My experience over the past weekend was with one of those big ocean liners.
I fired all 6 torpedoes, 2 aft and 4 forward, (Porpoise), and 200 rounds from my aft deck gun and never did sink it. It did look worse then a block of Swiss Cheese, tho. It had a bazillion holes in it.

I left it there and had to head back home for more ammo to complete my mission. I never did get credit for it.