Ptah just do what I did just now - run up to a convoy (escorted by two corvettes and a destroyer). Lure one of the corvettes to charge you, and use your superior speed to keep him at around 2500m - at that range he wont hit you but you can hit him. 40-50 rounds later he is a burning wreck. Then the destroyer come swooping in to check whats up. Engage him with your deck gun while twisitng and turning - get up close and personal and take out all three of his gun turrets.... He slinkers off away from the convoy the coward (all ablaze)
(Mind you, my sub has taken a HEAVy beating at this stage, but apart from a destroyed FLAK gun nothing critical - oh and firing your 88mm into his Depth Charge racks in the rear makes for a very satisfying explosion.)
Then rush the convoy, sink ships at leisure until the second corvette comes close enough to actually hit remotely close to your boat - then engage and destroy him with 30 or so of your deck gun rounds... Then laugh demonically while you sail around the convoy reloading and sinking, reloading and sinking.... 2 corvettes, 1 T3, 1 T2, 3 C3's, 1 C2 later set sail for home..... (To bad I only had 10 torps left when I encountered the convoy...)
Mind you, totally unrealistic and my sub is yellow all over (took two days to finish repairs and my experienced gun crew died..... but still).
This was March '41 with GW and Drifters Realistic Damage mod at 100% realism - so I feel like a battlecruiser kapitän right now
Wont do it again tough - it kinda spoils the immersion. I'll revert back to get in, sink a couple of ships - get out, while avoiding the escorts.