[UKönig]

I try to place my shots directly amdiships whenever I can. Otherwise, aim for just under or slightly behind the main funnel. Destroy the boiler room. When that's flooded, the power is cut off and the ballast pumps stop working. From that point, water goes in, but not back out. They don't last too much longer after that.
Another good place to aim is a spot under the main mast, forward. Two hits, 1 fwd, one aft in the engine spaces are usually enough to sink all but the most stubborn ships.
On occasion, I have had to feed 3, 4 and in one rare case, 5 torpedoes against one target in order to get demonstrable results (I hate that).