1 A portion of the keel in early submarines which could be detached in emergency to give additional buoyancy. By releasing this portion of the keel, usually weighing up to about 20 tons, a submerged submarine had a chance of returning to the surface if it could not regain positive buoyancy by any other means. It was released by a mechanism within the pressure hull. As the submarine developed into more reliable forms, the drop keel was omitted from the design.
2 A term often used to describe a centreboard or dagger plate.