Non-dimensional pitch (P/D)

Non-dimensional pitch is the pitch P expressed as the ratio P/D to diameter D, removing scale so geometrically similar propellers share one number. It is the abscissa-family parameter in open-water charts: each P/D gives a distinct pair of K_T(J) and K_Q(J) curves, and efficiency eta_0 = (K_T / K_Q)(J / 2 pi) peaks at a J that climbs with P/D. Standard practice reads P/D at 0.7R because that radius carries the representative loading. A controllable-pitch propeller varies P/D in service to hold the engine on its load curve across draught and weather, where a fixed-pitch wheel is locked to one P/D.