Hydrophone (aka passive sonar) extends further than Sound ( ASDIC) extends further than visual.
That means within the outermost circle, they can hear you on hydrophone - if you're making noise and they're gonig slow enough). Within the middle circle, they can find you with ASDIC, with the exception of the blind areas astern). Within the inner circle, the lookouts can spot you relative to what weather conditions allow.
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Raptor
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