Decarbonising transoceanic shipping

Wing sails

A typical cargo ship is in operation for 20 – 25 years, which makes fleet renewal a long-term process. It also means that achieving fuel reductions within the timeframe of a few years can only be done if the new technology can be retrofitted on the existing fleet.

The new crop of rigid sails can indeed be retrofitted to current ships. While the fuel savings are typically greater when the ship is designed with sails, the sheer scale of today’s global fleet means retrofitting is critical.

Sails work on the same aerodynamic principle as aircraft wings. When air flows over a wing, it moves more quickly across the upper surface than the lower one, creating a pressure difference that generates lift. A rigid sail works in much the same way – it is essentially a wing turned upright. Where a wing’s lift drives an aircraft upward, the lift on a sail is redirected into forward thrust, propelling the ship ahead.

Current names in the sector include BAR Technologies and their Windwings, Oceanbird and their Wingsails, GTWings, and Smart Green Shipping.

Both wing sails and rotor sails are usually designed to fold, to avoid interference with cargo handling, bridge clearance, and to reduce air draft in ports.

Rotor sails

Rotor sails, or Flettner rotors, have been known for over a hundred years. However, only recent innovations have made them viable. They are currently the main alternative to wing sails.

Flettner rotors are cylinders that can spin around their vertical axis. When a vertical cylinder spins, one side turns with the wind moving past it, while the other side turns against it. On the side moving with the wind the airflow speeds up; on the opposite side it slows down. This difference in velocity produces a pressure imbalance. This imbalance generates lift. Since the cylinder stands upright, the lift acts horizontally, driving the ship forward instead of upward.

Norsepower, Dealfeng, and Anemoi are among the main players in the rotor sails segment of the market.

Other options include suction sails, such as Bound4Blue’s eSAILS and kites, such as Airseas’s Seawings, and soft sails such as developed by TOWT.

The economics of sails

In any sector adoption of new technology depends on its economic viability. At the moment the main financial benefit comes from reduced fuel consumption. Wind propulsion, whether primary wind or wind-assist technology, offers considerable fuel savings compared to traditional ships.

Current wind-assist ships can reduce fuel consumption by 30% or more, depending on the number of sails and the route. For rigid wing sails this means the initial cost can pay off within 4-6 years; for the rest of the ship’s lifetime these fuel savings mean pure profit.

There is however another factor to shipping profitability: regulatory compliance. Recent international legislation has been moving towards forcing shipping companies to reduce emissions and non-compliance can be costly indeed.

The FuelEU Maritime Regulation (Regulation (EU) 2023/1805), in force since 1 January 2025, sets maximum limits for the GHG intensity of the energy used by ships calling at European ports, regardless of their flag.

The International Maritime Organisation (IMO) is expected to finalise the IMO Net-Zero Framework in October 2025. Once approved, this will set mandatory marine fuel standard and GHG emissions pricing for shipping. In its current draft form the regulations envisage two tiers of penalties for non-compliant ships, with the cost rising with higher emissions.

Outlook

Shipping accounts for almost 2% of global greenhouse gas emissions (GHG). Despite noises from some shipping companies tied to the fossil fuel economy, the shipping sector has been focussing on cleaning up its act.

Wind propulsion continues to gain traction in the shipping sector and outside. For instance, some manufacturers of consumer goods have begun to build their brand image based on zero-emission shipping and are ensuring their goods are shipped on wind-powered ships.

Public opinion is also shifting. A large-scale study by the University of Oxford in 2024 has shown that 80% of people globally want their governments to take stronger action to tackle the climate crisis. 86% want their countries to set aside geopolitical differences and work together on climate change.

The combination of these factors signals a fair wind for the future of wind-powered shipping.

Photo credits:

• Berge Olympus with installed Bar Technologies Windwings © BAR Technologies
• Brands Hatch with installed Bar Technologies Windwings © BAR Technologies
• Orcelle Wind with installed Oceanbird Wingsails © Wallenius Wilhelmsen
• Oceanus Aurora with installed Norsepower Rotor Sails © Norsepower