Abstract: A moving-fluid-operated rotor for the utilization of a portion of the energy of a moving fluid, comprises a rotor rotatable about a vertical axis. The rotor has four blades, or wings, which may, for convenience, be described as semicylindrical elements. The first and second elements are of equal diameter and are arranged in the form of an S-shape with the diametral planes passing through the two edges of the semi-cylinder being coincident for the two wings. The other two wings are semi-cylinders of larger radius than the first two. They have their diametral planes coincident with those of the first two elements, and they are each substantially coaxial with one of the first two elements. Wind flow into the open ends of the first and third wings passes along the curvature of these elements and is redirected into the corresponding curvatures of the second and fourth wings, so that the wind exerts a forward pressure on one set, and a reverse pressure on the opposite set, providing additive torque to the rotor.