Leonardo da Vinci proposed a double hull to protect ships from underwater attack.
The concept placed a second layer of material inside a ship’s outer hull. If the outer layer were damaged, the inner layer could help keep water out and preserve buoyancy. Leonardo included the idea among his maritime and military studies during the Renaissance.
A double hull has a practical safety advantage beyond warfare: it creates a barrier between the sea and the ship’s internal spaces. Modern tankers and some other vessels use double-hull construction to reduce the risk of oil spills after collisions or groundings. Leonardo’s proposal was not a modern tanker design, but the basic protective principle is recognizable.
The invention should not be confused with Leonardo’s imagined diving equipment or with later submarine designs. His maritime notebooks contain many proposals, including paddle wheels, watercraft, and underwater devices. The double hull was a structural protection system rather than a machine for traveling beneath the surface.