Wind propulsion: reviving sails for a new era

A vessel installed with a Norsepower Flettner Rotor. Credit: Norsepower

The rise in emissions regulations, such as the 2023 Carbon Intensity Indicator (CII), FuelEU and the Energy Efficiency Existing Ship Index (EEXI), has significantly increased interest in wind-assisted propulsion technologies. 

Bringing to mind images of the grand armadas and sail ships of old, wind power had, until recently, been consigned to the pages of history. Now, it’s coming back into its own thanks to the industry’s decarbonisation efforts justifying the business case.  

This is because – according to Finnish cleantech company Norsepower – wind-assisted propulsion can reduce fuel use and emissions requirements by at least 5%-25%. 

There are five key wind-assisted propulsion technologies that are leading the way in the marine sector. There are rigid sails, which work to the same principle as those on an aeroplane, but are vertical rather than horizontal. 

There are also soft sails, kites, and Flettner rotors, all of which feature cylinders that rotate at high speeds as the wind hits them, generating thrust. Finally, there are suction wings, also known as turbo sails, which were developed by Jaques Cousteau in the 1980s.

“These are rigid sails that have a ventilation system inside, which sucks the air around the wing, creating additional thrust,” explains Andrew Hurford, a senior specialist in marine, maritime asset and operational performance at Lloyd’s Register. 

While some of these wind-assisted propulsion technologies have been around longer than you might think, innovation has mainly accelerated in more recent years. 

“Generally suction sails and Flettner rotors are similar to their original concept, but the focus has been on optimising aerodynamic elements such as top plates and body shape,” Hurford explains. “Evolution has come more significantly in control systems incorporating measurement and advanced data, and modern materials that help weight, strength and long-term corrosion resistance.” 

According to Hurford, for a common case in the same global wind condition, Flettner rotors and suction sails can deliver a similar performance and typically have a higher efficiency of fuel savings per square metre of installed system. Rigid sails may offer slightly higher fuel savings in some cases, but they come with operational challenges such as larger sail size and higher installation costs.

“Note that although different technologies may have the same performance, there could be difference to get that performance in terms of surface area and number of devices,” he says. 

Retrofitting and new builds Wind-assisted propulsion solutions can not only be factored into new builds but also be easily retrofitted onto an existing fleet. They’re also suitable for almost any type of ship, from roll-on/roll-offs (RoRos) through to tankers and bulkers. 

“We recently saw the deployment of five rotor sails on a 400,000-tonne bulker, which is essentially one of the largest ships on the world from a dead weight tonne point of view,” notes Gavin Allwright, secretary general of the International Windship Association (IWSA).  

“We’re also going to see three rigid sails being deployed on two 220 metre-long Accor Group cruise vessels. They’re looking to operate at 70% wind, going at 17 knots.”

Which technology to choose though, should be decided on a case-by-case basis, as there are many factors to consider. 

Hurford advises that you need to consider your asset strategy, vessel age, routing, and operational constraints – such as visibility and deck space – as well as the effort involved to install, power requirements and fuel savings. 

“Having the appropriate space to install a system can be a challenge, and you may have to reinforce parts of the deck,” notes Allwright.  

“Placement is also an important consideration, as line of sight for navigation must not be obscured. Probably the biggest limitation to consider with this technology is the wind itself. If you’re not following wind patterns then you’re not going to get the most out of your solution.” 

But while the industry is getting its head around the pros and cons of each of these different technologies, interest in wind-assisted propulsions solutions continues to slowly rise. 

According to figures from the IWSA, over 50 large vessels deployed with wind propulsion systems are already in operation, and the organisation predicts that this number will hit 100 by the end of the year, doubling every year going forward.

In terms of adoption, the industry is starting to see the installation of multiple rigs on a ship, and a move away from individual ships to fleet orders. 

“It’s common to get repeat customers and negotiate large fleet-wide deals,” notes Heikki Pöntynen, CEO of Norsepower. “Most recently we closed the biggest deal in wind propulsion history with LDA/Airbus for their three-vessel RoRo fleet, each with six rotor sails. We also closed a deal with Union Maritime to fit four product tankers with two rotor sails each.” 

IWSA has identified 2026 as the real inflection point where the market takes over, as 100 is a significant psychological number.  

“Before fully embracing a new technology, the industry always asks for four or five refence points for each segment and we should have these when we reach the 100 deployment mark,” Allwright notes. 

Beyond 100 installations, market forecasts indicate that orders will rapidly accelerate, notably in the bulk and tanker vessel segments.  

“Analysis of top-end potential identifies nearly 14,000 candidate vessels over the next 26 years,” says Hurford. 

Production lines are already being ramped up. For example, Norsepower recently opened its own factor in China due to huge market demand. 

“We’re already starting to see production lines ramped up by our members, which will enable the premanufacture of these systems, so that they’re deliverable within a few months as opposed to up to a year,” says Allwright. 

“Lead times coming down and a learning curve kicking in will mean costs coming down, creating this virtuous circle that will support the growth of installations.”