Well here's a true story:
On my second mission to the Solomon islands I come across one large modern tanker, unattended. I've had problems with these mammoths before so I take no chances and fire all six of my bow torps at her. They all hit home, one after the other. That only slowed her down a bit and she's got a gaping hole on the side and a small fire.
I cut in front of her and fire all 4 of my stern torps, no misses. Another gaping hole and I can now literally see through the ship.
No worries, she's still proudly making 5 knots! I'm a little angry now and meanwhile my bow torps are loaded so I turn the boat around and fire another six, all hits.
She's now listing heavily, she's ablaze and has slowed down to 3 knots but she doesn't mean to go down, so I wait a while to see if she will oblige. And sure enough, she's picking up speed again! I couldn't believe it!
Same drill again, I turn around and fire another four to finally see her slowly leave the surface...
20 torpedos...