With Large Tankers I usually fire just one torpedo at first, with magnetic trigger set to 1 metre beneath the keel, and aim for just behind the forward bridge area. This will almost certainly cause enough damage to stop her in her tracks, if she doesn't break her back. if still afloat, one more eel with impact trigger set to around 4 metres fired at the engine room / fuel bunker area should send her to the bottom, with a nice fireworks display for your crew to enjoy!!
Obviously, if it's a convoy situation with escorts hanging around, it pays to fire off both torps together, with only a 8-10 second gap between, before getting the h*ll out of there!