Leonardo da Vinci designed a double hull to help a ship resist flooding after damage. His notebooks include a proposal for two layers of hull material separated by a space, an idea resembling the protective double-hull systems used on some modern vessels.
A second hull can provide a barrier if the outer layer is punctured, helping keep water away from the interior spaces. Leonardo's broader studies of naval engineering also considered pumps, pontoons, and methods for moving vessels through water.
The design was a conceptual Renaissance solution, not a standardized shipbuilding method adopted by Leonardo's contemporaries. It should not be confused with a catamaran, whose two hulls are separate side-by-side buoyant bodies rather than nested protective layers.
Modern double hulls have become especially important in tanker design, where the additional layer reduces the chance of cargo escaping after a collision or grounding. Leonardo's sketch is an early example of applying layered protection to ship construction.