Leonardo da Vinci proposed a double hull as a protective ship design intended to help a vessel remain afloat after damage.
A double hull places a second layer of structure inside or around the main hull, creating a space between the two surfaces. If the outer hull is breached, the inner hull can limit the spread of water and preserve buoyancy. Leonardo’s notebooks contain ship-related studies that include this protective principle.
The idea was far ahead of ordinary Renaissance shipbuilding. A double hull requires additional materials, careful construction, and a way to manage the enclosed space. Modern designers use related principles in some ships, including tankers and specialized vessels, although modern double-hull engineering is much more sophisticated.
Leonardo’s proposal should not be confused with ballast, which adds weight to improve stability, or with an ironclad, which protects a vessel with armor. Its central purpose was compartmental protection against flooding.