How To Size A Propeller Shaft

D = Shaft Diameter in inches (mm)

S.F. = Safety Factor*

St = Yield Strength, Tensional Sheer in lbs/in2 (Mpa)

N = Shaft Speed in RPM

* According to ABYC, safety factors of 2 are considered adequate for boats intended for light pleasure service.  Pleasure boats intended for heavy service and all diesel or gas turbine pleasure boats should consider safety factor approaching 5.  Commercial and high performance craft should consider safety factors approaching 10.