Excuse me, but "take out the escorts first" is really arcade crap and doesn´t belong here.
It is not true that destroyers could spot a U-Boat before the U-Boat could spot them. It was common practice to shadow a convoy for hours, sometimes for DAYS, because this was the way the wolfpack worked: one U-Boat shadowing, leading the others to the convoy. It was no problem for U-Boats to shadow convoys without being seen.
Thanks for your replies anyway.
Cheers, AS