Gu Yajuan, CCS Shanghai Rules & Research Institute

With increasing global attention on greenhouse gas emissions from the shipping industry, wind-assisted propulsion technology, as a clean and renewable energy solution, has gradually garnered significant interest from the maritime sector. This paper analyzes the principles and characteristics, development and current status, and challenges of wind-assisted propulsion technology for ships, and explores the prospects for its widespread application.

Principles and Characteristics of Wind-Assisted Propulsion Technology

Wind-assisted propulsion technology refers to the use of wind energy as a driving force for ships to reduce the load on the main engine, thereby lowering fuel consumption and carbon emissions. The energy-saving effect of wind-assisted propulsion depends on the combination of the power system and generally falls into two forms: one where wind power serves as the primary propulsion  and the engine power plant as the auxiliary propulsion, and the other where the roles are reversed. To achieve the optimal combination of energy saving, emission reduction, and safety, the choice of system depends on the route and climatic conditions. When wind power is the primary propulsion, energy saving is the primary goal, but it is highly susceptible to climatic factors. In ocean-going navigation where adverse sea conditions are common, safety can be compromised. Therefore, it is generally applied to relatively safer routes such as inland waterways or near-sea routes. For ocean-going vessels, the engine power plant usually serves as the primary propulsion which is relatively safer and can still achieve considerable energy-saving effects.

Wind-sail-assisted ships can achieve fuel savings by maintaining constant speed while reducing the main engine output power, or by increasing speed while keeping the main engine output power unchanged. Currently, the main wind-assisted propulsion technologies include sail technology (such as rigid wing sails, soft sails, etc.), kite systems, rotor sails (Flettner rotors), and wind turbines (using wind energy to generate electricity). After long-term practical applications on ships and comprehensive assessments of technical maturity, ship type and route adaptability, efficiency, and benefits, the shipping industry mainly focuses on the application of rigid wing sails and rotor sails.

Wing sails mainly include rectangular wing sails, triangular wing sails, Walker-type sails, and SkySails. The common characteristic of these sails is that their cross-sections are all wing-shaped, with differences in the longitudinal section shapes. For example, the longitudinal section of a rectangular wing sail is rectangular, while that of a triangular wing sail is triangular or nearly triangular.

Wing sails can directly convert wind energy into propulsion force, avoiding the energy conversion losses in traditional power systems. Moreover, modern wing sails are usually equipped with automatic control systems that can automatically adjust the angle according to wind direction and speed to maximize the use of wind energy and improve energy efficiency.

Rotor sails consist of a tall cylinder with an electric motor inside to drive it to rotate at high speed. When a ship is sailing, the rotating Flettner rotor generates a thrust perpendicular to the wind direction, thereby assisting the ship’s forward movement. This thrust can significantly reduce the main engine’s output power, thereby lowering fuel consumption and carbon emissions. Rotor sails are easy to install and adaptable. The wind-assisted rotor structure is independent and can be used not only for newbuilding installations but also conveniently for retrofitting existing ships. It is also suitable for ships with ample deck space, such as oil tankers, liquefied gas carriers, bulk carriers, ore carriers, roll-on/roll-off ships, and ferries, and has a broad application prospect.

Domestic Development and Application Cases of Wind-Assisted Propulsion Technology

As early as 1985, the Wuhan Water Transport Institute and Nanjing Shipping Company jointly developed a 200-ton arc-shaped wing sail-assisted ship, achieving an energy-saving effect of 50%. The Guangdong Provincial Shipping Research Institute installed a wing sail on an 80-ton cargo ship, which saved 42.2% of fuel compared to similar sail-less ships in winds of 3 to 4 Beaufort scale. In the 1990s, Ningbo Marine Company Limited developed a 2,500-ton “Mingzhou 22” sail-assisted container ship, which featured a stainless steel arc-shaped sail with an area of 120 square meters. The sail was operated by an automatic hydraulic control system and sailed between Japan, Ningbo, Xiamen, and Hong Kong.

Since 2016, the China Ship Scientific Research Center has established a project team to conduct research on wind-assisted propulsion technology. The team has successively carried out numerical calculations, scheme design, energy efficiency assessment, prototype development, and engineering prototype development for rigid wing sails and rotor sails, ultimately achieving demonstration applications and technology promotion on full-scale ships. The center has conducted extensive numerical studies on factors affecting the energy-saving effects of wind-assisted propulsion (such as wind speed, wind direction, rotational speed, diameter, height, and shape characteristics) and has established a comprehensive design method for wind-assisted propulsion systems. This method can provide technical support for designing wind-assisted systems for ships in the future. Faced with the technical requirements of large-scale and high-speed engineering prototypes, the No. 702 Institute and CMES-Tech (Shanghai) creatively proposed various solutions for wind-assisted transmission, perception, and control, and launched technical solutions for large bulk carriers, large liquid cargo ships, PCTCs, and other ship types.

 The 308,000-ton Very Large Crude oil Carrier (VLCC) “Kaili,” independently developed and built in China, is the world’s first VLCC to adopt a sail device. The rigid sail installed on the “Kaili” is 39.68 meters high and 14.8 meters wide, with a maximum outer diameter of the rotating base of 5.3 meters and a central cylinder diameter of 4.5 meters. The system comprises a rotating mechanism, mast, and airfoil-shaped components. Both the sail size and the tonnage of the ship equipped with it were innovations in the domestic and international shipping industry at that time.

The rigid wing sail installed on the “New Aden” has a height of nearly 40 meters and a total single-wing area of 1,200 square meters. The sail structure is made of lightweight, high-strength, and highly marine environment-resistant carbon fiber composite materials, making it the world’s first VLCC to adopt a carbon fiber rigid wing sail. The intelligent control system can achieve one-click deployment and retraction of sails, automatic rotation, and other functions. It can automatically adjust control strategies based on real-time data of navigation environment and motion status, enabling the sail to automatically find the optimal angle and achieve intelligent fine control, effectively improving the efficiency of wing sail propulsion.

The “HYSY226,” equipped with the first set of marine rotor sails  independently developed and manufactured in Asia, was delivered and put into operation in early 2024, marking a groundbreaking achievement in the first successful implementation of rotor sails on commercial ships in China.

The 4,950 DWT “Jun Bai 56,” which was launched at the end of 2023, is the first oil tanker in China to be equipped with a rotor sail. The ship was built by Zhejiang Tenglong Shipbuilding Co., Ltd.

In June 2024, Norsepower signed a cooperation agreement with UK shipowner Union Maritime to equip 12 of its 18,500 DWT product tankers with rotor sails, including four newbuildings constructed by Fujian Southeast Shipbuilding Co., Ltd. and Wuhu Shipyard Co., Ltd.

In October 2024, CSSC Chengxi Shipyard Co., Ltd. (hereinafter referred to as CSSC Chengxi) completed the manufacturing of a five-mast foldable rotor sail, which is also the second batch of foldable sails completed by CSSC Chengxi. The rotor sail was designed by Anemoi Marine Technologies, a UK wind-assisted propulsion technology supplier. CSSC Chengxi optimized the production design, reducing the overall weight of the steel structure components by 30%. COSCO Shipping Heavy Industry Co., Ltd. completed the installation of this type of sail on the 400,000 DWT ore carrier “SOHAR MAX” under VALE, which is also the second ship retrofitted with sail undertaken by the shipyard in 2024. Previously, it had completed the retrofitting of the world’s first Capesize bulk carrier “CAMELLIA DREAM” with a rotor sail.

Norsepower provided a rotor sail system for a 7,500 cubic meter LCO？ carrier built by Dalian Shipbuilding Offshore Co., Ltd. At the same time, CSSC Hudong-Zhonghua Shipbuilding (Group) Co., Ltd. signed a cooperation agreement with Anemoi Marine Technologies to jointly design rotor sails for two liquefied natural gas (LNG) carriers.

On November 26, 2024, Norsepower’s new factory in Dafeng District, Yancheng City, Jiangsu Province, was inaugurated, further strengthening the delivery capacity of rotor sails.

Challenges Faced by Wing Sails and Rotor Sails in Shipboard Applications

From the above situation, it can be seen that as two innovative wind-assisted propulsion systems, wing sails and rotor sails have been preliminarily implemented in various aspects of China’s shipbuilding industry, including product development, standardization, installation on existing ships, and construction of newbuildings. However, they still face many challenges in practical applications.

Improvement and Enhancement of Technical Level

Although wing sails and rotor sails have a long history of application on ships, they have not been widely promoted and applied. This includes design and integration, materials and structural strength, automation and control, mechanical and electrical systems, ship stability and safety, crew training and operation, energy efficiency and wind energy utilization assessment, and optimization of onboard layout. Stakeholders need to continuously conduct in-depth research and closely cooperate in all aspects of design development, installation and construction, standardization, and equipment use to continuously improve and enhance technical levels and performance.

High Installation and Maintenance Costs

The installation and maintenance costs of wing sails and rotor sails are relatively high. Shipowners need to comprehensively evaluate various aspects, including initial investment, the impact of CII implementation on ship navigation restrictions, the carbon tax/fuel tax on shipping that the EU and the International Maritime Organization (IMO) will introduce in the future, and the reduced fuel consumption costs, before making decisions. As product development units, they should continuously optimize materials, equipment, and systems. Shipyards should also continuously optimize installation processes to continuously reduce product and installation costs.

Technical Maturity and Reliability

As emerging technologies, the maturity and reliability of wing sails and rotor sails still need further verification. In practical applications, these technologies may face various unknown challenges and problems. Therefore, continuous research and testing are key to ensuring their successful application. Shipowners and manufacturers can cooperate on pilot projects and actual navigation tests to accumulate experience and improve technology.

International Cooperation and Standardization

The promotion of wing sails and rotor sails requires the joint efforts and standardization of the international community. Differences in regulations and standards among different countries and regions may pose obstacles to the global application of these technologies. Therefore, it is necessary to formulate unified regulations and standards through international platforms such as the International Association of Classification Societies (IACS) and the IMO to promote the global uptake of these technologies.

As innovative wind-assisted propulsion systems, wing sails and rotor sails have significant environmental and economic benefits. However, they still face many challenges in practical applications, including design and integration, materials and structural strength, automation and control, crew training and operation, energy efficiency and wind energy utilization, mechanical and electrical systems, ship stability and safety, regulations and standards, costs and return on investment, technical maturity and reliability, environmental and ecological impacts, and international cooperation and standardization. Through continuous technological research and development, policy support, and international cooperation, these challenges are expected to be gradually resolved, thereby promoting the widespread application of wing sails and rotor sails in the shipping industry and contributing to global environmental protection and sustainable development.

Future Development Trends of Wing Sails and Rotor Sails

As two innovative wind-assisted propulsion technologies, wing sails and rotor sails are increasingly being applied to ships. Many companies and research institutions both internationally and domestically are actively exploring the application of these two technologies and have achieved significant results. However, as innovative wind-assisted propulsion systems, the application of wing sails and rotor sails on ships still requires continuous innovation and development.

1.Technical Improvement

In the future, the technical improvement of wing sails and rotor sails will mainly focus on the following aspects. First, in terms of material innovation, lighter and stronger composite materials should be used to improve the structural strength and durability of the sails. Second, in terms of automatic control, more intelligent automatic control systems should be developed to improve the propulsion efficiency and adaptability of the sails. Third, in terms of integrated design, the sails should be integrated with other ship systems (such as the power system and navigation system) to optimize overall performance.

Application Expansion

With the progress of technology, the application scope of wing sails and rotor sails will be further expanded. First, multiple sets of wing sails or rotor sails can be installed on a ship, forming a multi-sail combination to improve the utilization rate of wind energy and further reduce fuel consumption. Second, the application of wing sails and rotor sails will gradually be extended to different types of ships. Driven by the pressing need to reduce greenhouse gas emissions from international shipping, these technologies are currently mainly applied to mainstream international sailing ships. As the technologies develop and mature, and costs continue to decrease, they will gradually be promoted and applied to various types of ships. Third, the Flettner rotor-wing sail combination is a wind energy utilization device that combines the advantages of both rotor sails and traditional wing sails. Rotor sails generate lift through rotation in crosswind conditions to propel the ship forward, while wing sails use the direct thrust of the wind. The combination of these two technologies aims to maximize the utilization of wind energy from all directions and further improve the efficiency of wind energy utilization.

3、Policy Support

As the pressure and difficulty of reducing greenhouse gas emissions increase, governments around the world will provide more policy support for wind-assisted propulsion technologies. First, subsidy policies will be implemented to provide subsidies for ships using wing sails and rotor sails, reducing their initial investment costs. Second, relevant regulations and standards will be formulated to promote the widespread application of wing sails and rotor sails. Third, increased investment in research and development of wing sails and rotor sails will be made to promote technological progress and industrial development.

In the future, with the advancement of technology and the implementation of supportive policies, the application scope of wing sails and rotor sails will be further expanded, bringing more opportunities for energy saving, emission reduction, and economic benefits to ship design and operation.