Nominal wake

Nominal wake is the wake field measured in the propeller plane with the propeller absent, the velocity deficit the hull’s boundary layer and potential flow leave at the disk before the propeller’s own induction modifies it. It is mapped as the axial, tangential, and radial velocity ratios over the disk, usually from pitot rakes or LDV in a towing tank or wind tunnel, and shows a strong axial-velocity peak at top dead center on a single-screw hull where the boundary layer is thickest. The volumetric mean gives the nominal wake fraction. The propeller actually operates in a thinner effective wake, because its suction speeds up the inflow; the gap is bridged in the ITTC-1978 analysis.