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Wind-Assisted Propulsion

Wind-assisted propulsion (WAP) is the use of sail or rotor devices fitted to a conventionally-powered ship to generate auxiliary forward thrust from the apparent wind, reducing the main-engine fuel consumption required to maintain a given service speed. The four principal technology families in commercial use are: Flettner rotors (rotating cylindrical sails using the Magnus effect to convert apparent wind into thrust); rigid wing sails (aerodynamically-shaped vertical aerofoil structures); soft sails and suction sails (modern fabric-based sails using boundary-layer suction to enhance lift); and towing kites (high-altitude tethered kites flying figure-of-eight patterns ahead of the ship). As of end-2024 approximately 80 commercial wind-assist installations are in operation worldwide, predominantly on bulk carriers, tankers, ro-ro vessels and ferries on routes with favourable wind patterns. Typical fuel savings range from 5 to 15% on long-distance trade routes (e.g. trans-Pacific, North Atlantic) and 15 to 25% on routes with consistent crosswind or following-wind patterns (e.g. some North Sea and Baltic routes). Wind assist is recognised as an innovative energy-efficient technology under MARPOL Annex VI Regulation 21 with a corresponding EEDI credit; the credit is calculated using the IMO Innovative Technology Guidelines (Resolution MEPC.244(66)) and provides a measurable reduction in the Required EEDI and EEXI compliance burden. Wind assist also provides material CII rating improvement, contributing to the SEEMP Part III operational plan and the CII corrective action plan. ShipCalculators.com hosts the principal computational tools: the Flettner rotor thrust calculator implements the Magnus-effect thrust calculation; the Flettner rotor drive power calculator computes the parasitic electrical demand; the rigid wing sail thrust calculator implements the wing-sail aerodynamics; the towing kite pulling force calculator computes the kite tension; the Flettner rotor wind-assist calculator and wing sail / kite / soft sail calculator provide the integrated savings analysis. A full listing is available in the calculator catalogue.

Contents

Background and history

Pre-2010 wind propulsion context

Wind has been the primary maritime propulsion technology for most of human history, with sail-powered shipping dominant from antiquity until the late 19th-century transition to coal-fired steam. The transition was largely complete by the 1920s for cargo shipping, with the last major sail-cargo trade (the Australian wheat trade by Finnish-flag windjammers) ending in 1949.

Through the second half of the 20th century, sail technology development was confined to recreational sailing yachts. The 1973 oil shock briefly revived commercial interest (the Japanese-built Shin Aitoku Maru of 1980 was the first modern sail-assisted commercial vessel, with computer-controlled rigid sails saving 10 to 15% fuel) but the experiment was not commercially scaled.

The modern wind-assist revival began in the 2008-2010 period driven by:

  • The 2008 oil price spike (peak USD 147/bbl crude).
  • Growing climate-related interest in shipping decarbonisation.
  • Commercial maturation of computer-controlled aerodynamic systems.

2010 to 2020: technology re-emergence

The 2010s saw the re-emergence of all four principal wind-assist technology families:

  • Flettner rotors: revived by the German-Finnish company Norsepower founded in 2012; first commercial installation on MV Estraden (Bore Ltd ro-ro) in 2015.
  • Rigid wing sails: revived by the Dutch company Lade AS and the British company B9 Shipping in the early 2010s; first commercial wing-sail installation on the bulk carrier MV Afros (Cargill-chartered, 2018).
  • Towing kites: revived by the German company SkySails GmbH founded in 2001; first commercial installations on the MS Beluga SkySails (2008) and the MV Theseus (2011).
  • Soft / suction sails: revived by the French company Bound4Blue founded in 2014; first commercial installation on the bulk carrier Pyxis Ocean (Cargill-BAR Technologies, 2023).

2020 to 2024: commercial scale-up

The 2020 to 2024 period saw substantial commercial scale-up driven by:

  • The 2020 IMO 0.5% sulphur cap and rising bunker fuel costs.
  • The 2021 EEXI and CII regulatory pressure on existing ships.
  • The 2024 entry into force of EU ETS Maritime adding direct cost to fuel emissions.
  • The 2025 Net-Zero Framework approval at MEPC 83.

By end-2024 approximately 80 commercial wind-assist installations were in operation. Notable installations:

  • Norsepower Flettner rotors: ~30 installations across Bore Ltd, Maersk Tankers, Sea-Cargo (Norway), Equinor offshore vessels, Berge Bulk, and others.
  • Anemoi Flettner rotors: ~12 installations across Yara, Tufton Oceanic.
  • Bound4Blue eSail (suction sails): ~6 installations including Pyxis Ocean (BAR Technologies + Cargill chartered).
  • BAR Technologies WindWings (rigid wings): 2 installations on Pyxis Ocean (Cargill).
  • SkySails towing kites: ~3 installations (commercial momentum lower than rotors and sails).
  • Smart Green Shipping FastRig: 1 installation on a UK-flag bulk carrier (2024 commissioning).
  • Oceanbird (Wallenius Marine + Alfa Laval): under construction for 2026 launch (the first ground-up wind-primary deep-sea cargo vessel).

Technology families

Flettner rotors

A Flettner rotor is a vertical rotating cylinder fitted to the deck of a ship, typically 18 to 35 metres tall and 3 to 5 metres in diameter. The rotor:

  • Is rotated by an electric motor at typical 100 to 250 rpm.
  • Generates lift through the Magnus effect: when air flows past a rotating cylinder, the boundary layer is asymmetric and creates a perpendicular pressure differential.
  • Produces forward thrust when the apparent wind is from the beam (90° to course); thrust falls to zero on dead-ahead or dead-astern winds.

Typical Flettner rotor characteristics:

  • Maximum thrust: 50 to 200 kN per rotor in optimal wind conditions.
  • Drive power: 50 to 200 kW per rotor (parasitic electrical demand from auxiliary engines).
  • Net fuel saving: 5 to 20% on long-distance routes with prevailing crosswinds.
  • Capital cost: USD 1.5 to 3 million per rotor installed.
  • Payback period: 3 to 7 years at typical bunker prices and EU ETS exposure.

The Flettner rotor thrust calculator implements the Magnus effect thrust calculation; the Flettner rotor drive power calculator computes the parasitic electrical demand.

Rigid wing sails

A rigid wing sail is an aerodynamically-shaped vertical aerofoil structure fitted to the deck, typically 30 to 45 metres tall. The wing:

  • Has a symmetric or cambered cross-section similar to an aircraft wing.
  • Rotates around a vertical axis to align with the apparent wind.
  • Generates lift through aerofoil aerodynamics, producing forward thrust.
  • Typically has computer-controlled trailing edge flaps to adjust lift coefficient.

Typical rigid wing sail characteristics:

  • Maximum thrust: 100 to 300 kN per wing in optimal wind conditions.
  • Drive power: minimal (only for rotation and flap actuation, ~5 to 20 kW).
  • Net fuel saving: 5 to 20% on long-distance routes.
  • Capital cost: USD 2 to 5 million per wing installed.
  • Payback period: 3 to 6 years.

The rigid wing sail thrust calculator implements the wing-sail aerodynamics. Notable installations:

  • MV Pyxis Ocean (Cargill, 2023): two BAR Technologies WindWings on a 80,000 DWT bulk carrier; ~14% fuel saving on the maiden voyage Singapore to Brazil to Denmark.
  • Oceanbird (Wallenius Marine, 2026 launch): first dedicated wind-primary deep-sea cargo ship with 5 telescoping wings of 80 metres each.

Soft sails and suction sails

Soft sails include modern fabric sails reinforced with battens; suction sails add a boundary-layer suction system that increases the effective lift coefficient by approximately 50%. The leading suction sail technology is the Bound4Blue eSail, which uses a vertical fixed cylinder with internal suction.

Typical suction sail characteristics:

  • Maximum thrust: 50 to 150 kN per sail.
  • Drive power: 30 to 80 kW (suction blower).
  • Net fuel saving: 5 to 15%.
  • Capital cost: USD 1 to 2.5 million per sail installed.
  • Payback period: 3 to 6 years.

Towing kites

A towing kite is a tethered high-altitude kite (typically 200 to 400 m² surface area) flown ahead of the ship in a figure-of-eight pattern. The kite:

  • Operates at altitudes of 100 to 500 metres above sea level where wind speeds are typically 1.3 to 1.5x deck-level.
  • Generates pulling force in the apparent wind direction, transmitted to the ship via a tether.
  • Is launched and recovered by a deck-mounted control system.

Typical towing kite characteristics:

  • Maximum pulling force: 20 to 80 tonnes (200 to 800 kN).
  • Drive power: 20 to 50 kW (control system + winch).
  • Net fuel saving: 5 to 15%.
  • Capital cost: USD 1 to 2 million installed.
  • Payback period: 4 to 8 years.

The towing kite pulling force calculator implements the kite tension calculation. Notable installations:

  • MV Beluga SkySails (2008): first commercial application; reported 10 to 15% fuel savings.
  • Airseas Seawing: deployed on Airbus Ville de Bordeaux (2024) for ro-ro service between France and the United States.

Performance and economics

Fuel-saving estimation

The actual fuel saving from wind-assist depends on:

  • Route prevailing winds: routes with consistent crosswind or following winds (e.g. North Atlantic, North Pacific in some seasons, Indian Ocean monsoon trades, Southern Ocean) achieve higher savings; routes with predominantly head winds achieve lower savings.
  • Vessel type and superstructure: higher freeboard and lower superstructure interference improve performance; bulk carriers and tankers typically perform better than container ships (which have high deck cargo).
  • Service speed: lower service speeds improve relative wind angle and increase percentage saving; faster ships gain less.
  • Number of devices: multiple rotors or sails provide better total thrust but also higher capital cost and parasitic load.
  • Operational profile: continuous deep-sea voyages benefit more than short port-to-port operations with frequent manoeuvring.

Real-world reported savings from operational installations:

VesselTechnologyTradeReported saving
MV Estraden (Bore Ltd ro-ro)2× Norsepower rotorsBaltic / North Sea8 to 12%
MV Afros (Cargill bulk)1× Anemoi rotorTrans-Atlantic bulk6 to 10%
MV Berge Mulhacen (Berge Bulk Capesize)5× Anemoi rotorsIron ore Pilbara to East Asia12 to 18%
MV Pyxis Ocean (Cargill bulk)2× BAR WindWingsVarious dry bulk routes14% (maiden voyage)
MV Shofu Maru (Oshima coal carrier)1× MOL hard wingCoal Australia to Japan8 to 12%

Innovative Technology Credit (ITC) under EEDI

MARPOL Annex VI Regulation 21 recognises wind-assisted propulsion as an innovative energy-efficient technology eligible for a credit in the EEDI calculation. The credit:

  • Is calculated per the IMO Innovative Technology Guidelines (Resolution MEPC.244(66)).
  • Reduces the calculated attained EEDI by the wind-assist contribution.
  • Applies to both new builds (EEDI) and existing ships (EEXI).
  • Is verified by the classification society at the EEDI / EEXI verification stage.

For a typical 80,000 DWT bulk carrier with two rigid wing sails achieving 12% average fuel saving, the EEDI credit:

  • Reduces the attained EEDI by approximately 12% (the average annual saving).
  • Brings the ship from approximately 5% above Required EEDI to approximately 7% below Required EEDI.
  • Avoids the need for additional EPL or ShaPoLi under the EEXI regime.

CII improvement

Wind-assist provides direct improvement in the annual CII attained:

  • A 12% fuel saving translates into approximately 12% reduction in attained CII.
  • For a bulk carrier with attained CII of 5.5 (D rating, 10% above Required), the wind-assist 12% reduction brings attained CII to 4.84 (close to the C/B boundary).
  • The improvement is sustained year-on-year as long as the system is maintained and operational.

The SEEMP combined operational measures calculator implements the combined effect of wind-assist with other operational measures.

Capital cost and payback

TechnologyCapital cost / unitTypical units / vesselTotal capexAnnual fuel saving (USD at $600/t bunker, 10,000 t/yr fuel)Simple payback
Flettner rotor$1.5-3M2-4$3-12M$0.6-1.2M (10-20%)3-7 years
Rigid wing sail$2-5M2-4$4-20M$0.6-1.2M (10-20%)4-8 years
Suction sail$1-2.5M2-4$2-10M$0.3-0.9M (5-15%)3-6 years
Towing kite$1-2M1$1-2M$0.3-0.9M (5-15%)2-4 years

The payback period falls significantly when EU ETS Maritime cost (EUR 60-100 per tonne CO2 avoided) is added to the fuel saving. For ships trading in EU waters, the EU ETS contribution can represent an additional 20 to 50% of the fuel-cost saving.

The Retrofit Payback calculator implements the payback calculation for arbitrary technology investments.

Notable installations and case studies

Cargill / BAR Technologies WindWings

In August 2023 Cargill, the world’s largest commodity trader and a Sea Cargo Charter signatory, fitted two BAR Technologies WindWings to the MV Pyxis Ocean, a Mitsubishi Corporation-built Kamsarmax bulk carrier (80,962 DWT). The maiden voyage from Shanghai to Brazil reported approximately 14% fuel savings; subsequent voyages on Atlantic and Pacific routes have shown 6 to 19% savings depending on wind conditions.

Cargill plans to fit a further 8 vessels with WindWings by 2027 across the dry bulk fleet, focusing on Capesize bulk carriers on the Atlantic and Pacific iron ore and coal trades.

Berge Bulk Capesize fleet

Singapore-based Berge Bulk has fitted Anemoi Flettner rotors to multiple Capesize bulk carriers including the MV Berge Mulhacen (5 rotors) on the Pilbara to East Asia Iron Ore corridor. Reported savings of 12 to 18% are among the highest for any wind-assist installation, reflecting the favourable trade winds on the Pilbara to East Asia route.

Norsepower rotor sail rollout

The Finnish company Norsepower (founded 2012, partly owned by Cargill since 2024) has the largest installed base of Flettner rotors. Notable installations:

  • MV Estraden (Bore Ltd, 2015): first commercial installation; 2 small rotors.
  • MV Maersk Pelican (Maersk Tankers, 2018): 2 large rotors on a Suezmax tanker.
  • MV Sea-Cargo Aurora (2021): 2 rotors on a Norwegian ro-ro.
  • Several Maersk Tankers vessels (2022 to 2024): rotor retrofits across the Maersk product tanker fleet.
  • Equinor offshore support vessels (2023 to 2024): rotors on offshore platform supply vessels.

Oceanbird wind-primary cargo vessel

The Oceanbird project, led by Sweden’s Wallenius Marine in cooperation with Alfa Laval, KTH Royal Institute of Technology and SSPA, is the first ground-up wind-primary deep-sea cargo vessel design. Specifications:

  • Length 200 metres, beam 40 metres, capacity ~7,000 cars (ro-ro design).
  • 5 telescoping wing sails, each 80 metres tall when extended (collapsing to 40 metres for harbour operations and bridge clearance).
  • Service speed 10 knots in optimal wind, 7 knots in moderate wind.
  • Auxiliary diesel engines for emergency power and harbour operations.
  • Estimated 90% emission reduction vs equivalent conventional ro-ro on the trans-Atlantic route.

First vessel ordered 2023 for delivery 2026 to 2027. The trans-Atlantic Atlantic route (Europe to US East Coast) is the planned trade.

SkySails towing kite revival

The German company SkySails Group (founded 2001) returned to commercial operations in 2023 to 2024 after a hiatus, with new commercial installations on:

  • MV Augusta Brave (Brave Tankers, 2023): towing kite on a Suezmax tanker.
  • Airbus Ville de Bordeaux (Airseas Seawing, 2024): kite on a ro-ro Airbus parts carrier between France and the United States.

The towing kite technology has lower commercial momentum than rotors and sails because of operational complexity (launch and recovery in adverse weather) but provides higher savings per unit cost when conditions are right.

Operational considerations

Crew training

Wind-assist installations require crew training on:

  • Operating the wind-assist control system (typically integrated with bridge nautical equipment).
  • Adjusting rotor speed or sail trim for optimal performance.
  • Safe operation in heavy weather (typically rotors fold or sails reef at Beaufort 7-8+).
  • Maintenance procedures (regular inspection, gearbox / motor servicing).

The 2010 STCW Manila amendments were updated in 2024 to include wind-assist operations in the bridge-team competence framework, with corresponding revisions to IMO Model Course 1.07 (Radar Navigation) and 1.34 (ECDIS).

Routing optimisation

Wind-assist benefit is highly route-dependent. Specialised wind-aware routing software (provided by major weather routing services such as Applied Weather Technology, MeteoGroup, StormGeo) integrates:

  • Real-time weather forecasts.
  • Vessel-specific wind-assist performance polar (lift coefficient as function of apparent wind angle).
  • Routing optimisation to maximise wind savings while balancing ETA and fuel cost.

The weather routing savings calculator and the weather routing fuel savings calculator implement the routing-savings calculation.

Heavy weather handling

Most wind-assist devices have safety procedures for heavy weather:

  • Flettner rotors: stop rotation and feather; reduced thrust but safe operation at Beaufort 8+.
  • Rigid wing sails: rotate to feather (parallel to wind); reduced thrust but minimal heeling load.
  • Soft / suction sails: lower or fold; suction blower stops.
  • Towing kites: recover to deck; reduced operation in heavy weather.

The handling procedures are documented in the ship’s SEEMP Part I and the corresponding STCW competence framework.

Manoeuvring impact

Wind-assist devices can affect ship manoeuvring:

  • High-side-area devices (rotors, wings) increase wind resistance during manoeuvring at low speeds.
  • Some devices fold or stow during port operations to reduce wind load.
  • Bridge wing visibility may be reduced; new builds typically design bridges to maintain clear lines of sight.

Insurance and class certification

Wind-assist installations are classified under the class society’s Innovative Equipment Notation (or equivalent), with periodic surveys and maintenance requirements. Major class societies (DNV, Lloyd’s Register, ABS, Bureau Veritas, ClassNK) all maintain wind-assist guidance documents.

P&I insurance has accommodated wind-assist as standard equipment from 2020 onwards, with no significant premium impact.

Future outlook

Adoption projection

DNV’s Maritime Forecast to 2050 (2025 edition) projects:

  • By 2030: ~500 wind-assist installations on commercial vessels worldwide (vs ~80 in 2024).
  • By 2040: ~5,000 installations covering ~10% of the global ocean-going fleet.
  • By 2050: wind-assist becomes standard equipment on most newbuild bulk carriers, tankers and ro-ro vessels (potentially 30 to 40% of the global fleet).

The principal driver is the rising cost of fuel under EU ETS Maritime, FuelEU Maritime, and the IMO Net-Zero Framework GFI standard from 2027. As fuel costs rise, the payback period for wind-assist falls, expanding the addressable fleet.

Emerging technologies

Several emerging wind-assist technologies are at demonstration stage:

  • Telescoping rigid wings (Wallenius Marine, Anemoi): rigid wings that telescope down for harbour operations and bridge clearance.
  • Distributed Flettner rotors with active control: smaller rotors distributed across the deck with coordinated control for optimal thrust pattern.
  • Hybrid kite + rotor systems: combining a high-altitude kite with deck-mounted rotors for redundancy.
  • Vertical-axis wind turbines (VAWT): alternative concept generating electricity rather than direct thrust; lower potential but easier to integrate.

Regulatory evolution

The IMO is considering further enhancement of the Innovative Technology Credit framework under the EEDI Phase 4 review (expected 2027 to 2028). Wind-assist credit boundaries may be expanded.

See also

References

  1. IMO MEPC. Resolution MEPC.244(66) - 2014 Guidelines for Calculation of the Energy Efficiency Design Index (EEDI) for Innovative Energy-Efficient Technologies. IMO, 4 April 2014.
  2. IMO MEPC. Resolution MEPC.245(66) - 2014 Guidelines on the Method of Calculation of the Attained EEDI for New Ships. IMO, 4 April 2014.
  3. International Windship Association (IWSA). Annual Wind-Assist Industry Report 2024. IWSA, London, 2024.
  4. Norsepower. Annual Performance Report 2024. Norsepower Oy, Helsinki, 2024.
  5. Anemoi Marine Technologies. Anemoi Performance Data. Anemoi, London, 2024.
  6. BAR Technologies. WindWings Performance Data. BAR Technologies, Portsmouth UK, 2024.
  7. Bound4Blue. eSail Performance Report. Bound4Blue, Barcelona, 2024.
  8. SkySails Group. SkySails Annual Report. SkySails, Hamburg, 2024.
  9. Wallenius Marine + Alfa Laval. Oceanbird Project Status. Stockholm, 2024.
  10. Cargill. Pyxis Ocean WindWings Operational Report. Cargill Ocean Transportation, Geneva, 2024.
  11. DNV. Maritime Forecast to 2050 - Wind-Assisted Propulsion Section. DNV, Oslo, 2025 edition.
  12. ABS. Wind-Assisted Propulsion: Technical Guide. American Bureau of Shipping, Houston, 2023.
  13. Lloyd’s Register. Wind-Assisted Propulsion: Practical Implementation Guide. Lloyd’s Register Marine, London, 2024.
  14. ClassNK. Guidelines for Wind-Assisted Propulsion Systems. Tokyo, 2024.

Further reading

  • IWSA. Wind-Assisted Propulsion: Technology and Adoption Annual Reports.
  • DNV. Maritime Forecast to 2050. DNV, Oslo, 2025 edition.
  • Wallenius Marine. The Oceanbird Project: Wind as Primary Propulsion. Stockholm, 2024.
  • B9 Shipping. Wind-Assist Economics Workbook. B9 Shipping, Belfast, 2024.