Well, a torpedo weights about 1,6 tons. If you put this number into this:
http://calculator.tutorvista.com/imp...alculator.html
that would result in something like the enrgy of 1.5kg of TNT.
To put that in perspective here's a video of 2kg of TNT:
Sure, it wouldn't sink the boat, but it might damage the hull, destroy the screws/towed array/planes etc.