http://www.savefile.com/files/1630195
Essentially the ships of the period, merchant and warship, had similar characteristics when it came to torpedo vulnerability. If you torp a merchant similar to this one illustrated under the stack, you can actually see the engine through the hole in the side. I hit one on both sides and could see the boiler and the engine. This 'test' ship was carrying frieght though to prevent a detonation. However, more modern frieghters will use explosive fuels and may give you more bang for your buck.
Some BBs had extra armor (anti-torpedo bulges) to counter torpedos, but a magnetic detonation would bypass this, only contending then with the double hull on the bottom. It would be worth it, in the right conditions, to attempt a magnetic detonation under the stack and any deviance to either side would be acceptable because the BBs don't use coal and they have munitions. Other than that, I would say place two torps in the same spot to blow through the bulge and then into the squishy engine room. By the way, I have seen a few people say it isn't realistic that a torp that doesn't make contact with the ship would do so much damage, but that isn't true. Just imagine what the shockwave would do to any personnel or equipment in that section. I would easily kill crewmen, rupture pipes, as well as start multiple fires. Like being inside a giant bell when it was rung. I know I wouldn't want to be there when the boiler goes!
http://www.savefile.com/files/1630221
**As a side note, the ship illustrated at the beginning of the post was carrying POWs headed to camps, was sunk by an American sub and when they pulled 2 oily europeans from the water, the Captain of the sub found out that around 600 POWs went down with it and would never take a command again. True story. Well...now that everybody is cheered up, lets sink some ships!