'96 Harris Kayot - first pontoon boat I've owned.

It has a Force 75ELPT with trim. Everything is mechanically sound. Motor
runs sweetly.

The motor was original with the boat. It was owned by an older gentleman and
professionally maintained. It's mounted as low as possible as per the
mounting holes, however it doesn't look like a particularly long-shaft unit.

On a lake with a small chop and only 3-4 aboard, distibuted evenly about the
boat, I'm having a problem with frequent/intermittent cavitation any time I
run over 2800-3000 rpm.

I can only run the motor with the trim all the way down. If I raise it at
all, I get severe cavitation.

I've looked over top of the stern while we are underway. It appears that the
cavitation plate is just about flush with the water in the bottom of the
wake. Is that a little high, shouldn't more of the shaft be in the water? If
so, there aren't any more mounting holes to drop it down to, and the bracket
is sitting right on the mounting transom.

A friend suggested a cupped prop--would I be wasting my money?

Thanks.
Dean