With the disappearance of large sailing ships from the cargo shipping industry in the mid-20th century, technological advances in sailing technology remained largely confined to the yachting sector. Admittedly, some engineers continued to explore ways of transporting goods using the power of the wind. This is evidenced by the idea of the Dyna-Rig, which was developed in Hamburg in the 1960s. However, new materials such as carbon fibres were primarily tested on racing yachts.

With the climate debate becoming increasingly urgent, a new search began for systems capable of harnessing the wind to propel ships. The International Windship Association IWSA has established itself as a leading lobby organisation in this field. In most cases, the focus is on wind assistance to reduce fuel consumption. Ideas for harnessing the power of the wind as automatically as possible are highly favoured. These technologies thus position themselves as the antithesis to the idea of transporting cargo manually and without an engine, which presupposes a far-reaching ‘partnership on equal terms with nature’.

The Flettner rotor

Anyone who plays a ball sport knows that a rotating ball curves. This is due to the Magnus effect: the rotation accelerates the ‘airflow’ around the ball on one side and slows it down on the other. This creates a pressure difference that deflects the ball sideways.

As early as the 1920s, the German inventor Anton Flettner came up with the idea of mounting cylinders resembling advertising columns or chimneys onto ships and setting them in rotation using motors. A few ships were built with these Flettner rotors, demonstrating that the principle worked. The idea then largely fell into oblivion until the 2000s. The wind power company Enercon commissioned the construction of the ‘E-Ship 1’, which was equipped with Flettner rotors alongside fossil-fuel-powered engines from the outset.

Although the hull, rudders, anti-fouling coating and so on were already optimised for wind propulsion during the design phase, the fuel savings achieved by the Flettner rotors on the E-Ship 1 amount to only around 15 per cent.

The disadvantage of Flettner rotors is that the force is always generated at right angles to the wind direction. If, for example, the wind is coming from behind, the Magnus effect, which acts sideways, does not contribute to the ship’s propulsion. For this reason, the Flettner rotor is only suitable as an auxiliary propulsion system. This is particularly true for ships operating on routes with predominantly crosswinds.

 

The Aero Rig

The Aero Rig consists of a free-standing, rotating mast that is firmly connected to a horizontal boom extending in front of and behind the mast. A conventional furling jib is mounted on the section in front of the mast, whilst behind the mast there is a conventional mainsail with full-length battens. The jib sheet is routed onto a short traveller so that it automatically switches sides when tacking. The angle of attack of the entire system is controlled by a single line, the mainsheet. Both sails are thus always trimmed to a position that, on a conventional yacht, corresponds to the efficient trim on a close-hauled course. Wind tunnel tests have shown that this rig design is aerodynamically superior to the traditional Bermuda rig (McDonald et al., n.d.). As the jib is located in front of the pivot point and thus develops a counterbalancing force to the mainsail, there is very little tension on the mainsheet. Consequently, a large sail area can be handled by a small crew.

This principle, developed for yachts, is also used on cargo ships. One example is the ‘Neoliner Origine’, a 136-metre-long Ro-Ro vessel owned by the French shipping company Neoline. On the two-masted Neoliner Origine, the mainsails are not made of fabric but consist of solid panels. These Solid Sails were developed by the subsidiary of the French shipyard Chantiers d’Atlantique bearing the same name. Neoline estimates the fuel savings at “over 80 per cent” at an average speed of 11 knots (compared to a purely motor-powered vessel travelling at 15 knots).

The Neoliner Origin is currently the largest ship to use sails as its primary source of propulsion. The mainsails of the two Aero-Rig systems consist of solid panels which, when reefing or furling the sail, fold like an accordion and stack up on the boom.

 

Inflatable wing sails

Press the button, the mast rises, pumps inflate the sail – and then nothing more. The system regulates itself. The principle of balancing the force of the wind through sail area in front of and behind the mast is utilised to such an extent with Michelin’s inflatable wing sail that no sheet is needed to set the sail optimally to the wind. Like a weather vane, it can rotate freely on the unstayed mast and trims itself.

The mast is telescopic and the sail is inflated using air pumps, creating a symmetrical profile. This generates more propulsion than a conventional fabric sail. The system was developed by two inventors on Lake Geneva. A prototype was tested on the ro-ro vessel ‘Pelican’ in the Bay of Biscay, and the French Directorate for Maritime Affairs, Fisheries and Aquaculture has ordered such a system for a deep-sea patrol vessel.

An inflatable wing sail on a French patrol vessel.

 

Rigid wing sails

If the wind-driven system is constructed from rigid materials, profiles similar to those of an aeroplane wing are possible. When the apparent wind direction is favourable, these provide significantly more propulsion than textile sails. By dividing the wings, an effect similar to an aeroplane extending its landing flaps is achieved: lift is further increased.

This high efficiency is utilised on extreme racing catamarans, which lift out of the water on hydrofoils and, due to their high speed, generate a high proportion of headwind in the apparent wind. Such racing machines can reach speeds of up to 54 knots (100 km/h) using wing sails.

Racing catamarans with hydrofoils demonstrate the efficiency of the technology.

 

As the sails generate some of the wind themselves whilst underway on most courses, calculations by Hamburg-based engineer Peter Schenzle suggest that an efficiency of well over 100 per cent is possible.

 

By taking the detour via wind turbines and green hydrogen, well over 90 per cent of the energy contained in the wind is lost. Direct use of the wind, on the other hand, allows for efficiency levels in excess of 100 per cent.

 

However, this high efficiency is only achievable with racing vessels that combine a low ship weight with a large sail area. When used as an auxiliary sail on cargo ships, the situation is exactly the opposite: a high ship weight is combined with a relatively small sail area. In this case, the aim is simply to support the engine.

Oceanwings, the company that builds such systems, quotes an average saving of 1.3 tonnes of fuel per day per installed wing system. On its website, Oceanwings shows an animation of a container ship with six wing sails installed. Let’s assume this ship burns around 150 tonnes of heavy fuel oil daily. In that case, the saving of 7.8 tonnes amounts to around 5 per cent. In other words: whilst every tonne of fossil fuel not burned is a gain for the climate, even highly efficient wing profiles make the 35 per cent CO₂ saving cited by Oceanwings on its website appear far too optimistic.

More effective is reducing speed: if the ship burns 150 tonnes per day at 24 knots, this would be around 40 per cent less at 18 knots. In that case, the savings from the wing sails amount to 8.7 per cent.

But this is immediately countered by the objection of a long-serving fleet manager at a major Canadian shipping company: if you reduce the ship’s speed, you need more ships to carry the same volume of cargo. Consequently, within the overall maritime freight system, the savings achieved by the individual ship would be lost again straight away. The key to real emissions reduction therefore lies in the volume of cargo: transport less...

Back to the 35 per cent reduction in CO₂ cited by Oceanwings. These are empirical figures from the ‘Canopée’. This specialised vessel for transporting Ariane rocket components, at 120 metres in length, is still within the size range of a true sailing ship and is equipped with four wing sails with a total area of 1,452 m². Her design was optimised for wind utilisation right from the construction stage. However, overall, it is likely to be difficult to achieve savings exceeding the 10 per cent mark on large ships using retrofitted auxiliary sails, as technical limitations are encountered.

 

The Canopée is a hybrid vessel specially designed for the transatlantic transport of Ariane rocket components; although its wing sails enable it to consume up to 35 per cent less fuel, it still relies on fossil-fuelled engines as its main propulsion system.

 

This is nevertheless economically attractive, as increasingly stringent regulations from the IMO and the EU will see greenhouse gas emissions increasingly subject to a CO₂ levy. Even a reduction in fuel consumption in the single-digit percentage range can bring financial benefits to the shipowner. However, the caveat here is that taxes on greenhouse gases have not yet been finalised, at least within the IMO, and the US under President Trump is putting up fierce resistance at the forefront.

 

Suction wing sails

From a distance, they look like chimneys and could be mistaken for Flettner rotors. Yet the operating principle is different: the lift on a wing depends on the flow on the leeward side – that is, the side with negative pressure – being laminar. If the flow breaks away, only vortices remain. With small angles of attack, this risk does not exist, but the lift is also small. To ensure that the flow remains parallel to the surface of the wing sail even at larger angles of attack, the idea was conceived of extracting the air from there. This effectively draws the flow closer to the wing, thereby avoiding the vortices that generate no lift – or propulsion.

When the fans that blow air out of the top of the profile – thereby drawing air in through the openings on the leeward side – are switched off, the thick profile simply creates turbulence. The system only becomes effective once the fans are switched on.

 

Such systems are manufactured by companies including Econowind and bound4blue. On its website, the latter emphasises that, thanks to the suction effect, the efficiency is six to seven times that of a conventional sail of the same area. The suction-wing sails have now been installed on several ships.

 

The 183-metre-long chemical tanker Bow Orion has been fitted with 22-metre-high suction-type wind sails. According to the trade press, tests on the sister ship Bow Olympus revealed a potential fuel saving of 15 to 20 per cent. The system is said to remain effective even when sailing extremely close to the wind (15 degrees).

 

Towing kites

What works for kite surfers can also help to pull a cargo ship. A key advantage of such steering kites is that they harness wind energy at a height of several hundred metres. Up there, the wind is more constant and, above all, stronger. The kite must fly a figure of eight, as this allows it to generate its own ‘headwind’, which significantly enhances the effect.

As with other wind-assisted systems for ships, the aim is to automate the system. This is because shipowners do not want to lose the advantage of lower fuel costs by having to pay for a larger crew. Various companies such as Beyond the Sea, Kite Dynamics, Skysails Marine and Airseas are competing to see who will ultimately bring the most reliable and efficient system to market.

Airseas is a subsidiary of the aircraft manufacturer Airbus and uses its parent company’s aerodynamic expertise to develop kites for ships.

One advantage of towing kites is that they can be installed relatively easily far forward on the foredeck, where they do not get in the way during cargo loading and unloading in port. As things inevitably go wrong at sea at some point, the kites are likely to be used mainly during the day. Unlike wing sails on board, which can be repaired or taken down at night under floodlight, a kite hundreds of metres away is harder to control in the dark.

An online tool allows the climate benefits and economic viability to be calculated. It indicates that, for the time the kite is in flight, fuel savings range from 3.5 to 8 per cent depending on wind strength. The software concludes that the key factor for climate benefits is how often and for how long the crew deploys the kite.

 

Physical limited possibilities

Until now, shipowners have sought ‘efficiency through size’: the larger the ship, the less fuel (and thus fewer greenhouse gas emissions) is required per tonne transported. However, as only a few ports can accommodate such 400-metre ships – partly due to draught – the economically viable limit has been reached.

For ships that use sails – of whatever type – as their main propulsion, the limit lies significantly lower. The ‘Preussen’, built in 1902, was 147 metres long. That is likely to be the limit for traditional sailing technology. If modern systems can convert the wind into propulsion more efficiently, the limit could probably be pushed towards 200 metres. However, if one goes beyond that, the sail area required and the forces involved would be too great and too difficult to control to move such a massive hull through the water. Consequently, for the size of most intercontinental cargo ships, wind power can only be considered as a supplement to propulsion that will, for the foreseeable future, still rely on fossil fuels. And as a rule, the fuel savings are unlikely to exceed 10 per cent.

It is also striking that the few larger sailing freighters built to date rely on variants of conventional sailing technology:

• The 81-metre-long ships of the Phenix class operated by the French shipping company TOWT feature two Bermuda rigs mounted one behind the other, with a total sail area of up to 2,500 square metres.
• On the ‘Grain de Sail II’ (51 metres in length), a schooner rig was chosen.
• And the 136-metre-long ro-ro vessel ‘Neoline Origin’ sails with two aero-rigs offering a total sail area of 3,000 square metres. Whilst the mainsails are so-called solid sails and consist of panels, the reason for this is less about aerodynamics than the notion that such sails have a longer lifespan than those made from textile materials. However, aero-rigs with mainsails made of fabric would function aerodynamically according to the same principle.
• The as-yet-unbuilt container ship from Veer – 100 metres in length, with a capacity of 152 twenty-foot equivalent units (TEU) – also relies on textile sails with its Dyna-Rig, a modern variant of the square sail.

The Oceanbird or Orcelle Wind project takes a different approach: a 220-metre-long car carrier for 7,000 vehicles is set to save up to 90 per cent in fuel consumption by using wing sails as its main propulsion. The ship, planned by the Swedish shipping company Wallenius Wilhelmsen, is being funded in part by €9 million in EU grants. The visualisations initially showed telescopic wings along the ship’s central axis. Later, the design shifted to split-wing sails mounted in pairs on the port and starboard sides. This shows that the project is not just about transporting cars, but also about developing a prototype. We are still in the early stages.

The evolving prototype: initially, the Oceanbird with retractable wing sails appeared on the designers’ screens (top), but the decision was later made to opt for split-wing sails (bottom).

 

Economic obstacles

However interesting and promising the individual projects may be, they do not solve the actual climate problem: the enormous volume of 11 billion tonnes of freight transported annually. Quite apart from the fact that, for example, the mobility model of motorised private transport contributes massively to the violation of planetary boundaries: why must cars be transported intercontinentally? Would it not suffice to use vehicles produced on one’s own continent?

The demands on maritime transport arise on land: the need for reliable delivery dates (just in time…) or, for high-value goods, the shortest possible delivery times. Consequently, few shipowners are interested in sails as the primary propulsion. On the contrary: major shipping companies are expanding into air transport, which is a hundred times more harmful to the climate, in order to satisfy customer demands (time is money…). Alongside the physical and technical limitations, these economic interests are the biggest obstacle to making wind – a climate-friendly, free fuel – the primary means of propulsion.

Even as an auxiliary propulsion system, wind power has so far been adopted only to a very limited extent: according to the relevant IWSA list, in the final quarter of 2025 only 86 ships were equipped with such systems, and 12 were about to have them installed. Consequently, only around one in every ten thousand ships is capable of saving fossil fuel in at least the single-digit percentage range.

 

References

Neal McDonald, Damon Roberts (o.J.): AeroRig® - The Rig of the Future. The International HISWA Symposium on Yacht Design and Yacht Construction, https://yachtmonalisa.com/Hiswa-AeroRig-Symposium.pdf 

Ship Universe (2026): Kite Propulsion Systems on Ships in 2026: Pros, Cons and Savings. https://www.shipuniverse.com/tech/kite-propulsion-systems-on-ships-in-2026-pros-cons-and-savings/ 

Tanker Operator (2025): Odfjell sees 15-20 per cent energy saving from suction sails, more than expected. https://www.tankeroperator.com/news/odfjell-sees-15-20-per-cent-energy-saving-from-suction-sails-more-than-expected/14835.aspx 

Wallenius Wilhelmsen (2023):World's first wind-powered RoRo vessel secures EUR 9M in EU funding. https://www.walleniuswilhelmsen.com/news/worlds-first-wind-powered-roro-vessel-secures-eur-9m-in-eu-funding