What water-powered propulsion device did Leonardo da Vinci design for boats?

Leonardo da Vinci designed a paddle wheel as a water-powered propulsion device for boats.

His sketches show a wheel fitted with paddles that would push against the water as the wheel turned. The motion could be supplied by people operating a mechanism inside the boat, converting human effort into continuous rotary propulsion. Leonardo explored the idea during his wider studies of water movement and machines.

Paddle wheels were not exclusive to Leonardo, and ancient as well as later engineers considered similar methods. His importance lies in the detailed mechanical treatment he gave the concept, including gearing and the arrangement of the paddles. The proposal was not a documented commercial boat engine.

A paddle wheel differs from a propeller because it pushes water with blades mounted around a rotating wheel at or near the surface. Paddle-wheel boats became practical much later, especially during the nineteenth century, when steam engines supplied steady power.