The new age of sail
26 February 2005
NewScientist.com news service
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
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
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
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
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
. 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
By 2010 cargo vessels will account for
three-quarters of Europe's sulphur dioxide emissions
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.
From issue 2488 of New Scientist magazine, 26 February 2005, page 44