Leonardo da Vinci proposed a double hull to protect ships from damage below the waterline.
The concept placed an additional layer of structure inside the outer hull. If the outer shell were pierced, the inner layer could help keep water from flooding the vessel immediately.
Leonardo explored the idea while developing naval and military engineering projects in the late 15th century. His notebooks contain many studies of water flow, boats, pumps, and methods of moving vessels through canals and rivers.
A double hull is not the same as an armored ship. Its primary purpose is buoyancy and damage control rather than stopping cannonballs. Modern ships, including tankers and passenger vessels, use related double-hull principles, though their materials and construction differ greatly from Leonardo’s wooden proposal.