“…The blades have been observed to vibrate with similar modes in both air and water when struck. The natural frequencies of blade vibrations are lower in water, however, because of the mass effect due to the inertia of the water set in motion with the blade and also because of hydrodynamic damping (25,28,34). The frequency for a given mode of vibration in water may be less than in air by the order of twenty-five percent (44) and is a function of the frequency itself in addition to other factors.…”