The old rule of thumb that shaft diameter should be one fourteenth of propeller diameter is often surprisingly accurate, however we can calculate it properly.

Formula

• D = cube root ( ( 321,000 x SHP x SF ) / ( St x RPM ) )

SF = the Safety Factor, this should be 3  for pleasure yachts and between 5 and 8 for commercial or racing applications. Passenger carrying or towing vessels should be 8.

St = Torsional Shear, put simply this is how much twisting force the shaft will take, this is dealt with in more detail on the Material page as it is a mechanical property of the shaft material.

In our example vessel the SHP is 42 @ 2000 RPM, we will use a Safety Factor of 3, and since I have selected Tobin Bronze as a shaft material the Torsional Shear is 20,000 p.s.i.

This computes to an ideal shaft diameter of one inch. This compares nicely with the 14 inch Ideal Propeller Diameter and the old rule of thumb.