First of all: Thanks everyone for the answers, especially RockinRobins for the quick rundown of how the method actually works. Since i took the video I posted as the start point I was under the impression that you take the TDC out of the equation and try to get exact 90° angles.
I now see how stupid this is since the TDC can mitigate a lot of the errors you make in the setup and will use this method in the future.
However: The method I described should still work as long as you are exact and after long testing i finally found the answer.
I was about to rip my hair out after trying it exactly like RockinRobins said but still got no hit. It wasn't because I set anything wrong.
It was the
Torpedo Depth. Either I am very unlucky or it's normal that they all run too deep. I alway set the depth to a few feet above the targets draft but never got a hit (I thought the were passing in front of the target but after checking the attack map again they were all more or less dead on).
As soon as I set the depth to about 5 feet I got hits, with the method I tried first and with the actual Dick O'Kane RockinRobins described.
Now I can finally start my actual carreer and not get insane after repeatedly not getting any torpedo hits.
Well, I still have a lot to learn, especially about attacking convoys and avoiding screens but at least I can reliable sink single merchants now
So, thanks again for the help everyone!
Edit: I also figured out the problem with the savegames. Seems like the game doesn't like it if you load a savegame when you are still in the sub. When I quit the game and then load the savefile, everything is fine.