The new age of sail

26 February 2005

NewScientist.com news service

Mick Hamer

THE coming of steam sent the world's great sailing fleets into decline. The internal combustion engine finally finished them off. So it would be a strange twist of fate if the age of sail was resurrected by what amounts to a child's toy.

For several weeks last summer, a team of German engineers sailed back and forth across the Baltic Sea playing with a large inflatable kite. The engineers, from the Hamburg company SkySails, were testing the potential of high-tech kites to pull a ship across the ocean by hitching a ride on winds high above the waves.

The idea isn't to propel a ship by wind alone - a conventional diesel engine will help it along on days when the wind is blowing from the wrong direction, is too strong or dies away entirely. But since the kite reduces the need to use engines, the team at SkySails believes it can halve the amount of fuel a ship burns.

This is just one of the ways in which sail power is being revived - and it's not the first. In tests more than 20 years ago, Japanese engineers equipped several ships, including a bulk carrier and a tanker, with masts and sails. The projects were eventually shelved, but this time round there are sound reasons why wind could win through.

Click images to enlarge.

For nine years a team of naval architects in Copenhagen, Denmark, has been working on a completely new design: a 50,000-tonne cargo ship whose diesel engine will be augmented by a set of high-tech sails set on six masts. Canvas is definitely out. Aerofoils are in.

Denmark is already a world leader in developing wind turbines for power generation and is keen to capitalise on this expertise to develop wind propulsion for ships. In 1995 the Danish Environmental Protection Agency began financing a major research project into windships. Led by the naval architects Knud E. Hansen, the research not only produced a new design of ship but also looked at how the ship could make the most of wind power and the cargoes it would be best suited to carrying. Now the team is about to embark on full-scale trials.

Though wind power has obvious commercial advantages - wind is free, after all - traditional sailing ships were never cheap to run. They needed a large crew to operate the rigging, the sails themselves had to be replaced and repaired regularly, the canvas had to be stored somewhere on board, which reduced the space for cargo, and the variability of the winds made schedules unreliable. On top of that, the economics of sail were steadily undermined in the 20th century by the rising cost of labour and a decline in the real price of oil. Engines won out, and even hybrid wind-assisted ships never became widely used, says Brian Lavery of the UK's National Maritime Museum in Greenwich. "They didn't go in for multi-skilling in those days," he says. "You would need one crew to operate the rigging and then stokers to run the engines."

So why should modern hybrid sailing ships fare any better? Part of the answer is that the economies of running a ship have changed again. The small crew needed on a modern ship, combined with the low wages they are paid, means that the cost of fuel as a proportion of total running costs rose from 10 per cent in 1900 to between 25 and 60 per cent by 2000.

Modern windships can also take advantage of new technologies and materials that weren't available in the days of sail. Wind tunnel tests on different types of rigging and sails quickly showed the Danish team how poorly traditional sails perform. A sail is more than a simple sheet of fabric. To propel a ship it needs to take up an aerofoil shape, and that only happens when the wind fills it. If the wind is too light, or it keeps changing direction, the canvas flaps uselessly and generates drag rather than propulsion.

So the Danish team came up with an alternative that exploits materials borrowed from the aerospace industry. Using high-performance steel for the masts does away with the need for stays to hold them upright. The sail itself is made of fibreglass, with a profile like an aircraft wing (see Diagram) . Flaps on the sail's trailing edge generate extra thrust when extended, but can be retracted to minimise aerodynamic drag - important when using engine power alone.

Wind-tunnel tests showed this design to be twice as efficient as the sails on a traditional windjammer. Even more importantly, the sail generates thrust when the ship is sailing close to the wind. Simulations suggest that the vessel will be able to make progress under sail even when the wind is blowing as little as 40 degrees off the bow, which is an excellent performance for a large sailing vessel. With a fresh breeze of 9 metres per second at 100 degrees - blowing only slightly from behind - the sails alone can propel the ship at 13 knots (25 kilometres per hour.

Unlike traditional sails, these fibreglass wings will not need a large crew to operate them, the designers say. They can be controlled hydraulically from the bridge, and because they never need to be lowered there is no need for storage space that would eat into the cargo capacity. The downside is that in light winds, with the ship under diesel power, they exert aerodynamic drag - even with flaps retracted - which negates some of the fuel savings from having them there in the first place.

The SkySails kite suffers no such handicap. The idea is to harness the winds higher above the ocean with an inflatable aerofoil - a kite designed to fly at a height of 100 to 500 metres, towing the ship on a cable fastened to the hull.

At 500 metres, winds are often stronger and less variable than at sea level, and can differ in direction from those immediately above the waves by 10 to 15 degrees, according to Barry Gromett of the UK's Met Office in Exeter. "Although these differences are not huge they could be really useful," he says.

SkySail's aerofoil is designed to maximise thrust whatever the wind conditions. It uses a computer autopilot and patented wind sensors coupled to the ship's steering system to calculate the kite's optimum position. Then the autopilot manoeuvres the kite using motors in a control unit suspended beneath it to change the trim of the aerofoil by adjusting the tension in its control lines. The kite can move along a rail around the hull to maximise its towing efficiency and a winch on the ship adjusts the length of the kite's main line to fly it where the wind speed and direction are most favourable

Last year's trials in the Baltic, aboard an 8-metre model of a cargo vessel, were mostly carried out in unfavourable conditions of weak and variable winds. Nevertheless, they showed that the SkySails kite can generate 1 to 1.15 kilowatts for every square metre of aerofoil. "In favourable winds it would generate a lot more thrust," says Stephan Wrage, founder of the company. The kite is designed to be retrofitted to ships of almost any size, but SkySail's largest version, with an area of 2000 to 5000 square metres, will generate propulsive power equivalent to a large ship's engine, he says.

Since the kite is controlled by an autopilot, Wrage says it will not need many extra crew to handle it. Compressed air will be used to blow it up when it is deployed and when not in use it is deflated, so storing it should not be a problem either. But as any kite flyer will tell you, launch and recovery are likely to be a little more complicated. SkySails says it will be an automated process, but won't reveal details until its engineers fit their first system to a ship next year.

Selecting a course that maximises the benefits from the wind is an important part of any successful voyage under sail. Here modern windships have another key advantage over their predecessors: they will have access to far more accurate weather forecasts to help get the best from the wind.

In the old days of sail, a ship left port if the wind was in the right quarter. The course of a voyage was largely a matter of luck, the ship changing course to use whatever winds there were. Nowadays the UK Met Office and its counterparts elsewhere provide shipping companies with route forecasts - principally to avoid bad weather. It would need only a small extension to the service to provide forecasts of the best winds, says Gromett.

The Danes have already looked at different strategies for setting a course. By studying records of winds and weather forecasts during the 1990s, they calculated how much power the wind would contribute during a voyage. One of the least successful of the strategies they modelled was to calculate the best course given the weather forecast at the start of the voyage, and then stick to it. Following this inflexible approach meant changes in the wind during the voyage robbed the ship of almost all the advantages of wind power. In fact, it was little better than ignoring the wind direction and simply sailing the most direct course to the destination. Far better, they found, was to re-plan the course every 24 hours based on the latest forecast.

Another factor affecting the overall viability of a wind-assisted ship is that its performance will vary from ocean to ocean. In the North Atlantic the average wind speed is 8 metres per second. On routes such as Rotterdam to New York the ship would save up to 27 per cent of the normal fuel bill at its design speed of 13 knots, the Danish team calculate. But in the Indian Ocean, winds are far lighter. On routes such as that from the Ras Tanura oil terminal in Saudi Arabia to Mumbai in India or Singapore, there is not enough wind to maintain a speed of 13 knots without using the engine on full power. The usually light winds combined with the aerodynamic drag of the sails when sailing under power would mean fuel bills were actually higher than with a conventional ship. Even under more favourable conditions, they found that over its lifetime, the windship would be about 10 per cent more expensive to run than a conventional ship when the extra cost of construction is included.

Wind of change

These results proved a major setback and work on the project stopped in 2000. But since then the cost of marine diesel, which closely follows crude oil prices, has soared to nearly three times its 1999 levels, and now the project could be moving again, with backing from Scandinavian governments and the European Union. And this time, environmental benefits are being factored into the equation too.

In the last couple of years it has become clear that marine diesel engines are having a greater environmental impact than many experts had believed. These engines release pollutants such as sulphur, nitrogen oxides and PM10 particles. In particular, marine diesel oil contains 2.7 per cent sulphur - more than 500 times what is allowed by the EU for diesel sold for cars and trucks. By 2010 it is estimated that cargo ships will account for three-quarters of all Europe's emissions of sulphur dioxide. In the US, the Environmental Protection Agency is stepping in with new regulations to help improve air quality around large ports.

Could this signal a sea change for sail? "It will now be profitable both environmentally and economically to build the windship," says Anders Carlberg of Knud E. Hansen. Other new sailing ship projects are already in the works, one in Germany and one in Japan. Carlberg and his team estimate that full-scale trials of their design will start within three years.

It is not just the oil price that has moved in the windships' favour. The Danish team is confident that it will be able to design a more efficient vessel. Jesper Kanstrup, Knud E. Hansen's senior naval architect, says that the original designs concentrated on minimising the amount of space the engine and sails took up to maximise cargo space. "They weren't designed for fuel economy."

Reducing the design speed of the windship from 13 to 11 knots, for example, would cut fuel bills by a third on both the North Atlantic and the Indian Ocean routes because the engine wouldn't have to work so hard. Ships this slow would only be suitable for non time-critical cargoes, typically bulk goods such as grain, timber and bauxite, which together account for only 20 per cent of cargoes worldwide. Though this would restrict the use of wind-assisted ships, there should still be a big enough market to establish the technology commercially. However, the shipping industry is conservative, Kanstrup warns, and it will need a lot of convincing before it adopts sails.

SkySails has its eye on a rather different market. "One surprising result from the trials was the vessel's stability in heavy seas," Wrage says. Unlike conventional sails, the kite tends to stabilise the ship instead of making it heel over. This is partly because it is tethered to a rail close to the vessel's centre of gravity, and partly because the horizontal tug of the kite is counterbalanced by the vertical pull it generates, which tends to hold the vessel upright. "The sail acts like a damper so the ship moves smoothly, which will prevent passengers being sick." This is significant because Wrage sees cruise liners, and the growing number of cargo ships that carry passengers, as important markets for the technology.

For Wrage the next step is to move from a model to a full-size craft and he thinks the system's superior seakeeping will be attractive to the owners of large and expensive motor yachts. After all, the last thing any modern sailor wants is to spill their gin and tonic.