When we talk high speed we are looking at 20+K rpm engines going 180-200mph in airplanes. For this you want a small high pitch prop. For your airboat you want to match the prop to the rpm and speed you are looking for. I would guess your engine would be in the 15K rpm range and your expected top speed would be no more than 70-80mph. This would mean you're looking more for a torque prop than a horsepower prop. I would try a 12X6 or 11X7 prop first and go from there. I would use a wider blade prop like a Master Airscrew to pull more wind.