The ultimate future fuel may mean going back to the past to take advantage of carbon-free wind energy.

The shipping industry has been sceptical about modern sail concepts, but there is a proven technology — first developed in the 1920s — that has shown demonstrable fuel savings in real-life tests on commercially operating ships in the past few years.

Finnish company Norsepower has fitted Flettner rotors, which it calls rotor sails, on three vessels — the 9,700-dwt Bore ro-ro Estraden (built 1999), the dual- ­fuel, 57,565-gt cruise ferry Viking...