What Leonardo da Vinci invention used two layers of hull to improve a boat's resistance to damage?

Leonardo da Vinci designed a double-hull boat with two layers intended to improve resistance to damage.

His notebooks include a boat concept with an inner hull and an outer hull separated by a space. The arrangement resembles a protective shell: if the outer layer were breached, the inner layer could help keep water away from the vessel's interior. Leonardo explored the idea while studying naval engineering and methods of constructing strong, maneuverable boats.

A double hull is not the same as a catamaran. A catamaran has two separate hulls supporting a deck, whereas a double-hull vessel has an inner and outer shell around the same main body. Modern tankers and other ships use double-hull construction for safety, but their materials, regulations, and engineering are far more advanced. Leonardo's sketch was a conceptual design, not evidence that he built a full-scale double-hull ship.