Flettner rotors

Flettner rotors, or rotor sails, are vertical cylinders which spin and develop lift due to the Magnus effect as the wind blows across them. Rotor sails must be mechanically driven to develop lift and propulsion power, and manoeuvrability is restricted by wind speed and direction. Working on a ship, the force created will generate thrust. Onboard vessels, such rotor propulsion are often called Flettner rotors after the German innovator being the first to install such a system onboard a ship at the beginning of the 1920s. Rotor sails may significantly reduce the energy consumption of a ship, but it cannot be used as main propulsion.

Applicability and assumptions

Rotor sails are a supportive propulsion system possible for both newbuild and to retrofit on existing vessels, primarily for the ships and trades that benefit most from wind assisted propulsion. There are no major feasibility barriers when disregarding performance. However, due to operational issues, rotating cylinders generating thrust are applicable for vessels with a sufficient free deck-surface and it is important that no objects block the accessibility to free wind. Containerships may be evaluated in the case a design with elevated/retractable rotors mounted above the container cargo. Operational height limitations must be taken into account, for example in case of interfering with operational related structures or infrastructural barriers related to the route if fixed rotor sails are installed. However, there are also solutions of rotor sails that can be folded and thus be installed even if such barriers are along the trade route.

The rotor sails generate forces in the horizontal plane, forward and sideway forces. As to make sure the seagoing properties of the vessel remains good, it must be prepared and planned for the healing movement influencing the stability and the strength of the cylinder foot must be properly supported as it is subject to high stresses.

The Flettner rotor principle is in general working in sideways winds and is depending on wind and vessel speed. The effectiveness of the rotors is therefore dependent on trade route and weather conditions.

In the recent years several successful retrofits of rotor sails have been carried out, and more and more ship owner are considering implementing this technology on both new build and existing vessels.

Cost of implementation

The range of installation cost for a Flettner rotor is $400,000 – $1,000,000 (USD) depending on the model (size) of the rotor.

Size of a typical delivery with multiple Rotor Sails also starts from $1,000,000 – $5,000,000 (USD).

The corresponding maintenance cost is assumed to be 2 % of the installation cost.

Reduction potential

The reduction potential for rotor sails is normally estimated at 3 – 15% of the main engine fuel consumption. In some special cases, a 25% reduction over time has been reported. The reduction potential depends on sailing pattern, speed, season (wind, sea), number of rotor sails, and size of rotor sail. The reduction potential can quite accurately be estimated before the measure is installed.

Other References

1. Wind rotor for ships: Success story of sea trial for Norsepower
2. Evwinds, 2013, The rotor sails on the ENERCON-developed "E-ship 1“ allow operational fuel savings of up to 25% compared to same-sized conventional freight vessels
3. Pearson, D. R., 2014. The use of Flettner rotors in efficient ship design.
4. Sea-Cargo, 2021. Reducing fuel consumption and CO2 emissions
5. Seddiek, I.S. and Ammar, N.R., 2021. Harnessing wind energy on merchant ships: case study Flettner rotors onboard bulk carriers.
6. Hochkirch, K., Bertram, V., 2022. Wind-assisted propulsion: Economic and ecological considerations.