Boat Plows....Why?

We just picked up an older boat for a lake that we fish often. Its an older aluminum beast, 14 ft, transom measures just over 16 inches and it has a beam of 64 inches. So I'd consider it a deep and wide. Probably rated for a 20 or 25 hp.
I've put my Yamaha 9.9 on it which has a shaft length of 18 inches.
No matter how far out I mount the motor from the transom the boat plows. I've put the gas can right next to the transom, keep as much weight rearward as possible yet the boat still wants to nose dive when I go about 1/2 throttle.
It wallows all over the place, I can't keep it in a straight line.

You guys any ideas?

My best guess would be not enough horse power to get the boat up on plane.

Perhaps insufficient transom angle? If the tilt adjustments on the motor itself cant resolve it, try experimenting with shims to position the motor as it is mounted on the transom?

The boat goes up on plane in a heartbeat but immediately goes beyond even at half throttle with my 9.9.
Yesterday I shoved a 3/4 inch shim behind the mounting bracket (on the outside of the transom) to increase the angle and it helped somewhat but the problem still exists to a point where the boat is completely unmanageable.

Would a longer shaft motor make a difference??

Have you tried going in the opposite direction with your adjustments? Bring the motor closer to the transom and adjust the loading.....Some boats are just weird.

What is the pitch on the prop ?
It should be stamped on it.


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Sound like it's trimmed down too much, trimming the prop closer to the transom will only make the experience worse. A longer shaft will not make a difference, you would only get better water pressure and better hole shot. I would keep trimming the motor up.

Trimming it up hasn't worked.... Might be that you are "past the point" and it is time to try the other direction. As you increase the gas, it tries to raise the bow, but it might be set beyond the point where the motor can do it.
Load the boat as normal ( level), then find the " horizontal line" on the motor cowling and adjust the trim until that line is parallel with the water line.

Quote:

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Originally Posted by sawbill

Would a longer shaft motor make a difference??

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It could.
From the info I got so far on this thread a longer shaft may help.
If your motor shaft is too short when the boat pops on plane it may now be getting starved for water to push causing the nose to drop back down.
When the boat is on a trailer with the motor set at medium trim if you look along the plane of the bottom of the boat is the prop fully lower than the bottom of the boat. If it is not than you need a longer shaft motor.

The cavitation plate should be level with the bottom of the boat.
It is possible that the motor is not reving to it's potential, [to high a pitch prop] check the pitch of your prop, if too high your motor then does not rev. up to it's highest capability.
Look around perhaps beg or borrow a prop with the next lowest pitch , this may solve the problem, it has for others.
Another problem that occurs, is when the boat is under way, and if the floor/keel is weak or broken/cracked, this allows the boat to flex at the floor just ahead of the transom and causing it to plow.