A Low-carbon Future – is the Answer Blowin’ in the Wind?

The IMO has set ambitious targets on reducing greenhouse gas (GHG) emissions from shipping. These targets will be partly met by improving efficiencies in vessel operation, but it will almost certainly require a huge change in how vessels are fuelled and powered. One of the potential solutions relates to harnessing wind-power.

Sail power on its own will not be the magic solution in achieving a zero-carbon means of propulsion. But if harnessed, wind-assisted propulsion can reduce a vessel’s fuel consumption, resulting in lower GHG emissions. The difficulty of course is harnessing the power of the wind, which seafarers have been tackling for millennia.

Modern wind propulsion comes in a variety of forms and some are already being used on cargo ships at sea.

Rotor Sails

Mounted on the open deck of a vessel, cylindrical rotors harness the thrust created by the ‘Magnus effect’.

When subject to wind, a difference in the air speed on either side of the rotor (essentially a large motor-powered rotating cylinder) results in a pressure differential. This pressure differential creates a force that propels the vessel forward. It is the same reason why a spinning football curves in the air.

The basic concept was created a century ago and, in 1924, the schooner Buckau was the first vessel to be fitted with ‘Flettner’ rotors. High installation costs and low fuel prices at that time led to a lack of further interest.

There are currently only a very few vessels operating commercially with rotor sails. Perhaps the most high-profile of these is the LR2 product tanker Maersk Pelican, which was fitted with two Norsepower rotor sails in 2018. Lloyd’s Register monitored the operation and reported fuel cost savings of 8.2% during the first year of operation.

This is consistent with research by BMT, who state that the installation of two rotor sails on a modern vessel could achieve fuel savings of 10%, taking into account minor losses in its effectiveness through increased heeling moments from sway forces and the extra rudder drag from weather helm.  Gavin Allwright, Secretary of the International Windship Association (IWSA), is more optimistic, reporting 20% fuel savings on a retrofitted general cargo ship. He also predicts that savings of more than 30% are achievable by using wind-assist on optimised newbuilds.

Some rotor sails have a tilting mechanism so it can be stowed horizontally, useful for when carrying out cargo operations.

Image Credit: Norsepower

Suction Wing Sails

A suction wing sail is a non-rotating aerofoil, where the leading edge adjusts automatically to the optimum angle relative to the wind. The profile of the wing then uses aerodynamics to create thrust that propels the vessel.

An example is the ‘ventifoil’ from Dutch company eConowind. They install retractable wing-shaped sails; either directly to a vessel’s structure on the open deck, or in a containerised unit. If conditions are unfavourable, the ventifoil can be retracted.

Image Credit: eConowind

Towing Kites

Systems such as SkySails use a towing kite with rope attached to the vessel’s bow by a rope. The kite is steered by the control panel on the bridge so it can manoeuvre in the air ahead of the ship to generate propulsion.

Rigid (Hard) Sails

The profile of a rigid sail resembles that of an aeroplane wing and operates in a similar principle. A hard sail system is to be fitted on MOL WIND CHALLENGER; a coal carrier expected to be delivered in 2022. It is claimed that the forecastle mounted telescopic rigid sail will reduce the vessel’s GHG emissions by about 5% on a Japan-Australia voyage, and about 8% on Japan-North America West Coast voyage.

Wind Trouble

Naturally, wind propulsion only works if there is wind that can be harnessed. And despite ships using the power of the wind for as long as time, the modern solutions are very different to those of the past. We are very much still in a learning phase and each of the different wind-propulsion options brings its own risks.

Maintenance: Sail systems will require maintenance and fixed systems could have an adverse impact on a vessel’s efficiency if they aren’t in operation and cannot be stowed.

Cargo operations: Non-retractable and fixed systems could hamper cargo operations such as limiting crane reach or conveyor positioning. They may also be vulnerable to damage during cargo operations, especially with crane movements.

Cargo capacity: As sails are mounted on deck, the space needed could impact the cargo carrying capacity of the vessel

Size: Sails with large surface area could affect visibility from the wheelhouse, increase vessel’s windage and the additional weight needs to be considered.

Find out more on the website of the International Windship Association at: www.wind-ship.org