Realistic submerged attack

[hewsu]

Im am currently trying to make a multiplayer mission that would be both realistic and fun. I have so far managed to create a time trigger so that I can only take the most action packed part of a submerged attack: the shooting of the torpedoes and skip over the survive for the next 2+ hours of depth charging.

But I have a problem:

It is way too easy to hit and sink as many ships as you have torpedoes for. Especially as I have included the speed and heading of ships in the mission description (since you probably would have those up allready when you are ready to shoot)

Why was it so hard for a U-boat to sink loads of ships once it got inside a convoy? I checked the Uboat vs. ships sunk figures in convoy battles from http://www.uboat.net/ops/convoys/battles.htm and mostly it was near to 1 to 1 ratio.

Did they only manage to get to the outskirts of the convoy? Or were they detected when they made the first shot and were driven away fast? Or maybe submerged attacks against convoys werent very succesfull when it came to sinking ships? (Surfaced night attacks were quite common I believe)