Ship Vibration • SOURCES of SHIP VIBRATION External Sources [Hydrodynamic loading by direct action or induced by the ship motions] (1) Hydrodynamic loading on Propellers Blades in non-uniform flow (freq. = RPM x No. of blades). More pronounced for low propeller submergence and in shallow water (2) Unstable cavitation of blades (3) Vortex induced forces (not on propeller) Eg. Brackets that holds the propeller (4) Slamming Load – short duration forces but give rise to high frequency forces.