What Causes Cavitation?

[Wcsweb]

I had my boat out on the lake today for the 1st time it has a 1988 20ft proline 115 V4 OMC Seadrive.

I'm very familier with jet pumps and Cavitation and I'm trying to figure out what's going on.

OK, here is what it does.

About 3/4 throttle just before its about to plane out she revs up higher but slows down. If I can get her to plane before it spazes on me it almost feels like it gets traction again.

The other thing it does when I hit full throttle always just before it planes the motor like looses all power. Sometimes if I pull the throttle down to about 3/4 it will run but still cavitation and die's out when I go into it full throttle.

Do I have 2 separate issues going on here or is it one issue causing the other?

Thank you for any knowledge you can share.

 

[ebry710]

Re: What Causes Cavitation?

http://en.wikipedia.org/wiki/Cavitation for a photo

Ventilation and Cavitation
Ventilation occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving.

Anti-ventilation Plate: Large plate cast into gearcase housing directly above propeller. Helps reduce surface air from being pulled into blades.

Cavitation: The aeration (bubbling) and boiling effect of water caused by creation of a low pressure area. Generally caused by a solid shape (propeller blade) passing through the water, in such a position and speed, that a low pressure area is formed due to the inability to move through the water in nonresistant manner. An example is, a propeller blade that has a rough edge would not cut efficiently through the water, thus creating a low pressure area. If the pressure drops below the vapor pressure, a cavitation bubble will form in that region. These bubbles will collapse when they reach the higher pressure region of the blade. This causes a rapid change in pressure and can result in physical erosion. You may notice burns (erosion) at some area on the face of the blade.

Common rules of thumb:

Ventilation can lead to excessive slippage.
Cavitation can lead to ventilation and/or slipping.

I had this question back in March. This was the best explanation at that time.

 

[Wcsweb]

Re: What Causes Cavitation?

Thank you I was looking for a little bit more specifics.

What would cause my symtoms and what part or parts that can go bad in an outboard motor that would cause cavitation?

 

[Dunaruna]

Re: What Causes Cavitation?

You are experiencing slippage caused by VENTILATION or a spun prop hub.

If it is ventilation, air (from the surface) is introduced into the prop area, usually caused by a motor that is mounted too high or trimmed incorrectly.

If it is a spun prop hub (very common), the prop blades are spinning slower than the prop hub, this is caused by age and torque. Use a waterproof marker, put a line on the prop blades and a corresponding mark on the hub. Go for a WOT run. When you get back to the dock, see if the marks are still aligned.

 

[ebry710]

Re: What Causes Cavitation?

You are experiencing slippage caused by VENTILATION or a spun prop hub.

If it is ventilation, air (from the surface) is introduced into the prop area, usually caused by a motor that is mounted too high or trimmed incorrectly.

If it is a spun prop hub (very common), the prop blades are spinning slower than the prop hub, this is caused by age and torque. Use a waterproof marker, put a line on the prop blades and a corresponding mark on the hub. Go for a WOT run. When you get back to the dock, see if the marks are still aligned.

I agree with duraruna.

Overpowering or wrong prop causes cavitation. It's kinda like doing "burnies" on a hot rod.
Ventilation is cause by exhaust gases or surface air causing an air bubble around a prop.

To me, either one is just a symptom if this problem so try duraruna mark and look at your engine/trim setup.

 

[Dunaruna]

Re: What Causes Cavitation?

. edited due to brain fart moment

 

[F_R]

Re: What Causes Cavitation?

Thank you I was looking for a little bit more specifics.

What would cause my symtoms and what part or parts that can go bad in an outboard motor that would cause cavitation?

The distinction always comes up, ventilation vs cavitation. From a practical standpoint though it makes little difference. It is air around the prop, just a matter of how it got there. In your case, I would hazard a guess that you have a high performance boat or a wannabe high performance boat, in either case the motor jacked up as high as you can get it. Race boats run with the propeller half way out of the water. That gives every last drop of top speed, but race boats are also dang near impossible to get up on plane because they get so much air around the prop from the surface. It's a trade-off, you wanna go fast or you wanna go without all the slipping?

 

[emdsapmgr]

Re: What Causes Cavitation?

Follow Dunaruna's advice and check for a spun hub on the prop first. If you find the hub is not the problem, look at the prop. What type prop are you running? Aluminum props don't have the cup and rake required to bite with the engine mounted high on the transom. Even the SST props are borderline. A good stainless prop like a raker or a renegade bass will bite. Personally, I prefer a high mounting and run a high tech prop to compensate. When you are on plane, and trimmed out, how far under the antiventilation plate is the water flow?

 

[Wcsweb]

Re: What Causes Cavitation?

Thank you for all the replies. I would say its a 1/2 to a foot under water when it does it and I'm trimmed all the way down. Here is a pic of the set up.


I think it is a aluminum prop.

 

[bowman316]

Re: What Causes Cavitation?

as a rule of thumb,
If you anti-ventelation plate is at or below the bottom of the transom, then your motor is not too high. You cant the plate about level with the bottom of the boat, and lower will not hurt.
make sure the nut holding the prop on is as tight as you can get it.

 

[emdsapmgr]

Re: What Causes Cavitation?

You will start to have prop ventilation problems with a high engine mounting when the water runs an inch or more below the antiventilation plate-when the boat is planed out and fully trimmed.

 

[Opus_X]

Re: What Causes Cavitation?

I had a similar problem with my prop. I have the Flo Torque 2 hub system and if you also have this hub system you need to make sure the hub is completely seated all the way into the prop. otherwise your prop may be slipping.

 

[Silvertip]

Re: What Causes Cavitation?

When talking about engine height, we mean the position of the antiventilation plate in relation to the bottom of the boat. The AV plate is the flat plate directly above the prop. Trimmed down, that plate should be flush or an inch or so above the bottom of the boat (at least to start with). We can't tell its actual position from the picture but it would appear with the engine tilted down that it is too high. But then the interesting tilt unit provides some setback so all bets are off as to what height works best.