Alright, lots of answers, thank you.
Don't worry about the technical stuff, i got the game working, it was a pain, because it's on steam so you don't choose where it goes , and i'm on windows 32 bit, so no x86 program files folder
i tried a lot of solutions before it worked on my rig, but i made it.
Back to the game itself, i think i got it now: missed my approach on a small convoy, i was able to surface outside their view range, get ahead of them, then get in position and submerge without being detected.
Oh and thank you for suggesting the 6 min rule (6'30 in my case, because metric system
).
So far i'm getting to a nice spot now, 85k tonnage on 4th patrol and it's not over yet. Didn't get any destroyers yet, but an escort carrier was kind enough to accept my torpedoes ^^
Later the same day, i prepared an attack on a task force... thank god i saw that jap flag, i was about to get a second carrier, but friendly this time.
Of course there are still a few misses (or duds), but it's mainly my fault, and that damn german tdc is... not user friendly.
Next question: i have an officer's ability that reduces the chance of duds. But if i didn't have him, are there ways to reduce the percentage of dud torpedoes?