With assistance from Albert Betz (German physicist), Jakob Ackeret (Swiss aeronautical engineer), Ludwig Prandtl (German engineer), and Albert Einstein (German theoretical physicist), Anton Flettner constructed an experimental rotor vessel (Buckau later renamed the Baden-Baden) in October 1924 at the Friedrich Krupp Germaniawerft.[11] The Buckau was a schooner refitted with two rotating 50-foot cylinders and was the first vessel built with a propulsion system based on the Magnus effect.[11] Flettner came upon the idea while at the beach with his wife.[2] Flettner used sand, flowing over his rotating hand, to describe the Magnus effect and realized its potential for sail propulsion.[7] The Baden-Baden crossed the Atlantic in 1926.[3] It could outsail normal schooners under moderate to heavy winds, but it was destroyed by a storm in 1931.[12] A commercial ship, the Barbara Rotor ship, was also built and sailed to the United States.[12]