International shipping runs largely on highly polluting “bunker” fuel, which is why the industry is under increased pressure to play its part in tackling climate change by reducing emissions. Drawing on a type of “spinning sail” invented almost a century ago, ships could reduce their fuel emissions by 10 percent. The spinning, or rotor sail, helps propel the ship because when wind passes the sail, the air flow accelerates on one side and decelerates on the opposite side, creating a thrust force perpendicular to wind direction. The unusual sails are being trialled by Maersk, one of the world’s biggest shipping companies.