Question about Type VII ballast tanks

[Feldpost]

Originally Posted by Onkel Neal

...The negative tank seems smaller that I would have expected.

What i red; were the negative-tanks (Untertriebszelle? - about 2x2m²) an aid to get the boat in stronger sea under the water line (assist to flood the main-cells especially in heavy waves from stern). I am sure there were also used for alarm-diving, but they have to been blown-free with compressed air as soon the destination depth was reached... (Source: "Unterseebootbau" page 26)