It's a big boat, with a high freeboard when surfaced, so I would say ballast volume is going to be in the ball park of about 1.5 litres. I'm sure OTW do a module for this, possibly very similar to the one they do for the Type VII as the boats are of a similar size and both twin screwed.
OTW and Sheerline modules both use water pumps, however although there are differences in the way these systems work. OTW vent the air from the tank into the dry space of the cylinder, and use a positive displacement pump which works in either direction, so the water is pumped in and out.
The Sheerline system pumps water into a sealed ballast tank with a centrifugal water pump, this pressurizes the air inside the ballast tank, to surface a pinch valve is opened and the air pushes the water out, the pump only works one way.
Coming along to the Dive-in at Brockwell Lido on the 10th October? There will be boats present with all these systems, plus Engel boats, scratchbuilt etc.
Andy