In my Karlün days we never wasted torpedos on escorts. They were soly used on the transports.
What i did was the overtake and slip in through the usual gap in the forward quater of the convoy. Then i would shoot rear torpedo first, then the front torpedos. Then move submerged at 20 ft. and Flank speed for about 1-2 min's in the direction of the convoy. That would most of the time have the side DD's and rear DD's react to my first attack and steam in on the "last known position" that i was racing away from. But they would rearly get close enough to get a soundlocation fix on me before i slowed down again. AKA i was in the clear for a new attack while they droped DC's on the wrong spot and generaly just made a mess up for the convoy and slowed it down letting me get into an even better position.
Racing at 20 ft. was allso my favourite tactic in the overtake it self in the event that one of the DD's protecting the side of the convoy detected me somehow. If a DD started to beam in on me i would first change course to directly away from the DD (usualy a 80 - 90 deg turn away from the convoy) and race ahead on Flank speed. Then i would dive at the moment he made his first gun shot at me. Dive to Periscope depth. Still in Flank and then make a turn back to convoy heading. I would then periodicaly check the DD's distance to me and when he had closed to about 4000 meters i would decrease the speed down in steps to 2 kts. and then slowly tiptoe away. This maneuvre would make the DD "calculate" a "proberble spot" for me that would be off with 90 deg's and the speed vector of 19 kts compared to my real course and the submerged Flank speed. The submerged Flank speed were sufficient to take me out of his search pattern and leave him searching for nothing while i could continue on convoy course (albeit on 2 kts untill i was either far enough from the DD or he left to join the convoy again).
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