It makes no sense to specify this thread on Battleship tecnics and where a torp would cause a support beam to flip over, break through the decks, destroy the main toilettes and then fall down killing the chef or something.
As far as I know on both ends of the world battleships had thick armor in the area around their waterline, where they COULD but not HAD TO survive around 4 Torp hits. (there can always be a weak spot in the material and its giving in).
To sink a battleship, I recommend several torps at where the magazines are to cause an explosion with brute force ("explode, dammit!") or to check the draft of the target to hit it below its armor platings (which is hard without magnetic fuse because you risk a dud.)
Sinking BBs and CVs with one torp is possible, but not common as you have to be very lucky for this.....who would try and fire just one torp at such a big bucket???
I never try to save torpedoes when encountering a BB or CV ("Oh this one is nice, but I'll fire just to fishes because the other two are for the next one.") because -like steed said- if you just cause damage, you'll earn nothing.....and by the way I think no Commander in real life would have thought that way -you have what you have. ;-)