[Corsair]

The way I used them most until now is loaded in the stern tube (or tubes depending which sub) set 5m deep and magnetic. Surface at safe distance from DD but still close enough so he sees me and start chasing full speed (which means homing will work) Dive to periscope depth, shoot when DD is 2000/3000 m away. Dive deeper at slow speed.
Almost every time a hit, usually the screws gone on the DD becoming a sitting duck.
Turn around, align for a bow shot, periscope depth and finish it off.
Used this most of the time in the Med when you often encounter 1 or 2 merchants with one escort.