Maritime propulsion market

It seems like there’s no competition for Airseas, is there any?
The larger ship traction kite part of Skysails seems to have been swallowed by Airseas.
@PierreB what’s your impression of ?

Yves Parlier develops also , which looks to be an available yacht market.

Concerning larger ship traction ( I have not yet any opinion. See also .

For AWES knowledge about these kites could be useful.

Thank you! This hasn’t been listed yet.

The Peter Lynn article has been discussed extensively:


I’ve been lurking here for a little while, but this is my first post so I think I should introduce myself.

For the past 20 years I’ve been working in automotive aerodynamics, with a pretty diverse mixture of consultancy, motorsport, and road vehicles. Most recently I was in charge of aerodynamics at the electric vehicle manufacturers Tesla, and Canoo. Now I am trying to start a new venture myself, with the goal of (re)introducing renewable energy to the cargo shipping industry.

There are already various groups trying to do the same, since the need is pretty desperate, but a difference with my approach is to use electricity as the common energy medium, and bringing together multiple energy harvesting technologies to contribute to an overall supply.

Where this is relevant to this forum is that I’ve been looking into the various approaches to using kites to generate on-board electrical power, and/or direct propulsion (similar to SkySails). There’s clearly a lot of knowledge and experience on here, and I’m hoping to get some feedback on parts of this idea.

I’ve read various papers, Prof. Schmehl’s compendium, and various things online, and used them in conjunction with published guidelines in the shipping world for evaluating wind power for propulsion to come up with a simplified model to try and understand the feasibility of this application.

So far, what I’ve found is that we would be less limited by the ship’s resultant apparent wind direction than a propulsion kite would be, but really we’d still be forced to stow the kite for any head wind at all (+/-90deg from the ship’s velocity), because otherwise we’re likely to increase the fuel usage of the ship by more than we’d save. Maximizing the net power harvested vs the tether tension would be key, and could cheat a few degrees into that zone. I’ve read several very different ratios for that in the literature, depending on many contributing factors. What is the feeling for how close we could realistically get to the theoretical 4/27 power harvesting figure for a lift-based kite in the near or mid-term (~3-5 years & 5-10 years)?

Another big concern is that the V^3 power relationship means that in theory, at least, we could harvest a very large range of powers from the typical wind distributions observed on the major shipping routes. This would present a substantial system constraint, in that there’s a key trade-off with the sizing of the energy storage system, and whether you opt for bingeing on the sporadic peaks, or gradually accumulating the more common low-medium wind power levels. From what I’ve read, mass issues make scaling up to MW levels very difficult, and would likely raise the cut-off at the lower wind speeds to the point where we wouldn’t be able to access the tails of the wind distribution that we might need. Is the feeling on here that this is an intrinsic element to the kite-reeling + groundgen approach, or an area where materials/operational advances might allow single systems that can access wind powers from perhaps a hundred kW up to the low MW range with reasonable efficiency?

I’m sorry for making my first post so long, and one trying to ‘take rather than give’, but as a noob I have nothing really to offer yet!

Thank you!


Hi @Rob_Palin, and welcome to the forum.

For my limited knowledge about maritime propulsion by using AWE, direct propulsion (as for SkySails) seems to be more suitable, in spite of its inherent limit by apparent wind: expensive storage is avoided, direct propulsion leads to less losses by conversion processes. Moreover MW range are already envisaged or even used (a 200-400 m² ram kite, see the attachment SkySailsPower_Brochure_EN.pdf (2.2 MB) ), while reeling-kite use implies a loss of apparent wind of 1/3, and as a result a tension force being only 4/9. Adding the recovery phase during half of the time, and the expense of energy. As a result the power with a reeling-kite would be a tiny fraction of the power with a direct propulsion by using the same kite.

The 4/27 power harvesting in reeling mode is considered in regard to the area swept by the kite, not the kite area which leads to a similar efficiency whatever we use reeling or flygen (Loyd’s formula) of which the drag of the turbines aloft leads also to a loss of apparent wind. Practically AWES are far below this limit, even flygen AWES which sweep far more than the required swept area by the power harvesting factor (kite area X about 4-8). However flygens like Makani or FlygenKite does not undergo the 4/27 limit, but undergoes 16/27 (Betz) limit. As a result one can implement more (flygen) kite area for a same swept area, but I am not sure it is useful, at least for now. However a flygen system generates continuous power, so it could be more suitable and efficient to produce electricity. But I think a direct propulsion is far simpler, efficient, and cheap.

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Hi Rob, welcome to the forum!

Can you link some of the things you’ve read so we can start from a shared understanding?

Only a portion of the members here visits regularly so you could try sending someone a personal message if you’d like their input, or contact them outside of this forum. Here is a list of members @rschmehl, the editor of the compendium, is a member.

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You could look at some plots and things I said on this page: Peter Lynn's Can kites be economically viable (for propelling commercial shipping in mainstream applications)

Not to reply to a few of your concerns:

I believe in hybrid onboard generation / traction for ships. This provides maximum flexibility and allows the ship to stay still on sea (unanchored in wind and moderate currents), powered by wind, or even go directly upwind. I think this “autopark” feature could be quite useful/neat, fixing a yacht to shore is not easy.

An efficient kite can easily be useful even with headwind. Take the apparent wind speed and then find the maximum upwind angle of the give, given by its combined kite and tether lift to drag ratio. I believe one would be looking at rigid kites with medium to short tethers for this purpose. As the scale (wingspan of the kite) is smaller than utility scale electricity production, the mass scaling issues would be less dominant.

The requirement of the kite system would be that the pull must not be acting in any way against the speed of travel. In the end, a hybrid system would continuously optimize electric power generation and direct pull relative to the desired path/movement.

Another limitation is that the kite does not generate a lot of power if it is at the edge of the apparent wind window. I think you are correct to assume that the minmax problem is dominant (the minmax problem as coined by Peter Lynn, related to the maximum pull being much higher than the minimum pull). I think these issues may be somewhat fixed by digital control, but this leaves you vulnerable to the system actually working at all times…

The real problem is that modern expectations require the ship to sail fast, while average winds are not always very fast. So even if the wind is facing directly dross to the path of the ship (expectedly optimal), the apparent wind may face mostly directly as headwind.

The hybrid electric/pull ship may alleviate some of these effects if you could run for a while only on batteries. “A while” could be a long time if running on foils, which may be possible in “yacht” scale.

There are a few things that such a design would benefit from that electricity production have problems with: First, you could use the electric propulsion to generate nice apparent wind for launching the kites. Second you have lots of open space, and probably no other kites to worry about. Third, the wind gradient is pretty sharp at sea, so you don’t need long tethers to get the higher winds.

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Let me also add that I think battery driven shipping, with in-transit charging, could be a solution for cleaning up the shipping industry. You would have mobile power plant ships collecting offshore wind, that could latch on to other ships and recharge them during transit.

We are still quite far away from realizing this though. Better options may appear before that happens. First step would anyways be to create either a basic onshore kite power plant, or a smaller (yacht) kite hybrid boat for the consumer market.

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Hi Rob,

KiteShip worked on ship kites with Matson’s engineers. The final conclusion was that shipping companies could not afford the risk that a traction-kite tether could foul the propeller in the middle of an ocean far from the specialized divers required to unfoul. SkySails has eked out a fine track record demoing ship kites, and the Airbus delivery ship is the latest specialty Beta user.

There are previous 100% AWE availability concepts based on seaborne harvesting by specialized ships that then support towing/charging/refueling of uninterrupted shipping. As electric shipping slowly emerges, options improve.

A best bet is to team up with SkySails and/or Airseas, who have overlapping technologies and business relations. There are also several nice yacht-kite ventures. Ship kites have been a slow market to develop, but the future need is real.

Vehicle aerodynamics is not very similar engineering to power kites, so you face a steep learning curve, but its definitely worth it, Good luck to you!

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Direct propulsion like this seems to be a simpler solution.

If on-board generation from the wind power is desired, a small hydro-turbine could be implemented.

But the kite propulsion for cargo ships faces to a major bottleneck which is the requirement of keeping the dates of embarkation of the freight and its routing whatever the wind. Diesel engines must run regularly, otherwise the temporary gains brought by the wind are canceled out by higher engine consumption due to periods of relative calm.

Hi Pierre,

Thank you, it’s great to be among this group of knowledgeable and passionate people!

My initial motivation for targeting electrical generation over direct propulsion was thinking that it would be less directionally limited, and limiting. By that I mean I thought it could allow energy to be harvested from the ambient wind from almost any angle, while not requiring tacking from the ship, with the journey time and operational hassles that come with it.

There was also concern about the ultimate power capacity, having seen the ~2MW from SkySails, and compared it to the ~17MW of a large number of cargo ships, or even the 80MW of some big container ships. We would need massive scaling to be a substantial powertrain contributor, but we could reasonably replace the 400-900kW diesel generators they use for electrical ancillaries (albeit with onboard energy storage/buffering).

After sanity-checking that idea, I’ve found that an electricity generating kite would still be limited to only 180 degrees of the wind rose, since almost any tether force that can be resolved into the ship’s drag direction requires more energy input from the ship than we can harvest from the wind. There was a second part to this hit when I also realised that the freedom I’d anticipated in disconnecting the inclination angle from a propulsion direction doesn’t offer any real value. :anguished:

Where I am right now is trying to figure out whether a hybrid approach makes sense, whereby we use a kite for direct propulsion whenever the wind is suitably aligned, but switch it to groundgen when the wind is unfavourably aligned, but still within the rear half of the wind rose. That’s where growing my understanding of the relative merits & limitations of each comes in, as it will inform where & how a transition between the two approaches could happen, and, honestly, whether the idea has any ‘legs’ at all!

I need to get a better intuitive understanding of the lowered Betz limit for groundgens, as I don’t think I’m grasping it properly, even if it seems like it’s not even close to a limit for functional groundgens at this stage of development.


@Windy_Skies (sorry, I forgot the replies to a thread are serial, not indented)

I’m not sure if this upload will work, but here’s an image of my bibliography so far. If there are any key sources I’m missing, or there are interesting caveats or developments on any of these papers, I’d love to hear about them!



I agree entirely about a transition to electrically driven shipping, partly enabled by offshore power farms where ships could recharge. There are tons of challenges to overcome to get there, but it makes a lot of sense. Having been a strong EV advocate for a while, and owning one myself, I know for sure that there is a flaw in the mindset of needing to start a journey with all the energy you could possibly need to complete it. In the context of cargo ships, and viewed from a Physics perspective, often their biggest obstacle is literally that there is too much energy in the environment through which they must pass. If we can find ways to make use of that energy - as they did for many centuries, but now using modern technology - it just makes straightforward sense.

As for a small system on a yacht, there is a team in the San Francisco area already doing that, and we may try and synergise with them, but so far we’ve been looking at bigger ships, since there’s huge pressure on them at the moment to make massive and meaningful changes in the immediate future. Sadly, though, their only plan of action at the moment is to slow down to roughly 50% of their current average speeds, which is pretty crazy, as they’re already saying they’ll need to double the number of ships. Our goal is to provide them with tools to reduce their fuel use without compromising on their other demands.

The hybrid system you described is essentially what I’m thinking of now, and I’ve mentioned just now in my reply to Pierre. The company I’ve started is called “THEME”, which stands for Technologies for Harvesting Energy from the Marine Environment, and the thinking is that a renewable energy solution at sea can (or must!) make use of multiple energy sources to really make a dent in fossil fuel use. A part of that idea was to utilise electricity as the primary “currency” of energy on a ship, with each source feeding into a common storage system. There’s no reason to limit it there, however, and we’re totally open to splitting methods for power and propulsion into separate systems. Basically, it’s whatever might work, with any possible consolidation coming once we know the individual components work!



Though the apparent wind changes direction towards headwind with apparent vessel speed, the apparent wind speed also increases. I would argue without any proof, that if you had a lossless kite system, you could make some power for any wind, even headwind.

So what are the losses? If we just consider the stuff in-air, it should amount to the kite drag and the tether drag. The kite’s glide number states the cost in drag losses of generating energy. The tether drag adds to this in the approximation:

C_{D,\Sigma} = C_D + \frac{C_{D,t} d l}{4 S}

with C_D being the drag coefficoent of the kite, d being the tether diameter, l being the tether length, C_{D,t} being the drag coefficient of the tether (value approx 1-1.5) and S being the surface area of the kite.

Note that the high efficiency dictates that you need a large wingspan on your kites (look to sailplane designs)

You might see that the sum drag coefficient is constant as long as the ratio S to d l is constant. This means that the glide number can scale to any wing size.

If the vessel speed is large, drag must be in order to be able to produce energy still. A kite should not have a tether lengh longer than approx (very approx) 10-20x wingspan. This would make it ineffective (tether drag starts to dominate the equation) You cant get around this much as tether strength is given by UHWMPE properties and the lift coefficient of a plane normally does not exceed 2.0 (and drag coefficient normally being a certain fraction of this).

I think this is where Skysails are failing to meet the requirements of fast vessels and non-crosswind wind conditions by choosing a draggy soft kite.

If you went this route, you still had to cope with the minmax problem. For a super efficient kite, its like an exploding rocket. It needs to be flown extremely carefully, or the generated forces will quickly destroy the kite or tether.

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There was an old demo of a wind turbine powered boat travelling straight into wind faster than sail boats of the time could.
It is mentioned in this quick summation of
As is @dougselsam

As for the minmax problem… As long as you can deploy and recover in short order, having a limited performance window may not be a huge problem.

Designing for rigidity and tension across the whole system is going to help.
A mast has the negative effects of adding healing moment to the vessel.

There is a lot of space in the ocean…
An underinvestigated proposition is using auxiliary foil craft as part of the rig. (bit like a waterskier riding along out the side) A line between even 2 craft could provide a great base for stabilising kite operation.


I would think a windmill just like AWE has massive downwind drag. The windmill probably has good efficiency (tip speed ratio \lambda > 10 The problem with a windmill I would think was related to the moment applied to the hull and high center of gravity


Race for Water demo yacht seems to be the closest match to your target marine AWES concept of multiple electrical inputs (solar, wind/H2/gen).

Its takes enormous capital to scale up and validate kites for commercial shipping. Existing players have burned around 200M USD without opening the market. SkySails’ hope is to transfer its sea experience to land, where operations are cheaper and easier.

SkySails had no choice but to adopt soft power kites, since power-comparable ~2MW rigid kites do not exist, and would suffer from frequent irrecoverable crashes, as current crash statistics for smaller AWES wings suggest.

Airseas’ estimation is 20 % less fuel consumption. Neither Airseas nor SkySails mentions a hybrid generation from kites. Both are considering only towing, and rightly so, the engines supplying already on-board electric generation. That said propulsion of cargo ships by towing kites is not for tomorrow, if it comes someday.

There’s so much useful information here - thank you everybody!

I was trying to reply to each individual, but I encountered a posting limit, so I’ll be more concise this time.

As I see it, there are 3 primary challenges to what we’re trying to do.

  1. Technological
  2. Financial
  3. Cultural

I doubt there are silver bullets anywhere to any, and certainly not all of these, but do think that there are ambient conditions now that are more favourable than at any point previously. Unless there’s some huge technological leap, I think wind-assist is as far as we can go while still achieving the timeliness that modern international commerce requires, so some kind of stored onboard energy will still be a necessity. That could be via Hydrogen fuel cells, Ammonia, or even some sort of biofuel, but there’s a long way to go before any of those is either practically feasible, or even much of an actual step forward from an environmental perspective. Our goal will be to find ways of harvesting the conscience-friendly energy already in the system to minimize the challenges faced on the storage/media front.

In some ways, money is as much of an ‘energy’ as any other source (albeit with different laws of thermodynamics), and right now there is a surge in interest in green investments. Some of that is just because of opportunism, some because there’s a growing recognition in the zeitgeist that old approaches need to end, and some because of government mandates. Our aim would be to ally with some of the people who have already invested so much time, effort, & money in AWES R&D, rather than trying to figure it out ourselves from scratch. It’s a solution that the world urgently needs, so it should be possible to get support for it.

The cultural battle is, I think, the hardest, but that’s where the external pressures can play the biggest part. Shipping is under assault to clean up its game, whilst also evolving into the “Amazon Prime” era of timescales and costs. They’re running out of green-washing stories, and need to make more tangible progress. We can target people at the top with opportunities to make feel-good stories with some more substance, while they are busy investing the billions required for the full replacement of fossil fuels worldwide.

Stepping back from big picture stuff to the specifics of towing vs generation, and wind/kite directionality, I find it is quite complex, and I might be misunderstanding it. From what I’ve read, and calculated from the International Maritime Organisation’s (somewhat flaky) guide for wind power evaluation, only tail-winds greater than the ship’s speed can yield a net energy benefit for a vessel. Assuming a kite tether would be aligned directly downwind and at a shallow incidence angle, then any resultant wind angle that points this tension rearward means that we would need more power from the ship’s engine to maintain the situation. Unless that wind condition is still very fast(?), we’re not going to be able to generate enough electrical power to be a net win over an engine-mounted alternator, or independent diesel generator. I calculated that we could operate at perhaps 75-80 degrees headwind (from the ship’s perspective of 0 being dead-ahead), but we’re pitting the net efficiency of pulling the kite along to generate electricity against doing it via an ICE-based generator.

Dry bulk cargo ships on average move at 14.7kts (7.6m/s), which is slower than container ships (~19-25kts), but they make up as much as 80% of the worldwide fleet, and they have more flexibility over their deck space. If we can replace their use of diesel oil for electricity generation, then that’s a win. If we can reduce their use of heavy fuel oil for propulsion, and/or electricity generation (there’s a mix of alternators on main engines, and stand-alone generators), that’s also a win. Ideally, we’d do both! :slight_smile:


It is not really very difficult. I tried to explain with some drawings. You really don’t need tailwinds in order to sail if you have an efficient kite/sail. The “moon cresent” shaped thing on the drawing represents how much power the kite generates at various positions in the wind window. Of course the drawing is only 2D so does not take altitude into account.

The first figure shows zero wind (no production of energy possible). The second shows a strong headwind but slightly angles towards the directio of the vessel motion. The third figure shows how an efficient kite can utilize a larger part of the power window.

I hope this clears things up.

The figure should also show quite clearly that you can choose a lower upwind pull (directly from the kite’s line), or a stronger electricity production perpendicular to the ship’s path. But as conversion from wind to propeller thrust carries huge losses, I believe a combination of direct pull and electricity generation will always be optimal.

Slice.pdf (15.6 KB)