Optimal impeller tip clearance

4000 RPM SEEMS VERY FAST FOR A BOAT PROPELLER. IF YOU LOOK AT THE PITCH OF THE PROP YOU SHOULD BE ABLE CALCULATE A VERY HIGH SPEED AT THAT RPM. THE ANSWER TO YOUR QUESTION WILL HAVE TO BE DETERMINED BY THE AMOUNT OF TURBULENCE AT THE END OF YOU PROP. AT GOOD STARTING POINT IS 15% (.3375")OF THE RADIUS. THIS WILL LOWER THE EFFICIENCY IN A DUCT SO MY APPROACH WOULD BE TO ALLOW THE MINIMUM CLEARANCE AND USE A CONVEX SHAPE ON THE VORTEX. YOU WILL THE NEED TO SIZE YOUR EXHAUST DUCT TO ACHIEVE A LAMINAR FLOW AND I WOULD USE A STANDARD FLOW CHART FOR HYDRONICS TO CALCULATE THE REYNOLDS #( TURBULENCE ) IN THE DUCT AT FULL SPEED AND KEEP IT BELOW 3000. CONCAVE WOULD BE STANDARD BUT I BELIEVE SPEED IS YOU GOAL AND IF SO, THE CORRECT CURVE ON THE VORTEX WILL CAPTURE THE MOST ENERGY AND DIRECT IT FOR ADDED THRUST.