Expert: Paul Soderman - 11/13/2009

Question
 I have an idea for a low drag boat hull design.  To avoid water interference in testing, I am considering a low HP pusher prop for power (swamp buggy style).  Question, for the same horsepower (say 16 HP), boat weight, etc., which will propel the boat faster (is more efficient), a boat propeller in the water or an airplane pusher prop setup.  What ranges of speed will the thrust/drag for air be better than water.
 Any hints on propeller size/pitch/blades design?
Bob

Answer
Bob
That is a good question. We know that the useful power output from a propeller (air or water) is TV (thrust time velocity of vehicle).  So let us fix TV. The question then is which is the most efficient way to deliver that power - a boat propeller or pusher prop.  Since water is so much more dense than air, the thrust generated by a small boat propeller would be equal to that of a pusher propeller only if the pusher propeller had a very large area.  Tip speed would go up also.  A pusher prop with large area would have large drag and torque required.  So, without going through the numbers my feeling is that the boat propeller is more efficient than the pusher propeller and would win the race.  Maybe that is why we only see pusher props on swamp boats in shallow water.

Stick with off the shelf propeller designs.
Paul