Which propeller should you fit on your yacht or motor boat?
Selecting the optimum propeller to obtain the desired speed with good efficiency is not an easy task. As well as the basic choice of type from the many different configurations available (some of which are shown here) there are further choices to be made including number of blades, pitch, diameter, blade size and shape, not forgetting the gearbox reduction ratio. To make matters worse the books on the subject introduce all sorts of complicated concepts such as real and apparent slip, cavitation, wake factor, rake, skew, and various coefficients, indicated by confusing Greek letters. Then having got through all that you find that it is unlikely that propeller efficiency will ever be better than 60%, and then don’t forget to leave adequate tip clearance, ensure the blades are strong enough and use a suitable shaft taper and keyway.
The fundamental problem in selecting a propeller is to obtain an appropriate match between the engine power output and the ability of the propeller to transmit that power. If this is achieved, the engine will be able to provide the maximum horsepower at the right RPM for maximum power output from the propeller. In this case, the variables, which must be chosen carefully, are primarily the pitch and diameter of the propeller. If there is too much pitch or diameter then the engine will be overloaded and will not reach the optimum RPM for best power output, leaving valuable horsepower unattainable. If there is insufficient pitch or diameter then the engine will be insufficiently loaded allowing it to “race” over its RPM limit, possibly destroying itself. In this situation, even if the engine can run in a lightly loaded state at high RPM, the propeller would produce little thrust and overall efficiency would be low.
I can advise you and carry out all the necessary calculations to find the correct size and to optimise the selection for maximum efficiency or speed.
If you want to know more about the theory, then the following book is recommended: The Propeller Handbook by Dave Gerr, published by Nautical Books, ISBN: 0713657510, described on the cover as “The Complete Reference for Choosing, Installing and Understanding Boat Propellers”.