Well, my whole career was pretty stupid...
I started with a type II. No kills on my first patrol. On the second, one torpedo kill (coastal merchant) and one flack gun kill (tugboat). I fired two torpedoes at a C2 cargo (she was not in a convoy, but had a destroyer escorting). Both fishes hit the ship squarely in the middle, and it caught fire, but it hardly slowed down at all. I was out of torps and I had to let that one go. I later encountered another coastal merchant. Emboldened by my success against the tug, I pulled right along side it, as it was zig-zagging (not smart) and gave it all I had with the flak gun. We were almost touching at some points, and I had to pull hair raising turns to avoid being plowed over by the ship. I caused all the cargo on its deck to catch fire and explode, but she sustained no other damage. I expended all my ammo and high-tailed it to base (but not before being bombed by several Hurricanes - thankfully I received no hits).
On the third patrol I fired a torpedo for no reason (accidentally hit Enter

) but later I found and torpedoed a Norwegian merchant, (his lights were out) and he caught fire but he continued on his way. I came across another C2, fired my last 3 fish, 2 hit under the funnel, and he was dead in the water, but still afloat. I waited for hours... I expended all my flak ammo, and then proceeded to ram him. No effect whatsoever. I went home without a single kill.
On my fourth and last patrol, I loaded up with 5 TIIs (electric). I had to expend 4 of them on one small merchant.

I decided to sneak into Hartlepool at night to send the last fish into a stationary ship. I came across a fishing boat in the port itself and sank him with the flak gun (I could not resist such an easy target).
Now here comes the really dumb part: as I watch the fishing boat go down, out of nowhere comes this DD with spotlights bearing down at ramming speed, shells whizzing past my head. I go to periscope depth and gun the engines. I narrowly escape him and the depth charges. Then I maneuver silently, for a while, and raise the observation scope for a look. Whoops - he's right next to me, and I got his attention. Searchlights are on me again and he's turning around. I lower the scope, but now I'm out of options. He starts pinging and nails me with depth charges. The sub settles the few more meters to the bottom, and he charges me again, ending my career.
What did I learn: don't push your luck with DDs, stay out of shallow water, ensure the victim never sees you, 1st mission objective is to stay alive, 2nd is to sink targets.