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Outboard Motor Height
LSF Member
Joined:
07/18/2013
Posts: 517
Just information sharing, cuz I wouldn't have believed the difference if I had not done this myself.

I have an Alumacraft Lunker LTD V16 with a Yammy 25hp 4-stroke. The boat always had a lot of bow lift on take off, and took a while to plane out, and a top speed of around 20mph with 2 people, normal fishing gear, 10-1/2 pitch OEM prop, at about 5300RPM, my motor is rated at 5000-6000RPM max. Have been contemplating upgrading to a 40-50hp, but I spoke with a boat mechanic and he suggested I check my motor height in relation to the hull.

He said the ventilation fin, the fin just above the prop, should be equal with the hull. So if you put a straight edge on the hull extended back to the motor, it should be inline with the ventilation fin. Well, mine was about 2" lower than that. My motor had 2 sets of mounting holes, coincidentally 2" apart. So, I raised my motor that 2", and put a spacer in on top of the bow, just because. That made my ventilation fin about 1/8" above the hull line, close enough.

This past weekend I went out Crappie fishing, and to test out the new setup. Well, was I shocked. First - no bow lift hardly at all, and it plane out very quickly. Boat ran on plane awesome, and when I looked down it was going 24mph, a gain of 4mph. But, I glanced at the tach and it was at 6300rpm, over the max limit. I then put it into some tight turns and also had ventilation spin out, which is a problem. But I could not believe 4mph speed increase and 1000rpm increase just by raising the motor.

I had an OEM 11-1/4 pitch prop, so I changed to that prop, and I maintained everything, speed at 24mph but dropped to 5800rpm, so more in the correct range. But still had ventilation issues when tight turning.

I was told by the mechanic that a 4 blade prop will help with the ventilation, and I priced one out. But if I go with a new prop I will go Stainless Steel, and trying to figure out the pitch of the prop going from aluminum to stainless, and from 3 blade to 4 blade it too much for my brain.

So I removed the 2" spacer, put in a 1" spacer, dropped the motor down, and drilled new holes in the motor mount plate in the middle of the 2 that were there, to split the difference. Most motors have the holes spaced already at 1" about, not sure why Yammy did not have that hole to begin with. I then put the original prop back on. Now I just have to get it out and test it again. Hoping it drops the RPM below 6000, maintains the other benefits, but gets rid of the ventilation.

Really just sharing the info in case others kind of have the same issues. Checking the ventilation plate height with the hull is pretty easy. But, different hull designs, different set back distances, and other dimensions factor in as well. But it definitely helped in my application.

Posted on: 5/9 9:08:28
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Re: Outboard Motor Height
LSF Member
Joined:
01/09/2008
From Mn
Posts: 3135
The FIN above the prop is a cavatation plate and should mounted 1 inch below the boats transom bottom.
Lower than 1 inch creates drag,higher creates ventilation (sucking in air)reducing prop push

Posted on: 5/9 10:20:19
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Of all the things I've ever done,
I aint never had Too much fun.
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Re: Outboard Motor Height
Staff Team
Joined:
02/19/2008
From Bloomington
Posts: 3481
"you learn more here by mistake than you do on purpose"

Good to know!

Posted on: 5/9 13:26:27
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Yes, Yes that is a 39" Pike.
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Re: Outboard Motor Height
LSF Member
Joined:
12/14/2014
From Duluth
Posts: 181
X2,,,I wonder how many of us went out and ck'd ,,,like me. Thanks for throwing it out.

Posted on: 5/9 14:56:12
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