Very much depends on what mod are you using. Assumming youre running stock damage model, one well aimed torp is often enough to set the ship ablaze. IF using NYGM tonnage war, things get much more complicated. Id say 2 torps, a magnetic triggered one in the bow to slow the ship down and one somewhere in the sten. Sinking is asured but it takes a whole lot of time to notice its taking on water.
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