blow
An instance of using high-pressure air to empty water from the ballast tanks of a submarine, increasing the submarine's buoyancy and causing it to surface.
blow: compressed air forcing water out of submarine ballast tanks
A blow is the operation of forcing compressed air at high pressure into the ballast tanks of a submarine to expel seawater and reduce the vessel's weight, making it positively buoyant and capable of surfacing. The air is supplied from the submarine's high-pressure air system, typically stored in flasks at pressures between 3,000 and 4,500 psi depending on the class and design of the boat. When ordered, air enters the bottom of each tank below the waterline, driving the water out through vents in the top of the tank structure.
The blow operation is a critical and noisy maneuver. As air expands within the tanks and forces water overboard, distinctive rumbling and rushing sounds escape the vents, making the submarine acoustically detectable to passive sonar at considerable distances. A full blow of all main ballast tanks (MBT) typically takes 30 to 60 seconds depending on how deeply submerged the submarine is and the design pressure of the tanks. Partial blows are used to control depth and trim during ascent.
Variants and limitations
Emergency blow is a rapid purge of all ballast tanks simultaneously to achieve an immediate ascent from depth, used in casualty situations or during shallow-water operations when speed of surfacing is critical. It consumes a large volume of stored air and can be performed only a limited number of times before the high-pressure air reserves are depleted. Some submarines have an additional emergency air system separate from the main blow system to ensure at least one full emergency blow capability.
Blowing in cold or polluted water can cause issues with tank vent corrosion or blockage. Partial blows are essential during normal submerged operations to maintain neutral buoyancy and compensate for changes in water density, fuel consumption, or cargo weight. A submarine that cannot blow is unable to surface and becomes a significant casualty.
The term derives from the simple descriptive action: blowing air into the tanks. It is distinct from venting, which is the passive flooding of tanks through open vents, and from blowing tanks