When the destroyer is starting to search You [or start attack You], you should dive to 140 - 150m at 5-7 knots. If destroyer is very close and moving fast then You should set speed to max, hard turn left or right, and dive to 150 - 200m. Also You can dive under some ship. When You are at this dept [150 - 200m] try to move slow about 4 - 8 knots. When pig is start dropping bombs on you, simply move faster and escape from attack zone. When he loose You, then You should move slowly
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Also remeber to check [better on map] depth under Your keel.