Modern ocean-going vessels are mostly propelled by diesel or diesel-electric engines with mechanical connection to the propeller shaft. Now these engines can get an assist from sails installed on the ship’s decks. These are not fabric sails with rigging. The sails being installed are rotor sails. To an observer the sail looks like a large, vertical pipe. When in operation these vertical pipes are spun using power from the mechanical plant in the ship. The spinning cylindrical sail works together with the wind in utilizing Magnus effect to push the ship forward.
These sails work in a manner similar to the wing of an airplane. An airplane’s wing is shaped so that the air passing over the top of the wing exerts less downward pressure on the wing than the upward pressure from the air that is passing under the wing. This creates an upward lift applied to the wing and to the aircraft.
To show this take a narrow strip of paper about 1 inches wide, and using both hands hold it by the near corners with the thumb and fore-finger of each hand. Blow across the top of the paper. Even a gentle blow causes the piece of paper to rise. The air pressure over the top of the paper is less than the air pressure below the piece of paper, so the paper rises up. This also applied to the spinning rotor sails. The rotor sails are smooth vertical cylinders. They do not “catch” the wind; rather, they work by air pressure.
If you were a sea gull flying above the ship and looking down you would see the vertical cylinder as a spinning circle. Let’s say that the ship is sailing North, or towards the top of your screen. For this example, the wind is blowing from the West; from your left to your right. The cylinder is spinning in a clockwise direction so that the North edge of the spinning circle, as you look down on the spinning cylinder (remember you are a gull above the ship), is rotating to the East and so is moving in the same direction as the wind. The South edge of the spinning cylinder is rotating towards the West, in the direction that the wind is coming from, and so is spinning against the wind. This creates a higher pressure on the South side of the spinning cylinder which exerts a pressure in the northerly, or forward, direction. This is the wind assist for the ship heading North.
The rotor sails can be spun in either direction, clock-wise or counter clock-wise to take advantage of favorable winds. The winds do not have to be 90-degrees to the course of the ship as the wind and the Magnus effect can be used even with the wind off the forward or after quarters of the ship. Unfavorable winds that cannot be utilized are those that are blow from approximately twenty degrees on either side of the bow or stern of the vessel.
The pressure difference between the high-pressure side and the low-pressure side of the spinning, cylindrical rotor sail is magnified by the Magnus effect. Based on the influence of this force, studied and described by Heinrich Magnus, a spinning object causes the air on one side to exert a force on the object so that it moves in a direction perpendicular to the axis of the spin. The use of the Magnus effect in rotating sails was first demonstrated in the 1920s. At that time it was not found to be more efficient than the normal means of the ship’s motive power.
An article in The Guardian in 2016 describes the installation and efficiency of rotor sails developed and installed by NorsePower on the MS Estraden. Using these rotor sails as a wind assist on its routes across the North Sea resulted in a reduction of 6% in fuel costs.
In 2018, an article on the Maritime Executive website described a more recent installation of rotor sails on the Maersk Line tanker, Maersk Pelican. The installation was further reported in 2018 in the World Maritime News, which stated that Maersk sees a potential fuel savings of up to 10% in using the rotor sails for wind assist. When wind conditions are favorable the rotor sails can provide sufficient forward thrust which allows the ship’s primary engines to be throttled back. This results in less fuel burned resulting in less combustion exhaust from the engines.
The electrical power that is used to spin the rotor sail is less than the energy required by the ship’s normal propulsion to generate the forward thrust generated by the rotor sail. When the savings in fuel and the reduced emissions is multiplied across many ships and many thousands of miles of ocean transport, the reduction of fuel burned and the reduction of the resultant pollution is significant.
https://www.maritime-executive.com/article/two-wind-power-rotor-sails-installed-on-lr2-tanker