Propeller “singing” phenomenon is defined as the resonance between the natural frequency of the propeller blade tip and the vortex shedding frequency at trailing edge of the blade. Propeller “singing” phenomenon as a result of the above resonance of propeller blade tip, creates a very intensive levels of radiated noise. In general the singing phenomenon occurs when the frequency of the response related to the vibratory amplitude of the blade coincides with vortex shedding frequencies of trailing edge of some blade sections.

It is not unusual for same propellers in service to produce a periodic tonal noise that can be heard in the steering gear room, stern tube area, engine room or even in the accommodation. The “singing” noise can take a variety of forms ranging from a deep sounding grunting noise to a high-pitched noise. The phenomenon rather than being anything particularly harmful is a matter of annoyance to the human ear and so has a negative effect on comfort levels on-board operating vessels.

Here a case study is presented, where an annoying noise was reported by a Bulk Carrier engineers, in the engine room and steering gear room area. Audio files recorded by crew at different locations (stern tube area, steering gear room, emergence fire room) was send for assessment.

Hear a sound sample.

Initially the location with the best audio quality in form of signal to noise ratio was identified, and ship was requested to repeat the recordings, from that location (i.e. emergency fire room) in 3 different well separated shaft speeds (i.e. 65rpm, 75rpm and 85rpm). Our plan was to analyse the audio signals and to investigate if that noise was at a constant frequency through the shaft wide speed range or it is a frequency which is affected by shaft speed. Constant frequency in most cases is related to resonance conditions, and in that case that most usual cause for that is the resonance between the local natural frequency of the propeller blade tip and the Karman vortices generated around the trailing edge of propeller blade (“singing” propeller phenomenon).

The audio files at the same day, was recorded by crew and sent for analysis. At FNT we filter the background noise of audio files in order to improve the signal to noise ratio, and we analyse them based on the power spectral density (psd) of each recording.

Based on the analysis, we found that the above frequency was constant in three measurements (800Hz). The most probable cause for the existence of that frequency is “singing” propeller phenomenon. In order to verify the above, a measurement of propeller blades, natural frequency has to be performed in a dry docking, or a mathematical model to be used to estimate it through Strouhal number and water kinematic viscosity.