Leonardo da Vinci proposed a double hull as a shipbuilding feature intended to reduce the risk of sinking.
A double hull places a second layer of structure inside or around the outer hull, creating a space between them. If the exterior is damaged, the inner hull can help keep water away from the vessel's main interior spaces.
Leonardo included double-hull concepts among his studies of ships and water engineering. The idea shows his interest in compartmentalization and in designing machines that could continue functioning after damage.
Leonardo's proposal should not be confused with a ship's keel, which is a primary longitudinal structural member, or with a bulkhead, which divides the interior into compartments. Modern double hulls are especially associated with oil tankers and safety regulations, but the general protective concept is much older.