Leonardo da Vinci proposed a double hull for boats to reduce the danger of hull damage.
His naval designs included an inner and outer hull separated by a space. The arrangement was intended to give a vessel additional protection if the outside planking were struck or breached. The principle resembles the compartmented protection later used in some ships.
Leonardo was interested in naval engineering as well as art, anatomy, architecture, and military technology. His notebooks contain ideas for boats, paddles, locks, and other water-related mechanisms. These designs were usually exploratory studies rather than documented production vessels.
A double hull does not make a ship unsinkable. Its effectiveness depends on construction, watertight compartments, materials, and the location of damage. The idea should also be distinguished from a catamaran, whose two hulls are separate side-by-side bodies rather than one hull enclosed within another.