The depth charge was fused to go off at a depth of thirty feet
Shackletons carried the same Mk. XI.depthcharge.
It is a mere 55 years since I did the MOTU course, and was taught about them.
The hole which let the water into the pressure capsule which fired the pistol was covered by a screw cap with a rubber seal, which kept the water out. This cap had vanes on it, so that when it fell into the slipstream the cap would unscrew and fall away, uncovering the hole. To prevent this from unscrewing prematurely, the vanes had a length of soft copper wire threaded through them. A loop at the end of this wire sat in the open jaws of the fusing unit. If the aircraft ditched, or the D/C was dropped safe, or as in this case, fell into the ogg, the wire would stay in place, the vanes would not rotate, and the water would not enter the hole. Only if the Nav "fused" the D/C would the fusing unit jaws close, so that the wire would be pulled from the vanes and the cap would unscrew to let the water in.
!. It is amazing what you can remember when you don't need to.
2.I really was paying attention to Pete C+++++y's lecture.
3. Can't remember where I left the car keys.
4. The depth charge couldn't possibly have gone off.
5. I reckon I would have swum half a mile in about 2 minutes in those circumstances.