[onelifecrisis]

Sometimes I'll use 2 torps on a big ship to be reasonably certain that it'll sink and sink quickish. For example, if the convoy is heading into shallow waters and will reach them before I can reposition for a second attack. Or when I'm just feeling impatient.

Otherwise, when there are deep waters all around, I'll usually fire 1 torp at each target (impact, aimed at the bow - this sinks 2/3 merchants in my experience) then escape, flank the convoy, and come in for a second attack. On the second attack I use 2 torps per target to make sure they sink, then escape, then surface and head backwards along the course of the convoy towards where the first attack happened, because any ships which didn't sink on the first attack will usually be found limping along some distance behind the convoy, alone, and can be finished off easily.

It doesn't always work. Sometimes a ship takes 2 torps and doesn't sink. Sometimes an injured ship heads straight for the nearest port instead of carrying on along the convoy course. But mostly this technique works well for me.