Keep in mind that all subs were trimmed to dive as quickly as possible,meaning that they had very little positive bouyancy. One Torp hit anywhere would destroy whatever positive bouyancy the sub had. A few US subs were hit by circular running torps,(at least 2 boats). Neither survived the hit.

And I can't remember reading about any sub surviving a torp hit.

Not being argumentative, but I'm not sure where the thought process came from that has permeated the bouyancy knowledge here. basic submarine doctrine from now to the turtle is to always find your neutral bouyancy. Thats the norm even US boats in WWII trimmed for neutral in a surfaced and submerged condition. Its the only safe thing to do. The thermal and salinity layers that exist in the oceans do not have a regular location. So if you were triming light (like the current SHIII GWX model) and went into water that would induce a positive bouyancy, or were crash diving and hit that layer, it could/would slow your decent downwards to the point you could not go unless you injested seawater to make your boat heavier. Remember, a "layer" can be temperature AND salinity which the more salinity the lighter your boat will behave. The flip side could be trimming heavy and diving into a layer that will make your boat act heavy. (I assure you that when I've conducted my trim dives, I've sailed right into a layer and needed speed to keep from sinking to the bottom, and floating to the top, sometimes as much as 20k lbs of seawater was needed to be injested or pumped) In doing this you could find yourself going straight to the bottom because the combination of negative layer and negative bouyancy would be more then the engines could pull out. I honestly dont know if the Germans conducted trim dives daily like US boats were required to if they spent alot of time on the surface, but I am sure that when they dove to do a sound check, they checked their trim as well.
Just for an FYI, all this trim adjustment is conducted with trim tanks, not ballast tanks. If you'd like an explanation between the two, just ask.

Frank :cool:

um.. just about every sub in WW2 sunk by a torpedo -broke in half. ;) Think about it.