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Rotor ship Buckau. Historical image of a ship powered by Flettner rotors (tall tubes). Rotor, or Flettner, ships use the Magnus effect for propulsion. The Magnus effect is a force acting on a spinning body in a moving airstream, which acts perpendicularly to the direction of the airstream. (KEYSTONE/SCIENCE PHOTO LIBRARY/LIBRARY OF CONGRESS/SCIENCE PHOTO LIBRARY)