I once attacked a small convoy sometime in early '40 that was being escorted by four armed trawlers. I noticed they were there one at a time; engaged one, sank it, another appeared; rinse and repeat. Once all four were down, I blew up the seven or so ships in the convoy with torps and the deck gun. I almost didn't get the last one; had to resort to hitting it with star shells until it finally exploded
Had to shake off a bunch of destroyers on the way home, though; DC'ing ruined all my guns and periscopes...needless to say the repair crews weren't very happy
I have not since had any equivalent luck.
-Qairyl