you're right they should leave when they run out of DCs... but if you would remain at 20 meters or so and let him do his run above you from stern to bow, when he passes over head go to periscope depth and fire a spread at him without doing much calculating... go to the plot solution on map screen and aim a 2 or 3 degree spread of as many as 3 or 4 torps in his general direction make sure all are on magnetic fuses at about4 meter depth... hopefully at least one will find him.
thats what i do in that situation even on manual targeting you can make something happen... it will cost you a few torpedos but you get out of the situation.
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