What boat-protection feature did Leonardo da Vinci propose using two layers of hull?

Leonardo da Vinci proposed a double hull as protection against damage to a ship’s outer surface.

The idea placed a second layer inside the external hull, creating a protective space between the two skins. If the outer layer were breached, the inner layer could reduce the chance that water would immediately flood the vessel. This is the same broad principle later used in some modern ships and oil tankers.

Leonardo included the concept in his studies of ships and naval engineering. His notebooks explored ways to improve vessels, pumps, locks, and methods of moving water. The double-hull proposal was an engineering idea rather than a documented modern-style shipbuilding project.

A double hull is not the same as a watertight bulkhead. A bulkhead divides the inside of a ship into compartments, while a double hull adds a second shell around the vessel’s exterior.