Thrust deduction fraction (t)

The thrust deduction fraction $t = (T - R_T) / T$ measures how the working propeller raises hull resistance by lowering pressure over the afterbody, so the propeller must deliver thrust $T$ greater than the towed resistance $R_T$. Typical values are $t \approx 0.12$ to $0.25$. With the wake fraction it forms the hull efficiency $\eta_H = (1 - t)/(1 - w)$, which usually exceeds one on single-screw ships and is a real propulsive gain.