That's kind of like the "how many licks to get to the center of a tootsie pop" question.
Thing about the Mark 14, started the war with 600 pounds of TNT, changed to 668 pounds of Torpex, which had 150% of the explosive power of TNT - so the equivalent of a 1000 pound bomb. The default in game feels like the early war TNT model, but even with that it's possible to sink a BB with one hit if you get the sweet spot;
You can adjust the blast power and damage radius in the \Library\Torpedoes_US.zon file, there's an assortment in this mod;
http://www.subsim.com/radioroom/down...o=file&id=4380