The Problems of the Singing Propeller

Abstract

The unusual noise emanating from what is known as a “singing” propeller is the direct result of the vibration of the blades. These are disturbed by the irregular hydrodynamic actions of cavitation and vortex shedding and induced by the extreme variations in the wake. It is demonstrated experimentally that a blade with a sweeping plan form and sharp edge, suppresses cavitation and vortex action within the working range of the propellers. Sharpening the blade edges round the periphery, in a wedge-shaped manner, has been applied successfully as a cure to many singing propellers. In addition to suppressing the disturbing forces, experiments show that the sharp edge provides considerable increase in hydrodynamic damping. The response of the blade to tip and edge excitation is considered. Blade flutter is examined, but it is concluded that such a condition is unlikely. Notes emitted by singing propellers suggest that many modes of flexural, torsional, and lateral vibration are present. It is shown that the lateral types respond easily and their importance is indicated by a number of noisy propellers which have exhibited fatigue cracks conforming to the lateral modes of vibration. It is recommended that the process of sharpening the blade edges be adopted, as this suppresses the exciting forces and damps the response of the blade.