Blister

Blister: The familiar Navy name, from its appearance, for the "bulge" or longitudinal protuberance, somewhat like an elongated cushion, on the hull of Monitors and battleships below the water-line, to minimize damage to the structure of the hull from a mine explosion or torpedoes. It formed an outer skin or exterior hull plating, thus saving the main structure of the vessel from damage. "Bulges" were first fitted in building the Royal Sovereign class, which were discontinued after the Great War.