mounting height, to move or not to move

yazoo

Recruit
Joined
Jul 1, 2013
Messages
1
I have a 15 ft flat bottom aluminum boat (what I call a suped up jon boat) with a yamaha 9.9 4 stroke on the back. This thing runs great, but as with anything else, I wouldnt mind getting a few more mph out of it. As the motor is currently mounted, the anti vent plate is about 1.5 inches below the bottom of the boat. My question is if I move the motor up to where the anti vent plate is even with the bottom of the boat or maybe just a bit higher will that speed me up much more, or do I just need to leave well enough alone? The motor does a fine job and is very fuel efficient, but so many people have told me to move the motor up. I just want to know if it will be worth the time invested. Thanks in advance for your help.
 

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
Re: mounting height, to move or not to move

Page 3-2 of the Yamaha owner manual says to mount it from 0--25mm(1 inch) below the bottom.

If you don't have one, you can down load a manual from their web site.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: mounting height, to move or not to move

Your prop size,rpm and gps speed will tell us if there is room for tinkering.A tinytach will get the rpm some cells have a gps app.
Your prop size and gps speed will help a little.
Wont hurt to raise it but may not gain much.Just lay a 1" stick on the top of the transom and tighten the clamps.
Just be sure they have a good grip.
 

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,605
Re: mounting height, to move or not to move

When you raise an OB engine, what you are really doing is reduction its' drag and therefore top end speed goes up. BUT, the amount of improvement is not predictable and each situation is different. You can try what "steelespike" suggested but make very sure you do tighten down the engine before trying it again... Let us know your results...
 
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