Leonardo da Vinci proposed a double hull as a form of protection for ships.
A double hull places a second layer inside or around the main outer hull, creating a space between the two surfaces. Leonardo’s maritime studies included ideas intended to make vessels stronger and more resistant to damage. The principle later became important in ship construction, particularly for protecting against leakage after collisions or grounding.
Leonardo worked during a period when wooden ships were central to trade and warfare. A second hull could add structural strength, although it would also increase weight, cost, and construction complexity. His notebooks contain many investigations of water, boats, pumps, and naval machinery.
A double hull is not the same as a submarine hull. Submarines may use pressure-resistant inner and outer structures for different engineering reasons. Leonardo’s proposal is best understood as an early ship-safety concept, not as evidence that he designed a working modern tanker or submarine.