Shaft horsepower (SHP)

Shaft horsepower (SHP), also called shaft power P_S, is the mechanical power transmitted along the propeller shaft, measured by a torsionmeter as SHP = 2 pi N Q, with N the shaft rotational speed and Q the torque. It sits between brake power at the engine and delivered power at the propeller in the powering chain: SHP equals brake power minus gearbox loss, and delivered power P_D equals SHP minus the stern-tube and line-bearing friction (shaft transmission efficiency typically 0.97 to 0.99). SHP is what sea-trial torsionmeters actually read, so it is the reference for trial powering and for converting the tank-predicted delivered power into an installed engine rating.