Hi,

the main parts of the ballast tank are:
- flood valves (sea inlet valve or Kingstones) at the bottom of the tanks;
- vent valves at the top of the tanks;
- compressed air inlets in the top part of the tanks;

When the sub is surfaced, the ballast tanks are empty (that is they contain air). To prevent the escape of the air, the vent valves have to be closed.
When you would like to dive, you open the vent valves. The air escapes through the vent valves and the sea water flood the tank through the bottom flood valves.
The sub dives. When boat is under water, you close the vent valves.
Now, if you want to surface, you admit the compressed air to the ballast tanks (blow the tanks). The high-pressure air force the water out through the flood valve at the bottom - the buoyancy increases and sub is surfacing.

--
Regards
Maciek


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