prop cavitation

R

Roadrunner

Guest
Hi,

I just had a new to me Merc 115 outboard put on my 18 foot Hourston. The dealer installed it and I expected that they would have done it correctly. It turns out I needed a longer shaft (This one is 20 inch). The cavitation plate is 1/2 an inch above the bottom of the hull (the dealer said this is not a problem). It runs good for the most part but if I turn hard or go over really big wakes I get bad prop spin and have to stop and get going again. It currently has a 3 blade 17 pitch prop. I am getting 5300 RPM at full throttle. Is there any way to reduce the cavitation further or do i just have to live with it. The dealer is not assuming any responsibility.

Any suggestions would be greatly appreciated.
 
A 4 blade prop reduces cavitation. Try one first before buying to make sure it can solve or reduce your problem.
 
If that dealer sold you the motor he should be hung by the knackers and drug behind your boat in shark infested waters.
I have a friend that bought a motor and boat like that and when we went back to the dealer he apologized profusely and proceeded to install a better motor with the right shaft length at a price that could not be beat.:D:D

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There are usually 3 mounting holes on the motor mount. Has the dealer installed the engine in the lowest mounting position (top hole)? If not, they can at least get it mounted a few inches lower.

1/2 inch above the hull bottom sounds about right. Ideally, the cavitation plate should be slightly above the waterline while the boat is on plane. Too low and it sprays water into the transom, too high and you can cavitate.

The recommended rough guideline for the height is 1" up for every 12" back your engine is from the transom - i.e. cavitation plate should be 1" above the hull if it is 12" away.

If the engine cavitates too much, you will ruin the engine pretty quick as it over-revs. I'd be taking it back and getting it done right if the measurements don't work out.

TenMile
<'((((><
 
It is mounted as low as it can go. It only cavitates on very sharp turns (which I can avoid) and really large wakes (tugboat wake last night) but I agree I should be able to take it back. I will try again.

Thanks for the advice.
 
At the worst you can get an adjustable transom bracket that should put you in enough water to stop cavitation.:D

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Try to put on those dol-fins - they could do the trick for you if the dealer does not want to cooperate.
 
Thanks for all of the advice. I went with a 4 blade stainless with lots of cupping and it had solved the ventilation issue. The prop I had on was actually a 19 3 blade. The new prop is a 17 4 blade.
I can get more than my max RPM's with this prop - I am guessing 5800-6000 and my engine tops out at 5200. Given all of the good advice about props on the other thread I will stick with this one and not exceed 5000 RPM.

It is surprising to me but I am getting more speed with this prop. I guess it gets more grip in the water.

Thanks again.
 
Who the dealer that sold you the wrong motor, You should have gotten a longer leg so the water can't wash back into the motor exhaust if you stop quick, also the cavation plate should be even with the bottom of the hull as per manufacture installiation instructions
 
PS I've have a mercury 115 on a 17 double eagle very similar and the exhaust is very close to the water with a 25' in leg , and check mercury installation manual .
 
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