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WingSails, sails of the future!

This website used to be dedicated to improving sailing in general, like propulsion and stability, focusing on what makes the sailing vessels different from any other vessels – on sails.
Better to say -  wingsails.
In recent years, the focus has moved to America’s Cup. Why? Because the America’s Cup is supposed to be a fertile producer of brave new ideas and trickle-down technologies that would eventually bring a new, modern class of sailboats. Not necessarily radical and lightning fast as the AC beasts, but certainly better than old fashioned boats and yachts that are still proudly offered on the market.
America’s Cup failed in this mission so far.
The same old concept is still prevailing. All sorts of sails on the market are thin, single ply, therefore inefficient.
Is anybody aware of a stock sailboat rigged with thick, double-surface, soft wingsails?

The history of unbelievably slow progress of sail design is long.
When in 1925 the regatta committee of the New York Yacht Club heard of L. Francis Herreshoff's new patented rotating mast design, it promptly passed the rule prohibiting "revolving masts, double luffed sails, and similar contrivances". What a poor vision and knowledge of sailing principles! This madness lasted for decades, but nobody took responsibility for hindering sail development for nearly a century.

The development of the so called Princeton sailwing started roughly 70 years ago. In spite of its aerodynamic efficiency approaching that of a hard wing of the time, it had not been appealing to the America's Cup pundits for decades to come.

Let us take a quick look at the evolution of wingsails.

In the 60’s, a new developmental class has been established, introducing high performance rigid wing-sails on catamarans – the C-Class, often referred to as the "Little America's Cup".
In spite of similarity in names, the big America’s Cup remained reluctant to ‘similar contrivances’.

Naturally, the C-Class rigid wingsails could not have any impact on everyday sailing, as there was nothing in them that could have trickled down – except the very idea of aerodynamically better shaped sails.

In the 70’s, wingsails were introduced to a particular field of interest, challenge and expertise – sailing speed records. As these vessels sailed on one tack only, they had a luxury of designing ‘perfect’ wingsails either for port or starboard tack. As there was no need for these wingsails to be reefable or stowable, they were made rigid. In any case the venturers did not even think of trickling down technologies. They were well aware that a record could not be broken by old fashioned thin sails, and their only chance was in a superb sail. For a 'sailing game where seconds and millimetres count' a scientific approach and knowledge of wing sections were a must.

Here are some pictures of land yachts (Iron Duck, Windjet, Greenbird). Their wingsails’ slenderness, in aerodynamics called the aspect ratio, was something unusual to see on yachts.
Apparently, such wingsails were copies of wings for sailplanes/gliders, the most efficient flying machines ever.

 

Another feature of a high efficiency wingsail is its cross section (or wing section).

This is how it should look like approximately:

Pictured below is a nicely shaped wingsail for a Swedish sailing speed record attempt on water. Its maximum thickness and its position are clearly visible – obviously  these guys knew what the most important issue in a 'sailing game where seconds and millimetres count' is!

 

This is Vestas Sailrocket, the current speed sailing record holder; again, it is visible that its wingsail's maximum thickness is not at the mast, but some 30% aft.

No doubt a modern sailboat should be rigged with a wingsail, whose efficiency would provide more power (speed) and stability. Such a wingsail must not be rigid, but practical, reefable and stowable – in other words soft. It must also be asymmetrical on either tack, i.e. of reversible camber.
Sounds tricky? Not really…

A rigid wingsail was first used in the 27th America’s Cup in 1988, in a successful attempt to defend the Cup. It was designed by the same people who previously played with it in the Little America’s Cup. No wonder it definitely proved to be superior to thin sails, so it took its place again in 2010, for the 33rd America’s Cup. The mission was accomplished, the Cup was triumphantly won, and the rigid wingsail established itself as indispensable in the campaigns that followed.
Unfortunately, its propagators did not see the forest for the trees – or they did, but most probably for personal reasons proceeded along the ‘rigid’ track. The wing’s two vertical sections were symmetrical, which is, by definition, far from perfection.  They were connected by hinges, leaving a slot between them, which allegedly improved the wingsail’s efficiency. If it was so, such a slot would be introduced and visible on, for instance, gliders’ wings – but no flying wings use such a ‘contrivance’. Perhaps a rigid wingsail with a slot shows better performance than one without it, but it can only be attributed to their low aspect ratios.  The slot actually divides the wingsail into two wingsails.
In any case, such a wingsail proved to be much faster than a thin, cloth sail, and advanced to the 34th America’s Cup.
To prevent challengers of devising lighter, soft wingsails, Oracle Team USA set the AC72 Class Rule 10.12:  The weight of the wing in wing measurement condition shall be not less than 1325 kg, and the center of gravity shall be not less than 17.000 m above the wing base plane. Insane.
This way they limited its weight and the center of gravity from below - to keep it heavy!!!
The wing they produced was clumsy, complex, fragile, heavy and very expensive. Ironically, they said they wanted to limit expenses other contenders might have incurred in their design and production race for the AC34, but obviously gave advantage to their extremely wealthy team. The result? A series of capsizes, including pulverizing their own $10 million AC72 yacht in San Francisco Bay.

For the 35th America’s Cup, the defender stuck to the same philosophy, as lighter wingsails were not allowed. Team NZ outwitted the Oracle US team, won the Cup and hinted that this idiotism might finally end.

Indeed, soon after the triumph, ETNZ announced: “An underlying principle has been to provide affordable and sustainable technology ‘trickle down’ to other sailing classes and yachts. Whilst recent America's Cup multihulls have benefitted from the power and control of rigid wing sails, there has been no transfer of this technology to the rigs of other sailing classes.”
True, the Oracle’s ‘cost-saving technology’ went to history.
Artemis Racing, after a painful experience with a rigid wingsail, had a similar line of thought:
“The boat and race format must be seen as bringing the sport of sailing forward and inspiring young and future generations.”
On the other hand, the ETNZ design coordinator defended the new, stringent class rule: "In some ways, designers would love to work without the constraints of a rule, but in practice it's important to have a comprehensive set of constraints to keep costs under control; if no limits were set, the wealthiest teams could gain advantages by out-spending their competitors using rare materials, extremely complex systems, and countless iterations of components.” 
Have we heard the same rhetoric before?

An interesting statement from ETNZ followed:
"As with all America’s Cup class developments weight is always a big issue, but especially so with the AC75’s because of their self righting ability it is important to keep weight aloft to a minimum."
That was the first time that anybody addressed the problem of heavy wingsails after a decade of Oracle’s coercion. Sailors, racers, record-breakers, champions, hall_of_famers, famous designers, boatbuilders, sail makers, contenders (including ETNZ), sport comentators, investigative journalists – none of them have ever questioned either the AC72 Class Rule 10.12 or the AC50 Class Rule 12.10  that prevented ‘keeping weight aloft to a minimum’."
In the age of science and technology, such an important issue remained obscure. Unbelievable!

The ETNZ soft wingsail was pompously announced as ground breaking, state-of-the-art technology sailing rig, produced by design geniuses.

As said before, a double-luff sail was patented by L. Francis Herreshoff nearly a hundred years ago. Numerous other ‘double-surface’ designs include Princeton sail-wing, Wharram’s Tiki soft wingsail, etc ... so, what is ground-breaking here?

The new wingsail is certainly aerodynamically superior to conventional single-ply sail, but its section could not be found in any of numerous catalogues of tested and proven wing sections. ETNZ’s design geniuses apparently did not pay attention to things like the wing’s maximum thickness and its position.

If an aircraft had such wings, it would fly as an old fashioned hang-glider. In forty years, non-powered aircraft designers have nearly reached perfection – just think of sailplanes, hang-gliders, paragliders and kites. Sailing rig designers lag behind them by fifty years.

Sir Jim Ratcliffe, the backer of the INEOS Team UK, "believes there is plenty to benefit from cross-pollination and in a sailing game where seconds and millimetres count, he's determined to explore every avenue to see Britain finally win sport's oldest trophy".
A wingsail of the Selig 1210 section pictured high above would undoubtedly outperform Princeton-like double-luff, nearly one-design wingsails adopted by all the AC36 teams. It is science and cannot be disputed.  But which particular wing section would be the best choice for AC75 yachts? Unfortunately, it seems the AC36 design teams would be incapable of answering this fundamental question. Or they do not care about seconds and millimetres indeed ...

The following graphics were taken from the  America’s Cup official Youtube channel’s video (The AC75 | Designed to Fly), in which the head of ETNZ design and AC75 class rule team stated that both the wingsail and the foil wing have much the same cross-sectional shape as aircraft wings.
This is a possible AC75 foil wing cross section:

True, the foil (underwater) wing is made solid, therefore can be given any of ‘tested and proven wing sections’  of aircraft wings.
But what about the soft wingsail and the chief designer’s false statement about its cross-sectional shape?

This graphic they provided  is just an artist’s impression, as there is absolutely nothing to support the necessary curvature of the leeward (upper) skin.  In real life the leeward skin of such a poorly designed wingsail is nearly flat (as can be seen on videos), making the wing much thinner than it is claimed, with the maximum thickness just at the mast, as opposed to the true aircraft wing’s optimal thickness position at about 30% behind the leading edge.
So, this is how little Johnny imagines a reversible camber, asymmetrical, thick, aerodynamically shaped, soft, light wingsail of the future.
Many serious people, researchers, scientists and inventors, have been engineering such a wingsail.  And then comes little Johnny with his team of design geniuses, instantly imagining a ‘wingsail much the same as aircraft wing’, creating a proper class rule and imposing it to all the contenders – with or without their consent or even understanding.

Wingsails below are definitely soft and light, but they are better shaped (read more efficient), which is crucial in a 'sailing game where seconds and millimetres count'.
Proudly produced in our Wing Sails Co. modest shop!

 

Aerodynamics of Sails and Its Implication on Performance and Stability of Sailing Vessels


Upwind sails are devices for generating horizontal aerodynamic force. In aviation terms this force is known as the LIFT force and the name of the generator of the aerodynamic lift is the WING. All airfoils work on the same principle, called the Bernoulli principle. Essentially, the generated lift force is a result of different pressures on the lower and upper sides of a wing, or the windward and leeward sides of a sail. The Bernoulli principle (est. 1738) states that the pressure depends on the speed of a fluid - the lower the speed the higher the pressure, and vice versa. At low speeds, asymmetrical wings are much more efficient than symmetrical ones and are designed (by nature and then by humans) in such a way that their upper surface is cambered (airflow wise) and the lower surface relatively flat. When a wing is set under a small angle of attack, the airflow above the wing speeds up dramatically in relation to the flow below the wing, creating a suction zone at the upper side and a pressure zone at  the lower side. This pressure difference is what keeps birds and non-powered aircraft (paragliders, hang gliders, sailplanes) as well as subsonic and supersonic planes in the air. The same process develops when a sail is set under a proper angle of incidence to the wind. If the sail is vertical, the resulting force will be horizontal and not necessarily parallel to the vessel's centerline, depending on the point of sailing. The vessel's keel and rudder actually break the total aerodynamic force into its driving (parallel to the centerline) and heeling (perpendicular to it) components. The ratio between these components should obviously be as high as possible and this is what makes a sailboat design a piece of art.
Sails that are too small cannot produce sufficient driving force. Larger the sail, larger the drive – inevitably penalized with a stronger heeling moment.  This in turn necessitates wider hulls and heavier ballasts to make vessels more stable and safe, and their sailing rigs as vertical as possible to avoid dissipating of the aerodynamic force. Ironically, heavier hulls with more wetted surface are slower, need larger sails...and the story goes on.
Can we ‘reverse the spiral’?
Traditionally, the problem of sailboat stability has been treated at the level of hull, improving various (multi)hull shapes, keels, ballasts, outriggers. For keel-less boats, beach cats and light craft, the main stabilizing method has been the crew balancing.
But let us move from this low level and go higher, to the root of the problem - to sails again.
The above-mentioned Drive/Heel ratio (D/H) is variable and depends on the point of sailing. However, there is a characteristic value for each sail that clearly determines the D/H ratio at a given point of sailing, and it is called (borrowed from the aviation terminology again) the Lift/Drag ratio (L/D).
This ratio goes from 5:1 in modest sails to 10:1 in America's Cup sail rigs. Now comes the strange part: the L/D ratio in wings of some sailplanes goes up to 60:1! Why is this so?
Wings are designed according to the science of aerodynamics and numerous catalogues (NACA, Goetingen, RAF, Wortmann, Eppler, Quabeck, Clark ...) with hundreds of efficient wing sections, each of them described by the amount of camber, the thickness, the position of maximum thickness, the leading edge radius and the coefficients of lift and drag. Wings of nonpowered, low speed aircraft (could be compared to sailrigs) have very high aspect ratios (the ratio between the span and the average chord of the wing) of up to 30:1 that significantly reduce the induced drag.
On the contrary, a traditional single ply sailrig doesn't have any 'aerodynamic' thickness and its leading edge (mast, even a 'wing' mast) is disproportionally thicker than the foil itself, creating a lot of turbulence just behind it.
The position of maximum camber is much further aft from its leading edge, their aspect ratios being usually between 3:1 and 6:1. Such a sail generates much smaller aerodynamic lift force per unit of sail area than an 'ideal' sail and needs to be much larger than the ideal one in order to produce the same driving force (from now on I'll call an 'ideal' sail a wing sail because there is simply no such design that is better than a design of the wing). A larger sail in turn increases the friction drag component. Because it is single ply, it must be trimmed closer to the centerline (the angle of attack of the wind has to be bigger to avoid 'luffing') which increases the form drag. We can see that all of the aspects of drag are inevitably higher for a membrane sail than for a wing sail - so is their sum total. The aerodynamic drag is usually thought to be a nuisance that only decreases the speed of moving through a surrounding fluid, but for sailboats it is also responsible for the heeling moment (see diagrams 1 and 2).

This is something very important that somehow eludes most designers and sailors (including specialists from WUMTIA - Wolfson Unit for Marine Technologies and Industrial Aerodynamics in Southampton, UK), in spite of their customary familiarity with the name and work of C.A. (Tony) Marchaj, who in his "Sailing Theory and Practice" wrote:
"We can conclude immediately from either pair of these equations that the drag not only lowers the driving force FR, but also increases the harmful healing force FH."
Does increased heeling moment of  a sailing vessel decreases the vessel’s  stability? If two identical hulls were rigged with different sails of different aerodynamic properties, and sailed the same course in the same conditions, one heeled 20 degrees and another 40 degrees, which one is more probable to capsize or to get rolled by a breaking wave? Answers to these questions seem obvious, or at least should be, but even if one would dare to ask such a weird question, who could possibly give the answer? In spite of routine stability calculations including determination of the maximum heeling moment, the sail yacht stability is measured and expressed through its Range of Stability. It basically pertains to a capsized keelboat which is supposed to return to upright (eventually), if not heeled beyond certain angle. In fact, this ability has nothing to do with sail aerodynamics, but with the vessel’s buoyancy, ballast, center of gravity and righting arm. How is this kind of ‘stability’ measured for dinghies and beach cats? Perhaps by number of crew righting the boat?
The total aerodynamic force FT, as a vector, is determined by its magnitude and direction. Ideally, this direction would be the same as that of the vessel (even theoretically impossible, except at a certain broad reaching angle) - practically, the force is being rotated back toward the stern and the major factors that contribute to this are: the angle of attack at which the sail works, the positions of maximum camber and thickness, and the total drag. A wing sail is absolutely superior to a conventional sail in all of these points and its total force direction is much closer to the bow.
Some experts claim that the driving force FD of a wingsail is twice the larger than that of a conventional sail of the same size, with twice the smaller heeling force FH. Essentially, the 33rd America's Cup has proved far superior wingsail performance, and made history.
From now on, the AC design teams will continue to "toil over hot computers to get that extra hundredth of a knot", but this time on new multihull platforms rigged with wingsails (AC45 and AC72).
One might rightfully ask why regular boats and yachts would simply not be rigged with  wings instead of Bermudian sails, if they are that superior?
There are at least two reasons that make this difficult:
First, one can always tell which side of a wing is the upper side and which is the lower side (except for some special purpose symmetrical wings). Contrary to this, sailboats tack and gybe, receiving wind from either the port or the starboard side. That means that a wing sail has to be adjustable (i.e. flexible) in order to provide an optimal, asymmetrical aerodynamic shape on either tack.
Secondly, a wing sail has to be light, much lighter than aircraft wings. Increasing the weight aloft can easily annul the aerodynamic superiority of a wing sail, or even render it unusable. For instance, the U.S. patent No.3, 332, 383 mechanically solved variability of a wing sail camber, but such a sail would probably weigh hundreds of kilos (the BMW Oracle's wing weighs 3.5 tons!) and a ripple would be enough to flip it over.
The search for a soft, light, simple and foldable wing sail has not been fruitful until recently. Now there are a couple of designs that fulfill the requirements mentioned above. Between the two sailcloth panels there is either a lateral or a longitudinal light structure that maintains the thickness of the sail while giving it a near optimum asymmetrical aerodynamic shape. Details about technical/structural solutions for these sails would go beyond the scope of this article.
 
Now a few words on downwind sailing:
If traditional upwind sails are conventional, traditional downwind sails are pre-conventional. That is how sailing started and how upwind sails were derived from downwind sails. Downwind sails as we know them work on a different, much simpler principle than the Bernoulli’s one. The resistance of a large sail area spread out before the wind (basically the form drag) is much higher than the resistance between a hull underneath and water. The end result is running before the wind.
There is an interesting development here. An airfoil under a proper angle of attack to the wind (10 - 20 degrees, Bernoulli effect) will produce an aerodynamic force roughly twice the resistance force produced when the same airfoil is placed at a right angle to the wind (conventional downwind sailing). Why isn’t this effect utilized in downwind sailing? First, rigging wires (stays) don’t allow booms to travel beyond 90 degrees - at this point the boom would hit a stay.
Secondly, long and heavy booms, if they could pass the beam point when they rotate toward the bow (like freestanding 360 degrees rotational masts), could be difficult to haul back. Freestanding (unstayed) masts have been a reality for some time, thanks to technological advancements (the Team Philips catamaran, the British entry into The Millennium Race, had two freestanding masts over 40m high!). Combined with wingsails, they bring sailing to a quite amazing perspective.
Wingsails do not care which direction the wind is blowing from. When in neutral position, a wingsail will feather. When set under a proper angle of attack, it will start producing an aerodynamic force (see diagram 3).

It is only up to a sailor to use this force for propelling the sailing vessel in a preferred direction. The only difference between upwind and downwind sailing from this standpoint is that the aerodynamic drag slows down upwind sailing and speeds up downwind sailing - yet the sailor might easily be unaware of this effect. Even tacking and 'gybing' are substantially identical operations.
Stayed rigs on large boats and yachts still have some merits, but light craft are actually much easier to rig with freestanding rigs, both technically and practically, either on shore or on the water.
Happy Sailing!