Just 99 vessels worldwide carry wind-assisted propulsion today, according to DNV data — yet the technology is spreading faster than at any point in its modern history, with Maersk, Odfjell, Wallenius Lines and MOL all moving in the past two months.

The roster of adopters now spans every major cargo segment. Maersk is preparing to fit a rotor sail on a container ship. Odfjell has ordered wind-assisted propulsion for a second tanker. Wallenius Lines is adding it to an LNG-capable dual-fuel vehicle carrier. MOL has launched an LNG carrier already equipped with the technology.

The DNV fleet breakdown shows where the niche has taken root so far:

• Bulk carriers: 25 vessels
• General cargo ships: 24 vessels
• Oil and chemical tankers: 24 vessels
• Ro-ro cargo ships: 11 vessels
• Gas tankers: 8 vessels

Those numbers remain tiny against the global merchant fleet. But the pipeline of installations, orders, launches, approvals and trials logged in recent months points to widening commercial application rather than isolated demos.

Why are owners fitting sails now?

The commercial logic is fuel economy under regulatory pressure. Shipowners face tightening CII ratings and emissions targets, and wind-assisted propulsion delivers additional thrust that cuts fuel burn — an old answer to a very modern problem.

The technology is not a substitute for engines. Ships still need fuel to hold schedules on long hauls, manoeuvre in and out of ports, and keep moving in unfavourable weather. Fitting a sail does not turn a conventionally fuelled vessel into a zero-emission one.

Its appeal is simpler: burn less of whatever fuel the ship uses. That fuel-agnostic quality matters in a market where no winning low- or zero-emission fuel has emerged. Wind-assisted systems work alongside engines running conventional fuel, LNG, methanol, biofuel or other fuels. Wallenius is adding the technology to an LNG-capable vessel, while Veer.Voyage has designed a sail-powered container ship backed by hydrogen fuel cells.

For shipowners hesitating over bunker choices, that flexibility de-risks investment in efficiency now.

Who installed what?

The recent installations cut across vessel types and technology suppliers:

• Bound4blue fitted four rigid suction sails on Marflet Marine's 50,000-dwt chemical tanker
• Leonhardt & Blumberg installed a system on its general cargo vessel Hansa Drejoe
• OceanWings equipped the bulk carrier Maria Topic

Class societies moved in September to bring new designs to market. Lloyd's Register assessed SunRui's technology, ClassNK granted approval in principle to OceanWings' rigid wingsail systems, and Bureau Veritas assessed a new suction sail design from Bound4blue.

What else moved in alt fuels?

A.P. Moller-Maersk bunkered a methanol dual-fuel container ship with an unspecified quantity of 100% bio-ethanol at the Port of Houston. Barge operator Kirby delivered the stem, sourced from biofuel producer POET, to the Tangier Maersk in a ship-to-ship operation.

The European Commission must set a "quantifiable limit" for methane slip from LNG-powered dual-fuel ships under its sustainable investment criteria, according to Opportunity Green's reading of an EU General Court ruling — a decision that could tighten the investment case for LNG dual-fuel tonnage in Europe.

In India, the Ministry of Ports, Shipping and Waterways estimated production costs at the planned e-methanol plant at Deendayal Port in Kandla, Gujarat, at $750/mt. The ministry called that a "much more competitive rate" against the global average e-methanol production cost of $1,300/mt — a spread of $550/mt that could anchor green-fuel supply for bunkering in South Asia.

None of this means sails are about to sprout across the global fleet, and 99 fitted vessels will not move freight rates. But while the industry works out what it will bunker in the decades ahead, one of shipping's oldest engineering principles can help ships use less of whatever fuel eventually wins.