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This paper examines the hydroacoustic processes involved with "singing propellers" aboard marine vessels. Methods are presented to determine the potential for a resonant response of a propeller to a vortex shedding excitation-a phenomenon known as "singing." Methods are provided to determine the likely shedding frequency and structural natural frequency for propeller blades. Diagnostics procedures to determine the presence of singing are explored. Measured and theoretical differences between the blade's natural frequency response in air and in-water are explored. Treatments are identified to change the vortex shedding frequency or to de-tune the structure. Case histories are detailed showing the potential magnitude of the problem and effective solutions.
Keywords: propellers; noise; vibration hydrodynamics (propulsor)
1. Background
THIS PAPER examines the hydroacoustic processes involved with a "singing propeller" aboard marine vessels. A "singing propeller" has a tone generated by the interaction between a Karman vortex shedding mechanism, usually from the trailing edge of the propeller blade, and the blade's natural mode of vibration. The "singing" manifests itself as a high sound radiated into the water, which can interfere with ship's sonar systems or affect the detectability of the vessel. It is often coupled via the waterborne path to a high noise level inside the vessel. This tone can be objectionable to the crew and passengers. Furthermore, a singing propeller can cause stress cracks or catastrophic failure of the blade. Methods are presented to predict or measure the existence of this potentially harmful resonant response. It is also noted that other ship protuberances can also "sing" such as rudders, bilge keels, shaft supports, keels, and so forth.
Empirical methods are provided to determine the likely vortex shedding frequency and structural natural frequency for propeller blades. The hydrodynamic conditions and propeller geometric parameters needed to predict or diagnose the occurrence of a singing condition are presented. The controlling factors are the Strouhal number, thickness of the trailing edge, and the flow speed over this edge.
Diagnostic procedures to determine the presence of a singing phenomenon are explored. Natural frequencies of the blades are often measured in air rather than when waterloaded. A propeller in air would have a higher blade natural frequency than the same blade in water due to the small entrained fluid loading effect. Measured and theoretical...