Leonardo da Vinci proposed a double hull by placing one ship hull inside another for added protection.
The concept uses an outer shell and an inner hull separated by a space. Such spacing can help protect the interior from impacts and reduce the chance that damage to the outside will immediately flood the vessel. Leonardo included this kind of shipbuilding idea among his studies of naval engineering.
The design belongs to a broader family of Leonardo's maritime inventions, which also included pumps, boats, and methods for handling water. His notebooks show that he approached ships as systems of buoyancy, structure, propulsion, and defense rather than as simple wooden shells.
A double hull is not the same as a catamaran. A catamaran has two side-by-side hulls that both contribute to flotation, while a double-hull arrangement places a protective outer hull around an inner one. Modern tankers and passenger ships use related safety principles.