From my discussions with DerStosstrup:
U-boats were not trimmed for negative buoyancy at periscope depth. Instead, they were meticulously balanced to achieve neutral buoyancy at periscope depth at a slow speed with the bow angled slightly down.
Before a patrol, a U-boat's weight was adjusted in a test called the
Trimmversuch ("trim attempt"). This established an amount of reserve buoyancy and established the starting point for the dive logbook
(side note:- an example of this logbook is shown on the crew/boat management screen [F7] in GWX KC). The boat was brought down in a static dive in the harbour until the top of its periscope shears (the housing for the periscope) were sticking above the sea surface. This establishes an amount of reserve buoyancy to the tune of the amount of pressure hull protruding. The trim attempt is generally stopped at just under 2 tonnes of reserve buoyancy, but > 0.
To maintain periscope depth (11-13m), the crew would need to set a slight downward angle on the boat. As the boat moves forward, water flows over the dive planes, creating a downward force to counteract the positive buoyancy. Without forward motion, the dive planes lose effectiveness, and the boat would very gradually rise to its condition established in the trim attempt, with scope shears just poking out.
As the boat dives deeper, water pressure compresses the hull, reducing displacement and decreasing buoyancy. To compensate, a Type VII U-boat would need to drain approximately 1 tonne (1,000 litres) of water from its regulating tanks per 100 metres of depth. If we assume a loss of buoyancy of about 5% of that, which would be about the loss going just to periscope depth, that’s the equivalent of about 50 kg of weight. If, during the trim attempt, the boat had established say 0.5 tonnes of reserve buoyancy, and it loses only 50 kg due to the constriction of the hull at periscope depth, then it stands to reason that, in order to maintain depth, we need some speed and we need to angle down a bit. And this is exactly what we see in the diving regulations. Without forward motion and dive plane adjustments, the boat would gradually rise back to its trim attempt state.
^ The left-hand scale of the Papenberg gauge in the control room (U-995) is marked with reserve buoyancy graduations; the blue water level added to the photo indicates 0.5 tonnes of reserve buoyancy against the outline of the scope shears.
Gameplay context:
Positive buoyancy was a feature of GWX3 and is retained in GWX KC. The mod doesn't perfectly simulate every aspect of U-boat buoyancy — after all, SH3 isn't a full physics simulator — but it captures the essence of positive buoyancy and dive plane dynamics. And so it's fair to say that what the team achieved leans in the right direction for players who seek to add more elements of realism to their gameplay.