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Football Field-Sized Kite Powers Latest Freighter
coondoggie writes to tell us that a new freighter set to launch in December will be receiving a hefty dose of power from a kite the size of a football field. The 460-foot ship, owned by the Beluga shipping company, hopes to see as much as a 50% drop in fuel consumption during optimal conditions. "The SkySails system consists of a towing kite with rope, a launch and recovery system and a control system for the whole operation. The control system acts like the autopitot systems on an aircraft, the company says. Autopilot software sends and receives data about the sail etc to make sure the sail is set at its optimal position. The company also says it provides an optional weather routing system so that ships can sail into optimal wind conditions.The kites typically fly at about 1,000 feet above sea level, thereby tapping winds that can be almost 50% stronger than at the surface. "
The difference between conventional sails and the kite (from the article) - the wind 1000 feet up is going to be stronger than the wind at the surface. Also, with this technology, it looks like it is easier to retrofit the existing fleet of ships with a sail than to add a conventional sail.
Motive power is only the largest fraction of consumption on a ship. On all ships, auxiliary equipment must be powered. This ranges from the small consumers, such as navigation equipment and lighting, to the large consumers, such as reefer containers and engineering subsystems. A 10,000 TEU Maersk liner might have 250 reefer slots, and that sucks a lot of power, as does the bunker fuel heater (though usually steam, but still energy from the engine).
Then consider that engine efficiency doesn't scale linearly with fuel consumption, and that propellers on large ships are fixed, not constant speed. This means that a ship moving at 17 knots HAS to make, say, 83 RPMs (for a big Sulzer). So, the kite might provide 50% of motive power, but the ship will only be able to cut the fuel pumps 20%-25%, and can't cut RPMs at all, else the prop starts dragging and cavitating.
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yeah, according to the skysails website, and shown in a live action promo video, the launch and retrieval of the sail is completely automated, as is the steering. a person should be able to operate the whole thing by pressing the launch button in the control room to start it, and press the retrieval button when done. i recommend watching the video, it was interesting and good to see in action.
...all cock-blockery aside...
Also, the lever arm is shorter. A conventional mast needs to be very tall to get the most sail area, which is a lever from the water line to the top of the mast. This means that the wind is producing more torque on the ship, causing it to tilt ("heel") from the wind. Since the kite attaches directly to the deck, its lever arm is only from the water line to the deck. Heeling makes a sail less efficient, but has no effect on the kite.
Furthermore, the kite has upward lift, which helps pull the bow out of the water. This makes it feel less of the effects of waves, smoothing out the ride a bit.
The biggest difference, though, is that it can be used in high winds, everything from 10-40mph. A traditional sailboat cannot sail in such high winds.
dom
Most large ships are hybrids.
Conventional generator powering an electric motor reduces wear and tear.
I know you're just being funny, but pirates have other and better means to locate possible targets, such as AIS (http://en.wikipedia.org/wiki/Automatic_Identification_System), which tells them not only exactly where the target is, but also where it's heading, how large it is, what kind of cargo it is transporting, how many crew members are on board, ... . AIS transponders are required on all ocean going vessels (except small yachts and stuff, though even that is probably about to change).
AIS-receivers plus a good VHF-antenna cost only a few hundred dollars so cost is not an issue to the pirates.
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This is essentially a giant spinnaker. The main disadvantage is that it is really only good for downwind propulsion, whereas a conventional sail can make some progress upwind at an angle.
Thought this might interest those who didn't RTFA (or didn't have time to trawl through the website looking for it):
http://www.skysails.info/index.php?id=71&L=1
What happens when the kite falls into water
From my experience of flying kites above 500 feet (perfectly legal in the U.S. as long as the kite is 5 lbs. or less and not a hazard) the wind doesn't die. I had more problems with the line and structural integrity of the kite. The line may break, the kite may collapse, or the winds may start blowing the wrong way long before you have to worry about a perfectly good kite dipping into the water.
Not true. This type of kite is more effective at steering into the wind than a conventional sail. A conventional sail always has a significant vector of force in the direction of the wind, and relies on the ship's keel to redirect that force. A kite can steer 90 degrees towards the wind, generating lift directly perpendicular to the wind direction. If a kite was attached to the center of a ship with a keel, I'd guess you could get close to 10 degrees of direct upwind. As it is, this is about products for cargo ships and yachts, and the kite pulls from the bow. The SkySails site says you can go within 50 degrees of direct upwind, 70 degrees with full power, which sounds realistic to me.
I do kite sailing in the winter here in Norway, and the kite shown in the article is almost identical, except for size of course, with the kite I use. (I have also windsurfed since around 1990.)
My kite is a Peter Lynn Venom II http://www.peterlynnkiteboarding.com/, this is a twinskin kite which keeps its airfoil shape due to internal air pressure: A set of small mesh openings in the leading edge allows air into the opening between the front and back side.
This form of kite is an airfoil, not a spinnaker, the difference is huge:
A spinnaker is effectively a large bag to catch the wind, while a kite works best by having air moving faster on one side than the other. Among other things, this means that a kite allows you to sail much faster at an angle to the wind instead of straight downwind.
Another nice trick you can do with a kite, unlike a windsurfing rig, it to let the kite loop around in little figure-of-eights: This makes the airfoil move even faster through the air, increasing the lift particularly during a lull in the wind.
Terje
"almost all programming can be viewed as an exercise in caching"
This is essentially a giant spinnaker.
d00d, a modern traction kite is to a spinnaker like a modern wind turbine to a 16th-century windmill. These are airfoils, and yes you can go upwind with them - ask any competent kitesurfer. Rest assured though that you are not alone in your confusion.
A traction kite develops more power per area than a sail for 3 reasons:
1. no spillage (reduction in effective area due to heeling);
2. stronger winds at higher altitudes (where SkySails is flying, winds are roughly double those near ground, generating 4 times as much force);
3. higher airspeed (up to another factor of 2) than ship speed when working (looping or figure-8ing) the kite.
Taken together, these mean that traction kites can have *way* (as in, up to a factor of 20 or so) higher power density than *efficient* sails. A spinnaker is not an efficient sail.
SkySail's projections are in fact rather conservative - these are German engineers after all. They've convinced me - in fact they've got my money riding on them.
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