In a german type VIIC there are 5 different forms of ballast tanks with the correct german vocabulary:
1) Tauchzellen = MBT
2) Regelzellen = Regulation Tanks (own translation) for fitting the boat to different watertemperature, salinity, consumption of supplies etc.
3) Trimmzellen = Tanks to keep the horizontal diving position (to compensate the heavy wight of the diesel engines f.e.)
4) Untertriebszelle = crash dive Tank
5) Torpedoregelzelle = are filled immediatly when the torpedo gets out, to compensate the lost of wight and prevent a liftup of the bow
The diesel fuel was also stored in the saddle tanks outside. The fuel was swimming on the seawater in the saddletanks. When the fuel gets empty over the time, more and more seawater gets through the openings on the bottom of the saddletank and the boat gets heavier over the time. So the Regelzellen have to blow out more and more to hold the correct position.
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