I normally do a Kretchmer, and go deep into a convoy , then shack up next to one of the slower merchies,real close, the escorts try to get you but don't, for fear of the merchant, so they circle round, and when they do...
In the early war years it possible to take out all the escorts,even with manual firing(300 meters roughly) ,sure you use alot of torpedoes,sometimes all but if its a calm day and the escorts are taken care of, you go up top and its a wolf in the henhouse...

just stay real close to the merchant.