I could build a frame of sorts with a flat panel elevated over and across two wind surf boards that weigh about 30 pounds each.

These boards without rudders or keel would slip and slide all around like an airboat I think.

This I think could be the very most performance from the very lowest HP that we could dream up.

The two boards I got because I wanted to build a human powered bicycle peddle boat
but lawn mower engines would make them fly

Yes theoretically the two boards with just the operator and the engine should fly...I wonder how much fighting it would be to turn if you're moving along at a good pace and then need to turn without the ability to "lean the boards" like you do during windsurfing ??? :confused:

you would use the air rudder just like the air rudder on an ordinary flat bottomed airboat. You would slide sideways a bit then the motor
is blowing you in a different direction. The surf boards are merely acting as pontoons.

If all things are good as far as power to weight and it does have the ability to plane I see a big problem. Without a rake in the front to push water the front could nose dive at any time and that would be trouble with a capital gone swimming.

Good point...I was'nt thinking about choppy water,bouncing, or even the operator moving up on the board would plunge the front into the water...