Problem with porpoising keel is straight

I have a 16' long by 72" wide aluminum flat with a 2008 50 mercury 2 stroke. Problem is anything over 25 miles an hour and the motor has to stay trimmed all the way in, or it will start porpoising. If you trim it up and just take the hops the boat will get up to 40, but it bounces and porpoises like hell. The motor is set with the vent plate about 1 inch below the bottom of the boat. The keel is straight, and the problem exists no matter how much weight you put in the front, the 30 gallon gas tank, and trolling batteries are all up front. The only way to make it stop is to "plow" the bow down into the water, which makes the motor noticibly bog, and kills the speed to about 30 and it still bounces just a bit. It has a mercury 3 blade 15p on it. Im wondering if moving the motor up level with the bottom will give the motor less of a "lever action" and allow it to push more than try to "dig" below the boat? If not would getting a cupped, and very low raked 15p prop fix it? Im hesitant to drill holes to install the hydrofoils, or drill into the hull to install smart tabs if there is a way to fix it with motor height and prop first. Thanks

Re: Problem with porpoising keel is straight

The motor is set with the vent plate about 1 inch below the bottom of the boat.

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others probably know better than me. but that might be the place to start at. i think the anti ventilation plate is supposed to be a shade above the bottom of the keel..

i'm no porp expert either, but i think some props may give more lift to the stern than others. that'd be the second place i'd look at.. might try to prop forum for that question.

i agree, a hydrofoil or tabs would be a mask for the issue at hand... may cure the problem, but don't address the reason why...

Re: Problem with porpoising keel is straight

You shoule raise your motor to where the cavitation plate is level with the keel...

Re: Problem with porpoising keel is straight

My boat is a 18'-72" jon with a 90HP and it does pretty much the same thing, just can't trim it up much at high speed without that porpoising. I don't run real fast much anyway due to price of gas so it's not a real big deal. Maybe this is just part of the jon boat experience. My boat is an Express with the Yamaha 90HP and it came as a "factory package" so I always assumed they would set it up correctly to start with. Hopefully you can modify your boat/motor set-up to improve your situation. Good Luck!

Re: Problem with porpoising keel is straight

Knightgang said:

You shoule raise your motor to where the cavitation plate is level with the keel...

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That should do the job

Re: Problem with porpoising keel is straight

The next hole up is about an inch, so might make it dead on even, but what if it places the motor above the keel some? It already vents when turning around, and you basically have to stop, let the prop get a bite again, then continue your turn. Probbaly just gonna be one of those do it and see what happens things.

Re: Problem with porpoising keel is straight

What boat do you have? What is the deadrise, if any? There is no particular standard as to where the anti-cavitation plate should correspond to the hull bottom. There is a starting point and that would be "even with" and then your tilt tube should be marked for future reference at your zero trim angle. Before you move your motor what prop is on it? Most all "unwieldy boats" can be corrected by balancing the load and running at the max rpms for the load you carry with a correct prop. Correct propping can cure all sorts of ills. I would begin with that before doing anything else.

Captain Chris

Re: Problem with porpoising keel is straight

Scubasky said:

I have a 16' long by 72" wide aluminum flat with a 2008 50 mercury 2 stroke.

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.....just a guess here but:
With that much horsepower and that wide a hull, it would seem to me that the slightess variance in the water's surface would have an immediate lifting effect on the hull in spite of the prop depth.
I see only two solutions !!!!! 1) Slow down, 2) speed up until only the prop is in the water !!!!
FWIW....Ed in 'ol Virginny

Re: Problem with porpoising keel is straight

Yea the water was pretty choppy today. The motor came with a powertech 9p partybarge prop which shot the boat out of the water like a rocket, and would probbaly kick arse pulling a tube. I knew this going in but anything past half throttle was just reving the motor no speed increase, but it did 27 mph at half throttle and just sipped gas. The party barge prop did not porpoise, probbaly due to the large diameter, and short rake? I am gonna hang on to that prop for pulling stuff.

Re: Problem with porpoising keel is straight

you have a barge, not made for fast turns, slow down. raise the motor, ad some weight to the bow.

Re: Problem with porpoising keel is straight

50hp seems a bit much for a jon boat.

Re: Problem with porpoising keel is straight

And there is a proper place for the anti vent plate. It should be 1 inch or there about ABOVE the bottom of the transon.

Bill

Re: Problem with porpoising keel is straight

I will add only one comment, SPEED KILLS, slow down & listen to the above posts. You are trying to make the boat do something it was not designed to do.

Re: Problem with porpoising keel is straight

Scubasky said:

Yea the water was pretty choppy today. The motor came with a powertech 9p partybarge prop which shot the boat out of the water like a rocket, and would probbaly kick arse pulling a tube. I knew this going in but anything past half throttle was just reving the motor no speed increase, but it did 27 mph at half throttle and just sipped gas. The party barge prop did not porpoise, probbaly due to the large diameter, and short rake? I am gonna hang on to that prop for pulling stuff.

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I just bought a similar boat with a 4-stroke. Trim in till it runs level and it planes great. If your motor bogs when you level out, maybe too much prop? Is that possible?

Re: Problem with porpoising keel is straight

Too much, as in?