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Originally Posted by glenno
Interestingly they tell you that a submarine at periscope depth doing 6 knots , you can hear the propellor cavitation (beats) . A submarine at 250 feet depth doing 6 knots will have no propellor cavitation (beats) .
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That's due to the water pressure - cavitation is the noise made by tiny vacuum pockets forming and then collapsing, usually around the propellor. As you go deeeper, the water pressure suppresses the formation of these pockets so you can run a bit faster before you start to cavitate.
Edit: Not a bad article which goes a bit deeper:
http://www.globalsecurity.org/milita...hip/silent.htm