I hate the fact that you can put two torps into a ship (C2 most of the time) without them slowing down....that seems rediculous to me.......just he sheer amomt of extra weight on board (water flooding in) should be enought to slow one of those bastards down..to maybe fall out of the convoy....the only sure way is to nail the engine room, prop shaft, or screw....but flooding does not seem to have an effect on speed.
another bug that has been metioned is the fact that merchants do not slow down in adverse sea states like they do in real life (your sub slows down...but the ships dont!!???)....I've read of accounts of ships in heavy seas not being able to say in formation and falling out of a convoy only to be picked off my lurking U-boats (like wolfs hunting ...spotting the weak and attacking)
do some reading about convoys at U-boat.net:
http://www.uboat.net/ops/convoys/