Leonardo da Vinci recommended a double hull to protect ships from side attacks.
His naval notes included the idea of separating an outer hull from an inner hull. The space between them could make a vessel harder to penetrate and could help limit the effects of damage to the exterior shell.
The principle resembles the compartmented construction later used in some ships and submarines. Leonardo's proposal should not be confused with the double-bottom rules used in modern commercial shipping, which have specific safety and pollution-prevention purposes.
Leonardo's maritime designs ranged from practical equipment to imaginative warships. Many were recorded as sketches rather than completed vessels, so the double-hull concept is best understood as a proposed naval engineering solution.