Leonardo da Vinci’s protective ship design used a double hull.
A double hull places an inner hull inside an outer hull, leaving a space between them. Leonardo proposed this arrangement as a way to reduce the danger of a vessel taking on water after an outer layer was damaged. The outer shell could absorb an impact while the inner structure remained intact, at least in principle.
Leonardo’s proposal was a design concept, not evidence that he built a modern ship with a double hull. The idea should also be distinguished from a catamaran, which has two separate side-by-side hulls rather than one hull enclosed within another. Double-hull construction later became important in shipbuilding and in oil tankers, where separated hull layers help reduce the chance of cargo escaping after a collision or grounding.