A century later, however, as the shipping industry confronts volatile fuel costs and climate change, rotor sails are making a comeback. But unlike conventional sails, which require no power other than the wind, rotor sails require something to keep them spinning. Rotor sails’ rise and collapseIn Boston, naval officers Joseph Kiernan and W. W. Hastings, put Flettner’s invention and theory to the test. With volatile oil prices and the urgency to reduce emissions, Flettner rotors have been making a comeback. Finland-based Norsepower, founded in 2012, has equipped 22 ships with rotor sails as of June 2025 with 17 more under contract.