I've only had 2 blow in half so far. One was a medium merchant that was struck just forward of the bridge. Cargo or fuel bunker must have been set off.
The other was a gunboat at anchor in a harbor. Even at shallow setting, the torpedo will go under the hull. I set the magnetic detonator and when the torp went under the hull... BOOM
Blew it in half and almost totally out of the water. The stern sank, but the bow section floated for a while before sinking.