Thanks for the explanation. Sailing on purpose in the blind is kind of spooky. I understand that dropping for instance a spanner in a sub that this sound can be heard at large distances due to the fact that the hull will act as a large transmitter antenne all around coupled to the seawater. Also seawater is a very good medium for the propagation of sound waves, about 1500 m/sec, while the losses are small and signals therefore will reach far.
Covering tools with rubber or some other material would be a solution to prevent this kind of man made noise of occurring.