Another evasive tactic I try to use is, as follows:
I came up inside a convoy and pop a couple of targets. That, of course, alerts the DEs. In looking around, I see that one of my hits has disabled a freighter. She is dead in the water. I mark her bow and stern on the attack map with "X"s and draw a line on each side. Then I dive. The DEs start looking for me as I creep away SLOW. As they begin to move away and I can make my way toward the disabled ship, I come up to periscope depth along side as close to the lines along one of her sides. Then I sit and wait for the DEs to start circling the area. Of course I use good periscope practice so as not to be noticed by the search lights (Night) or their being too close during the day. As a DE comes within range, I pop him. Then I wait for the next one. A little sneaky, but all's fair.

Old_tex