4. Propellers

Since a propeller blade is a load-bearing profile, everything said in the previous paragraph applies, but because it rotates, two new phenomena appear.

On the one hand, speed creates a Doppler effect on the frequencies emitted. Since blade tip speed is higher than ship speed, the propeller is generally the dominant noise source, since all hydroacoustic sources depend on U with a power of 4 to 6, unless a structural resonance produces exceptionally intense radiation.

However, the Doppler correction factor is (1 - M ² ) and can often be neglected.

On the other hand, noise will be created by...