Ships generate noise primarily by (a) propeller action, (b) propulsion machinery, and (c) hydraulic flow
over the hull. Propeller noise is associated with cavitation (Ross 1987, 1993), the creation of voids from
zones of pressure above the ambient. The collapse of these voids generates sound. Cavitation creates both
broadband noise and tonal sounds, as it may be modulated by blade-passage frequencies and their
harmonics (the blade lines). The broadband and tonal components produced by cavitation account for 80-
85 percent of ship-radiated noise power (Ross 1987). Propeller noise may be also generated by unsteady
propeller blade-passage forces. Additional ship noise results from propulsion machinery such as diesel
engines, gears, and major auxiliaries such as diesel generators.