A new age of sail begins

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.