The winds of change – how rotor sail technology can boost maritime decarbonisation efforts

A deep dive into flexible, energy-saving rotor sail technology, which can cut fuel consumption and emissions by up to 30% and aid vessel CII compliance.

In an age where modern ships are floating miracles of technology and engineering, the maritime industry is returning to its roots when seeking new power sources. Mariners have been harnessing wind power since Ancient Egyptian times, and today this renewable energy source is making a comeback on ocean-going vessels of all kinds, harnessed not with squares of cloth but with sleek, mechanical rotor sails.


Ancient Egypt’s pioneering mariners sailed the Nile, the Mediterranean and the Red Sea using the power of the wind, laying the foundations for an age where sails dominated shipping before the rise of steam power in the late 1800s. Today, with pressing climate challenges to address and tight emissions restrictions to adhere to, the maritime industry is once again turning to Mother Nature for a helping hand.

“Rotor sails, also known as Flettner Rotors, are modern mechanical sails – tall cylinders that, when spinning round, provide extra propulsive power for vessels and thereby reduce the amount of fuel consumed on board,” explains Elias Boletis, Director, Propulsion R&D at Wärtsilä.

“They are an exceptionally environmentally friendly way of generating additional propulsive force because wind power is fully renewable, and they are also an extremely flexible solution that can easily be integrated onboard a wide variety of vessel types without impacting their operational viability.”


Harnessing the power of the Magnus Effect

When the wind speed and direction is optimal, rotor sails, which are powered by electric motors, can be switched on automatically. When the wind flow meets the sail, an aerodynamic phenomenon called the Magnus Effect means that the air in front of the sail accelerates as it is pulled in the direction of the spin. This faster-moving air has a lower pressure while the air behind slows down and increases in pressure – and this pressure differential propels the ship forwards.

Rotor sails are well-proven in the field and have a high technology readiness level. Powering them takes very little energy, perhaps 30 to 50 kW, and the gains in propulsive power can be in the order of 10 to 15 times that.


Big savings from a simple, flexible technology

Installing rotor sails can result in a reduction of between 5 and 30% in fuel consumption and emissions. These gains are a welcome boost for ship owners as they develop their roadmaps for tackling stricter regulations. These include the Energy Efficiency Existing Ship Index (EEXI), which defines a minimum energy efficiency level for existing vessels, and the Carbon Intensity Indicator (CII) rating scheme, which gives vessels an annual rating on a scale of A to E, with A being the least carbon intensive.

Savings are dependent on the size and number of sails installed as well as the route sailed. But with CII and EEXI coming into force in 2023 urgent action is needed, particularly in segments such as bulker carriers where around 80% of vessels will be non-compliant if owners do not take any action to address their energy efficiency.

Wärtsilä is an authorised seller and servicing partner of the Anemoi Marine Technologies Rotor Sail System, which is particularly well suited to bulk carrier vessels.

Anemoi has developed its own fuel saving assessment method in order to develop reliable performance predictions for potential customer vessels. To give an example, this method predicts an annual fuel and emission saving of 13.5%, or 1,622 tonnes of fuel and 5,044 tonnes of carbon, for a 310,000-dwt crude oil tanker fitted with five rotor sails plying a route between Nigeria and China.

In June 2022 a joint development project studying Anemoi Rotor Sails was awarded Approval in Principle from Lloyd’s Register, who validated that a 210,000-dwt Newcastlemax bulk carrier operated by Oldendorff Carriers could cut its Energy Efficiency Design Index score by 29% by installing six 5 x 30 m rotor sails. But fuel and emission savings only tell part of the story.

Rotor sails are simple to install, have a compact footprint on deck and can be installed in such a way so as to not interfere with cargo loading and unloading.

“For example, Anemoi offers options for folding sails, which fold down towards the deck during cargo-handling operations, and rail-mounted sails which can be moved longitudinally or transversely out of the way of cranes or other lifting equipment.”


A transferrable investment in decarbonisation

“Once the vessel is ready the owner can choose to install the rotor sails in stages during appropriate port calls, using a ‘plug-and-play’ approach. And should the vessel’s operational profile change or if the vessel is sold, the sails can simply be removed and transferred to another vessel – a benefit which is simply not possible with some other energy-saving technologies. In this sense rotor sails are what you might call transferrable CAPEX,” shares Elias.


Integrate to accumulate

When it comes to optimising the energy efficiency of a new build or retrofit vessel, Wärtsilä takes a holistic approach to developing an integrated system that delivers significantly higher value than the sum of its individual parts. “Before installation a comprehensive feasibility study is performed in order to determine the optimal number and position of the rotor sails. With our integration expertise we can ensure that rotor sails are integrated in such a way that they provide maximum benefit in terms of propulsion and that they complement the main propeller and the other energy-saving solutions onboard. At the same time we can plan the integration in such a way that the rotor sails do not negatively impact the vessel’s course-keeping or manoeuvring performance,” Elias points out.


Optiwise project gives wind propulsion a lift

Launched in 2022, the EU-funded Optiwise project is focused on maximising the energy savings from wind propulsion technologies, with the aim of achieving average energy savings of between 30 and 50% compared to conventionally powered vessels. Wärtsilä and Anemoi are both participating in the project, with Wärtsilä as the OEM driving the technology integration at the vessel level, providing systems expertise and streamlining the technology innovation to meet the needs of the maritime market.

“At sea the wind is all around us and is a free source of energy when harnessed effectively. With rotor sails such as the Anemoi system, vessel owners can make big efficiency gains that could mean the difference between compliance and non-compliance with the increasingly tight efficiency targets on the horizon. The time is right to put the sails back into sailing.” Elias concludes.    

Learn more about rotor sails

Written by
Charlie Bass

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