Quasi-propulsive coefficient (eta_D)

The quasi-propulsive coefficient $\eta_D = P_E / P_D$ is the ratio of effective to delivered power, the product of hull efficiency $\eta_H = (1-t)/(1-w)$, open-water propeller efficiency $\eta_O$, and relative-rotative efficiency $\eta_R$. It captures every loss between the propeller shaft and the towed-hull power and typically lands near 0.65 to 0.75 on a well-matched single-screw ship. Shaft power then follows from $P_S = P_D / \eta_S$ after bearing and gearing losses.