sensor 74 recovers. 
The threshold cavitation water pressure is programmed into the electronic 
control unit 29 and is selected at a value slightly above the onset of 
impeller cavitation. The specific value of the threshold cavitation water 
pressure value depends on the configuration of the jet drive including the 
configuration of the impeller 38. The threshold cavitation water pressure 
value also depends on other factors including the power output of the 
engine 22, boat size and the like. For the embodiment of the invention 
illustrated m FIGS. 1 and 2, the threshold cavitation water pressure value 
is in the range of 7.5 to 8.5 psi below the nominal water pressure in the 
jet pump duct when the watercraft 10 is at rest. 
FIG. 3 also depicts an engine crankshaft rpm sensor 86. The crankshaft rpm 
sensor 86 is preferably a crankshaft position sensor as is known in the 
art. The rpm sensor 86 monitors the revolution rate of the crankshaft 23, 
and thus provides a measurement of the revolution rate of the impeller 
shaft 40. The rpm sensor 86 generates an rpm signal that is transmitted 
through line 88 to the electronic control unit 29. Based on the rpm 
signal, the electronic control unit 29 determines whether the impeller 38 
has cavitated. If the program in the electronic control unit 29 determines 
that the impeller 38 has previously cavitated, the electronic control unit 
29 automatically modifies the threshold cavitation water pressure value so 
that future impeller cavitation is unlikely. The ability to modify the 
threshold cavitation water pressure value is advantageous because damaged 
impellers 38 are more likely to cavitate than undamaged impellers 38. 
The pressure sensing access hole 78 through the wear ring 50 is located 
upstream of the location where the outer edge 72 of the impeller blades 70 
sweep around the inside surface 76 of the wear ring 50. It is desirable 
that the access hole 78 be as close to the upstream edge of the impeller 
38 as possible. Locating the access hole 78 farther upstream in the jet 
pump duct, such as locating the access hole 78 through the wall of the 
intake housing 30 into the intake duct 34, may be suitable in some 
applications but is less likely to provide an accurate prediction of 
imminent impeller cavitation. Locating the access hole 78 in the wear ring 
50 at the bottom of the wear ring 50 is desirable because that location 
provides the largest and most accurate water pressure fluctuations. 
However, depending on the hydrodynamics of the specific jet pump 26, it 
may be desirable to locate the water pressure access hole 78 through the 
top or the side of the wear ring. Placing the access hole 78 through the 
top or the side of the wear ring 50 may be advantageous in some systems 
because there may be less chance for the access hole 78 to fill with sand 
or the like. 
The foregoing description is a description of the preferred embodiment of 
the invention as installed in a personal watercraft. It should be readily 
apparent to those skilled in the art that the invention has utility to 
prevent cavitation in other types of marine propulsion systems. For 
instance, the invention may be used in marine jet drives for larger 
watercraft, in jet drives having vertically mounted impellers, in marine 
drives having propellers, and in highbred marine propulsion systems. It is 
recognized that other alternatives, modifications and equivalents of the 
invention may also be possible in accordance with the true spirit of the 
invention. Such modifications, alternatives and equivalents should be 
considered to fall within the scope of the following claims.