Robert:

Yes, I am familiar with that site, and the procedures you quote.

I think what J.T., Scott and I are saying is that on WWII U-boats it was desireable to get the negative tank empty at shallower depth rather than doing it at depth for a number of reasons, one of which is that doing it at depth would raise the internal pressure. It also uses high pressure air - can't use 225# air to blow a tank at 600 feet!

High internal pressure was not desireable on a WWII boat because it takes time to safely vent it down and allow the lookouts to get topside. Not forbidden, but not desireable. It was quite dangerous for a U-boat to spend much time with the hatch cracked slightly on the surface but without lookouts. Generally you swung the hatch open and climbed up seconds after the conning tower broke water, even as the boat was still not done surfacing.

As for blowing the negative dry, I'm not precisely sure what the U-boat procedure was. US fleet boats used negative as a variable tank, - maybe U-boats didn't and would want it dry. Not sure.

I've read wartime accounts of boats cracking the hatch as you describe, it had to be done occasionally, but it was avoided if possible.

Perhaps procedures changed a bit with post-war boats.

Congrats on being an silent service vet.