Emergency main ballast tank blow

An emergency main ballast tank blow is a procedure used aboard a submarine that forces high-pressure air into its main ballast tanks. The high-pressure air forces ballast water from the tanks, quickly lightening the ship so it can rapidly rise to the surface.

The partial failure of the emergency main ballast tank blow system, caused by icing of strainers in the air lines, contributed to the loss of USS Thresher in 1963. That accident resulted in substantial redesign of submarine emergency blow systems by the United States Navy.