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In a dream, the image came to me of a modern / future ship driven by ring / cylinder sails. (Well, technically I saw something different, but this is what I knew it was supposed to refer to...) That is, these would be short tubes oriented horizontally that could pivot. Is anything close to this design technically workable?

Clarification: The ship in my dream (would have) looked something like this.

sails lengthwise sails crosswise

Is there anything remotely like this that can work? Note that I'm only asking about SAILING ships. The propulsive force must come from the wind, not an engine. The sails I showed didn't constantly spin, they only pivoted when needed to change course or to deal with changing wind.

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    $\begingroup$ What you want is the Flettner ship: en.wikipedia.org/wiki/Flettner_rotor But the tubes need to be vertical, not horizontal. $\endgroup$
    – jamesqf
    Feb 14 at 18:14
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    $\begingroup$ Not talking about a powered rotor, but a passive sail. $\endgroup$ Feb 14 at 20:34
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    $\begingroup$ I'm thinking I saw pictures of such a ship, probably 50 years ago. Obviously didn't catch on. $\endgroup$
    – Hot Licks
    Feb 15 at 1:26
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    $\begingroup$ Is this not an engineering question? I don't see the worldbuilding angle. $\endgroup$
    – rek
    Feb 15 at 6:09
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    $\begingroup$ Just curious,what did you "technically" see? $\endgroup$ Feb 15 at 12:14

13 Answers 13

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I don't know if its exactly what you're referring to, but Rotor Sails fit your description. They are spun rapidly by the ship (via a motor), generating forward thrust via the Magnus effect. I believe you can turn them horizontal if desired, but they are typically vertical to minimize issues with docking or other narrow clearance situations.

Some companies have found that they can improve fuel efficiency with them over simply turning the propeller.

enter image description here

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    $\begingroup$ Great answer, I had no idea. Just one of the wild and wacky things which edge into the mainstream. $\endgroup$ Feb 14 at 16:49
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    $\begingroup$ Apparently I need to clarify. I asked for a sail, not a propeller or rotor. When I said the sails could pivot, I meant to catch wind from different directions, just as you change the positioning of traditional sails. $\endgroup$ Feb 14 at 20:33
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    $\begingroup$ They cannot be horizontal, because the Magnus force is perpendicular on the axis of rotation. To have the rotors rotate around a horizontal axis you would have to place them crosswise to the ship's axis, and that's not really feasible. $\endgroup$
    – AlexP
    Feb 15 at 0:04
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    $\begingroup$ As mentioned by AlexP, they can’t really be horizontal to generate forward thrust, though some aircraft designs have used the Magnus effect with horizontal Flettner rotors to generate extra lift, reducing the stall speed, and some ships have been designed that use them horizontally mounted below the waterline to provide additional stabilization in rough seas. $\endgroup$ Feb 15 at 2:34
  • $\begingroup$ I like the transfer of angular momentum to generate impulse but OP wants a coil for a sail so... maybe the coil acts like wind vane to drive the submerged propeller blades mechanically ;D $\endgroup$
    – user6760
    Feb 15 at 7:55
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Is anything close to this design technically workable?

Your boat reminds me of something called a hoop glider. It is similar in that it also uses hoops, but as wings. I think the glider demonstrates how this shape interacts with the air as a wing by creating lift and therefore most likely would never work well on a boat as a sail.

Hoop Glider

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    $\begingroup$ "by creating lift and therefore most likely would never work well on a boat as a sail." But that's what sails do, in certain configurations. A sail perpendicular to the wind acts like a parachute, a sail parallel to the wind acts like a wing. $\endgroup$ Feb 15 at 0:01
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    $\begingroup$ At some points of sail, ship sails work by creating lift, just like the wings of a sailplane; but in the case of sailing ships, the airfoils are arranged so that the lift acts in the horizontal plane to propel the ship forward. $\endgroup$
    – AlexP
    Feb 15 at 0:01
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    $\begingroup$ One interesting takeaway from this is the possibility of hoop sails letting the ship work kind of like a hydrofoil. You probably couldn't get enough lift to get it completely out of the water, but you may be able to measurably reduce drag. $\endgroup$
    – Turksarama
    Feb 15 at 1:09
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    $\begingroup$ @JamesMcLellan I was talking about "lift" in the horizontal direction, IE, a wind blowing along the sail creates a force perpendicular to the sail that moves the boat. $\endgroup$ Feb 15 at 20:23
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    $\begingroup$ @TristanKlassen By upwind, I mean less than 90 degrees to the wind. As best as I understand, sailors call that upwind. There is indeed a small region right around directly up wind (size depends on the ship and sails) where they cannot sail. $\endgroup$
    – Cort Ammon
    Feb 16 at 2:07
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Given that there is no aerofoil section, I think this would have great trouble sailing into the wind.

With the wind directly behind the boat, the cylinders would have to be at right-angles otherwise the wind would whistle straight through them. Even at right-angles they would be less efficient than a flat sheet of canvas such as on an old square-rigger.

I think it would sail okay on a broad reach. That is with the wind coming at right angles to the side of the boat. The cylinders would then have to be at 45 degrees to the keel.

Ultimately, the best way to find out is to build it. It must have a keel and a rudder. You can buy a cheap model sailing boat and just change the sails. Put it in the bath and blow it with a fan. Make sure not to drop the electric fan into the bath.

Maybe adapt one like this.

enter image description here

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    $\begingroup$ Just a thing : Make a control experiment first. You should make a try with the old sails, before replacing them with the cylinder ones. Should help knowing if the boat you bought can actually move with your configuration, and with proper measurements, you can compare performances, too ! Of course, the quality of the comparison is up to your craftmanship. $\endgroup$
    – Tortliena
    Feb 15 at 11:07
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    $\begingroup$ Also do runs with no sails, to help rule out pushing the boat (or water) itself. Which is going to be an issue with lighter craft and smaller spaces. $\endgroup$ Feb 15 at 18:21
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    $\begingroup$ "Given that there is no aerofoil section" -- Did I say there couldn't be one? $\endgroup$ Feb 17 at 23:16
  • $\begingroup$ @Tristan Klassen - Oh, I was going by the paper model. Of course I understand that it would be difficult to make an accurate model from a dream. Maybe there's a modification that would help. BTW can the cylinders move independently? Do they have to be at the same angle? $\endgroup$ Feb 18 at 1:25
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It works in principle but not in practice.

You are just creating a pair of normal sails per cylinder. but is it incredibly inefficient, most of the rings surface generates no propulsive force. You can't tilt the individual surfaces, more importantly there is no way to furl the sails, so you can't stop and your sails and likely ship will be ripped apart during a storm. Even a single lateen sail is going to generate way more thrust, way less dangerous, and way easier to use, plus you can fit more of them in the same deck space, so those will quickly replace yours.

It is also can't be built as shown without modern technology, the rings are stable becasue they are tiny, on something ship sized they will either collapse under their own weight or be so heavy they generate more drag than thrust.

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  • $\begingroup$ "can't be built as shown without modern technology" well, I was specifically talking about a modern / future ship, not fabric sails but something rigid. $\endgroup$ Feb 17 at 23:17
  • $\begingroup$ Then as I said you end up making them very heavy and thick which makes the whole ship very top heavy, and reduces the effectiveness as a sail even more. $\endgroup$
    – John
    Feb 18 at 1:23
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" short tubes oriented horizontally that could pivot. Is anything close to this design technically workable? Is there anything remotely like this that can work?"

Maybe these were not purely sails!

This could be a hybrid system. How closely did you examine the 'sails' in your dream?

They could have been extra-large Air Multiplier™ Dyson fans. that also make use of the wind to get a massive boost from entrained air.

enter image description here

enter image description here

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  • $\begingroup$ This does not appear to answer the question. Does it matter how the OP came up with the idea? They are asking about the feasibility of the sails. $\endgroup$
    – BMF
    Feb 15 at 23:10
  • $\begingroup$ @BMF - I see what you are saying. I suppose I was answering a sub-question: The OP asks, " short tubes oriented horizontally that could pivot. Is anything close to this design technically workable? Is there anything remotely like this that can work?" So my answer to that is Yes. I agree that it is not exactly a sailing boat but maybe it could make use of the wind direction in a novel way by pivoting the fans. It would be a wind-assisted hybrid and maybe much more stable than a pure sail rig. $\endgroup$ Feb 15 at 23:51
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    $\begingroup$ Ah, I see what your answer is supposed to mean now. I think maybe you should explore it a bit further. Or add onto it. On first read, it reads like a quick zinger hence my first comment. $\endgroup$
    – BMF
    Feb 16 at 0:43
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    $\begingroup$ @BMF - I've edited. $\endgroup$ Feb 16 at 1:59
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    $\begingroup$ I'd also like to point out that the fan blades are contained within the base of that unit, and suck air through the bottom grille before pushing it up and through the system that 'amplifies' it. Using these would require large engines, probably ones large enough to push the boat around with a propeller. $\endgroup$
    – Gigaflop
    Feb 16 at 18:34
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Yes, it can work. It deflects wind in some direction.

The problems that may arise:

  1. The shape - the usual sail is kept in its shape by the tension of few ropes. The ropes feed back the force of the wind. Here, you will have to make your "sails" more or less rigid or invent some other way of keeping them cylindrical in the presence of a sidewind. Will work for a model scale, not sure about 10 feet boat. The whole sail will be quite heavier than the flat sails and you don't really want your center of mass higher in a boat.

  2. The efficiency. Will be less than the usual sail.

  3. The footprint. A sailor can go around the usual sail. Here, in most pivot positions, one will have to go either under the sail (and you want to keep it low, see above), or thru the opening (if it is that much rigid to support a sailor's weight).

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  • $\begingroup$ I guess it wasn't clear in my original post, but I knew this had to be a rigid sail -- as I said, modern / futuristic sailing ship, presumably inspired by half-remembered designs I've seen. $\endgroup$ Feb 17 at 23:20
  • $\begingroup$ Go for it, then. When I said lower efficiency, I meant lower efficiency per mass. In exchange, you will have means of pulling the boat up, lowering the water drag. $\endgroup$
    – fraxinus
    Feb 18 at 7:35
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The sails are mag(net?)ic!

(Not magic as involving spells or individuals with power, more like altered physics.)

Along the sails sit some kind material with a self repelling Force. I imagine it works somewhat like magnets with their north poles facing the centre of the cylinder, pushing it outwards to maintain its shape.

To pull this off with real magnets on our real Earth you would need very strong magnets.

One way to solve this is to not use magnets but some magical substitute. The best kind of magical object would repel other similar objects in all directions, as strong magnet-like two-sided forces would be rather dangerous I think.

Another way, if you want to keep magic out of it, would be to have this take place on a planet with much lower gravity. Who said your dreams must take place on Earth? Why not a smaller planet with a dense atmosphere and abundant magnets? (The resulting magnetic field around the planet would also help explain how a small planet could maintain its atmosphere!)

So. Let's assume you have managed to shape the sails, how to use them for propulsion?

There are many nice answers on how they need to be shaped to generate lift, so I'll not dwell to much on that. One thing I haven't seen is a suggestion on how to change their shape. My suggestion is to simply have ropes inside channels along the rims, which you can pull tighter or loosen, like the strings on a hoodie. This way you could have them more or less conical as needed, and so they would work even with the wind in your back, and you can even fine-tune how much force you get. Of course you could also turn them as needed to propel you in the direction you want to go.

One advantage, like @Turksarama notes, to normal sails could be that you could also generate some lift and reduce the drag of the water. In a low-gravity setting, a ship might even be able to briefly take off if it's not too heavily loaded and the winds are right!

If you opt for magic (which IMHO is the cooler option) I would suggest that the ring of repelling Force also makes the air inside the cylinder slightly thinner than the ambient air. This would speed up the air moving through and thus enhance the lift. Note that the people using the ships don't have to be magicians; they don't even need to consider the Force magical! They just know that in certain places you can find self repelling nuggets. Maybe the cylindrical sails are fancier than normal sails so that rich people use them to show off, maybe they are common. I've never been to your dream world, so I won't pretend to know.

I think this is a really cool idea!

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Yes but...

As other answers have said, you need the sails to form an aerofoil section, and wind passing over the aerofoil generates a force perpendicularly to it. On a plane wing, the aerofoil is horizontal and the force is vertical. On a boat, the aerofoil is vertical and somewhere outside of straight down the middle of the boat, so it generates a horizontal force with some forwards and some sideways. The keel opposes the sideways force, so you only get forwards motion.

If you shaped the ring appropriately, you could give it an aerofoil cross-section to the ring sides. That should create appropriate forces in the same way as a sail, as the wind passes over it.

Having parallel sides with opposing directions of aerofoil is going to be a problem though. The force generated by one side is going to be opposed by the other side. Flaring the sides out, so the ring cross-section is maybe 45-degree line, will give you some assymetry which would give you different net forces (because each aerofoil will present a different angle to the wind).

So it's physically possible. What it isn't going to be is efficient, unfortunately. You can certainly build one though.

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  • $\begingroup$ Who says the airfoil has to go in opposite directions on opposite sides? I don't think that's how annular-wing aircraft work. $\endgroup$ Feb 15 at 15:28
  • $\begingroup$ @TristanKlassen On a plane, a symmetrical wing uses angle of attack to create lift, instead of camber. The lift from the wing is not at all sensitive to the direction of the vertical component, and all you need is a bit more power from the engine and/or accept a lower groundspeed. A sail is profoundly sensitive to the angle of attack though, and if you try to use an uncambered aerofoil then it's going to be next to impossible to tack upwind. $\endgroup$
    – Graham
    Feb 15 at 16:19
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Lift and drag forces both can be used across your sail to provide propulsion.

enter image description here

enter image description here

Assuming no vertical angle-of-attack $\alpha$ (won't assume that in bit), the profile of your sail providing force perpendicular to the surface (per mast) is twice the sail diameter high and the full width of the sail wide.

$A \approx 2Dw$

The total amount of canvas is $A = \pi Dw \approx 3Dw$.

You learn something interesting from just this. In this configuration, you've lost roughly a third (33%) of your canvas available for thrusting.

What have you gained?

By the same reasoning, roughly two-thirds of the canvas air is now available for lifting.

The equation for lift is:

$L = {1 \over 2} C_L \rho_{air} (v_{air} - v_{ship})^2 A$

The density of air ($\rho_{air}$) is 1.1 ${kg} \over {m^3}$, and the lift coefficient ($C_L$) of a sail can be up to 1.4 (see chart below). So, the equation above becomes.

$L = (0.5) (1.4) (1.1) (v_{air} - v_{ship})^2 A = 0.77 v^2 A$

For a ship like a Caravel with about 100 square meters of canvas, A becomes about 66 square meters and the equation simplifies a little further to -

$L = (0.5) (1.4) (1.1) (v_{air} - v_{ship})^2 66 \approx 50 v^2$

Thrust from lift, neglecting using drag for thrust, is the same

$T = (0.5) (1.4) (1.1) (v_{air} - v_{ship})^2 66 \approx 50 v^2$

Drag, the force countering thrust to slow the boat, comes from skin friction and displacement. In a Caravel shaped boat, mostly displacement.

$D = {1 \over 2} \rho_{sea} v_{ship}^2 A_{profile}$

Where the density of sea water is 1,200 ${kg} \over {m^3}$ and the boat profile is the width (2 meters for a Caravel) times displacement (also 2 meters for the Caravel), and the equation above becomes-

$D = {1 \over 2} 1,200 v_{ship}^2 4 = 600 v_{ship}^2 4 = 2,400 v_{ship}^2$

enter image description here

Let's try this out at a few speeds

In leisurely 15 knot ($\approx 7.5 {{m}\over{s}}$) winds, $L \approx 50 v^2 \times 2 masts$, you get about 5,600 Newtons of lift per mast. That'd offset the weight force of about 560 kilograms. And, for a 50 ton ship, isn't really significant.

Might be good to work out cruising speed and compare it to a ship with flat sails.

At cruising speed, the thrust being generated by the wind is being totally balanced by drag force (so that the ship is neither accelerating nor slowing) $T = D$

For our ship $T = 50 (v_{air} - v_{ship})^2 \times 2 masts; D = 2,400 v_{ship}^2$

I don't have any way other than trial-and-error to solve for that, so maybe take it with a grain of salt that your cruising at 1.2 $m over s$, or about 2 knots.

A Caravel, our reference ship, would be travelling in the same winds at $T = 77 (v_{air} - v_{ship})^2 \times 2 masts; D = 2,400 v_{ship}^2$ at about 3 knots (or 50% faster)

Things start to get interesting in high wind

With a strong 45 knot wind (22.5 m/s), things start to become interesting.

$L = 50 v^2$ \times 2 masts$ = 50,000 Newtons. Or, 5 metric tons of weight. For a 50 ton ship, it's about 10%.

The drag of the boat hull is based on it's width (2 meters) multiplied by how deep it's sitting in the water (also 2 meters). The tonnage of the boat is the displaced 1,200 kg/m^3 sea water = 2 x 2 x 10 x 1,200 ~ 48,000 kg.

Lift, pulling some of the weight off the boat, means the boat will sit higher in the water. About 10% (0.2 meter) higher in this case.

This has 2 benefits :

  • Can sail in shallower water
  • Can move faster

In this particular case, being 10% higher in the water increases speed by about 5%. We're closing on the performance gap with the comparison ship : 8 knots (us) vs 9 knots (comparison).

What about dangerous conditions?

At 60 knot winds (30 m/s), the boat is 10% (0.5 meters) higher and still a little slower than the comparison ship (11 knots vs 12)

How about a lighter ship?

As you can see, lift vs. weight is how this effect scales. Decreasing both our reference ship and ship's tonnage by half to 25 tons, the cylindrical sail finally starts to outperform it's competitor. The ship sits much higher in the water at 60 knots winds (0.6 meters total displacement) meaning invisible coastal shoals are much less of a threat. It also sails faster: almost 19 knots in these conditions, vs the competitor at 16 knots.

Going further

Modern competition boats, I read, are about 6 to 12 tons. In this case, it seems, you're nearly on top of the water and much faster than the competition at 30 knots vs 26.

Lift is getting cut as the relative wind ($v_{air} - v_{ship}$) decreases with increasing boat speed. Would need to re-formulate how lift works to really explore this area.

I think there are other questions about stability and how to control the thing, but it seems like a very interesting concept!

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  • $\begingroup$ Interesting, though I lack the knowledge of sailing to say whether any of this is correct to upvote or downvote. $\endgroup$ Feb 18 at 0:00
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If you're willing to allow a little flexibility, sort of.

Make the sails as parachute foils, arranged as in a standard rectangular ram-air parachute, but with the edges extending down to the deck (to make most of a circle) and enormous. Add a line from the front of the sail down to the deck somewhere in front of the sail. Then you can:

  1. Generate thrust downwind by leaning the sail forward (essentially making a square sail high up), or convert that to movement in any direction below a beam reach by using the ship's keel to cancel out most of the lateral force.
  2. Steer by pulling down on either side of the parachute, exactly as skydivers steer.

And that gives you pretty much everything you need for a functional ship. Indeed, you can buy something vaguely similar today.

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If you really tried, you could make a boat sail like that, but you wouldn't want to. Compared to conventional triangular sails, it presents many problems and no benefits.

  • Most of the "sail" doesn't do much work, since it isn't vertical.
  • Difficult to maintain a good airfoil shape. The rigging would be quite complicated.
  • A lot of unnecessary weight aloft, that the keel has to counteract in order to keep the boat upright. That makes the boat displace more water and move more slowly.
  • Looks like it's difficult to adjust the sail area for varying wind conditions.

It's kinda like how you could make a car with wheels that aren't round... but why bother?

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  • $\begingroup$ Rather than traditional sails, maybe this should be compared to other configurations of rigid sails? $\endgroup$ Feb 18 at 0:02
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Essentially you'd need some form of way to apply force to the middle part of the annulus/loop. Two highly exotic ways to do that would be:

Some acoustic propeller thing:

You could use the interaction of acoustic waves with the wind to "suck" momentum out of the wind and into the sail frame. You'd need some kind of very powerful acoustic emitter to send the waves, and a highly tuned absorber, but I think you could create a kind of adaptive sail, maybe even one with a kind of "standing wave" or even some wave pattern which rotates similarly to a propeller (therefore requiring less energy to sustain). Of course it seems to be highly inefficient, require massive acoustic energies, and possibly deafen any sailors onboard, but it would look really cool and may be slightly possible using some yet-unresearched soliton physics

An ionizer & electromagnet:

If we could get a large concentration of ions in the air flowing through the loop, then we could apply a magnetic field perpendicular to the wind to sail on it in a very similar fashion to regular sails. Well, we can ionize the air itself using something like very powerful lasers at the loop entrance (which just like the magnetic field would be invisible to the naked eye), but again this is a pretty weak effect and would probably require massive amounts of energy to produce meaningful thrust. Again this would be hazardous to any personnel on the ship, this time due to ozone poisoning, but it would look cool and might even work.

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  • $\begingroup$ Ionization is a good idea, but you're thinking about it the wrong way around: ionize the masts instead of trying to ionize the air. See my answer for an implementation that is at least scientifically feasible (albeit not necessarily sufficiently efficient). $\endgroup$
    – Will
    Feb 16 at 23:44
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EWICON: Electrostatic Wind Energy Conversion

Your "sails" could function as ion wind generators, based on a principle first demonstrated by Lord Kelvin's Thunderstorm.

The idea is to use masts shaped somewhat like tennis rackets along which you expose (sea) water to the open air. Wind action physically removes water droplets from the masts, inducing a static charge in the masts that can be utilized as electric energy, for example to power the vessel's propeller through an electric engine.

A major advantage of this scheme is that efficiency is more or less the same from any wind angle.

EWICON in its current form is a recent innovation originating at the University of Delft in the Netherlands, where the technology is being actively developed as a means of power generation.

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