Ask SAIL: Replacing Propellers and Diesel Engines

Algirdas Vilkas, Mt. Kisco, New York asks:

Over 40,000 Atomic 4 engines were installed in new boats during the last century. Repowering is not that hard, but a new propeller may be requiredI have a 1977 Pearson 28 and have replaced the old Atomic 4 gas engine with a three-cylinder Universal 20hp diesel engine. Do I need to change the original propeller that was driven by the Atomic 4? If so, how do I determine what propeller is correct for the new diesel engine?

Nigel Calder Replies

The Atomic 4 is rated for 20hp at 2,500 rpm, with a peak power rating of 30hp at 3,500 rpm, whereas the Universal is rated for 20hp at 3,600 rpm. The critical missing piece of information concerns the reduction gears used with the two engines. Basically, if the propeller shaft speed at 20hp is about the same for both engines, then the existing propeller should work. If there is a significant difference, the propeller will probably need to be replaced. The best reference for working through the sizing process in detail is Dave Gerr’s Propeller Handbook. The steps are clearly laid out and are easy to follow.

Nigel Calder is an author and expert on boat systems and diesel engines

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