What defensive feature did Leonardo da Vinci propose for warships?

Leonardo da Vinci proposed a double hull as a defensive feature for warships.

The concept placed an additional layer of timber inside or around a ship's main hull. If an enemy projectile pierced the outer layer, the inner layer could help prevent flooding. Leonardo included the idea among his naval and military engineering studies, where he explored ways to improve ships used in warfare.

A double hull does not make a vessel invulnerable. Its effectiveness would depend on the thickness, spacing, and construction of the layers, as well as the size and location of the impact. The proposal was also different from later modern double-hull designs, which are widely used for structural strength, buoyancy, or pollution prevention.

Leonardo's naval work included many practical and speculative projects. The double hull is notable because it applies layered construction to damage control, a principle that remains important in ship design.