I've sunk 29,846 tons so far using the Dick O'Kane technique. It works for U-Boats exactly as it does in the Fleet Submarine. It's deadly accurate and you set up the whole attack usually at least 15 minutes before you fire torpedoes. You can bake your victory cake while you wait for the target to arrive!
I've hit 5 of 7 torpedoes fired so far and sank 2 tankers and a cargo ship, all American.