An Unusual Onboard Experience: Singing Propeller

Do Propellers Sing?

I know, it sounds very odd but the answer is YES, sometimes they sing. The singing propeller is a hydro acoustic phenomenon which I personally experienced when I was serving as a chief engineer on a suezmax crude oil carrier owned by a very reputable tanker operator. Some propellers in service produce a high-pitched noise, often referred to as Singing. This sound typically is a clear harmonic tone much like a humming or ringing wine glass. For those who are not familiar with the described sound, it sounds like this;

The ship was en route, it was a ballast voyage, the sea was calm. We first noticed the high pitched sound, a few minutes after we decreased the main engine r.p.m. to abt 68 r.p.m. It sounded like metal parts in stern tube rubbing each other without a proper lubricating oil film and as though caused this very annoying sound. We put a lot of effort in a very short time to recognize this annoying sound. The team checked every single parameter in E/R related to the main engine, shaft, intermediate bearing, stern tube and steering gear. No abnormality found, the worst case, everyting seems normal but the sound was still live. After increasing main engine r.p.m the sound so-called singing propeller just started to fade out, then comfortable silence. Subsequent reporting our stressful experience to the office, a bunch of components carefully examined which might be related to that sound. Condition assessment carried out with all departments.

Having an expert with fancy industrial microphones and accelerometers outside on the aft deck for investigation of the sound is one of the many efforts made during the investigation period. A sea trial was setup up to investigate the sound. In the beginning, there was no other sound than what was expected. Reaching a shaft speed of 68 RPM and the targeted sound was not audible. Increasing the shaft speed 76 RPM and the sound was still not audible. Decreasing the shaft speed to 68 and the targeted sound was audible.The sound was audible outside on the aft deck and in the steering gear room, but not in the engine room. We installed a vibration monitoring system to the stern tube and main shaft in order to record every possible data related to the sound. After a few weeks with full of effort and concentration from every involved party, the term of "Singing propeller" has been introduced.

The singing propeller is more of an annoyance than anything harmful, the causes of singing are not completely understood. Many theories have been put forward to account for the phenomenon of Singing, but it appears to be affected by critical factors for which the theories 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 position of propellers. Or had been replaced by an identical spare Propellers which were found silent. Also the less number of blades the less cases of "singing".

Fluid circulation and vortex shedding

Have you ever been driving in an automobile and their vertical radio antenna begins to vibrate and produce a sound? If so, the tone you heard was caused by the way fluid (air, in this case) will circulate around the rod and set up alternating eddies. The pitch of the tone is the frequency of these alternating vortices.

A “singing” propeller has a tone generated by the interaction between a Karman vortex shedding mechanism from the trailing edge of the blade (though the excitation from the leading edge should not be excluded) and a blade natural frequency. As the free shedding vortex frequency approaches a blade natural frequency, they synchronize or lock in, and the propeller “sings” at its blade’s natural frequency. The blade natural vibration is usually in the form of transverse motion along the chord of the blade.

Consider the accompanying graphics, which illustrate the development of the tone. The fluid flow starts to curl around the body (A). Eddies (vortices) are created behind the body (B). Any asymmetry in the flow direction or in the shape of the body will cause the separating vortices to set up sequential eddies. The force of the unbalanced vortex on the body will impart a sideways force on the body – further promoting flow and shape asymmetry, and the development of alternating vortices (C). Finally, a well-behaved system of alternating eddies and forces is established, resulting in the audible tone of singing that we hear (D)

The propeller trailing edge as the singing body

The basis of using this model for propeller singing is that a rounded trailing edge corresponds to the circular body – an “equivalent cylinder” of sorts.This is illustrated in the graphic below (E).

Mitigation of singing

It has been demonstrated that some propellers sing while others do not, even though the propellers are of identical design. The singing propeller can have one blade or any number of blades singing. The blades can sing at identical or slightly different frequencies. These variations, even though the propellers are nominally the same, have caused much confusion regarding singing propellers, their detection, and cure.” Gutsche (1937) had one of the first papers regarding singing propellers.

These relationships also tell us that singing is a function of propeller diameter and rpm, vessel speed, and trailing-edge size (thickness) and “roundness”.We cannot do much about diameter, rpm 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.

The following graphic illustrates the desired geometry of an anti-singing edge, where points off low separation are spaced both in thickness and inflow-stream position [Saunders, 1957].

Many sources recommend that the anti-singing edge is applied from the 40% radius (0.4R) fully to the tip, or even slightly beyond [Carlton, 1993]. It has also been noted that erosion of the blade edge is a risk if the new edge were made too thin. There is also some evidence that cup can be an effective anti-singing technique. Cupping, however, changes the thrust and power characteristics of the propeller, where an “anti-singing edge” would not measurably alter performance.

References

A HydroComp Technical Report-Report 138

Raymond Fischer-Singing Propellers—Solutions and Case Histories