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Home🌌Off TopicsOff Topics Main Discussion › What can I do about cavitation on power boat
06-24-2018 09:10 PM  14 months ago
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datidun

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What can I do about cavitation on power boat
Having a bit of trouble with my new sports boat,it has a Yamaha 20hp outboard engine,I have had it just over a year and every time out on the water when I turn sharply I get cavitation and a nasty roar from the engine from the prop slipping on the air bubbles,today I tried it at full low trim and only having a speed of 5 knots and it over reved again, I spoke to the dealer and they sent me out a smaller prop, but its still doing the same thing, after doing research I read about cavitation plates and the the other thing to try is installing impeller inducer,can anybody please send me in the right direction,the boat was all bought brand new ready to go,what should I do?
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06-24-2018 09:20 PM  14 months ago
Gearhead

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I would think you would need a larger prop with less pitch, remember a boat prop and a plane prop work on the same principlesJim
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06-24-2018 10:07 PM  14 months ago
datidun

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The dealer said a smaller prop would be better and improve performance,which it has, except when turning, cavitation kicks in,the first prop was bigger.
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06-25-2018 02:34 AM  14 months ago
Gearhead

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I understand that, but a larger prop with less pitch is more efficient, another thing you can try is to lower the prop deeper in the waterJim
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06-25-2018 03:13 AM  14 months ago
spaceman spiff

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06-25-2018 07:51 AM  14 months ago
datidun

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The prop was at its full low trim, it was all the way down, it cant go any lower,is this cavitation a usual thing?
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06-25-2018 06:08 PM  14 months ago
Pistol Pete

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Could be a motor mounting height issue as well, so it may be too low.

The anti-ventillation plate should just about be even or just a tad below bottom of boat.

Easily measured with a yardstick sticking out from bottom of stern which should run on top of anti-ventillation plate.

Propeller shaft (trim) should be parallel to bottom of boat as well.

How many blades on prop? Going with 3 can also help.
~~Enjoying the hobby one flight at a time~~
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06-25-2018 06:44 PM  14 months ago
datidun

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Yes 3 blades on Prop,slipping usualy occurs when the trim is to high in this case its full low.
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06-25-2018 08:54 PM  14 months ago
Gearhead

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hey, I just thought of something LOL, look for the problem on YouTube LOL

well the link won't work,, so go to YouTube and search "about cavitation on power boat",, there's a bunch of videos on it
Jim
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06-26-2018 01:31 AM  14 months ago
Pistol Pete

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datidun...
slipping usualy occurs when the trim is to high in this case its full low.
I'm referring to motor mounting height as in it may be "mounted" too low or perhaps you purchased too long a shaft for the boat.

The anti-ventillation plate is right below the red line on the photo below.

Get a yard stick or equivalent, place flat part along bottom of boat and make sure it should be same as the red line in photo. This assures proper motor mounting height as well as proper tilt-trim. PARALLEL is the key word.

If the anti-ventillation plate is deeper in the water in relation to the bottom of the boat, it causes your issues.
~~Enjoying the hobby one flight at a time~~
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06-26-2018 08:41 AM  14 months ago
datidun

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Thanks Pistol pete, will do that,I have to take it in so it can be water tested by this guy and his says it may need a Wedge plate for the transom, Im not to sure about those, thanks Gearhead will have a look.
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06-27-2018 11:09 AM  14 months ago
datidun

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This seems to be a Ventilation problem and not a Cavitation problem after watching a lot of videos, there is no marks or any paint come off the prop its like new, I came across an outboard Hydrofoils that seems to help for props sucking in air/ventilation, anybody tried one of those?
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07-01-2018 10:02 AM  14 months ago
oballzz

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excessive trim?Music is probably the only real magic I have encountered in my life,it's pure and it's real,it moves,it heals..TP
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07-01-2018 05:01 PM  14 months ago
datidun

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What do you mean?
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07-02-2018 09:30 AM  14 months ago
oballzz

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take a pic of your boat transom,....on the trailer that will help usMusic is probably the only real magic I have encountered in my life,it's pure and it's real,it moves,it heals..TP
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07-02-2018 09:56 AM  14 months ago
datidun

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I did the water test with the mechanics, the answer is a ventilation problem no water around the prop causing her to slip and rev, I have to fit a Hydofoil which should be the cure but Im not keen on holes been drilled through the plate so close to the Anode, Im told by the dealer its normal and wont be a problem, I have seen the non drill ones but made for rather larger engines, wonder if there is a way to rather glue it on with something like Sikaflex marine adhesive very strong flexable stuff, would rather try something before drilling.
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