Photograph of the experimental 'Rotor' ship 'Buckau', on sea trials, 1924. Anton Flettner equipped the 680 ton schooner, 'Buckau', with two rotating cylindrical towers to test his Flettner 'rudder'. With the wind in the right place and the cylinders rotating, Flettner believed he could create a 'sail' that was 15 times more efficient than a canvas sail of the same size. Trials showed he was right, but the technology was not reliable enough to encourage commercial use.