Boat Plows....Why?

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Originally Posted by jaycee

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.

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sounds like the problem right there, the cavitation plate should be even or slightly below the hull, the motor will always try to run perpendicular, tilt it in to far, it will push the bow down, tilt it out the far, the boat will start to porpoise, if your prop is not letting your motor run at the max rpm the boat will drop off and plow,

Not correct about the cavitation plate. The cavitation plate while at WOT should be just out of the water. Where it lines up with the boat while not on plane does nothing. The comments about moving the trim back towards the transom are incorrect. If you are trimmed up too high your boat will start to porpoise "bounce up and down". You have not experienced this yet so either you lack enough horsepower lift the bow or you are not trimmed up high enough. I'm thinking your not trimmed high enough. Or the boat is just a tank and just runs like that


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When you say that the boat plows, you mean that the bow rides low and the stern high so the boat always seems to be going downhill, right?
Is there any chance that the hull has been deformed somehow?

When I say plow, I mean it wants to nosedive. The 9.9 Yam 2 stroke has plenty of power to get it up on plane so that's not a problem. And the boat is in good shape with no leaks, bangs or dents. That's why we want to get it up and running properly but mainly because the deep and wide is, or should be safer in the big rollers that that lake gets.

I've tried just about everything...shifting as much weight to the stern as possible, shimming the lower outside motor mounting bracket to increase the trim even more.
The boat is at a camp right now so checking things--cavitation plate orientation to hull, depth it sits above or below the transom, etc isn't possible.
But you guys have given me some ideas of what to try and what to look for.
Thanks to all who've responded.

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Originally Posted by sawbill

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??

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Originally I thought it may just need to be brought in closer to the transom, but reading your measurements, it might in fact be that there's cavitation. That size boat is going to displace a fair amount of water when it's load is at the very rear and moving at some speed.

A longer shaft motor may help, but I'd try dropping the trim right tight to the transom 1st.

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Originally Posted by sawbill

When I say plow, I mean it wants to nosedive. The 9.9 Yam 2 stroke has plenty of power to get it up on plane so that's not a problem. And the boat is in good shape with no leaks, bangs or dents. That's why we want to get it up and running properly but mainly because the deep and wide is, or should be safer in the big rollers that that lake gets.

I've tried just about everything...shifting as much weight to the stern as possible, shimming the lower outside motor mounting bracket to increase the trim even more.
The boat is at a camp right now so checking things--cavitation plate orientation to hull, depth it sits above or below the transom, etc isn't possible.
But you guys have given me some ideas of what to try and what to look for.
Thanks to all who've responded.

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All I got to offer is ,man that's weird. The problem is usually just the opposite of what you got. Problem is if you go to extreme in tilting that motor up bad things can happen in corners. Is the 9.9 long or short?

The shaft measures 18 inches. The transom is just a tad over 17 inches.
And you're right about mean things happening. At half speed it's a constant struggle just to keep the boat in a straight line because it wallows all over the place while trying to maintain some sort of trim.
I suppose all I can do is shim it more and see what happens then.

Is your cavitation plate below the hull bottom? If not then you will lose thrust when the boat starts to plane.

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Originally Posted by Bluebulldog

Originally I thought it may just need to be brought in closer to the transom, but reading your measurements, it might in fact be that there's cavitation. That size boat is going to displace a fair amount of water when it's load is at the very rear and moving at some speed.

A longer shaft motor may help, but I'd try dropping the trim right tight to the transom 1st.

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Wouldn't that make things worse? When you want to get on plane you trim the motor down, as you get on plane you trip the motor up to lift the bow in a traditional boat with power trim. To lift the bow in my Ranger I trim the motor up the lift the bow, I would not trim the motor down???