Hi Guys
Not knowing anything about the management of ballast tanks and air pressure inside a submarine I find all this fascinating stuff. So let me add another "log on the fire". The reference here is Gallery, "Twenty Million Tons Under the Sea". He is describing the sequence of events immediately following the first boarding party from USS Pillsbury on to U-505. Lt.jg Albert David and a few others have gone below leaving a man on deck. U- 505 is taking on water and is now so low in the water that the seas are breaking over the conning tower hatch and spilling down inside the sub adding to the problem. David orders the man on deck to close the hatch which he does while those below continue their work.

A few minutes later a salvage party under command of Cdr. Earl Trosino arrives and they can't get the hatch open which they believe is caused by a partial vacuum inside the sub. So they fish one of U-505's crew out of the water and he shows them "a little valve which let air into the pressure hull" thus equalizing the pressure inside the sub and allowing them to get the hatch open. I don't understand this. It seems to me that if the hatch were closed and water is entering the sub the effect would be to create a positive pressure inside the sub. What could have been occuring to create a partial vacuum? Any explanations?
Don B