> The ships in service show a reduction in fuel consumption, and thus a similar drop in emissions, of between 5% and 25%, says Tuomas Riski, the firm’s outgoing ceo. Each rotor costs around €1m, which he says can be repaid in fuel savings over three to ten years.

> After six months at sea the WindWings have cut fuel consumption by about 15%, although the company reckons three sails could cut average fuel use by 30% or more.

> Oceanbird reckons the sails could deliver 50-60% lower emissions compared with conventional vehicle-carriers.

> Airseas tested a Seawing last year on a k Line cargo ship. They are expected to reduce carbon-dioxide emissions by some 20%, says k Line.

These are consistently good results across a range of carriers. If the industry moved to these sails, the aggregate emissions drop would be significant.