Just failed my main campaign, too, taking on an enemy convoy in the North Atlantic above the UK. I had thought it was just a merchant ship and a warship (guess I wasn't paying attention).
I had already lost two torpedoes sinking a British/Canadian (their white variant of the flag looks the same, I don't know my history) coastal freighter in the sector I was supposed to be patrolling. I had three left (it was early 1940, and I still had the IIB ship). The convoy was too good to pass up, though!
So I finally caught up with the convoy. The warship opened fire, and I sank to periscope depth rather than try to take it head on. For some reason, I decided that letting the Weapons Engineer fire for me was a good idea. He missed. Or rather, the warship DODGED the torpedo by taking a three point turn. I was up against a very skilled crew!
I went to manual and decided to have a torpedo take them in the side (this was after I'd already dodged some depth charges). They saw this ploy immediately and swerved so they were now coming directly at me. Unfortunately for them, my torpedo was set to make a sharp curve. It wasn't quite the angle I was hoping for, but it would do!
But it went right underneath the British ship. My crew of 28 (including me) sighed in despair. Until the torpedo smashed into the propellor and blew the warship to pieces. We all cheered and I immediately ordered a surface so we could stalk the merchant with our final torpedo. We almost killed ourselves by surfacing directly into the burning husk of the warship on the way up, but U-6 got out with only some minor scratches.
There was a second warship. It fired a cannon round into our hull, flooding the diesel engines. We immediately took to the depths once more. The warship took chase. The crew fixed the electric engine rather quickly and off we were. It was a brilliant chase. At one point we even did a neat maneuver where we surfaced for eight seconds, repaired the diesel engine, and plunged into the depths again before the warship could fire its cannons. Eventually, the SO gave us the message that the warship was moving away. It was time to surface and get our bearings.
The top of the submarine smashed into the bottom of the warship. I had just enough time to notice (by which I mean the attack periscope was shorn off) and order a plunge to 20 feet before it started dropping depth charges on us. U-6 did pirouettes from those explosions, and the SO (as well as some spare guy in the bow quarters) was killed for his lies. I tried my best to get us back up, but it was to no avail. The U-6 sank to 85 metres (I assume that is where the ocean floor was) and we all drowned.