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Comment by bufferoverflow | Hacker News Reader

Aren't kite systems much more efficient and lighter and cheaper?

https://www.researchgate.net/figure/A-ship-sailing-with-the-...

From the article : "Ships can easily be retrofitted, literally overnight, with rotors activated by an on/off switch."

I think kite systems require more work, e.g. for launching & landing. When it gets taken down, you probably need to dry it, fold it, stow it, etc. So even if they are more efficient, lighter & cheaper, doesn't mean they're necessarily the preferred solution.

With the technology we have today, why couldn't self-drying, self-folding, self-stowing sails be used to make ships even more efficient?

Edit: there actually is a ship called the Oceanbird[1] with telescopic metal sails.

[1]- https://www.youtube.com/watch?v=pZVDs4n_a0M

Because that's a fairly hard problem comparing to just tacking two rotating columns.

There's a patent for a memory metal sail[1], telescopic sail mast[2], automatic retractable sail[3], and many others[4].

Also, there's the Oceanbird[5], a modern car-carrying ship with telescopically retractable metal sails.

[1]- https://patents.google.com/patent/US9481432B2/en

[2]- https://patents.google.com/patent/DE202011101489U1/en

[3]- https://patents.google.com/patent/CN103086280B/en

[4]- https://www.google.com/search?q=telescopic+sail+patents

[5]- https://www.maritime-executive.com/article/swedish-collabora...

All this complicated, fiddly tech, vs. two spinning bits of metal. I know which one I'd be more confident in...

Perhaps they could, assuming somebody is willing to make the necessary investment to develop what you describe.

However, it will probably be a complex system, and like most complex systems, it will be fairly fragile. Compare that to these Flettner rotors, which are essentially "a tube and a motor".

Ships can easily be retrofitted, literally overnight, with rotors activated by an on/off switch.

Kite systems seem far less practical but this can't possibly be true unless they're talking about a Sunfish. And even then, I'm skeptical. Or maybe they're using literally, as is the fashion, to mean figuratively.

Yeah that claim stinks to high heaven. You're just going to bolt these suckers down to the deck and plug in an extension cord? https://en.m.wikipedia.org/wiki/Rotor_ship#/media/File%3ACar...

That picture is not of an auxiliary retrofited sail, but of E-Ship 1 which is all sails. The retrofitted ones are much smaller relative to ship size and are used together with main engines.

Are they? How is maintenance? How sensitive are they in changes to wind? How much training does the crew need to operate either?

https://en.wikipedia.org/wiki/Wind-assisted_propulsion says

> In comparison to kite sails, Flettner rotors often offer considerable efficiency gains when compared to the size of a sail or kite vs. size of the rotor and prevailing wind conditions.

But I'm not sure if this size comparison makes all that much sense (maybe the performance per dollar invested would be more interesting?)

I'm genuinely curious about a comparison from an operator's perspective, and I also hope both approaches are being developed further.

Kite lines can kill others nearby. There is some animosity between windsurfers and kite surfers because of this risk.

Surely this isn’t an issue in the open sea, where tankers would mostly be using this.

Kite surfers are the assholes of the sea according to some sailors. I've seen them zip in and out of navigation channels and between boats. Also had a few get dangerously close and cross our bow as we were sailing on a reach.

Also, I do enjoy the sport. It's similar to endurance swimming and nearly every muscle in the body is worked. I found kitesurfers in some really remote banks and wondered how the hell did you get all the way out here.

Why not both? Flettner rotor works best when the wind is perpendicular and kite works best when the wind blows in the same direction to the ship's course.

If I understand the concept correctly the rotor does not only work as "sail" but also as "air propeller", as you need to input energy through its rotation (that gets multiplied by the "sail" effect of the Magnus effect). So that's an apples to oranges comparison, that does not bode well for the kite (as it is "passive", not effecting any compounding of energy).

Surprisingly, no. Last time this was posted there were some efficiency figures, wingsails being the best, but Flettner rotors still beating old-fashion sails and kites. And rotors are very likely cheaper than a sail/kite long term (they need complete replacement quite often because of rips and tears).

What I've read previously is that the kite systems took more training and skill to utilize, which was holding back adoption, whereas this sounds like it would be a much easier technology to use. Although, I do wonder if one could use both.

How to kite systems work any direction into the wind? Like anything from the beam or closer?

Pretty sure you need a tail-wind to use a kite system, this works in any wind.

Pretty sure you don't need a tail-wind. Regular old sails as well as kites can be used at various angles to the wind. See e.g. https://en.wikipedia.org/wiki/Point_of_sail

[edit] P.S. from my understanding of the mechanism, I'm not so sure these Flettner rotors are useful in a headwind. They work by creating a pressure difference, but in a headwind, you could only create a pressure difference between the port and starbord sides, not the front and back. Unless there's some mechanism I'm missing?

If, by headwind, you mean sailing directly upwind, then no, they cannot do that, but they can beat to windward like a sailboat (the direction of the rotation must be reversed when tacking.) According to Wikipedia, the rotor ship Buckau could sail within 20-30 degrees of the wind, though I suspect that is a misreading of the supposed source of that data, and that 50-60 degrees is a more plausible figure. Similarly, I suspect the claim that they can sail closer to the wind than conventional sailing vessels is in comparison to square-rigged ships.

https://en.wikipedia.org/wiki/Rotor_ship#Pioneers

https://books.google.com/books?id=9lqwg3iZyH0C&pg=PA656#v=on...

I would be surprised if a kite system could get much closer than 90 degrees to the wind.

Aslong is the wire points forward of left and right the net force is forward right?

In practice you probably want the wire +-45 degrees from the bow or something. That is where you would use a gennaker or spinnaker.

I feel like this would be a worse version of a symmetric spinnaker, which you generally sail in 100-180

To second the others, "reaches" are the fastest. I forget why, but I suspect it's like gearing, less force, more distance, for the same amount of work.

I can see the ecological disaster of loose kites in the ocean already

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