The right prop for a Classic 37' Yawl

[Gerry Wood]

Dear Mr. Woody,I have recently purchased a 1969 Classic 37' Yawl. The original motor was the Atomic 4 gasoline engine, 30bhp @ 3500 RPM, with, I'm guessing, a 2:1 reducer. The original prop was a 16X13X3.Prior to my purchase, a Universal diesel M4-30 was installed. 25bhp @ 3600 RPM with a 2:1 reducer, right hand. I must now determine what size prop to mount as a new prop was never fitted.Boat Info: LOA 37' 3 1/2", LWL 26'3", Beam 10'2 1/4", Draft 6', Hull Fiberglass, Displacement 18,000lbs, Keel Type Full,Prop Clearance from shaft center to Although I understand this boat is more a cruising boat than a racer, I would like the boat to perform well under sail. Having raced for twenty years I like a boat that slides well.I have seen both two and three blade props and don't appreciate the differences in each. How much performance is lost when choosing a two blade over a three blade? Is a large disc two blade almost as good as a three blade in terms of performance? What will be the loss in speed with a three blade compared to a two blade? Hope you can help,Gerry

 

[Kevin l. Woody]

New prop

Dear Gerry,I would agree that the reduction gear you have is most likely a 2:1. The normal 1:1 reduction was installed with a 12”. For the M4-30, I would suggest we stay with the same diameter of 16” but reduce the pitch to 10” for a two blade and 9” for a three blade. Your existing propeller should most likely be able to be reduced in pitch at a local prop shop. For your convenience I have listed our site, www.pyiinc.com , were you can post your vessel info on our recommendation sheet and have it run by one of our staff. A two blade will usually propel the vessel at apx. the same speed when motoring in flat calm water. The three blade being almost thirty percent greater in efficiency will allow the vessel to maintain its speed when it is confronted with heavy winds or seas. The three blade is also a much quieter operating propeller. I would say that almost 70% of the propellers we sell go out as three blades.Hope this helps. Kevin.

 

[Ernie April]

Propeller Handbook by David Gerr

David Gerr is an amazing marine architect. His book will provide all the info you need to match your engine and boat to the right prop. The below URL should take you right to it. (Copy and past it into the location box if not active)Ernie Aprilhttp://www.amazon.com/exec/obidos/search-handle-form/107-4812034-5256569