Whole 'nother puzzle. The marine engine will not turn an air propeller 30K rpm. It's desined to work with a boat prop to get those rpms. If it has long enough crank threads to put an air prop on it I would go ahead and use it though, I assume a full length tuned pipe? You'll want to try to get in the 20K rpm range and a 11X7 would be a good start, use a APC sport prop to test with as the carbon APC pylon props will set you back a bunch. When you get a pix post it for us, we like speed, any speed.