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.
Example: a propeller with a radius of 1 m rotating at 10 rpm has a tip speed of 63 m/s, i.e. a Mach number of 0.04...
On the other hand, noise will be created by...
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