Open-water diagram

The open-water diagram plots thrust coefficient $K_T = T / (\rho n^2 D^4)$, torque coefficient $K_Q = Q / (\rho n^2 D^5)$, and efficiency $\eta_O = (J K_T)/(2\pi K_Q)$ against advance coefficient $J = V_A/(nD)$ for a propeller running in undisturbed flow. It is the propeller’s characteristic curve, measured in a towing tank or read from a systematic series like Wageningen B, and it is the bridge between hull powering and the chosen propeller geometry.