outboard

I have a 1988 maxum 20' with a 150 blackmax outboard that is about 3" to short the cavition plate is about 3 or 4" above the bottom of the boat not getting a good bit in the water any ideas on how to fix this problem?

 

I am probably not the most qualified person on this forum to address your question. But just to get things started I will offer some ideas that may or may not have much merit.
The possibilities that come to mind are

cut down the transom enough to solve the problem

install a longer shaft on the motor

install a bracket outside the transom to mount the motor on ( I have heard reports that this can work out really well, but have no personal experience)

if it is a v bottom with a wide transom you could try moving the motor toward the starboard side to moderate the problem (this would have some effect on the steering but would beat having cavitation; it would have less effect on the steering than moving it to port)

if it is a welded aluminun hull with a v bottom you could make the tip of the v into a flat for the last few feet ahead of the transom.

I hope one of these notions may be of help to you.