In 1926 an unusual vessel arrived in New York after crossing the Atlantic. This was a converted sailing ship renamed Baden-Baden. Its two masts had been torn down and a pair of 15-metre-high revolving cylinders were mounted on its deck instead. Known as Flettner rotors, after Anton Flettner, the German inventor who came up with the idea, the rotors worked like sails. Not only were they extremely efficient, allowing the vessel to consume less than half the fuel an oil-powered ship of a similar size would use, they also let the craft tack closer to an oncoming wind than its original canvas rigging allowed.

The rotors were hailed as a great achievement before cheap oil caused interest to wane. But the cost of oil has been rising—and not just financially. Ships, which transport more than 80% of the world’s goods, account for some 3% of humanity’s greenhouse-gas emissions, a similar fraction to aviation. Ports are therefore imposing emission limits on marine craft. The International Maritime Organisation has set targets to reduce emissions to net-zero by “around” 2050. The imprecision arises because at present there is no easy way of getting there.