Spot on Pen!
@Gorskov,
GWX is all about getting into a good position and setting the trap. A U-boat is an ambush predator, not a chase and kill predator. The chase should be done out of sight of the convoy. You should get ahead of them and then lie in wait. I've lost count of the number of times I've sat at around 18m, silent and engines stopped waiting as the forward escort rumbles by. When you are in his baffles, slow ahead up to PD and then commence the attack when the targets are in range, order the boat to 100m still at slow, immediately after firing and then slide out the back of the convoy as the torpedo hits register. By the time they locate you you are long gone.
All the info on how to do this in on the boards here.
That said I've posted elsewhere on being able to do a surface approach at flank speed and survived after sinking a Large Merchant!
I also have had my share of getting creamed, usually because I make a mistake. Certainly not because the GWX AI is too hard.
I haven't sat down and accurately calculated it, but from practise and patience I now survive more than 90% of encounters in early war and more than 50% in late war, buddy!
What's your hurry?