Leonardo da Vinci proposed a double hull as a structural feature for stronger ships.
His ship studies included an inner and outer hull separated by a space. The arrangement was intended to improve protection and stability, and it resembles the basic principle later used in double-hulled ships. Leonardo's notebooks contain many investigations of water, vessels, and naval engineering.
A double hull can also provide a safety barrier: if the outer shell is damaged, the inner hull may remain intact. Modern tankers and some other ships use double-hull construction for this reason, although their materials, calculations, and regulatory requirements differ greatly from Renaissance proposals.
Leonardo's idea should not be confused with a catamaran. A catamaran has two separate hulls side by side, while a double-hull vessel has an inner hull enclosed within or protected by an outer hull. His notes contained concepts rather than a documented full-scale ship built to this design.