What reinforced ship design did Leonardo da Vinci propose by placing two hulls side by side?

Leonardo da Vinci proposed a double-hulled ship by placing two hulls side by side.

The design is associated with Leonardo’s studies of naval engineering and ship construction. Two parallel hulls could provide greater stability than a single narrow hull, especially in rough water. The arrangement also offered a broad platform and could reduce the risk of a vessel capsizing, although the drawings do not amount to a complete tested ship design.

Leonardo examined many watercraft problems, including buoyancy, propulsion and resistance. His notebooks contain both practical observations and speculative mechanisms, so every sketch should not be treated as a finished invention. The double-hull proposal is valuable because it anticipates a configuration used much later in catamarans and other multihull vessels.

A double-hulled ship is not the same as a submarine or a galleon. The defining feature is the pair of parallel hulls, not underwater operation or a particular historical rig. Modern catamarans use related geometry but benefit from materials, structural calculations and naval testing unavailable to Leonardo.