# Cavitation plate depth?



## Batt34786 (Apr 7, 2011)

Is there a rule of thumb on the depth of the cavitation plate on an outboard in relation to the bottom of the transom? If it's too deep, can it adversely affect performance/speed? My old Gheenoe has a small, narrow transom - looks similar to a NMZ, but transom appears to be shorter. I've got a Mercury 8 hp 4 stroke with 15" shaft (and hydrofoil) but it seems really slow.


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## AfterHours2 (Aug 2, 2010)

I try to keep my cav plate flush with the bottom of the transom. Too high and you get purpoising depending on your load and layout of boat. I've ran a short shaft 8 hp on a highsider for a short time and it would run around 15-17 with a light load. Not fast but it was very efficient on fuel. Since then I have upgraded to a 15 and the difference is huge. You might want to look into a little more hp if speed is what you are looking for. Good luck....


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## Brett (Jul 16, 2008)

The less lower unit there is in the water, the less wetted surface you have, the less friction you end up with.
Cav plate height depends on the hull shape and propeller.
Flat bottom hulls trap aerated water under the hull and this makes it difficult
for factory props to get a clean bite in the top inch and a half
of the water coming out from under the transom.
In this situation the engine is usually mounted so that the cav plate
(really the anti-ventilation plate) is 1/4 inch below the bottom of the hull.
On a vee hull the starting height is with the cav plate 1/2 inch above the bottom of the hull.
The reason being that the water coming out from under the center of the hull
has less aeration due to the vee shape.


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## Batt34786 (Apr 7, 2011)

Thanks, guys. I'll gps my speed in the current configuration and then after raising the motor and see what the delta is. Then I'll ask Santa to bring me a 15 hp Yammy.


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## paint it black (Nov 3, 2007)

Don't mean to steal the thread or nothing, but my cavitation plate sits an incher lower than the hull bottom. 
I was told it should be an inch above.
Would this improve performance? If I move it up I mean.


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## Brett (Jul 16, 2008)

Deeper means better bite at hole shot.
Higher up means better top speed by 1-2 mph.
Is it worth worrying about? How often or long do you run all out?
My typical speed on the water is about 3/4 throttle, the comfort zone.


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## paint it black (Nov 3, 2007)

> Deeper means better bite at hole shot.
> Higher up means better top speed by 1-2 mph.
> Is it worth worrying about? How often or long do you run all out?
> My typical speed on the water is about 3/4 throttle, the comfort zone.


I run wide open all the time.
I need the extra speed. 
My top speed decreased by 3-4mph when I went from an uncupped stainless 10 pitch to the cupped 9 pitch stainless.
I was hitting 29-30 on the regular. even hit 31.9 with the 10 pitch. I'm running 26-27 now. sometimes 28, and if the stars allign, 29.
I need more speed. I feel like I'm crawling.....lol


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## Brett (Jul 16, 2008)

So when are you installing the power jack plate, low water pickup and cleaver?


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## firecat1981 (Nov 27, 2007)

why did you swap props?


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## paint it black (Nov 3, 2007)

> why did you swap props?



I was running it with the 10 pitch and getting those numbers still well under the RPM range the tohatsu specialist wanted it running.
he wanted it around 6000rpm's. I was getting around 5400 rpm's out of the 10 pitch. 
So I had it repitched and cupped locally. what a mistake that was...... rpm's have gone up slightly but lost performance. 
Only upside is that I can still run 26mph with three adults on board. with the 10 pitch I was running 24-25 with three adults. but 29 all day with two adults.


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## firecat1981 (Nov 27, 2007)

I'd go back to the 10 piitch with a mild cupping and raise the motor an inch or 2. I'd bet you will break 30 easily.


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