Outboard mounted to high or to low?

ugadawg187

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Ok so i originally had a 20" shaft motor on my boat & the prop didnt sit below the bottom of the transom. I was told it should since its a 18ft bowrider & not a bassboat but when i would hammer down on the throttle the motor would rev up but barely move. I was told the prop was blowing out (cavitating) because it wasnt low enough so when i repowered the boat i got a 25" shaft motor which really gets the prop below the transom. Was that the right choice to make or was i way off to begin with?
 

JB

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Re: Outboard mounted to high or to low?

The prop must be below the bottom of the transom. Actually, we look at where the anti-ventilation plate (often call a cavitation plate) is relative to the bottom. Ventilation is what your prop was doing. . . .grabbing air and trying to run in the resulting foam.

The AV plate should be even with to about an inch higher than the bottom of the transom.
 

ugadawg187

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Re: Outboard mounted to high or to low?

You can kind of see from this pic how low the foot is below the transom. Is the shaft to long or would it be ok? Thx
 

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Fl_Richard

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Re: Outboard mounted to high or to low?

It looks a couple of inces deeper than it should be but not horrible. A 20 inch would be too short. Can you raise it up on the bolt holes a few clicks?
 

JimS123

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Re: Outboard mounted to high or to low?

Sounds like you are getting bad advice. Bowriders and Bassboats all use the same rigging concepts.

Just measure your transom and see what height it is.

As someone else said, the cavitation plate should be about 1" below the bottom. On some boats, the hull design will allow raising the motor a little to improve speed and economy.

Determine the performance curve for your existing rig. Plot rpm vs. mph. Then, raise the motor one notch and repeat. Optimum setting will give you max sped. Be sure that you don't get cavitation. If you do, drop one notch.
 

ugadawg187

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Re: Outboard mounted to high or to low?

When trimed all thw way down the cavitation plate tucks in right below the transom. They maybe only 1/2 inch of clearance between the two at best.
 

kenmyfam

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Re: Outboard mounted to high or to low?

How about a close up photo of what you have right now ???
 

ugadawg187

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Re: Outboard mounted to high or to low?

As soon as i get a chance to take a pic i will post one so you can get a better look. It may be a few days before i get one up the weather is completely nasty here.
 

ugadawg187

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Re: Outboard mounted to high or to low?

Okay so KENMYFAM i got the pics you wanted to show a better look at the transom. Hope these help.
 

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Expidia

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Re: Outboard mounted to high or to low?

Okay so KENMYFAM i got the pics you wanted to show a better look at the transom. Hope these help.

You are way off. That first plate just above where the prop goes should be even or 1 inch above the transom.

You might need to install a jack plate if you don't have enough holes to raise it up.

Oops, I did not see the other threads where you were asking Kennyfam's advice. Sorry, he's probably watching the kickoff!
 

hwsiii

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Re: Outboard mounted to high or to low?

Dawg, it looks way too low, but I an't tell from the photo.

Take a picture more like this and get down where the camera is level with the anti ventilation plate.

9. Anti-ventilation Plate height above the bottom of the transom of boat if it is an outboard in inches, use a straight edge or a board under the keel and sticking out to the anti ventilation plate for a reference, and take about 4 pictures for us to see.

AntiventilationPlateStraightEdge-1.jpg


If there is water coming up the back of the transom, that is a guarantee the motor is too low.


H
 

ugadawg187

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Re: Outboard mounted to high or to low?

Wow im confused some people are saying the cavitation plate should 1" below the transom & others are saying it should be 1" above. Which is right?
 

ugadawg187

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Re: Outboard mounted to high or to low?

Okay HWSIII i got what your saying & thanks for the pic it explains things clearly. I need to raise the motor up prob about 2 or 3 bolt holes so not a big deal. Thx for all your helps guys.
 

hwsiii

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Re: Outboard mounted to high or to low?

I am glad to be of assistance Dawg, and it should be about 1" above for maximum speed.


H
 

ugadawg187

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Re: Outboard mounted to high or to low?

I just wanted to explain the picture cause the motor isnt trimmed completely down & its not the best angle.
 

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Frank Acampora

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Re: Outboard mounted to high or to low?

My experience with several older Deep Vee Glastrons is that it is possible to run quite nicely with the cavitation/anti-ventilation plate one inch higher than the apex (point) of the vee. You DEFINITELY do not need it one inch or more below the transom-- just more drag and less speed for the fuel you burn. BTW: This is measured with the plate parallel to the bottom, not at full tuck in or trim out.

The cavitation plate is the one that runs all the way to the end of the engine. the one above it and only toward the front of the engine is an anti-splash plate and keeps water from running up the leg and into the splashwell at speed.

It is very difficult to see but from the photos, it still looks like the engine is a couple of inches too low. If you measure the transom from the apex of the vee to the splashwell, 21 inches or so is for a 20 inch engine length. (Needs to ne longer than 20 because it is on an angle)

BTW: That engine has a 5 inch extension on the midleg. You can easily convert back to 20 inch by removing the extension and getting a shorter driveshaft. (you may possibly need a new water tube too, but there may only be a 5 inch extension on that also) Buy one (shaft) from a junker though--the new ones are horribly expensive.

Whoever initially counselled you should be ostracised. Before buying a 25 inch shaft engine you should have been told to try a CUPPED prop. Some engine/hull combinations will not run with an uncupped prop.
 

ezmobee

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Re: Outboard mounted to high or to low?

I swear that motor looks like it has some sort of shaft extension on it. And yeah, definitely too low.
 

ugadawg187

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Re: Outboard mounted to high or to low?

The only reason i went with a longer shaft was that the last motor i ha on the boat had a 20" shaft & when taking off the rpm's would rev up but the boat would barely move & i was told the prop was cavitating or blowing out. I couldnt lower the motor on the transom as it was already in the last bolt hole slot. Now from what you guys say this motors shaft is to long so thats another problem. The last prop i had on the boat with the 20" shaft motor appeared to have scorched places on it from the exhaust caused from what i was told was it cavitating...
 
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