I've spent more time on U-boat.net than I care to admit, but I can definitely say merchants of all sizes routinely sailed home after a single torpedo hit. I even read of the bow and stern staying afloat AFTER breaking in half. Merchants weren't built for battle, but they were made to float. They were made to float while carrying as much cargo as possible. Sometimes a smallish hole in the middle just won't make it sink.
Torpedo damage and flooding is highly area dependent. Aim for the masts if you want a better chance of a one-shot kill: you might just open both compartments to flooding.
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