In plate 25, the air line to torpedo compensating tanks 2 and 3, would only be an overflow or overpressure line and could not be normally or never used to blow the water out of these tanks?
This line was used to vent/overpressure the compensating tanks during flooding these tanks either
from sea or from torpedo tubes (when tubes were drained), or to blown water from compensating tanks
to torpedo tubes (but not to the sea).
Why would you want to move water from the compensating tanks to torpedo tubes?
Was it to equal the pressure the tube before opening the outer door?
That's right. To open outer (muzzle) torpedo tube door, the tube had to be flooded and
connected for a while with a sea to equalize the pressure.
To flood the tube, ~370 l of sea water had to be taken (per one tube). If there were
four tubes flooded, over 1 ton of water had to be taken.
To avoid such great change of boat's weight (which was carefully compensated previously),
the water previously stored in torpedo compensating tanks was used.
The water was blown to torpedo tubes by means of pressurized air.
Thus the weight of the boat was not changed, and the change of the trim
was relative small.
After launching the torpedo and closing, the tube was full of water. Then the water from tube
was blown by means of pressurized air to compenating tanks.
I tried to describe process of flooding and draining the tubes in the following text
http://www.ubootwaffe.pl/component/option,com_flexicontent/Itemid,76/cid,66/id,337/view,itemsparagraph
2. Torepedo tube flooding and drainage installation--
Regards
Maciek