Leonardo da Vinci proposed a double hull as naval protection for a ship.
The concept placed an inner hull inside an outer hull, creating a space between them. Leonardo’s notes connect the arrangement with protecting ships against damage, especially damage that could allow water to enter. The separation also anticipated the general principle of using compartments or layers to improve survivability.
A double hull is not the same as a ship’s keel. The keel is a major longitudinal structural member, while a double hull describes two layers of shell or hull structure. Nor is it simply ballast, which is weight used to improve stability.
Modern tankers and some other vessels use double-hull construction for environmental and safety reasons. Leonardo’s Renaissance proposal was not a modern tanker design, but it shows his interest in applying layered construction to naval engineering.