Advice re cavitation

[stupiddog]

I have a 37' houseboat with a 130 hp johnson / 25" shaft. the prop is a 15 X 10. The engine is mounted on a bracket and is secured to the bracket just high enough to keep the engine bracket out of the salt water when sitting still at anchor.

Normally I do not have any issues with cavitation, but on occassion when it is rough in a following sea, I will get some cavitation when the stern lifts. Would adding a wing on the anti cavitation plate (one that extends back above the prop) help out for those situations? I am generally moving at around 6 to 7 mph and running the engine at around 2500-2800 rpm.

I am not looking to gain lift, which probably would not happen at those low speeds anyway, just minimize the cavitation. At no point does the prop really come close to breaking the surface. But at that slow speed (I call it sailing without tacking) the large prop can suck some air in a following sea. Any thoughts?

Thanks, scott

 

[steelespike]

Re: Advice re cavitation

Are you running a 4 blade prop?Is the prop in good condition?
I realize trim has little effect on that type application but have you tried different trim settings.
Is the bottom clean and smooth?
A plate of some sort may help.I wonder if a trolling plate in the retracted position would
work better than an unecessary foil?

 

[JB]

Re: Advice re cavitation

What you are getting, SD, is prop ventilation. The prop is grabbing air from the surface, making foam, and then spinning in the foam.

Where is the antiventilation plate (usually incorrectly called a "cavitation plate") relative to the bottom of your hull? I think I would run it at bottom level, or at least try it to see what results I get. Don't set engine height to any criterion other than prop and AV plate relationship to water level underweigh.

I don't know if a hydrofoil would help your situation. I would try re-rigging the engine first.

Good luck.

 

[sasto]

Re: Advice re cavitation

If you have trim tabs.....A few boats react well in a following sea with the trim tabs down. I know this throws logic out the window as you stated the stern lifts causing your problem. With the tabs down in a following sea, the force of water may hit the topside of a tab causing the stern to lower. I have found this technique works with some boats but is determental in others.

Good Luck!

 

[stupiddog]

Re: Advice re cavitation

I don't have trim tabs on this tank, but I see what you are saying. The prop is three blade in good condition. A trolling plate retracted just might be worth trying. Anything that extends back above the prop is what I'm thinking. And JD, you're correct, the prop is sucking from the surface.. so the condition is ventilation. I always learn something here!

I am looking for a solution other than rigging the engine lower because I don't want to have it sitting in the salt water all the time. On my first engine, which was rigged a bit lower, I ended up having a good bit of corrosion on the lower part of the bracket after several years. I actually ended up having to replace the brackets because I was concerned about the integrity of the lower portion that the trim tilt unit pin goes through.

The anti-ventilation plate is probably just above the bottom of the hull and sitting around 2 1/2 ft. back on the bracket. The boat has very little dead rise at the stern. Basically a fiberglass barge look from half way back from the bow. See pictures. When the water and fuel tanks are full, the bottom of the engine mount brackets barely clear the water. Lowering the engine would let the bracket sit in the water when fully loaded. The jack plate is fixed in it's position. If I could lower it under way, that would solve things.... too much $$ for one of those right now.

 

[steelespike]

Re: Advice re cavitation

While the prop looks sound it does appear to have some wear.Instead of a new prop you could have a little pitch removed and some cup added.Find a good prop shop explain your problem.
be sure they understand the speed and wouldn't hurt to know your max wot rpm.Even though
you seldom run wot your motor needs to be able to reach its max rpm. 5,500 at wot.