Wind-Assisted Propulsion

Wind-assisted propulsion can be used on ships to reduce fuel consumption. Numerous techniques are available to support this. Previous C-Job research has identified Flettner rotors as the most viable option as they are robust, safe, and easy to operate. These Flettner rotors are installed on the deck to generate a forward thrust using wind. However, they are not the only option available to ship owners looking to harness wind.

What are flettner rotors?

Flettner rotors are tall, rotating cylinders mounted on a ship’s deck. They utilize the Magnus effect to generate forward thrust and supplement a vessel’s main engines. The Magnus effect is the force that acts on a spinning body in a moving airstream and creates an additional force on the object. Because the Magnus effect acts perpendicularly to the direction of the airstream, the optimum wind direction is 90 degrees to the direction of the sailing (a side wind).

German engineer Anton Flettner built the first ship that used this technique. This vessel successfully crossed the Atlantic in 1926. In the 1930s, a few aircraft were equipped with rotating cylinders to produce lift. Unfortunately, these cylinders caused so much drag that these machines were highly inefficient.

More recently, the Finnish company Norsepower has developed updated Flettner rotors for ships also known as Rotor Sails.

New build or retrofit

Flettner rotors can be installed in both new build ships and retrofit situations. There are a few important factors to consider:

• Vessel size and sailing speed. Rotor Sails are better suited to larger and slower vessels than smaller and faster vessels
• Amount of free deck space. Cargo on deck can interfere with airflow around the Rotor Sails. Therefore, ships such as bulk carriers, ro-ro vessels, car carriers, livestock carriers, tankers, and ferries would benefit most from Rotor Sails
• Sailing route. Long stretches of ocean sailing with consistent and predictable winds are more advantageous than numerous port stops seen in shortsea shipping. Transatlantic routes, therefore, would be well-suited vessels.

Ship design

The major goal of incorporating Flettner rotors in any ship design is to have as much benefit from the rotors as possible. With both new build vessel design and retrofits of an existing vessel, it is important to consider the wide array of factors that will create an optimized design. In most cases, allow the largest rotors as possible to yield greater fuel savings.

Larger Rotor Sails deliver more thrust and thereby greater savings in fuel consumption. However, larger rotors are also more expensive and influence factors such as stability, resistance, structural strength, and cargo capacity. This trade-off between initial investment, operational requirements, and potential fuel savings throughout the lifetime of the vessel.

Putting theory into practice

C-Job developed a 5,000 DWT general cargo ship concept design as part of its research into the application of Flettner rotors. This concept was part of the European SAIL project. SAIL is an international cooperation in the European Interreg IVB North Sea Region led by the Dutch province of Friesland.

An important outcome of the research is that a vessel sailing 100% on wind is not possible. This is due to the dependence on trade winds and is therefore not economically feasible within the commercial cargo transport sector.

Research continued to discover alternatives. The Flettner Freighter demonstrates the theory behind wind-assisted propulsion and puts it into practice. The cargo vessel concept design shows it can save up to 18% fuel on average with the installation of four Rotor Sails. This is when compared to conventional cargo vessels in the same DWT range.

The main innovation of the Flettner Freighter is the possibility to move the rear two Flettner Rotors in the longitudinal direction to accommodate for the best sail balance of the vessel and thus to optimally exploit the wind force and wind direction.