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Ship kites in to port

February 8, 2008 By Katharine Sanderson This article courtesy of Nature News.

As a the SkySails cargo ship finishes its maiden voyage, looks at the logic behind towing a ship with a kite.

Is it a bird? Is it a plane?

No, it’s a giant kite towing a cargo ship. What's more, it helps to save engine power and fuel. The company behind the kite, SkySails, based in Hamburg, Germany, claims that freight ships, and even cruise liners, could reduce fuel consumption by up to 50% under optimal conditions.

That sounds great – is it new?

Yes, on Tuesday the cargo ship MS Beluga SkySails sailed into the port of Guanta, Venezuela, completing its 2-week long maiden voyage from Germany, and the successful testing of the giant kite system. It is the first commercial vessel to use the SkySails kite system.

The kite looks just like a paraglider's wing. Is it?

Well spotted. Yes, the kite is shaped just like the wing used by extreme sport enthusiasts who 'paraglide' over hills — it is the same shape, and has a double-walled construction that fills with air to help provide lift. But it is much, much bigger: the average paragliding wing is about 30 square metres, and these boat-pulling kites have surface areas of between 150 and 600 square metres.

The maiden voyage of the MS Beluga SkySails used a 160 square metre sail to help pull the 132-metre-long, 10,000 tonne vessel from Bremen, Germany, to Guanta. The development of super light, strong materials for sport kites, surf kites and paragliders has driven the development of these huge ocean-going kites.

What happened on the maiden voyage?

Half way through the trip, as the ship reached the trade winds near the Azores, the computer-controlled kite was unfurled. SkySails hasn’t finished doing all the calculations yet, but estimates fuel savings of 10–15% during the time the kite was flying, as the ship cruised at top speed using less engine power. That is a saving of about US$1,000 to $1,500 in fuel costs per day. The company hasn't yet revealed how many hours the kite stayed aloft during the two-week voyage.

The company claims that savings of up to 50% can be made for short time periods, with an average saving of 10-35%. A kite with an 'effective tractive force' (which depends on the ship, its load and speed, and wind conditions) of 8 tonnes can pull the equivalent of 1,000 kilowatts of engine power, compared to the MS Beluga SkySail's main engine's 3,840 kilowatts of power.

The kite needs winds of at least 3 Beaufort (8–12 miles an hour or 7–10 knots — a gentle breeze) and can be used at up to 8 Beaufort (39-46 miles per hour, or 34–40 knots – a gale).

Does the ship have to be going the same way as the wind is blowing?

No, the kite can work over a range of angles away from the wind to provide power, just like a sail. In theory a ship can take a course up to 50 degrees skewed from the wind. Salty old sea dogs might recognise that this isn’t far off the capabilities of a yacht.

How does the kite compare to sails?

A 3-masted clipper ship might have a sail area of 2,000 square metres, 10 times the size of the kite, and move at top speeds of perhaps 17 knots on wind alone. But for massive cargo ships aiming to cruise at 10 knots or so, adding masts and sails isn't a very practical idea.

A kite is much easier to install and comes with a bonus: the higher the kite, the greater and smoother the wind speed. Closer to the sea’s surface, the friction, or drag, between air and sea slows things down and can cause turbulence, which decreases efficiency.

This kite can fly up to 300 metres above sea level. SkySails says that at 200 metres above sea level, the kite has twice the amount of energy available to it than on a 10-metre line.

Can I have a kite-powered boat for my own, personal use?

Yes. Small kites for yachts already exist, and are even used in races.

Are there any other uses of kites, apart from pulling ships, and loop-the-loops?

Yes, kites are also being developed to produce electricity. A kite attached to a ground-based dynamo and whizzing around overhead can generate electricity. Again, the development of materials for sport kites in recent years has seen this burgeoning technology begin to get commercial interest — Makani Power, a Hawaii-based wind power company, has recently attracted investment interest from Google to develop kites for electricity generation. Of course this is very different, and somewhat safer, than Benjamin Franklin's alleged attempts to use kites to harness electricity directly from lightning.

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