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22 MN January 2011 PROPULSION Yes, sometimes they sing: Some pro- pellers in service produce a high- pitched noise, often referred to as “singing.” This sound typically is a clear harmonic tone much like a hum- ming or a ringing wine glass. More of an annoyance than anything harmful, the causes of singing are not completely understood. Many theories have been put for- ward to account for the phenomenon of singing, but it appears to be affected by critical factors for which the the- ories make no allowance. For instance, in some cases when a twin-screw vessel has one propeller that sings, the noise is eliminated just by switching the position of the pro- pellers, or by replacing the propeller with an identical spare. Also, the fewer blades the propeller has, the less cases of singing are observed. Mitigation of Singing: the Anti-Singing Edge The singing is a function of propeller diameter and RPMs, boat speed and trailing-edge size (thickness) and roundness. In most of the cases, not much can be done on diameter, RPMs or speed, but we can modify the edge geometry. This has been the strategy for all efforts to eliminate singing. Most propeller professionals (and others) are familiar with the anti-singing edge, a chamfering of the trailing edge, typically on the suction side. The intent of this shape is to avoid the creation of curving flow eddies by cleanly separating the flow off of the blade. However the adoption of unduly thin edges can result in erosion or fracture of the blade near the edges. It is advised that an anti-sing edge be considered as a last resort to min- imize the singing of an existing propeller. www.ricepropulsion.com Do Propellers Sing? By Leonardo Montoya, Rice Propulsion The graphic shows the chamfering that goes from a 0.5 or 0.4 radius to the tip of suction side.
