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Originally Posted by tycho102
Reserve buoyancy is pretty much exactly as it's named. It's whatever extra buoyant force that a ship has above neutral buoyancy.
For subs these days, negative tanks have been discarded since dive times are pretty much....negligable. So a ship is made as neutrally buoyant as possible, and the ballast tanks as small as possible. Fill'em with water and the boat goes down, fill'em with air, and it provides 99.99% of the "reserve buoyancy" the boat has.
For surface ships, the reserve buoyancy is generally a couple times the ship's own dry weight. For subs, it's a bare fraction (~10%).
The measurement of reserve buoyancy is also directly linked to the amount of damage a boat can receive before going under...
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Basically, for subs, reserve buoyancy is determined by the size of the main ballast tanks. The bigger the ballast tanks, the greater the reserve buoyancy (assuming the overall weight of the sub remains the same). The ballast tanks on US subs are typically sized to give about 10% reserve buoyancy when filled with air (thus, a 9000-ton sub that takes on 900-tons of water in a flooding casualty would sink, if it didn't use its propulsion to 'drive' the sub upwards).
Russian-design subs typically have about 30% reserve buoyancy, meaning they can withstand more flooding before sinking.