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another question!

my 1968 16' 6" whaler has a 1991 yamaha 90hp engine. when i get on a plane she rides fine, but if i trim the motor up at all the bow begins to rise and fall despite the flat calm water. the more i trim the engine up the higher the bow rises and falls.

my only thought is that the engine is mounted too low on the transom, but this is a guess based on nothing.

any help would be great.

thanks,

dana

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Dana,

What holes is you engine mounted in? Top mounting holes?
You might want to take a look at your engine height. If the engine is too low in the water, the porpoise effect could show up.
Check the spray from your cavitation plate. If you have quite a spray from the cavitation plate, then your engine is too low. Raise the engine up 1 hole at a time and then check the spray from the cavitation plate.

You might also have the wrong prop. A bow lifting prop might create the same scenario.
I would start by checking the spray from your cavitation plate though. Cheaper to raise the engine than to buy a prop unless you need a prop of course.

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Joe,

Thanks for both responses. I am not real clear of mounting holes, and my whaler does not come out of the water for another couple months. I do know that the lower 2 mounting bolts are threaded into the engine bracket the top two are through bolted. If i was to change the engine height would that require drilling more holes in the transom, or can the engine move on the bracket without more drilling?

Thanks,

Dana

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Dana,

Your engine should have 4 or 5 holes at the top of the mounting bracket.
These are holes as opposed to the lower slots.
If the bolts go thru the top holes, then your engine is mounted all of the way down.
Each hole below that raises the engine 3/4 of an inch.

The lower blind holes are a problem for raising the engine.
My Montauk was mounted the same way so I never did raise the engine up.
However, I only had a 70 hp and never had the porpoise effect like you mention.

If you are certain that the engine is too low by getting a lot of spray from your cavitation plate, then you could raise the engine up to the second top set of holes and drill 2 new holes in the TOP of the bottom slots. Then you could fill the two old lower holes. The only way you could go back to the original mounting position is by using those two holes you just filled.

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No matter how high or low your motor is mounted, your boat will always start porpoising if you trim the bow too high. The trick is to trim her out for an even plane under most conditions. Is it electric trim or manual?

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Danedg,

she has electric trim, but even the slighest trim up creates this effect. after reading Joe's response i dont think i'll be getting into remounting the engine, however i dont think porpoising is the issue. i may be wrong, but i think of porpoising as when the prop exiting the water... please let me know if i am wrong.

thanks dana

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My definition of porpoising is when the bow sets up a rythmic rise and fall at speed.
I can do it at will by "trimming" the motor "up" too far. By trimming "down", the bow settles into plane nicely. By continuing to trim down, the bow will "plow", sometimes necessary in rougher water....
As for the prop exiting water, I'd call that cavitation. The prop creates a "cavity' of air around itself, rendering it useless.

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thanks for setting me straight. i guess she is not cavitating, but is porpoising. my only two options are (1) plowing, (2) porpoising, there is no inbetween. any amount big or small of trimming bow up creates (2). back to the drawing board, and prop looking.

thanks

dana

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In your initial post you said it rides fine until you start trimming...don't trim.

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Plus...Have you got a "fin" on the motor? They make a dramatic improvement in ride and stability...cheap compared with a new prop...or motor.