International trade was powered by wind for millennia, before falling from favour with the rise of fossil fuels. Now the decarbonisation imperative is putting wind back in the sails of shipping.
This article was originally published in the November 2024 issue of create with the headline “Sailing cargo”.
Maritime shipping is incredibly efficient. For every tonne of cargo a modern container ship transports, it will emit as little as 10 g of carbon dioxide per kilometre. Truck transport emits at least six times more emissions, while air freight is about 50 times more carbon-intensive.
Nonetheless, the vast scale of maritime freight transport means the industry currently accounts for almost three per cent of global greenhouse gas emissions. Without strong action, this proportion will swell rapidly as shipping volumes grow and other sectors decarbonise.
Achieving the industry’s target of halving emissions between 2008 and 2050 demands a wide variety of innovations, with green hydrogen, ammonia and methanol all being touted as replacements for bunker fuel. While future fuels will undoubtedly be crucial to decarbonising shipping, some engineers are revisiting wind power, with encouraging results.

Cutting the engines
When it comes to sustainable shipping, the most radical approach of all might in fact be the most traditional – relying only on sail power.
A small handful of shipping companies are doing this already. For instance, Fairtransport has been shipping cargo across the Atlantic since 2007 on the Tres Hombres, a 1943 brigantine, and now operates a small fleet of sailing ships.
However, their largest ship has a capacity of just 100 t, a tiny fraction of even the smallest modern container ships.
Recognising that traditional designs cannot offer close to the volume demanded by modern shipping, others are working on designing and building sail-powered container ships.
France’s Windcoop is developing an 85 m sailing ship with a 100-container capacity, 14-fold greater than the Tres Hombres – though still much smaller than the vast majority of the maritime freight fleet. Its two masts will be self-supported rather than guyed, to facilitate container handling.
Its projected average speed, however, is just eight knots, about half to a third of a traditional container ship.

Meanwhile, fledgling Canadian outfit Veer Voyage has plans for a sail- and hydrogen-powered container ship capable of carrying 150 shipping containers at 18 kn.
There have also been a number of attempts, some ongoing, to improve ships’ efficiency with high-flying kites.
The current leading proponent of this approach is the French outfit Airseas, founded by Airbus engineers. Their automated kite system deploys and operates a 1000 m2 kite mounted to the bow. By flying continuous figure-of-eight loops, the system aims to produce up to 100 t of traction, delivering fuel savings of up to 16 per cent.
It is in use on a 154 m cargo ship, while one of Japan’s largest shipping companies is also invested in the technology.
Different spin on wind
Innovators have long been exploring alternatives to sails. One of the oldest technologies harnesses the Magnus effect, in which a spinning body in an airstream creates an air pressure difference on one side.
Known as rotor ships (or Flettner ships, after an early proponent), these ships use tall cylinders mounted vertically, driven by an onboard power source. By spinning the cylinders in the right direction, these ships can harness crosswinds to create forward propulsion.
They were first developed a century ago, but interest revived in recent decades as a way of boosting the fuel efficiency of more traditional ships.
In 2018, Maersk installed two 30 m rotor sails on the Pelican, a 244 m crude oil tanker with a carrying capacity of more than 60,000 t. When operating, the rotors cut fuel use by 8.2 per cent. And last year, Airbus commissioned six large “ro-ro” (roll-on, roll-off) ships for transporting aircraft sections.

Solid state
The International Windship Association estimates that at least 20 commercial cargo ships are using a form of retrofitted wind-assist technology.
The current leading technology for modern sail power is rigid sails, for making traditionally designed and fuelled ships more efficient.
Numerous technologies are being developed and deployed, many featuring retractable or foldable self-supporting structures to capture the wind, with automated control to maximise performance.
In 2022, the 235 m Shofu Maru began shipping coal from Australia with the assistance of a Wind Challenger, a telescoping sail up to 53 m high and 15 m wide, mounted on the bow. Depending on conditions and routes, the sail can deliver average fuel savings of between five and eight per cent per voyage.
The Shofu Maru’s operator, Japan’s Mitsui OSK Lines, plans to launch another 25 vessels with the technology by 2030, and 55 more by 2035.

Since August last year, the 37 m bulk carrier Pyxis Ocean has been operating with two foldable wingsails, deployed on the starboard side so as to not interfere with loading and unloading operations.
Consisting of three adjustable elements (lead, main and trailing), each WindWing is more than 37 m tall. With a combined surface area equal to the two wings of three Boeing 747s, the technology has delivered an average fuel saving of 14 per cent, or three tonnes per day.
This September, four WindWings were retrofitted to the 300 m Berge Olympus bulk carrier, for a projected carbon dioxide reduction of almost 80 t per day, sailing between Brazil and China.

Meanwhile, French shipbuilder Zephyr et Boree has built what is likely the largest modern purpose-built sailing cargo ship, the 121 m ro-ro Canopée.
It sports four articulated wingsails that can provide up to 40 per cent of the total propulsion, and is shipping rocket parts from Europe to French Guiana.