Note: This is a “classic” Interesting Thing of the Day article from over 10 years ago. It has not been edited recently, so it may contain broken links, outdated information, or other infelicities. We plan to eventually update or retire most classic articles, as time permits.

We’re well into the 21st century, and one of the big things on the minds of the world’s technologists is improving propulsion. Cars, trucks, and buses are moving from conventional gasoline-powered internal combustion engines to hybrid engines, diesel engines running on vegetable oil, and fuel cells. Airplane manufacturers are designing better and more powerful jet engines. Submarines are being built with engines that require no moving parts. And rocket scientists are trying to figure out the best means of propulsion to use for sending spacecraft to Mars and beyond. High-tech solutions to get from point A to point B with greater efficiency and lower cost are appearing constantly.

And yet, sometimes the best way forward is to go back. Steam power for cars is making a comeback, for example. What was thought to be a dead-end approach a century ago has turned out to have some redeeming qualities after all, now that technology, materials, and engineering methods have caught up with it. The latest blast from the past, though, really blows my mind. The brightest and best in the field of marine propulsion have come to the startling conclusion that if you want a reliable, inexpensive, and efficient way to move ships across the ocean, you might try…the wind.

The twist is that instead of using conventional masted sails, you can send the whole sail aloft, tethered to the ship by a rope—just like a kite. Using kites to pull vehicles along isn’t new; it’s been done for everything from surfboards, skis, and rollerblades to buggies and small boats. Such kites are known as power kites or traction kites. But multiply the size of these kites by ten or even a hundred times, add some sophisticated deployment and control mechanisms, and you have something else entirely: a kite sail, which can pull a large yacht or even a cargo ship.

Mastless Transit

My first thought on hearing about kite sails was that the idea was reasonable enough, but why not just use regular, masted sails—a tried and tested technology? That just goes to show you how little I know about ships and sailing. Masted sails, apart from being complex to build and operate, place a number of constraints on a vessel. It must have adequate ballast to offset the weight and pull of the masts and sails, for one thing. If carrying cargo, the masts and rigging can seriously complicate loading and unloading, not to mention reducing available storage space. And for vessels over a certain size, you run into problems of geometry and physics; sails would have to be unworkably huge to move the ship at all, and could never propel it at the speed of a petroleum-powered engine.

Kite sails, by contrast, take up almost no space. They attach to the deck, and the only real space required is for the deployment mechanism—typically a telescoping mast that lifts the kite up into the wind and later retrieves it. The whole process is automatic. Once flying on its own, the kite isn’t entirely at the mercy of the wind. It’s shaped like a parafoil and has numerous steerage lines connected to a control unit. By selectively shortening or lengthening the lines, the controller can alter the shape of the sail—and thus its direction and pull. And because the kite sail isn’t constrained by a mast, it can fly high enough to take advantage of winds that are often stronger and less turbulent than those closer to the water’s surface.

Of course, kite sails do require the wind to be blowing, and you’ll need a healthy breeze to move a ship of any serious size at a reasonable speed. The wind should also be blowing in approximately the right direction—the closer to your direction of travel that the kite sail is being pushed, the more efficient the power transfer will be. However, depending on conditions and the kite sail design, the direction of the kite’s motion can be as much as 130° off in either direction from the desired direction of travel. In other words, unless the wind is blowing almost exactly the wrong direction, you’ll get some benefit from it. Some kite sail installations use a computerized system to plot the best route for your ship based on the direction of the winds in the area, which might take you on a somewhat roundabout course but at least saves fuel.

On Sail

Kite sails are not intended to replace conventional engines, but to supplement them. So why would the owner of a ship spend hundreds of thousands, or even millions of dollars, to retrofit an existing ship with a kite sail? Two words: oil prices. Manufacturers claim that kite sails can reduce fuel usage, and its associated costs, anywhere from 10 to 35 percent, and in ideal circumstances, by as much as 50 percent for a brief period of time. Given the rapidly rising cost of fuel and the attendant rise in shipping costs, that savings—if it proves to be real—would be enormous. But only time will tell. Although kite sails for large vessels have been successfully installed and tested, commercial deployment is still a couple of years in the future. Meanwhile, the technology is being put to good use on smaller craft, such as racing yachts, where it has been shown to be reasonably effective but where fuel savings are not at issue.

Needless to say, using less fuel will be better for the environment, too, though I doubt that greener shipping is foremost in the minds of those considering the use of kite sails. But it’s a case where economics and environmental considerations happen to coincide nicely. And a kite sail might even serve as an inexpensive backup engine, enabling a ship with mechanical problems to get to port. It’ll be interesting to see if the idea catches on. If it does, it’ll give a whole new meaning to the expression “Go fly a kite.” —Joe Kissell

More Information

The two major companies working on kite sails for large yachts and cargo ships are KiteShip and SkySails. (SkySails has a great video of a kite sail being deployed, used, and recovered automatically on a large cargo ship.) KiteShip also has a site called KiteTugs, which describes the kite sails they build for racing yachts.

Additional resources:

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Kids from 10–99 can build and test their own miniature kite sail designs with the Kite Dynamics kit.