Hi,

very good point. In fact, the flood valve is not necessary. However, sometimes it was used. For example, in type VII U-Boats, the main ballast tank 3 (under control room) had flood valves, while the main ballast tank 1 and 5 (at stern and bow respectively) had only flood holes.
The flood valves were used for security reasons - mainly in harbors. To prevent the accidentally sinking the boat (due to accidental opening the vent valves), the flood valves were closed. However, opening and closing these valves took long time, so on war patrol, they were always opened.

Slightly different thing are main ballast-reserve fuel tanks. Type VII U-Boats had two saddle tanks, which could be operated as fuel tanks, and after the fuel were used - as ballast tanks. They were fitted with flood valves, which were closed, when the fuel was stored in them. When the fuel was used, they were converted into ballast tanks - the vent valves were unsealed and flood valves were opened.

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Regards
Maciek


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