How to escape from destroyers, you ask?
Very carefully
Well, id recomend you change your entire approach to convoy attacks. Pick your high value targets, fire your torpedoes when in position and as soon as that is done start a dive to 150 meters while changing course away from the convoy. Depending on the proximity of destroyers choose the speed of your decent, personaly I wouldnt go over 4 knots in a VII or IX boat unless you are evading depth charges, and when at your target depth go silent and listen on the hydrophones for aproaching destroyers and take evasive action if nessesary, otherways make your getaway and take another pop at the convoy the same night if possible, if not shadow it and then hit it again the next night till you are able to or untill the convoy has been sent into the abyss.