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Author Topic: What to do when boat "Porpoises" under power  (Read 2749 times)

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Offline jcn59

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What to do when boat "Porpoises" under power
« on: November 07, 2010, 04:36:08 PM »
This forum needs some "jump starting",  huh?

I've been boating for over 50 years, but I forgot the answer to this problem, so I thought I would ask the experts.

I have a 14' aluminum boat rated for 25 hp. with an older 25 Johnson on it.  It's a pretty light boat with a pointy bow.   When I use over 3/4 throttle, the boat "porpoises", that is the bow flops up and down creating an unsettling condition.   The boat "planes" at around half throttle.

I'm thinking I have to move the transom bracket adjustment one notch closer to the transom to "tuck" in the motor to get rid of the porpoise effect.

Am I right or wrong?   Thanks!
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Offline Cheesehead

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Re: What to do when boat "Porpoises" under power
« Reply #1 on: November 07, 2010, 04:42:41 PM »
If you are saying, to manually adjusting the trim, you are correct.

Cheese
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Offline jcn59

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Re: What to do when boat "Porpoises" under power
« Reply #2 on: November 07, 2010, 05:05:29 PM »
That's what I was trying to say.  Thanks, Cheese.
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Offline bilmac

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Re: What to do when boat "Porpoises" under power
« Reply #3 on: November 07, 2010, 05:48:41 PM »
That helps. Moving some weight forward also helps. I like to put my gas tank in the bow if I can. Requires making up an extra long fuel line but for me it is worth it, I hate it when my bow floggs the water. Under some wave conditions everything you do won't keep the bow down, that's when some folks speed up to get out of the weather. I do the opposite and let the boat back down off the plane. Much safer that way to my thinking.
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Offline zoner

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Re: What to do when boat "Porpoises" under power
« Reply #4 on: November 08, 2010, 11:41:15 AM »
I had a 12' aluminum skiff that did that, even with the motor all the way in. So i made an extension tiller so I could run the boat sitting in the middle seat. It worked excellent
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Offline Rex in OTZ

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Running with a empty load.
« Reply #5 on: November 10, 2010, 06:51:27 AM »
Trimming out your load's CG helps, its just like Fall time combineing corn, we were hauling corn with straight trucks we had to watch closely when empty the trucks "like boats" can be all over the place when unloaded on washboard gravel roads, rushing to keep up with the combiners, adding a transome jack plate may help, them trim wings that mount on the underwater may work for you.
Adding a gas can up front  some sort of ballast weight up front (sandbag's?), mabe bring your wife or buddy and put em up front to keep the boat trimmed.

In my case its heading out with a load of gas and coming back heavy with meat.
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Offline jcn59

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Re: What to do when boat "Porpoises" under power
« Reply #6 on: November 10, 2010, 07:04:35 AM »
I tucked the motor trim to the last notch and it that doesn't do it, I'll move the gas tank to the front.  Thanks for your help all!
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Offline jcn59

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Re: What to do when boat "Porpoises" under power
« Reply #7 on: March 18, 2011, 06:31:04 PM »
So as I am contemplating putting my gas tank in front of the bow seat, I notice that there is no "flat" floor there to set the gas tank on, just the "Vee".   Anyone have any ideas on how to make a "perch" for the gas tank to sit nicely on?   Thanks!
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Offline Gun Runner

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Re: What to do when boat "Porpoises" under power
« Reply #8 on: March 18, 2011, 11:02:56 PM »
Had same prob. so got longer fuel line and moved tank fwd. put long leads on my trolling motor and put the batt, fwd already had the motor set on its lowest setting. Normally I fish alone so that puts my 225lbs and the motor the only things in the back of the boat. It seems to work good for me.

Gun Runner
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Offline bilmac

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Re: What to do when boat "Porpoises" under power
« Reply #9 on: March 19, 2011, 05:12:13 PM »
I made a "V" shaped shelf to put my tank on. Took lots of fitting.
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Offline jcn59

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Re: What to do when boat "Porpoises" under power
« Reply #10 on: March 19, 2011, 06:24:46 PM »
That's what I see myself making....but I fascinate about a drop shelf bolted to the aluminum front seat and maybe with rubber feet resting on the floor....or something like that.   Our lakes are still ice.
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Offline bilmac

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Re: What to do when boat "Porpoises" under power
« Reply #11 on: March 20, 2011, 11:56:06 AM »
Yup, it was a pain to male with all the curves and angles. Just make it sturdy.  I took my boat on the open ocean out of Valdez once when the chop was so bad that it broke it. It was made out of inch pine and wasn't much wider than the tank itself. Things in the bow can really take a pounding.
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Offline fatercat

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Re: What to do when boat "Porpoises" under power
« Reply #12 on: March 20, 2011, 12:16:14 PM »
you can buy wedges that look like door stops. this puts negitive trim on your engine. worked on my old boat. cheap fix.
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Offline 30-30man

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Re: What to do when boat "Porpoises" under power
« Reply #13 on: April 03, 2011, 03:47:20 PM »
Buy a dolphin or the Se 300 from Overtons and the problem will be solved.  I had an old Checkmate Enticer ski boat with a Suzuki 140 on it that porpoised so bad even the kids hated riding in it. It bounced no matter what adjustment I made. I didn't want to buy an expensive jack plate, so I went the cheapest way possible. I put the Se300 on it for around $50 and never had a problem after that.  It greatly helped my hole shot and I was able to turn a 23 pitch prop which help my top end as well.  It made a much different boat after that.
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Offline jcn59

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Re: What to do when boat "Porpoises" under power
« Reply #14 on: April 03, 2011, 04:47:05 PM »
Hadn't thought of that!  Bet I can make one of those lots easier than a shelf up front.  Maybe do both??

I have a dolphin on my big boat which works well~~
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