Sail Power for Ships

We tend not to think of shipping as a big contributor to climate change. But there are around 50,000 merchant ships around the world, and between them they emit some 800 million tonnes of carbon dioxide, more than four per cent of global CO2 emissions. If current trends continue, this will reach six per cent by 2020. Carbon dioxide is not the only pollutant from ships. Sulphur emissions are a particular problem – marine diesel contains a lot of sulphur as it is not subject to the same EU standards as diesel used for road vehicles – and ships’ engines also produce nitrogen oxides.

The Revival of Sail Power?

In bygone times, ships depended on sail power and caused no pollution, but for a long time now cargo and passenger ships have had diesel engines, and sail power has been largely the preserve of the tourism and leisure industry. Sailing dinghies have never gone out of out of fashion, whilst ‘tall ships’ have always had their enthusiasts and attract spectators wherever they go. But the late 20th and early 21st century has seen a renewed interest in wind energy as a potential power source for commercial shipping.

This is partly in response to sharp rises in the cost of marine diesel; partly due to environmental concerns; and partly inspired by keen consumer interest in food miles. Niche regional products that are exported around the world cannot claim low food miles, but they could claim ‘green’ food miles. A French company has begun offering a shipping service using traditional tall ships, and it’s proving popular with food and drink producers who wish to demonstrate their green credentials to customers.

Around 90 per cent of world trade is carried by sea, and the French Association of Shipowners has predicted that five per cent of this could be carried by wind-powered ships in future. Clearly, sail ships cannot match the speed of modern cargo vessels, but there are many types of goods which are not perishable and where speedy delivery could be of secondary importance.

New Types of Sails

When our ancestors built sailing ships and set out to sea, they did not have access to the technology and the research and development facilities that we have today. We have accurate weather and wind forecasting techniques, and we can plot navigational courses by computer. We have wind tunnels to test different types of sails and rigging and work out the optimum design, and we have new materials that perform better than canvas sheets.

One project in Denmark has designed a 50,000 tonne cargo wind-ship with hi-tech fibreglass sails mounted on steel masts. The sails have hydraulically-operated flaps and can be adjusted remotely from the ship’s bridge. By capturing as much wind energy as possible, it is envisaged that the sails will be able to drive the ship at 25 km/hr (13 knots) in a fresh breeze of 9 metres per second, blowing at 100 degrees or only slightly from behind. Propulsion by wind energy allows the ship to limit use of its diesel engine to those periods when the winds drop altogether, or blow completely the wrong way.

Another innovative concept being developed by a German company is to attach an inflatable aerofoil – made of waterproof textiles and resembling a very large kite or a paraglider – to a ship, to harness the energy of the winds high above sea level. The higher you go, the stronger the winds are likely to be. A computer autopilot system calculates the optimum angle, height and position of the aerofoil, and adjusts it by means of control lines and a winch. Like the fibreglass sails, the aerofoil – which can be retrofitted to ships – supplements the diesel engine, and it is estimated that it will capture sufficient wind energy to reduce fossil fuel consumption by between 10 and 35 per cent.

How Much Speed do we Need?

However hi-tech the sail, it cannot be effective if the air is completely still. The average wind speed varies from one ocean to another; the Atlantic, for example, has more wind than the Indian Ocean. On windier stretches of sea, there would seem to be a good case for using hybrid ships with sails as well as engines, thus reducing fossil fuel usage and cutting greenhouse gas emissions.

Increases in the cost of marine fuel has already caused many shipping lines to reduce the speed of their vessels; by running at 90 per cent of normal speed, most diesel-engined ships can expect to use around 25 per cent less fuel. This reduction in speed will benefit the environment too. If attitudes do gradually move towards accepting that speed is less important than the environment, sail power could indeed make a comeback.