depending on the mission, the destroyers may have been set with you as detected. If you start on the surface and they are within about 4000m they will detect you.
You can avoid them and lose them. Remember that when they drop charges you have a period of time after they go off to go flank, change depth and direction. 15 seconds after the last charge goes off, you better be engines off or at least ahead slow.
When they are closing in on you (and know your position) go at flank. They see you anyways so just go like hell. When they are within 1000k meters, cut your engines and go hard to port (or starboard). As soon as the charges roll, go flank, dive and turn the opposite direction (unless of course they are passing you on that side). Detection is based on depth, distance and sound.
Hope that helps.
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