Leonardo da Vinci proposed a double hull as protection against underwater attacks on ships.
A double hull places an inner watertight structure inside an outer hull, creating a space between them. If the outer layer is pierced, the inner hull may keep the vessel afloat. Leonardo’s notebooks include ship and naval-engineering ideas that used layered construction to improve survivability.
The principle is important because it separates damage resistance from propulsion or weapons. A ship can suffer an opening in its outside skin without immediately flooding the main interior. Modern tankers and some other vessels use double-hull construction for safety and environmental protection, although their engineering, materials, and regulations are very different from Leonardo’s proposal.
Leonardo did not build a modern steel tanker. His design was a Renaissance concept drawn in the context of military engineering. The feature is also distinct from a reinforced deck or a keel, which serve different structural purposes.