here is a porpoise class sub i just put 8 torpedos into her and visually she shows damage but even with 3 huge open holes in the hull it wont sink or even list!
http://gallery.filefront.com/keakar//1190378/
somehow this needs to be fixed and im sure its the same "bug" that causes most of the ship to ship collisions to not cause damage as well.
PS - its not just the porpoise its every single sub in the game, (US, GE, and IJN) the sen toku was made sinkable only by turning a freighter into a sen toku not the actual sub itself
there is a mod that makes the sen toku sinkable but it did this by moddng a freighter into looking like a sen toku not the actual sub itself.