Ideal Propeller Diameter is often less than Minimum Propeller Diameter, because while the Minimum Propeller Diameter is only concerned with moving the vessel no matter how rough the conditions are, the Ideal Propeller Diameter takes a more “human” approach and accepts that not all vessels will be fitted with propulsion systems of Trinity House standards, and most skippers will be quite happy with a proplsion system that works well in most conditions and running like hell for the nearest port (or staying there in the first place) when it gets dirty. After all, few leisure sailors venture out in dirty weather, whereas commercial sailors often must.

Ideal Propeller Diameter is also based upon the engine, and not the hull, whereas Minimum Propeller Diameter is based upon the hull, and not the engine.

Formula (exp = Exponent, or “raised to the power of”)

• D= ( 632.7 x ( shaft HP exp 0.2 ) ) / ( RPM exp 0.6 )

For our example vessel with 42 Shaft HP (or BHP left (after losses) at the propeller) @ 2000 RPM this gives us a figure of 14 inches diameter.