Stena claim that their Flettner rotors (spinny sails) reduce fuel use by 9%. https://stenaline.com/media/stories/introducing-stena-connecta/ https://stenaline.

Stena claim that their Flettner rotors (spinny sails) reduce fuel use by 9%. https://stenaline.com/media/stories/introducing-stena-connecta/ https://stenaline.com/media/stories/introducing-stena-connec... They are in commercial service already on the Irish Sea.