I find the most important thing is to make sure the torpedo doesn't start its pattern before arriving at the target I aimed at! It's almost as annoying as firing a spread and missing by 5m fore and aft.
You need some range to get a useful number of wiggles so I usually set low speed for the steam flavoured ones. I make the pattern run in the convoy direction. The ocean is a big place and firing at random is not a great strategy. Not sure what the best running depth is - based on primary target or as shallow as possible?
Any luck? Not usually. Maybe you should ignore the above
I tend to do better from "overs" and "shorts" with the straight ones. i get the impression they were not a huge success in real life either. But I'm open to education on that one as well.
If you are inside the convoy... how did you survive until 1943?