I'm no expert by any means but from recent experience, I would say for the larger merchants aim for the bow area and for the smaller merchants aim for the stern. Ninety percent of the time a bow shot will take down a large merchant, not necessarily instantaneously but they go down eventually. With small merchants/freighters a stern shot usually brings them to a stop and sometime you get a fireworks display and they go down immediately (usually when I hit a neutral
). In my opinion the smaller ships are more of a challenge to sink with one torp. I always use impact pistols and vary the depth setting depending on the conditions. On the occasions when I have come across warships such as the Nelson, I have managed to sink it with three torps aimed at the bow area. A Southampton Class cruiser on the other hand took two torps one in the bow and one in the stern.
Nemo