Hi Stiebler,
thanks for the reply! After thinking more about it, I realize that there is one thing I did not consider: the water pressure at great depth might be just too high to pump any water out
IIRC, in the original U-Boot handbook it's written at which depths the pumps are still working. I have to check it.
So, when you dive deeper, the pressure compresses the boat, you loose buoyancy, then you start to sink, and because of the high pressure you cannot compensate this by trimming. Sounds reasonable.
Cheers, LGN1