Cavitation Cause and Remedy

[cat man do]

Took the new tender out for its first run yesterday with just me onboard.
When I open her up the bow points to the sky, it accelerates well but just as it hops on the plane there is a massive amount of cavitation around the prop before biting in and off she goes up to 25 knots in what I would consider to be normal trim for a 4m tinnie with a 2 stroke 20hp on the back.

From stopped and powering up into a strong breeze, she bogs down, bow pointing to the sky, wouldnt get on the plane, initially making me think there was a throttle problem but as soon as I turned and headed downwind, she accelerated like a madman, cavitating again at about half speed. Once on the plane I of course turn and continue up into the wind.

Shifting 25 litres of fuel to the front of the boat (was at transom) helped dramatically with its behaviour (less sky pointing) but the cavitation was still happening on powerup and this is the issue.

The cause.
The prop while generally appearing to be in good shape, does have some burs and is not sharp on the "cutting" edge. Could this be the problem?
Perhaps I have the wrong prop?

Thoughts?

[neilpride]

Fit a couple of dolphin foils and say good bye to cavitation unles your engine fit to hig in the transom!

[ShaunJ]

It sounds to me like the prop is not pitched right. I would try a different prop with more pitch and see it that helps.
I guess you don't know the RPM's, that would tell you right away.
If the prop has a rubber hub, you may want to check to see if it is blown out.

[Bash]

Probably a trim problem. Adjust the pin forward so that the engine is further down. Do this one notch at a time to find perfect trim. And do this before you try something like bolt-on foils.

Basic rule: engine up, bow up. Engine down, bow down.

[atoll]

Originally Posted by Bash

Probably a trim problem. Adjust the pin forward so that the engine is further down. Do this one notch at a time to find perfect trim. And do this before you try something like bolt-on foils.

Basic rule: engine up, bow up. Engine down, bow down.

have same prob with my 25hp on my very light 3.2m rib,adjusting the leg fwd has more or less sorted the problem,though should really be using a 15 hp on such a light dinghy...............

[Cheechako]

Is it possible the hub in your prop is shot and not cavitation?
I tried dolphin foils because people were saying they were cool. Eventualy took them off, pretty ungainly etc and didnt see any effect. But then, I didnt have a problem in the first place! A 4 meter with 20 hp ought to get on plane immediately.... with or without foils I would think!

[cat man do]

Originally Posted by Cheechako

Is it possible the hub in your prop is shot and not cavitation?

I seriously doubt it.
When you look back you can see the big boil where the cavitation was.

I tried dolphin foils because people were saying they were cool.
Eventualy took them off, pretty ungainly etc and didnt see any effect.

Same here, I may still have a set in the back shed from several boats back.
They cost me money so I cant imagine I threw them out.

But then, I didnt have a problem in the first place! A 4 meter with 20 hp ought to get on plane immediately.... with or without foils I would think!

And it does.
Only time it didn't was with all weight at back and 20knots of wind on the nose (funnelling up river).
Once weight was shifted forward it was fine, but still cavitated at about half revs, just for a second or two before biting and off she'd go.
This is why I thought it may have been the burs on the bighty edge of the prop.

I'll file the burs and see how it goes on the weekend with the financier in the front seat.

[Cheechako]

Might just be too much HP for that light boat when you gun it..? If it's coming out of the hole good and going that fast... "Don worry boutit"

[rodney_k]

The anti-cavitation plate on the outboard should not be above the bottom of the boat. If the outboard is too high, the anti-cavitation plate will be above the bottom or keel and the prop will ventilate or cavitate.

[cat man do]

So after a bit of research down the boat ramp and asking a few questions it seems that foils, as some suggested here are the answer.
Getting on line showed that Permatrim Foil Stabilizers seemed to the popular choice that produced results.
Dragged out some ally plate and jigsaw and carved out a set and with some clamps and a hammer did some folding.
And it worked a treat.
Boat leaps on the plane now with me and 40 litres of fuel at the back and no weight fwd and actually rides level.
I can back off to about half throttle and stay on the plane and level.
Boat tracks straight even when letting go of the tiller.

Result.

[Wotname]

Well sorted !