As long as you don't cavitate you can consider yourself "silent".
In game your noise emanation increases very slightly every 4-5 knots of speed increase.
Except for the Kilo, you never are really silent, as the nuclear reactor can't be shut down...for example my normal speed wit a Seawolf is 15 knots, even when hunting...
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If you are going through hell... keep going (Winston Churchill)
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