Engine height

About inboard or outboard motors.

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rbrandenstein
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Engine height

Post by rbrandenstein » Sun Sep 09, 2018 6:16 pm

I built my Malahini with the standard 20" transom height.
My 70HP Yamaha 4-stroke has a height of about 21.5", so the cavitation plate is under water when planing.
The bolt holes in the bracket are 1" apart.
So, if I move it higher, I would either have the plate above the water (2 holes up) or slightly under water (1 hole up).
Is one move better than the other? Or, should I drill new holes to get it spot on?
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hoodman
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Re: Engine height

Post by hoodman » Sun Sep 09, 2018 6:37 pm

Easier to try the other holes before you drill new ones.
Matt

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Bill Edmundson
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Re: Engine height

Post by Bill Edmundson » Sun Sep 09, 2018 8:19 pm

Bob

I vote 1 hole up.

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PeterG
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Re: Engine height

Post by PeterG » Mon Sep 10, 2018 12:46 pm

One hole up sounds like the way to go, see how it works and go from there. Does it affect how the upper part of the bracket clamps to the transom? Not familiar with your setup but I'm thinking probably not, or maybe a 1" shim at the top edge of the transom. Look at your steering too, for any effects from raising the motor.
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rbrandenstein
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Re: Engine height

Post by rbrandenstein » Mon Sep 10, 2018 2:25 pm

The engine currently rests on the transom but I believe it is really held by 4 large bolts. It could be suspended above the transom if necessary. However, I would cut some oak blocks to put under the engine shelf.
I will have an issue with the steering. I had to cut a notch for the current configuration. If I raise it, I will need to take out a little more of my motor well border.
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hoodman
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Re: Engine height

Post by hoodman » Mon Sep 10, 2018 2:54 pm

Stupid question maybe but what do you hope to achieve by raising it?
Matt

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Hercdrvr
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Re: Engine height

Post by Hercdrvr » Mon Sep 10, 2018 4:07 pm

I’m guessing Bob is looking to reduce drag by getting the plate out of the water while on plane.
My engine bracket over the transom is no where near resting on the boat, its a good 2 fingers above. To get the cav plate completely out of the water while on plane I had to mount it that high. My motor is also about 21.5” on a 20” transom. Zoom in on the pic to see how high motor is off transom. I do have a porpoising issue with a heavily loaded boat at high speed, don’t know if motor height has anything to do with it but I just want you to know the truth about my set up. Installing trim tabs soon to fix porpoising and to be able to plane at a slower speed.
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Hercdrvr
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Re: Engine height

Post by Hercdrvr » Mon Sep 10, 2018 4:14 pm

Bob,
It will be close, but you might be able to raise the motor just enough to clear the back deck altogether. I don’t have a block under my transom bracket, no problems as of yet but it sure wouldn’t hurt if you install one.
Matt B

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Bill Edmundson
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Re: Engine height

Post by Bill Edmundson » Mon Sep 10, 2018 4:29 pm

Go to any marina. You'll find many engines "floating" on the bolts. Apparently no big deal.

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PeterG
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Re: Engine height

Post by PeterG » Mon Sep 10, 2018 9:08 pm

So I see the motors above don't even have the hand screw clamps on them like the older motors, that's a good thing. Keeps them attached better to the boat! And the brackets don't need to rest on the transom. Bob, when I was researching my Malahini build I remembered from your blog that you had the steering clearance issue crop up when you replaced your outboard. Because you pointed that out I was able to check my setup and avoid a similar situation. Turns out the Malahini design met the Mercury installation dimensions nicely, and my steering system will have the clearance. Thanks for the heads up!
As far as cavitation plate height at the transom, I default to whatever the outboard's installation instructions say. And that can vary a lot between brands and models of outboards.
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hoodman
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Re: Engine height

Post by hoodman » Tue Sep 11, 2018 5:28 am

I thought the cav plate was "supposed" to be even with the bottom of the boat. On my Geronimo the engine is just barely touching the top of the transom and the cav plate is exactly even with the bottom of the boat. Just got lucky.
Matt

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billy c
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Re: Engine height

Post by billy c » Tue Sep 11, 2018 6:06 am

hoodman wrote:I thought the cav plate was "supposed" to be even with the bottom of the boat. On my Geronimo the engine is just barely touching the top of the transom and the cav plate is exactly even with the bottom of the boat. Just got lucky.
Correct, that is the place to start. Then you might find better performance going slightly above that point reduces drag and still does not cavitate. On my older motors there were holes for water pickup above the plate so not possible to move the motor up
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rbrandenstein
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Re: Engine height

Post by rbrandenstein » Tue Sep 18, 2018 10:48 am

Just a followup.
My bolt holes on the motor bracket are 3/4" apart and not the 1" I estimated.
So, I moved the motor up 2 holes for a 1.5" extra height. This puts the cavitation plate on plane with the bottom. This should reduce the drag of the motor plate when on plane.

At G12 I tried it for the first time. I believe I am getting a couple of mph additional speed from the new height. I did have to notch my deck to allow the steering arm to clear. I also put a block of white oak on the transom to support the motor, but as others have mentioned, it is probably not necessary.

Thanks for all suggestions and comments.
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