Decarbonisation of shipping is underway. International Maritime Organisation (IMO) decarbonisation goals now call for net zero emissions by 2050, while even stricter regulations by regional bodies, like the European Union (EU), are putting a price on carbon emissions. Under pressure to reduce emissions, shipowners are exploring all options. However, the nature of ships and the length of typical voyages means that shipping is one of the ‘hard-to-abate’ sectors, where solutions like electrification are challenging to apply.

Other routes to decarbonisation are needed and alternative fuels like methanol, ammonia and biofuels are seen as the most likely long-term solution. However, these are in short supply and infrastructure is still years away from full development. Therefore, energy efficiency is seen as the most practical way of reducing emissions in the short term.

A range of energy efficiency measures are available today, and WAPS are already proving to be an attractive one. Recent images of elegant sails supplanted on modern vessels have captured the imagination of the maritime industry, and beyond, solving a modern problem by bringing us all the way back to our roots. As DNV Maritime CEO Knut Ørback-Nilsson said recently: ‘After all these years of technological development, who would have thought we would be sailing again?’

A history of WAPS  
Using wind for propulsion is, of course, not new and ships with traditional rigs have transported goods and people around the world for centuries. While more modern solutions, such as the Flettner rotor (see below), have been available since around the 1920s, the preponderance of fuel oil, coupled with decades of low fuel prices, meant that innovations like this were never really considered for the modern fleet. However, this situation has now been turned on its head by the need to decarbonise.

So far, the uptake across shipping has been limited. According to Clarksons Research, 40 vessels in the global fleet have sail systems installed today. Nonetheless, implementation is increasing and a further 53 are already on the orderbook, with indications that growth could be even stronger throughout the rest of the 2020s and beyond.

The future of wind propulsion in shipping could go beyond current levels of wind assistance towards a scenario where wind is the primary propulsion mechanism on commercial vessels. 

Different WAPS technologies  
A number of different versions of WAPS are available on the market today with all of these working from the same physical principle – using wind as a natural source of energy. They apply modern materials to old concepts, while also taking advantage of contemporary advances in digital solutions and automation to integrate fully with modern vessels. All can be either retrofitted on existing ships or equipped on newbuilds.

• The Flettner rotor, also known as a rotor sail, is the most commonly-installed solution on the market today. The rotor is a cylindrical structure that rotates around its own axis. In wind, this creates a low- and high-pressure area on each side of the rotor at right angles to the wind, generating ‘lift’ – propulsive force. The lift coefficient of a Flettner rotor is large; however, a relatively large drag force (or coefficient) is also created, affecting the aerodynamic efficiency of the sail. The Flettner rotor does not need trimming against the wind, unlike traditional sails and other WAPS, which makes operation relatively easy.
• Rigid wingsails utilise a typical foil structure to create thrust, like an airplane wing mounted vertically on the deck. While there are different products on the market today, or under development, most of these are made of hard shells and equipped with high-lift devices, such as flaps. The flaps delay flow separation and stalling, allowing for larger angles of attack against the incoming wind and increasing the generated lift. Wingsails have a relatively high aerodynamic efficiency due to low drag generation.
• Soft or hybrid wingsails are similar to rigid wingsails, except for the use of modern textile sail materials instead of a hard shell. The advantage is that the system weighs less than rigid wingsails, which is better for the stability of the vessel. When necessary, these sails can be reefed and furled, providing an added advantage in cases of stormy weather or the need for reduced air draught in port. 
• Suction sails, also known as ventilated foil or turbo sails, are oval shaped vertical structures with a moveable flap downstream of the sail and an electric-powered suction system. The suction system delays flow separation in the air, which increases the generated lift. Like the Flettner rotor, the lift coefficient of a suction sail is large.
• Other examples include soft sail systems, which have much in common with a traditional sail rig, and kites, which are deployed and fly in a certain path in front of the vessel.

The Pyxis Ocean bulk carrier is equipped with two solid wingsails; BAR Technologies' WindWings®, 123 ft tall
Photo: BAR Technologies

How effective are WAPS?  
The power produced from WAPS depends not only on the specific sail system, but also external factors. While the most important of these are the wind speed and direction experienced by the WAPS, the hydrodynamic efficiency of the hull can also affect the performance. For this reason, the most suitable WAPS type for a specific vessel is dependent on, for instance, average vessel speed, the operational route and weather conditions, but also practical considerations such as the availability of deck space and undisturbed air flow to the sail.

For example, Flettner rotors are probably more suitable for slower vessels, such as bulk carriers and tankers, while the relatively high aerodynamic efficiency of wingsails makes them better suited for faster vessels, such as RoRos (roll-on roll-off). Suction sails and wingsails may be advantageous if a vessel is operating with the wind closer to the bow.

Modern vessels equipped with advanced digital technologies, such as decision support systems and routing algorithms, are also more likely to maximise the benefits of WAPS.

Performance verification   
According to feedback from shipowners and operators, WAPS have already delivered annual fuel savings of between 5% and 9% for certain ships, with claims that this could be increased to 25% or 30%, or even higher, under the right conditions.

The many factors affecting the performance of WAPS installations and their implications on a commercially-operated vessel means that more knowledge is needed to truly gauge their impact on decarbonisation of the maritime industry, to trust the performance and thus increase the uptake of these technologies. DNV, through its Maritime Advisory business, is bridging this gap by performing third-party verification of WAPS installations onboard vessels being operated today.

Building knowledge and reducing the uncertainty around the performance by transparent assessment of the technology benefits all parties involved; from manufacturers obtaining a fair and independent assessment of their system, to shipowners and operators being informed about their investment.

Full-scale tests for the purpose of verifying the WAPS performance are performed in-service, while the vessel operates its trading route, by measuring relevant performance data during a dedicated period when the system is on, followed by a dedicated measurement period when the system is off.

Although very important, verified performance assessments are not sufficient to ensure continued emergence of the technologies. Naturally, ensuring safety during development and operation is a key role of classification societies. Some classification societies, like DNV, have published rules for WAPS and their onboard integration. However, it is important to continue development of class rules and statutory regulations to fill gaps, especially during development of systems that give a major contribution to the propulsion.

The WISAMO wingsail engineered by Michelin is an inflatable, telescopic and automated wingsail for decarbonisation of maritime transport, the 100 m² prototype shown here onboard MN Pelican
Photo: Michelin  
 

Full wind propulsion – the future?  
The future of wind propulsion in shipping could go beyond current levels of wind assistance towards a scenario where wind is the primary propulsion mechanism on commercial vessels.

Orcelle, a Horizon Europe EU joint research project coordinated by Wallenius Wilhelmsen and featuring several partners, including DNV, commenced in January 2023. This is seeking to develop and demonstrate a showcase for wind as main ship propulsion, where 50% or more of average propulsion energy in full year operation is to be provided by wind, as opposed to a fully fossil-fuelled engine. In ideal conditions, wind propulsion could even reach close to 100%, with the vessel’s engine only used when absolutely necessary, such as in insufficient wind resources, confined waters, for manoeuvring, and where necessary for the safety of the ship.

The project will demonstrate feasibility through two physical demonstrators, a one-sail retrofit and a multi-sail newbuild. The latter, known as Orcelle Wind, will be a RoRo pure car truck carrier (PCTC) vessel measuring 217 metres in length and 39.4 metres in beam, and will have a capacity to load approximately 7,300 cars or equivalent cargo (however, this may be subject to modification due to the ongoing R&D initiative).

CGI rendering of Wallenius Wilhelmsen’s multi-sail Orcelle Wind Ro-Ro vessel capable of carrying about 7,300 cars; the Orcelle project is funded by the European Union
Photo: Orcelle Horizon

For an innovation like this to reach the propulsion target it will need go beyond current designs, including achieving more efficient wing systems with automated ways of operation, developing new ship design and design processes that handle the complexity of new wind systems, and integrating new weather routing systems and logistics solutions.

Where next for WAPS?  
Wind-assisted propulsion is here to stay and is set to play a significant role in the future energy mix of the maritime industry. With many shipowners struggling to keep up with fast-moving regulatory requirements, and unsure about where and how to invest to achieve compliance, these technologies can – under the right conditions – provide real and tangible decarbonisation results.

• Further reading: ‘Shipping looks to the past for a cleaner future’. 2023 was expected to be a formative year for giant wind-powered vessels as they follow in the wake of coastal traders, ferries and other smaller ships that have been using sails for the last three or four years.
• Is the maritime industry’s approach to GHG emissions fuzzy for future regulations?