I usually play games purely for entertainment, so it doesn`t matter if I dont get the best results.
That´s the reason I usually use impact fuses, I found it more entertaining to see the target ship slowly cutting it`s speed, slowly starting to take on water and start to sink.
With magnetic fuses, I found there`s not as much entertainment on watching the ship go BOOM, split in two and sink rapidly.
Oh, I also hope that the ship dont sink after my 1st torpedo, I just want it to go to full stop, sink a little but not completely. Then I can maneuver myself to a perfect angle, surface and pretend to be in the scene of Das Boot, where they launch another torp to finish off the tanker.