The ship-protection innovation Leonardo da Vinci proposed to improve resistance to hull damage was a double hull.
A double hull places two layers of hull structure around part or all of a vessel, leaving a space between them. If the outer layer is breached, the inner layer can help keep water out. Leonardo included related shipbuilding ideas among his studies of naval engineering and watercraft.
The principle is not identical to modern tanker construction. Contemporary double-hull vessels use engineered compartments, structural standards, and safety systems developed over centuries. Leonardo’s proposal was an early conceptual example of separating the external skin from the main watertight interior.
Double hulls can improve protection, but they also add weight, cost, and construction complexity. The idea should therefore be understood as an engineering proposal rather than proof that Leonardo built a modern ocean-going ship.