26

In CNN's June 30, 2023 Giant kites could pull cargo ships across the ocean – and slash their carbon emissions

“What differentiates it from other wind solutions,” says Bernatets1, “is that the wing is not just pulled by the wind and countered by the ship.” Instead, it flies in figure-of-eight loops, which multiply the pulling effect of the airflow to give what he calls “crazy power.”

“Plus, we fetch the wind 300 meters above the sea surface, where it’s 50% more powerful,” adds Bernatets. The combination “explains why the power is tremendous for a system that is very compact, simple on the bow of the ship, and can be retrofitted on any ship, not just new ships,” he says.

1Airseas co-founder and CEO Vincent Bernatets

I'm hoping that my question about the aerodynamics and lift from a "giant kite" that "flies in figure-of-eight loops...300 meters above the sea surface" is sufficiently on-topic here.

When kites are used to generate electricity, you certainly need motion - perhaps it reels in and out or it lofts a rotating wind turbine or something else - you need substantial relative motion to generate substantial electrical power.

But a kite affixed to a ship and static relative to it can certainly provide mechanical power to the ship, at least when the wind speed is greater (and in the same general direction) as the ship's speed.

So... what's with the figure-eights?

Question: Why would a kite flying at 1000 feet in "figure-of-eight loops" serve to "multiply the pulling effect of the airflow" on the ship to which it is attached?

quiet flyer
  • 22,598
  • 5
  • 45
  • 129
uhoh
  • 5,808
  • 3
  • 32
  • 71
  • 1
    different but perhaps somewhat related in that the "magic" happens when two things at different altitudes in media moving in different directions are tethered in a clever way: In which situations can a Dual-Aircraft Platform (atmospheric satellite) remain nearly geostationary? – uhoh Jul 04 '23 at 00:51
  • I've temporarily added both [tag:flow] and [tag:laminar-flow] since "airflow" is mentioned. Once the answer becomes more apparent, tagging can be adjusted. Note that while they are used 18 and 12 times respectively, neither has any usage guidelines currently, and I have a hunch they belong on a lot more questions than that. – uhoh Jul 04 '23 at 00:56
  • 4
    A kite that is static relative to the ship is probably stalled, and flying in "8" loops might un-stall the kite (by increasing its forward velocity); if that's true, then it should generate more lift than a stalled kite. – Aditya Sharma Jul 04 '23 at 01:21
  • 2
    I don't really know either, so not writing an answer. But the kite moving to "fresh" air can have an effect also, because it doesn't build up a region of locally slower flow. Similar to how wind turbine power is relative to the swept area instead of the area of just the blades. – jpa Jul 04 '23 at 11:12
  • 3
    Highly related : https://en.wikipedia.org/wiki/Crosswind_kite_power, https://en.wikipedia.org/wiki/Airborne_wind_turbine. You get a lot more energy by flying in a dynamic pattern at high speed as opposed to simply maintaining a fixed position relative to the tether point. Have you ever watched a kite boarder? Note that the kite is not simply kept in a fixed position relative to the board. – quiet flyer Jul 04 '23 at 13:45
  • 1
    @quietflyer my observations of kite surfers is that the kite is pretty much stationary in relation to the surfer – Jpe61 Jul 04 '23 at 14:25
  • 1
    Consider the apparent wind on the kite, which is the resultant of the actual windspeed (relative to the ship) and the speed of the kite. The fast moving kite sees a much higher apparent windspeed, allowing it to extract more energy. (Similarly, the tip speed of a wind turbine blade is 5-10x the windspeed). – user_1818839 Jul 04 '23 at 23:55
  • 5
    @Jpe61 You haven't observed enough. When kitesurfing you 'park' the kite if you want to cruise, yes. To do something a more airborne you need more power, which you get this by diving the kite down and back up. Or by looping it, for even more power. In light winds, parking the kite isn't an option and the rider will need to constantly sine the kite up and down just to cruise. Some malfunctions can result in a kite that never stops looping, which is how people get thrown downwind and injured. Bottom line; a moving kite always provides substantially more pull than a parked one. – aroth Jul 05 '23 at 03:52
  • For sure @aroth, and now that you mentioned it, I must admit that yes, they do at times fly the kite in a pattern, apparently for extra pull. That will quickly become tiresome, so for longer cruises they will leave the kite stationary. – Jpe61 Jul 05 '23 at 19:45
  • 2
    A racing sailboat driven only by such a kite is undergoing sea trials. Many delicious technical details: https://www.yachtingworld.com/extraordinary-boats/armorkite-650-kite-surfing-yacht-sailing-126194 – Camille Goudeseune Jul 05 '23 at 20:27
  • @CamilleGoudeseune oh that is so cool! "The ArmorKite is extremely sensitive on the tiller, and also sensitive to the positioning of the crew, whose total weight can easily equal that of the boat itself. It’s important to keep a close eye on both longitudinal and lateral trim." I don't know what much of that means, but I get the feeling that if the point of attachment could be moved dynamically somehow (slides along a front-to-back rail) rather remain at a fixed point... – uhoh Jul 05 '23 at 22:40
  • 1
    @CamilleGoudeseune ...they might perhaps add even more control. I'm going to go buy a dual-line kite now, stand firmly on the shore and away from everyone else, and get in on the fun! – uhoh Jul 05 '23 at 22:40

5 Answers5

23

The giant kites mentioned aren't the quadrilateral kites kids fly. They are airfoil shaped parachutes with open noses that generate lift as they move through the air. Ram air pressure inflates them and maintains their shape. In skydiving lift works against gravity. (ignoring turns and the like)

enter image description here

Anchored to bow of a boat the airfoils must travel perpendicularly to the direction of ship motion in order to pull it forward. Here lift works against hull drag in the ocean. The farther the parachute gets from perpendicular, the less forward force it provides so it has to turn around to make another pass across the bow. The path that allows the wing to maintain continuous flight while generating the maximum forward pull is a horizontal figure eight. There is nothing magic about the shape of the path. Altitude is selected for best wind and size of parachute.

An airfoil generates more pull than a simple round parachute of the same area placed directly in front of the ship because it is flying at speed, one reason being the higher dynamic pressure experienced because of that speed. Looking at the weight each type can carry per unit area is one way to see this difference, recognizing that each type has a range of performance capability. A round T10 parachute with an inflated diameter of 25.7ft has an area of 480sqft and carries a maximum load of 350lb, or 0.72 lb/sqft. The ram air RA1 parachute has an area of 360sqft and carries a maximum load of 450lb, or 1.25 lb/sqft. In addition, the round parachute has a descent rate of 22 fps while the ram air chute's descent rate is 8 fps, so the wing will work in much lighter wind.

Pilothead
  • 20,096
  • 3
  • 54
  • 98
  • 1
    Possibly interesting for everyone here: https://www.diva-portal.org/smash/get/diva2:1270437/FULLTEXT01.pdf (esp. Fig. 2 on p. 9! So they could EASILY add a swash plate, and a much simpler one than what I suggested above!) and also https://mdpi-res.com/d_attachment/energies/energies-16-02603/article_deploy/energies-16-02603-v2.pdf – Sixtyfive Jul 06 '23 at 10:19
  • Re "Anchored to bow of a boat the airfoils must travel perpendicularly to the direction of ship motion in order to pull it forward." -- Suggested edit for clarification-- something along the lines of "An airfoil is designed to generate lift with minimal drag. To pull the boat using lift rather than drag, the airfoil must travel perpendicular to the direction of the relative wind experienced by the ship." Pertains to this related question: https://aviation.stackexchange.com/q/99840/34686. Technically, the kite could pull the boat using drag alone... remaining directly downwind of the boat-- – quiet flyer Jul 06 '23 at 13:54
  • Or for more detail see https://aviation.stackexchange.com/questions/99840/forces-acting-on-ram-air-parachute-for-pulling-ships#comment278607_99840 and following comment-- – quiet flyer Jul 06 '23 at 16:55
  • Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed. – DeltaLima Jul 08 '23 at 05:47
12

Have you ever flown a dual line kite? If you keep it steady it will climb until it reaches a steady state and until it is mostly horizontal and only pull minimally on the lines.

If however you turn it and have it go near the ground the kite will be more vertical into the wind and it will pull a lot more on the lines. And the entire goal of flying a kite on a ship is to get it to pull on the lines.

You could make the kite go in circles or ovals but that will end up tangling the lines. So a figure 8 is used instead.

ratchet freak
  • 27,428
  • 5
  • 79
  • 143
5

I think the reason is to maintain the optimum angle of attack, while also positioning the kite lines so they pull the ship forward. It probably only helps when the ship is sailing at certain angles to the wind.

Wings generally work best when at 10-15 degrees to the airflow. That probably works well for a ship sailing across the wind. The kite can be ahead of the ship, and the lines pull in the right direction.

However, when the ship is sailing downwind, putting the kite in front of the ship would leave it at 90 degrees to the wind, and deeply stalled. Moving the kite to the side would allow it fly efficiently, but then the pull wouldn't be in a useful direction. Flying figure-8s in front of the ship allows the kite's airspeed to keep the angle of attack below the stall, while keeping the lines pulling mostly forward.

Additionally, the faster the kite is flying, the more lift it generates. If the kite has a lift:drag ratio of 5:1, it could fly 5 times faster than windspeed. Lift is proportional to speed squared, so it could generate 25 times as much force than if it were stationary relative to the ship. When you fly a steerable kite, it's very noticeable how much more it pulls when moving fast!

Toby Speight
  • 1,101
  • 11
  • 22
Robin Bennett
  • 3,343
  • 16
  • 19
2

Think about it in terms of how much air your sail is interacting with:

  • The classical sail of a ship with the wind at its back can only interact with the air directly around the ship. The ship's speed is subtracted from the wind speed when computing the effective air speed of the sail, which is lower than the wind speed by consequence. That's not much of a reaction mass that the sail interacts with per second, and hence not a lot of force can be generated. As such, sailing in front of the wind is always rather slow.

  • A sailing boat that's sailing at a right angle with the wind adds its own speed to the wind speed (vector addition). This allows the effective wind speed around the sail to get much higher, allowing the sail to interact with a lot more air per second. And, the faster the boat, the greater the force the sail can generate!

  • An airfoil shaped parachute that's moving perpendicular to the wind is not restricted to the speed of the ship. As such, it can move even faster, dominating the relative wind speed with its own speed. Again, this vastly increases the amount of air it can interact with, allowing it to pull much, much harder on its line than any fixed sail could.

Flying the kite in figure 8 serves to keep the kite's speed as high as possible, and by consequence its effective wind speed and reaction mass per second to interact with.

The same concept of maximizing air reaction mass applies everywhere in aviation. It's the same reason why helicopters need to work harder when hovering than when they are in straight flight, and why air planes are generally more efficient the faster they fly. Because more reaction mass means that the reaction mass does not need to be accelerated to as high speeds to provide the same force, and thus that less energy is carried away by the accelerated reaction mass.

cmaster - reinstate monica
  • 121
  • 2
1

Of course, they could also try this, or this, but these are a bit faster. These have "crazy power", able to go many times faster than the wind. Reduction of drag (on ice) definitely helps.

But these make most efficient use of the wind for lifting force.

The kite flying "figure 8s" is combining its motion and the direction of the wind to create a relative wind most favorable for the creation of lift. Another way of doing this is to sail the entire ship around 45 degrees off of downwind back and forth with a fixed sail on a mast. This is what iceboats can do, enabling them to reach speeds several times more than the wind.

The kite technique allows the ship to stay on course, and harnessing the stronger winds above the surface of the ocean provide very promising advantages.

Robert DiGiovanni
  • 20,216
  • 2
  • 24
  • 73