DOSSIER

 

Different types of sail


The three main types of wind sail for ships seen so far are the Flettner Rotor, Wing sails and Suction Sails. DNV’s expert Johanna Tranell explained

The Flettner Rotor “is the most mature solution,” for wind power on ships, said Johanna Tranell, Engineer in Hydrodynamics and stability at DNVand a specialist on wind power.

There is a cylindrical structure which rotates around its own axis. In wind that creates a low and high pressure area which generates lift and drag forces. These are perpendicular to the wind direction.

The sail creates its own drag, and so it is most suitable for slower vessels, such as tankers, bulk carriers and ro-ros. The sail can be lowered to go under bridges, she said.

A second type, “wing” sails, are like an aeroplane wing mounted vertically on the deck. It moves the ship forward in the same way as an aeroplane wing gives the aircraft lift. Air has a longer distance to pass over the wing on one side, leading to reduced pressure on that side.

They can be made of hard-shell material, with lift devices such as flaps to generate more lift. These have higher aerodynamic efficiency, which probably makes them more suitable for faster vessels. Or they can be made of soft material such as a textile, which reduces the weight, and can help with stability. It can be easier to bring down and put on the deck.

A third type of sail is the suction sail also called “ventilated foil” or “turbo sail”. The suction sail looks similar to a Flettner Rotor, but it does not rotate. It is oval shaped and vertical.

There is an electricity powered air suction system. The suction delays flow separation in the air, which improves the lift.

At the date of the webinar in February 2024 there were 37 vessels in operation with wind assisted propulsion, with 52 in the order book.

There were 17 vessels in operation with Flettner rotors with a further 16 being built; 7 had rigid sails and 9 more being built; 10 had suction sails and 26 more being built.

In addition there is one vessel in operation with an inflatable sail, and two vessels in operation (and one on order) with a wind kite.

Owners were stating they were seeing fuel savings of 5-10 per cent, with some claims of 25 per cent and more for newbuilds.

“Some ship types are better suited for wind assisted propulsion than others; some sail technologies are better than others for certain ship types,” she said.

Different systems work best at different wind angles, so you need to consider the wind direction as well as the speed on the routes where your vessel trades.

You can run simulations of past voyages over multiple years, and at different times of year, using hindcast weather data, to tell you which wind conditions are seen most often.

If you are planning a system which requires electrical power, you need to consider if there is enough power supply onboard. And if you are generating more auxiliary power to run the sail, that will affect the overall fuel saving.

It is important to consider deck space, and the effect on stability, weight of the vessel, and if the structure can take the weight of the sails. Also, if the sail will obstruct a navigator’s view.

 

 

 

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