Effective wake fraction (w)

The effective wake fraction w is the fractional reduction of the inflow the working propeller actually sees relative to ship speed V, so the speed of advance is V_A = V(1 - w). It is termed effective because it is derived from a thrust-identity or torque-identity analysis behind the hull, not measured directly: the open-water J that reproduces the measured behind-hull K_T (or K_Q) fixes V_A, and w follows. Effective wake exceeds the nominal wake because the propeller’s own suction alters the field it ingests. Single-screw full forms run w around 0.25 to 0.45; fine twin-screw hulls run lower. In the propulsive coefficient w feeds the hull efficiency eta_H = (1 - t) / (1 - w).