SilentPrey is correct in his memory. A ship traveling at less than half its rated MaxSpeed (found in its .sim file) will not be heard when
you manually use the Hydrophone......even if it's within viewing distance. Your AI Sonar man will hear it, and report its position, but you won't hear a thing.
Mods that tinkered with ship speeds between waypoints didn't know this (I just found this out a few months ago) so, many times you won't hear the target when you should.
The stock game avoids this by not slowing down individual targets, or convoys as many mods do. The idea of changing a ships speed between waypoints put "unpredictability" into the game......but it also shoots you in the foot!! We sometimes don't know when to leave well enough alone.
