Could be dependent on a mod you're using, some have sinking mechanics built into the mod. Stock game the big 10,000 ton tanker usually takes two fish, sometimes one will do it, sometimes it takes 3 or 4.
This is realistic - an empty tanker sailing in ballast has a lot of airtight tanks filled with air. A full tanker has tanks full of oil or gas, which is lighter than water per gallon, so will also float after several hits. Unless the cargo is set on fire, of course, in real life some tankers took 7 or 8 hits and stayed afloat long enough to be saved, others exploded like an A-bomb from a single hit inducing a secondary explosion from the extremely flammable cargo.
Size of holes, I made the "bang for your buck" more as a "what we should have had" mod than any kind of realism. The history of US torpedoes is a dismal SNAFU of bureaucrats and defense contractors screwing up everything they got their hands on. Just before the war the Mark 14 was increased from a 400 pound warhead to a 500 pound warhead, increasing the bang but inducing the depth problems since they never compensated for the additional weight. Eventually US torpedo warheads increased to 600 and 800 pounds by the end of the war, meantime the Japanese had the Long Lance with high speed, long range and a 1000 pound warhead at the very beginning of the war.
So in my opinion the damage done by the stock torpedo is realistic, but unlike real life you get zero credit for damage, so if the target steams off over the horizon at 25 knots with 4 fish in him, you just wasted four torpedoes for nothing. So I cheat.
