mgalvin, with that much reduction he will have to run a much higher pitch especially if he reduces the diameter to clear the water. Simple example: in theory a 25" pitch prop at 3500 rpm should be flying the same speed as a 12.5" at 7000 rpm keeping the same diameter. I suspect he will be turning the prop well below 3500 rpm. Yes I know about slip, efficiency, etc. Maybe a 3 blade prop might be in order.

Ken