How many are there? If there aren't too many, my advice would be:
torp the suckers!!!
It works. They usually circle around you, and quite close too. Just set up a torp, preferably a magnetic keel shot, and look in the radio room to find where they are. If they come behind the stern/in front of the bow, quickly raise the scope and loose the torp!!!
It works with T1 (but you have to be really close) and T2 (bit more difficult to aim).
Another tactic is going ahead flank, and luring them behind you. If their behind you, heading the same course as you, you can sink them with a stern keel shot. It's being discussed somewhere on the forum.
I hope it helps for you!:p