Pitch-diameter ratio (P/D)

The pitch-diameter ratio P/D is the propeller’s pitch normalized by its diameter, the primary geometric loading parameter in open-water design. It is usually quoted at the 0.7R section for fixed-pitch wheels, where the representative blade angle sits. Merchant propellers run P/D from about 0.6 on heavily loaded tugs to 1.4 or more on fast, lightly loaded craft. P/D fixes the shape of the K_T and K_Q curves against advance coefficient J in a systematic series such as the Wageningen B-series, and design tools select P/D to place the operating point at peak open-water efficiency for the required thrust at the design J.