Peter Lynn's Can kites be economically viable (for propelling commercial shipping in mainstream applications)

To have a better discussion I will now number Peter Lynn’s claims:

[Skipping the onboard energy gen for now]
[Also skipping over “has not shown to be working yet” without evidence of someone trying]

  1. developing autonomous flying (kite auto-pilots) that will operate reliably in marginal and turbulent conditions is probably unachievable
  2. When there’s too much wind, above 40knots, kites cannot usually be flown safely.
  3. When there’s too little wind (less than 6 or 7 knots), kites fall into the water.
  4. Until there is 15knots or so they don’t develop useful pull.
  5. the ‘steady mid range winds’ that I have not often experienced in my kite flying life, not yet anyway, though I live in hope.
  6. It’s not believable that ‘just-in-time’, customers will accept 5 weeks plus or minus 3 weeks instead of the 10 days to-the-hour trans Atlantic delivery schedule that internal combustion power allows. And, such delayed and irregular schedules would be the reality if kites (or sails) were used now instead of diesel motors.
  7. As well as unacceptable reliability because of wind variability, there is a technical reason why kite power cannot be useful for commercial shipping- unless container ships change to submarine form so as to reduce their above-water-line drag profile. Aerodynamic drag is the killer. A largish container ship, with containers stacked up 15m and more above deck level can have side area above the water line of about 5000sq.m. Even if it’s kite is also 5000sq.m’s (5 x larger than any kite that’s yet been built, but probably possible), the ship’s superstructure drag will reduce the efficiency of the kite/line/ship system (measured by lift to drag ratio, L/D) to less than one, and options for upwind courses will fade away. When other other inefficiencies, such as hydrodynamic drag, are also allowed for, even if the apparent wind strikes the ship’s superstructure at 45degrees rather than the worst case of beam-on, upwind sailing will still not be achievable to any bankable extent.
  8. Fuel use per tonne km is perhaps 70 times better for a container ship than for a truck or train- they’re VERY fuel efficient.
  9. wind speed is never easy, and especially not over any sustained period. So, the wind speed will have to be 25knots or more if kites are to propel ships at the speeds the market demands.

[OK, so say kites are only used when the wind is appropriate by direction and strength?]

  1. Hmm, for downwind sailing, is the wind in a useful direction 35% of the time, and in a useable strength range for 10% of the time? Let’s be generous and say it’s going to work for one day in every 25.
  2. Are ships owners going to install the equipment, add extra crew, and supply specialist training for some kite assistance barely one day a month? And then there’s the associated reduced load capacity- and the downtime from running over the lines etc, and the risks to the boat itself and its crew and to other craft from out of control and escaping kites (which will happen sometimes, no matter what fail safe systems are installed) -more costs.
  3. Oil is so effective and efficient for ship’s propulsion that commercial shipping will be the last transport mode to switch to alternatives should oil prices ever climb to unaffordable levels again.
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I do believe pulling container ships with kites is a great idea.

Some ralated information: Skysails has had a working system for some years and has gone “bankrupt” but they’ve got some company structure that allowed for skysails yacht to stay alive and they are basically using all of Skysails IP. My assumption was that the reason was mainly because the people who own the ships are not the people who pay for fuel. Udo Zillman hinted at the possibility that the technology was not ripe. Airseas was recently founded. A spinoff from Airbus. They have done their research and concluded that that’s a worthwhile endeavor and we can safely assume that they know more than we do atm.

Some general claims by me about awes for ship propulsion:

  • (1) The sea is a good place for awes. Higher windspeed. No obstacles. More constant wind.
  • (2) Price of energy on a ship is higher than price of grid energy. Even if ship engines were as efficient as power plants, they need to transport their own fuel to where it is used. This costs energy and reduces load capacity.
  • (3) When pulling a ship downwind there are hardly any transmission losses.
    (Compare to generation of electrical energy and usage of the same)
  • (4) Kites should be used as auxillary power for up to 0.2 of motor load or pull the whole thing. This is to prevent the engines from running inefficiently.

Now on to Peter Lynn:

  1. Much research is done in that field. I don’t have definite evidence, but I believe it to be solveable.
  2. That’s probably based on his own experience flying his kite. I don’t see a physics reason why a specially designed kite could not withstand the speed. We’ve got flying things that go much faster.
  3. Nah. They’re landed.
  4. Don’t know cut-in speeds. Any pull is useful.
  5. Much steadier winds can be expected in higher altitudes at sea.

[to be continued…]

I agree that Peter Lynn’s comments are coloured by his thought that the kite must be a cloth structure mounted on a bridle. If you replace the kitesurfing kite with Kitemill’s high efficiency wing and VTOL, a lot of the arguments are moot.

The argument that still holds very well is: Apparent wind points more or less directly downwind if you sail at 12 m/s. And Kite power production is mostly effective downwind, where the strong forces to withstand tether drag will affect ship speed in a very bad way. If you generate power mostly crosswind or upwards, there is little power to harvest due to the cosine law.

These “facts” carry the most weight in this issue.

Flying a common kite behind ship would be useless. It has to be directly in front of ship or as close to that angle as possible.

More exotic system might work, not for traction, but direct mechanical transfer or energy generation. Lifting kite which flies almost directly overhead. It should even be able to fly against the wind. (Imagine a plane descending). On the tether is a vertical wind turbine. The drag from it is offset by the forward force of the lifter kite.
Now that I think about it… If the whole flying against the wind thing is ture, then one could just use that for traction.^^ brainfart.
That thing would have to have mad CL/CD. Lift acts mostly upward with a very small component pushing against wind direction while drag is still going directly in wind direction. Maybe a kite can fly that way but not worth considering for traction.
However if one puts the kite a bit downwind and lets it yoyo mostly up/down instead of mostly back and forth, one might gain some power.

Anyway - energy generation with wind going against ship course does not need to be possible for kite propulsion to make sense in other scenarios.

From the post:

Oil is going to become more expensive, and unpopular, and maybe even regulated, so alternatives are going to be looked at, like for example trains from China to Europe. Independently, a shipping company that can make shipping cheaper will have an advantage. So there should be a drive to develop alternatives in working markets.

An idea would be to build fuel efficient electric ships, perhaps partly driven by sails/kites, and have recharging/docking stations/ships along the shipping route.

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Or just a kite charged electric towing ship

Would prefer liquefied methane generated by kite power as fuel. Energy density much greater, investment cost for storage much lower. Considering the price portion storage is in electric cars, I bet it would be quite an extreme number for ships.
Existing ships could also use vegetable oil.

No matter what process one compares it to, losses are always much higher than just “tying the ship to the wind” and letting it be pulled.

Storing electricity doesnt have to be expensive if your batteries could weigh almost anything and space is a minor issue. Im thinking liquid batteries, cheap and lower capacity.

Anyways, my electric car is far cheaper than a fossile car, so with that argument every container ship would use batteries too



Dont take these pictures to seriously. But the plot shows the availabale power in a 1 m^2 kite in 10 m/s wind on a ship travelling 0…15 m/s in the direction 0…180 to the wind. 0 degrees is straight downwind. The glide number G of the wing is 5 and 10. The power is only produced in perpendicular to the vessel path so as to not cause any drag. And it’s a rotating windmill.

It is quite possibly entirely wrong.

If it is correct though, there is ample power available on downwind tacks as long as you’re moving slightly fast. When moving at 15 m/s, and the wind is 10 m/s, you’re generating power 3/4 of the time, depending on wind direction

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If anyone has a good map of global offshore wind resource and direction that would be nice.
This one is a great one for land:
This one is a nice visualization but not useful for data extraction:,29.91,429
(use mouseover at the scale)

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Wow.Thx. Here’s the paper. This will take some work to run and understand. Maybe one day this can run as a web app on

So many conversion losses. I wouldn’t be surprised if you’d end up with an efficiency in the low single digit percentages. Now sailing starts to look more interesting.

If you were able to produce LNG onboard a kite ship, its probably better to pump it over into another vessel so it could use it for fuel in that way. To make things more interesting, pump the fuel in transit

Why would you generate it on a ship?^^
It’s either slowing the ship down by drag or one could just as well use a pure traction kite. Either way gigantic transmission cost.
I was talking about Generating energy and converting it to LNG on platforms.

One good reason would be the fact that ships may travel the seas to find areas with optimal wind and current conditions.

The other reason could the be the possibility to refuel ships in traffic. Remember a cargo boat travels nonstop at 12 m/s.

The third reason could be that maintenance for a ship is a lot simpler than maintenance for a platform.

The fourth reason could be that by using some of the generated power for ship propulsion, the apparent wind speed increases, making it possible to produce with a heavy kite in lighter winds.

In fact, I don’t see many good reasons to put the power plant on a platform, other than to have an anchor connected to the bottom of the sea.

Ships are more flexible.

The only way to use kites that I can think of at open sea is if the ocean currents flow in opposite direction of the wind and large sails are deployed in the water (sea anchors) or the ship is anchored to the seabed.

Special tanker/battery boats might be the better solution, depending on a lot of things.

Why is it that maintaining a ship is simpler than a platform? A ship would need to be anchored as well and a platform would not have to be as steady as an oil rig. I agree- as long as it is anchored and not trying to go anywhere itself it might as well be a boat.

The fourth reason just produces losses. I know you were not suggesting a perpetuum mobile, but I don’t think getting a kite flying in wind lower than its cut in speed is useful. Little energy generated.

The ship does not need to be anchored. The keel of a sailboat is a very powerful device, once the ship is moving a bit forward.

The fourth reason is valid. Some of the kite’s power is used for propulsion of the ship (say 30%). This increases the effective wind experiences on the vessel, but also the directio of the wind will turn in a way so that the windmill drag will act as a breaking force to the vessel. So, depending on the angle you are able to fly your AWE rig efficiently to the side of the wind window (obviously less than 90 degrees due to losses and the cosine law), you still may harvest up to 70% of the power. The reason 70% may be better than 100% is that now the minimum wind speed is a bit higher, and you do not have to land your AWE wings.

Anyone experienced in kitesurfing will know this effect. Going on a tack well powered, once you stop moving, the kite falls out of the sky

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Haven’t noticed the update. Nice plots.
Why is there no energy production downwind? There would be energy production and fuel savings.
A fixed course ship would be able to benefit from kite power at least at the same angles as a sailboat.
(When using fancy things like dynamic soaring like seabirds, maybe even more)
A augmented ship will not have a keel though, which becomes relevant with large kites relative to the ship.

6. Becomes irrelevant when using auxillary kite power.
7. The submarine point is really stuid. Hydro drag will always be much larger than aero drag regardless of the hull shape. Yes, kites will need to be large to pull a whole ship and they will need to sweep a large area. But any auxillary kite power is usefull, too.
8. Fuel use depends on the size of the system. A truck is worse than a barge is worse than a train. However we don’t operate giant trains with 15TEU.^^ So yes- ships are fuel efficient.
Compared to trucks or trains they do lend themselves very well to kites. Much better wind conditions, no obstacles, fixed course, low travelling speed, long travels. The energy demand for auxillary ship propulsion on a large vessel exceeds any power generated by awes to date. So once a system is commercialized and a contract with a large vessel operator is made the system can be scaled and or multiplied very fast.
9. 25knots ~12m/s. That’s ship speed. No relative wind. Here he’s way too optimistic. o.O
10. I assume useful wind direction and strength is much more stable high up on the ocean.
11. These are technical difficulties that can be overcome. That’s what I wanted to address with my startup. Kites are operated by a seperate company which charges just a portion of the fuel savings.
12. CO2 emissions are an unpriced negative externality. If the world ever does agree to price it, oil will be very expensive. However unfortunately I don’t see this coming in the next 20 years.