Under normal circumstances you would never blow your Main Ballast tanks as it simulates in the game until your Main Induction/Snorkel Mast was clear of the water. The planes and ship's speed were used to drive the boat to the surface. There is one other large use those air flasks are put to as well, there is a special ballast tank that is only used for submerging and once submerged is used to assist in surfacing. That is what they called the Negative tank. It is called that because when full and the MBT vents were opened it gave the boat a negative bouyancy allowing it to submerge faster. Once fully submerged, this tank was blown until the boat had a neutral or a slight positive bouyancy. There were other variable ballast tanks forward and aft on the ship that were used to TRIM the boat. By pumping water forward or aft into and out of these tanks the boat could be balanced over it's center of bouyancy.
WW II sbumarines had a total of 7 Main Ballast tanks, of these MBT 1, MBT 2A, 2b, 2c, 2d, MBT 6A, 6b, 6c, 6d and MBT 7 were strictly used for diving and surfacing and were permanently open at the bottom. MBT's 3A, 3B, 4A, 4B, 5A, 5B were also fuel tanks and at the start of a patrol were full of diesel fuel. As fuel was used from these tanks a valve at the bottom of the tanks could be opened to allow sea water to enter the bottom of the tank. This is safe to do since diesel fuel is lighter than sea water and will float on top where the piping is to draw off fuel as it is needed.
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