Wing Sails

[venomousbird]

Hi there, I'm wondering if anyone can explain to me briefly the advantages and disadvantages of using a rigid wing sail. I'm wondering if they offer greater efficiency, and also, how they are furled (or not) in bad weather. Thanks.

[Autodafe]

Hi VB,

I'm not an expert on these, but I think you've got the main points already :)

Rigid wing sails are (if correctly designed) more efficient than soft sails, as long as the conditions match those they were designed for. To achieve an advantage requires a lot of care at the design stage, and preferably, plenty of cash for high tech materials.
Wing sails that perform well in a variety of wind speeds and apparent wind angles may end up having a lot of moving parts, with two or three adjustable slots in the wing.

And the big down side, as you alluded to, is that no one (that I know of) has yet come up with a convenient way to reef or furl them.
Additional problems that are particularly telling for offshore work include increased fixed windage and increased fixed weight aloft.

There are a few reefing/furling options that have been built, but these tend to be complex and/or heavy:
It is possible to reef some of the more complex wing sails without reducing area by adjusting the trim of slots in the sail.
Alternatively several wings can be installed, and then progressively lowered to the deck as wind increases.
Another option is to pivot the wing laterally around its center, and then to reef (or furl) you progressively lower the wing closer to horizontal, thus the sail is "furled" when the wing looks like a aircraft wing, and produces only vertical lift.

As a consequence of these factors their use at the moment is almost exclusively confined to day racing at the cutting edge of performance, such as the C class cats of the little americas cup, and to experimental designers who value the elegance and robustness of the concept more than the sailing.
At the risk of over-generalising:
In all other conditions soft sails are a better option as they can be lighter, cheaper, easier to access, change and repair, are easier to remove in extreme weather, and can be more easily trimmed to changing weather conditions.

[ancient kayaker]

Your points about rigid wing sails are correct. When higher efficiency is discussed it usually means the lift/drag ratio (l/d) which is considerably higher in a rigid wing. However this applies when the wing is operated at a low angle of attack or alpha, whereas sails typically operate at higher alphas.

L/d is crucial in an aircraft where the “wind” is coming from virtually dead ahead. L/d is not always of much significance in a typical sailboat which operates over a much wider range of aerodynamic conditions.

Nonetheless, a rigid wing can score big time over a sail when sailing close to the wind, which is why they are often used in very fast craft which, due to their speed, create an apparent wind that is shifted well toward the bow.

There are a few other factors. Although a rigid wing cannot be reefed or furled it can be designed to feather quietly, unlike a sail which will thrash around violently if let fly in a breeze. There are also several designs of “soft wing” that can be reefed and furled; they are not as efficient as a rigid wing since the profile cannot be as precisely controlled, so less critical, lower efficiency profiles are used.

[venomousbird]

All of the rigid wings I see are in the shape of aircraft wings, that is, a very low width and extreme length. Could a wing sail more similar to a gaff rig (triangular) be made? Perhaps one of this shape could be designed to collapse more easily, accordion style? (My gf adds, exactly like a large collapsible fan)

[ancient kayaker]

No reason why not, but I don't see why it would be easier to reef or furl. I assume you are speaking of a fabric sail here not a rigid sail.

However, with the caveat that I am not an aeronautical engineer, as I understand it broad low aspect ratio wings have higher drag and lower efficiency than high aspect ratio wings. Gliders where efficiency is crucial have extremely long and narrow wings, as do competition land sailers and ice yachts. For the same reason performance sailboats have tall narrow rigs. The main reason for having a wingsail in the first place is to obtain higher efficiency.

[venomousbird]

Very good points, thanks. My main interest in wing sails is the idea that they might be easier to control mechanically without having direct access to the sails in a closed cabin, vs being out on the deck. I was thinking along the lines of a traditional Chinese junk sail in a triangular shape, something closely resembling a fan in shape and function, but slightly curved like a wing. I figure a triangular shape could collapse down flat more easily than a longer wing shape.

[PortTacker]

Just the contrary is true.
Hard to control, narrow operating range, dangerous as hell in a storm...

There have been experiments - here's one:
http://www.yachtworld.com/boats/2001/Walker-Wingsail-Zefyr-43-Trimaran-1713367/Southampton/United-Kingdom

[ancient kayaker]

Post # 6 in this thread describes an attempt to put a “deltaplane” wing on a Tornado and other wingsail experiments.

http://www.boatdesign.net/forums/sailboats/highly-assymetrical-wing-sail-25362.html

PortTacker just to clarify, are you referring to wingsails in general or venomous's idea of a fansail? Fansails were quite common about 100 years ago for sailing canoes.

The boat in the link you give looks to have a wingsail with a control foil although the control foil looks rather large, or it maybe a wingsail ketch. The mainwing looks to have an adjustable profile or slotwing; reminds me of the kinf of thing used on racing land sailers.

[tspeer]

Hi there, I'm wondering if anyone can explain to me briefly the advantages and disadvantages of using a rigid wing sail. I'm wondering if they offer greater efficiency, and also, how they are furled (or not) in bad weather. Thanks.

Rigid wing rigs have these advantages over soft sails:
- The section shape and twist match the design shape
- Slotted flaps can develop a higher maximum lift coefficient
- Larger cross sectional moment of inertia for greater stiffness and strength
- Less drag at low angles of attack due to avoiding windward side separation

Rigid wing rigs have these disadvantages over soft sails:
- Inability to reduce area as the wind increases
- Heavier (generally)
- Difficult to transport and handle
- Must be "flown" 100% of the time, making mooring problematic
- More expensive (many more parts, labor to construct)

The higher maximum lift coefficient of slotted flaps has been a key factor in their success in C class catamarans, with their sail area limit that precludes the use of spinnakers off the wind.

AFAIK, the most widespread use of rigid wing rigs has been in landsailing. They have dominated the C class cats, but there are comparatively few of those compared to the larger number and greater variety of landyacht wings. The fastest of the rigid wing landyachts run neck-and-neck with the fastest of the wingmast/soft-sail landyachts, so one can't generalize the superiority of one over the other.

In bad weather, rigid wings are taken down and stored indoors. Landyachts at a regatta are typically "moored" by using dollies under the rear wheels that have their axles aimed at the front wheel. This allows the yacht to pivot freely about the front wheel, like a boat on a mooring, but the toed-out dolly wheels resist backward motion. It's not uncommon to see tracks in the morning that indicate the yacht has swung through 360 degrees during the night.

In principle, a rigid wing yacht could live on a conventional mooring. However, when a gust or wind shift hits the wing, it will develop lift and drive, causing it to sail forward and possibly capsize. Aerodynamic control of the wing using a tail to make it aerodynamically balanced allows the wing to react much faster than the hull can move, which will alleviate gust loads. However, the wing must be able to swing through 360 deg (and for an unlimited number of rotations) if the hull is to remained fixed. Aerodynamic control has been successfully used offshore in the form of the Walker Wingsail, although I don't know how extensively they have sailed offshore.

The experience of sailing a rigid wing has some big differences from sailing soft sails. A wing doesn't "talk" to you the way a soft sail does because it doesn't luff. Instead, it backwinds, which can slow you down quickly or even pitchpole backwards bow over stern. When tacking, if the wing is not rotated quickly enough to follow the hull, the angle of attack will become negative and the wing will backwind as the hull turns. If the wing is rotated too quickly during the tack, the wing will backwind. A wing that has positive control (manually rotated instead of being free to turn) takes some practice to tack properly. An aerodynamically stable wing that is free to turn is easier to tack because it can be allowed to feather itself, like a luffing sail. It can also be sheeted conventionally, pulling in on the wing and allowing aerodynamic moments to rotate it outboard.

Especially for high-speed craft like landyachts, flutter is a definite possibility. This is a self-sustained oscillation due to coupling between rotation of the wing about its pivot axis and the side-to-side translation as the wing rotates laterally about its mast step due to flexibility or slack in the rigging. Positive control is sometimes used to minimize flutter, but moving the center of gravity forward, preferably on or ahead of the pivot axis, is a more effective remedy.

The relevant telltales are on the lee side, where they are not visible through an opaque rigid wing. Phil Rothrock solves this problem by having a wind vane forward on the body of his landyacht, with the arms that form the references slaved mechanically to the wing so they rotate with the wing. This allows him to gauge the wing angle of attack as he's looking forward to see where he's going. Another form of telltale that would work with a rigid wing is the close-mounted wind vane at the leading edge, like is used by many wingmast sailors to locate the stagnation point.

Going back to your original question about efficiency, If a softsail rig is given the same height as a comparable rigid wing rig and designed to avoid separated zones, then there's no fundamental reason why it would be less efficient. Those are two big "ifs" though. The flip side is also true. If you design a rigid wing to have a low aspect ratio, it may not give you any performance edge over a soft sail.

The ability to adapt the soft rig to the sailing conditions can give it the edge over the rigid wing rig - adaptability through wing twist was the key to Cogito's success. The ability to add sail area or to reduce it through reefing makes the soft rig the better choice for most application.

[PortTacker]

This wing I designed and built myself. It's 1,5 meters, on a 1 meter x 1 meter landyacht.
http://www.box.net/shared/sgtbx438nl

The shroud attachment bracket forces twist of the upper sail, the top of the wing is a single piece, the rest of the foil forms articulate and also are free to pivot on the central internal spar. At the bottom there's an arm which, because the sheet attaches to it, forces the wing to articulate, but also has stops to limit the articulation. The entire thing weighs ounces, it's actually lighter than the conventional main and jib with booms on a graphite mast. It's flipped many times, no damage, it's stronger than I thought it would be!

It worked pretty well, the thing was wicked fast. Not much good in lower wind speeds. Once moving - accellerates quickly! But it often capsizes in gusts so quickly I cannot react, even though I using a very fast servo and a single arm for fast action. It generates enormous power compared to a comparable sized soft sail, first outing it broke wire shrouds, and it was easy for it slide the rear wheels sideways if you aren't careful. I raked it "less aft" and it quit doing that.

But if the gang gets back into landsailors again (as opposed to sailboats - a few of us built landsailors just for fun) I'll be building a new rig. I think a wingmast with a narrow soft sail behind it might give me more versatility (read: control) and good speed.
BTW, I never got up the nerve to try it on the trimaran! (also 1m x 1m)

[ancient kayaker]

I took a close look at a repro WW1 aircraft some time ago; the wing construction was fabric over ribs with a spar and a leading edge molding as is common for models, but the trailing edge was a cable under tension, which explained to me why so many early planes had scalloped TEs. I wonder if this construction could be adapted to a wingsail that could be furled.

[BigCat]

Check out http://bigcatcatamarans.com to see my version of this, and to see relevant links.

[Eweston]

I've been fascinated with the Idea of a clean airfoil as a soft wingsail for several decades. I just reworked it for the umpteenth time. The hoisting and furling is a little involved, but doable. I suppose the next step is a good sized working model.
The whole thing of a workable full sized sail seems as much art as enginering, even today. I appreciate that.

[yipster]

not advocating rigged or freestanding, wingsails, the ^ frame or any peticular sail
i am fascinated by wings and sails and so made a little study on them
was crazy enough to ask an inflatable boat exhibitioner at a boatshow if it would be possible
changing the form to an inflatable wingsail, than noticed alinghi expirimenting with inflatable battens
looked at the crab claw and delta wing, now imagining a > mast, maybe its something
havent done (m)any testwork but am looking into all aspects knowing simple is best
much is possible and name Ilan Gonen (http://www.omerwingsail.com/) as one who did make a reefable wingsail
in my gallery (http://www.boatdesign.net/gallery/showgallery.php/cat/500/ppuser/690) some sketches, links, pdf's

[Windmaster]

I have been experimenting and testing self-trimming wingsails for a number of years. Some of the foregoing comments are misguided and just conjecture without actual experience.

I have collected together details of my research on my website which is http://www.sailwings.net pleased to see any comments you may have.

[Eweston]

Don't know if you've run acrossthis (http://www.wingsails.com/cetiri.html) variation or not. I've seen some inflatables used with windsurfers also.
I'm still drawing and thinking on my latest version. I hope to complete it and if I can figure out how display it on this forum, do so.
In short a fully rotatable wing mast which would set directly on deck. The cloth, or other material of the aft sail, would be fully battened. This has been used, or used in attempts before.
I saw an earlyer thread discuss the uncertainties of what airfoil shape would work best. It would be a heavy and involved practical experiment to investigate this with a real soft wing sail. I envision something using lasers to detect wind movement over the sail.
I've figured that induceing an assementrical curve to the sail and adjusting its width would be flexible enough of an approach to investigate what does work.
This shaping would be based on four c section vertical struts. Two would locate the front of the cloth. Pulling one edge forward and allowing the other to slack off will produce the shape of an assemtrical airfoil. The two other struts would be located arround 1/4 to 1/3 aft. The struts will be attached to the battens. These struts will be pulled forward and inboard, or allowed slack to let the negative air pressure pull it outboard.
The four c section struts will use a luffrope (not real sure this is the proper term) to both allow attachment to the sails forward edge, sides and to be used to hoist the sail. Supporting these struts from above, while allowing sufficient movement and maintaining even tension on the halyards is fairly clear. But not finalized yet.

[Eweston]

Two interacting wings does have advantages. You could also add a slot/ small foil forward of the main wing. Its mechanism would take some thought though

[Doug Lord]

Here is an interesting potential development:

http://www.liveyachting.com/wally-omer-wing-sail-presents-wow-wing-sail-technology-for-large-cruising-yachts

[Eweston]

Thanks Doug
That looks like a very good simple setup. Performs too!
Having no luck loading images here. I did up a curvature mechanism model for my system and have three semifuzzy images on file here. I'm pleased with how it worked and see an improvement or two. It is possibly more than it needs to be.
Hope too see about progressing it. Right now Life is rompin and stompin on my time.

Ed W

[CT 249]

Well, it CLAIMS that it performs.....
According to the claims made for the Omer wingsail, if you put an Omer wingsail on a Hobie 16, it would probably beat the fastest C Class cats in the world.

That seems.....shall we say, rather unusual, given that the C Class is a vastly more sophisticated and much larger boat that already has a wing rig.

Apparently, if you put an Omer wing rig on a 470, you would beat the 49ers quite easily. Hmmmmmmm.........................

Similarly, according to the Wally/Omer site, if you put an Omer wing rig on an A Class cat, you'd beat the other A Class cats by about 30%. However, when a very good sailor put a more sophisticated wing rig on an A Class cat, they got beaten by soft rigs. So how did that come about? According to the site, the A Class would have been the fastest small boat around a course in the world if it carried the less sophisticated Omer wing, so why did it get beaten by other As???

Funny thing, Omer was promising to provide us with results of his testing against a standard sistership back in 2008. Oh, and that was just after a BDF poster had pointed out that his polars (as posted on Omer's website) were wrong and Omer agreed. It seems that he may have fixed them, but certainly there were errors with the publically-provided information released earlier.

Racing sailors are very easy to convince,. It goes like this - you turn up to a race. You win that race easily. People go geezers, that was FAST!!!!!!

I wonder how the testing went when the Omer wingsail went racing?

[Windmaster]

You'll notice that the picture is not a real boat but a made up computer one.
You always have to be suspicious when you see this because usually it's just someones dream and will never get built.

These kind of uncontrolled wings are difficult to sheet accurately because of the narrow angle of attack that works best for them.
A much better scheme is to introduce a controlling surface, a tail if you like, to automatically keep the angle of attack constant, thus relieving the helmsman of having to do it manually all the time for every little windshift.

This kind of approach was used on the Walker wings, and my own wings. Some of which you can see on Youtube under "controlled wingsailing".

[ancient kayaker]

I've been thinking about wingsails for a while now. I read fantastic claims and occasionally see something that seems to work but not often. Theoretically they should outperform a conventional sail close to the wind although perhaps not on other points.

I used to design and build model gliders and sailplanes, and the wings were usually built concave on the underside, undercamber we called it back then, not sure if that term is correct or still used. They were claimed to be superior to flat-bottomed wing profiles although I never established that in practice. However an asymmetrical profile is better than a symmetrical one, dramatically so at low speeds, but not for aerobatics.

Since a boat's wingsail must take the wind on either beam it must be either symmetrical like an aerobatic wing, have a variable profile which can get complicated, or use slots to increase lift. A conventional cloth sail provides the desirable asymmetry automatically, but has difficulty maintaining shape very close to the wind unlike a wingsail. However, since the actual speed made good upwind can be improved by sailing a little further off the wind, this is moot.

The wingsail does not provide the same feedback as a sail, although the control problem has been addressed by a tailplane to control alpha as mentioned by windmaster. Again it must be changed to take the wind on the opposite beam which adds complexity. This may be the optimum concept for upwind work, short of going to the extremes illustrated in the last America's cup.

Leaving the question of sailing close to the wind, on a reach the drag and its contribution to leeway are less important, and downwind drag is actually beneficial. Sailing at these points the wingsail could be expected to be handicapped, and in most cases I know about, that seems to be the case. The best thing for downwind should be a parachute, and a fabric wing can approximate one of those quite well.

Things change for a really efficient hull and sail combination, especially if the boat is good enough to benefit from tacking downwind and can go faster than the wind on some points. As the boat begins to "make its own wind" as the saying goes, the wingsail should come into its own, but modern sails are still hard to beat and have advantages for reefing and furling. For that reason, I expect to see combinations of rigid wing and fabric sail virtually take over in competitive sailing unless prohibited by the rules. p.s., Tom Speer's (post #166) at http://www.boatdesign.net/forums/sailboats/americas-cup-whats-next-31489-12.html notes control forces for a wingsail can be significantly less than for a fabric sail.

The nail in the coffin as far as I am concerned is, to benefit from the reduced drag of the wingsail, the rest of the boat must be also designed to reduce aerodynamic drag, or there is little point in the exercise. I suspect this is a major reason why wingsails often deliver disappointing results. I was planning to try out one on a kayak, until I realized that, sailing upwind, the drag of my body and head sticking up above the deck would completely overwhelm any gain in efficiency I might get from the wingsail.

[Eweston]

I believe a tail was folded into Bernard Smith's work in the early sixties. The local library system had a copy of his "40 Knot Sailboat". Interesting, but as a proa design, tacking was an involved thing.

Hopefully Omar's sail will eventuall show it's worth, or lack thereof.

Consider the wasted effort to produce a Halfnium nuclear grenade. A bad idea from several points of view, evidence submicroscopic. Yet an amazing amount of federal cash was spent on it.

Despite a fair amount of thinking and calculating, the best ways forward right now may just show us were we are mistaken. More real world experience is needed.

Terry, the thing that a rotatable wing would allow is to produce dynamic lift from an aft wind. This is different from the using a parachute, spinnacker, mainsail ~ 90 degrees to the apparent wind.

I hope I can get back to this sometime soon. A worthy endevor, and a whole lot less damaging than a nuclear handgrenade.

[messabout]

Windmaster; you may feel free to question the knowledge or experience of any one on the forum except TSpeer. He is our resident guru who is very much respected and believed.

[Windmaster]

Windmaster; you may feel free to question the knowledge or experience of any one on the forum except TSpeer. He is our resident guru who is very much respected and believed.

Quite right. I agree. Didn't mean to question anything he said. Hope there was no misunderstanding.

[ancient kayaker]

I ... Terry, the thing that a rotatable wing would allow is to produce dynamic lift from an aft wind. This is different from the using a parachute, spinnacker, mainsail ~ 90 degrees to the apparent wind. ...

Quite so; they can all be operated at low alphas at any point of sailing. Be careful playing with those grenades now!

In my early thoughts about wingsails I read Peter's accounts of experiments using tailplanes. It seemed to me it would simplify control and increase safety. For competitive sailing the constant alpha would be a disadvantage and the tailplane would need to be varied over a continuous range rather than have two fixed positions plus a neutral setting. For sailing directly downwind - before I heard about tacking - I assumed the wingsail would be used 90 deg to the AW for maximum drive, and was considering an arrangement similar to the dethermalizer used in competition model sailplanes, at least many years ago.

The technique of tacking downwind renders that unnecessary as well as being far quicker. If the boat is fast enough to beat the wind then there is no need to gybe either.

[tspeer]

...In my early thoughts about wingsails I read Peter's accounts of experiments using tailplanes. It seemed to me it would simplify control and increase safety. For competitive sailing the constant alpha would be a disadvantage and the tailplane would need to be varied over a continuous range rather than have two fixed positions plus a neutral setting. ...

Tails and direct control are not mutually exclusive. Some friends and I built a landyacht with a tail-controlled rigid wing (http://www.tspeer.com/landyachts/Lydia/LydiaPhoto.pdf). The controls were arranged so that the pilot could control flap and tail separately or gear the flap and tail together as one control with a variable ratio between them. The tail control worked very well. It was able to control the angle of attack in lighter winds than the yacht could sail, and in fact became a gigantic Windex. The gust response was very good, immediately sheeting out as the gust increased the apparent wind angle. Actually, the gust response was too good - when the gust hit, the yacht did not get the punch of acceleration that it usually did.

Because the tail boom passed directly over the pilot's head, pilots found there was a third method of control we designers hadn't thought of. Pilots discovered their preferred way of sailing was to neutralize the tail and simply reach up and grab the tail boom to position the wing. Balanced by the tail, it only required finger-tip pressure to move the wing. By manually controlling the wing, the pilots were able to get exactly what they wanted from it.

[Paul B]

Terry, the thing that a rotatable wing would allow is to produce dynamic lift from an aft wind. This is different from the using a parachute, spinnacker, mainsail ~ 90 degrees to the apparent wind.


I'm not sure what you mean by this. When sailing "with an aft wind" with a kite or a spinnaker you do produce dynamic lift. Many people don't understand how sails like spinnakers are used.

[Paul B]

You'll notice that the picture is not a real boat but a made up computer one.
You always have to be suspicious when you see this because usually it's just someones dream and will never get built.



Not only that, but the perforance gain claims seem pretty outlandish.

I look forward to seeing some two boat testing.

[ancient kayaker]

... pilots found there was a third method of control we designers hadn't thought of. Pilots discovered their preferred way of sailing was to neutralize the tail and simply reach up and grab the tail boom to position the wing. Balanced by the tail, it only required finger-tip pressure to move the wing. By manually controlling the wing, the pilots were able to get exactly what they wanted from it.

Now that is very interesting, if the tail can be fixed things get simpler. The arrangement for controlling the boom would have to be able to push the boom out as well as pull it in, but in practice it would not be a problem in a high speed craft as the apparent wind would not be far off the bow at speed. It suggests an offshoot of BMW Oracle's computer control requiring less power to operate the sail, allowing smaller, battery-powered servos. Might bring it within the range of us poor folk!

[tspeer]

Now that is very interesting, if the tail can be fixed things get simpler. The arrangement for controlling the boom would have to be able to push the boom out as well as pull it in, but in practice it would not be a problem in a high speed craft as the apparent wind would not be far off the bow at speed. It suggests an offshoot of BMW Oracle's computer control requiring less power to operate the sail, allowing smaller, battery-powered servos. Might bring it within the range of us poor folk!

You might want to consider a belt or chain drive. A large pulley or sprocket on the wing is connected a smaller capstan mounted to a motor. Basically the same way an autopilot is connected to a steering wheel. It's simple, works in both directions, and you can get a wide range of gear ratios between the motor and wing. Wing angle sensors can be driven by the belt/chain, too.

With aerodynamic control balancing the wing, not much power would be required. In fact, a comparatively weak motor would be designed to be overpowered by the tail when the wind was high or gusting. Basically the maximum torque from the motor would correspond to maximum achievable heeling moment from the wing, because the hinge moment about the pivot axis from the wing and tail would correspond to the lift on the wing.

Direct control and tail surface control are not mutually exclusive, either. Rapid changes in wing trim could be done with the direct control. The tail angle would be used as a trim control to alleviate the load on the direct control. The tail would be driven by integral compensation, naturally washing out the direct control. That would preserve the direct control power for transient control, reduce power consumption, and avoid overheating an electromechanical actuator. The tail actuator would not have to operate rapidly, which would reduce its power consumption and weight, too.

In the event of failure of either the direct control or tail control, the other can act as a backup, providing functional redundancy in the system. With some judicious limiting in normal operation vs abnormal situations, and fail-passive design, the surviving control method could be given the ability to overcome the crippled control.

[ancient kayaker]

You might want to consider a belt or chain drive. A large pulley or sprocket on the wing is connected a smaller capstan mounted to a motor ...

Pretty much waht I had in mind for a first experiment in a small boat, but connected to a simple handle wheel or tiller. Worth a try: I'll add it to my list of things to do, but the list is getting awfully long ... so don't nobody go holding yer breath now ...