On Fri, 19 Oct 2007 17:37:17 +0700, Bruce in Bangkok
wrote:

the H.P. and the gearbox reduction
give the diameter of the propeller

Not really. Diameter of the prop is limited by tip clearance of the
blades. Generally speaking you should use the biggest diameter that
will still give 10 to 15% clearance. Too little clearance will cause
vibration as the prop thrust interacts with the hull and/or aperature.

There is a way of backing into the prop numbers based on the estimated
hull speed and slippage factor assuming that the engine will deliver
the required power to maintain RPMs.

Required horsepower is a function of displacement and drag. If the
prop pitch is too great, or the available torque is too low, the
engine will not rev high enough to deliver its rated power.