To the question about the negative tanks and buoyancy:
Correct, the negative tanks’ purpose was to break the surface tension quickly to get below. The difference was only seconds - the VIIB for example had no negative tanks and wasn’t at a huge disadvantage. These tanks were then blown out anywhere from 8 to 12 m as their purpose had been fulfilled. When the boat was trimmed at periscope depth daily, trim was achieved typically at KF, with a 0.5° - 1° “down bubble” (coarse trim angle changes with trim tanks fore/aft, fine maintenance of trim angle by the aft planesman). The negative tanks were not used to adjust buoyancy, that was the role of the regulating tanks and bunkers (called “regulators”). The contents of those were maintained on the surface, and changes in weight etc. over the course of the patrol would be logged in the dive logbook, and the necessary adjustments made to the regulators daily. The daily trim dive was a validation of the accuracy of the dive logbook (chief engineer and his control room mates were evaluated on this).
The result of the slight down bubble trim would be ever so slight positive buoyancy if momentum, and thus depth control, were lost. The sinking is not realistic.
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