Harnessing wind power with rotor sails

In February 1925 a ship named the Buckau began its maiden voyage from Danzig (now Gdańsk, in Poland) to Scotland. The vessel was wind-powered but its sails – if you could call them that – were radically different to those previously seen at sea. Instead of sheets of canvas, two large metal spinning columns stood upright on the deck. Designed by Anton Flettner, a German engineer, these were rotor sails – and rather than catching the wind as a traditional sail would, these ‘Flettner rotors’ made use of the Magnus effect, in which a rotating object generates propulsion by creating a pressure differential as wind passes through it. 

The technology was poised to take off; then came the Great Depression. Diesel fuel was cheap and environmental factors were of no concern, so there was little incentive for shipping firms to invest. It would take nearly a century before interest in the concept was reignited. This time driven by pressure within the maritime industry to decarbonise, and a new openness to low-cost, green alternatives to fuel.

It is why companies such as Anemoi have emerged, a key player among a new generation of rotor sail producers. Having started out as a research-driven spin-off from a shipping company in 2007, it has since provided 11 sails for three ships; and after entering the commercial market in 2020, Anemoi is currently producing 30 more across eight vessels, including retrofitting the world’s largest bulk carrier, with many more in the pipeline. The 70-person team is headquartered in London, with test and engineering facilities throughout Britain and manufacturing in China. Its focus to date has been large bulk carriers and tankers – a segment with about 20,000 ships – but it recently won a £1.2 million British government grant to develop its sail design for medium-sized vessels, too, a segment with an even broader customer base.  

At its helm is Kim Diederichsen, who joined as CEO five years ago. A master mariner, Diederichsen has spent much of his career steering vast cargo vessels and oil tankers round the globe, deciding 20 years ago to take this experience into leadership work at the intersection of technology, energy and shipping. It is a pivotal moment for the sector, too. Shipping accounts for about 2 per cent of global energy-related CO2 emissions. In order to be on track to meet net-zero targets by 2050, the industry needs to reduce its emissions by 15 per cent in the next six years. Rotor sails, which enable a vessel to switch to wind propulsion whenever the conditions are right, can reduce fuel consumption on a voyage by up to 30 per cent. Wind propulsion is not only winning over shipping, but represents a vibrant new industry of its own. It is estimated to grow from about £300 million to £2 billion by 2050.

“What’s happening right now is that fossil fuels are being phased out for transitional fuels,” says Diederichsen, nodding to the use of fuels such as ammonia or hydrogen, or maybe even nuclear at some point in the future. “But all these fuels come with limited availability and cost. Wind is not only a transitional support to propel a ship, but wind will always be there. Irrespective of fuel type, wind can reduce the consumption of whatever the future fuel will be.”

In the years that followed Anemoi’s beginnings in 2007, it trialled different technology prototypes with its first full-sized rotor sail being installed in 2015 on a land-based test facility. Three years later the MV Afros, a dry bulk carrier, sailed from China to Australia with the help of four Anemoi rotor sails, winning industry acclaim and setting the company up for market launch. Its biggest challenge – and one that Diederichsen was recruited to tackle – was how to shift from a research-based to commercial operation. It meant rapid scaling; hiring 40 new people and setting up a production facility in China. The entire supply chain now involves more than 200 people. The next step will be mass production, rolling out sails for entire fleets at a time.

Though the technology is a century old, Anemoi has had to overcome numerous technical hurdles to make a product that is fit for modern shipping. One of the key obstacles was how the rotor sails would be fixed to ships – particularly as they need to be retrofitted, or fixed onto ships that may already have cranes on the deck. Anemoi currently offers five sizes of sail, between 21 and 35 metres tall, that have been designed as movable assets, suitable to be redeployed to different vessels according to need. “We had a strong belief historically and data-wise, that a spinning cylinder and an airflow will generate thrust,” says Diederichsen. “Where we find ourselves today is that we use an existing technology that we know works, but we have overcome the way to install them.” 

What is particularly striking is the way wind is being adopted more broadly – there are many wind technologies including rigid sails, suction sails and kites. Driven to support a green transition, Diederichsen does not see the different technologies as a threat. “My view, and also that of the rest of the team, is that we love wind. So, we would very much like to see wind technology in general have an uptick.”