Anyone familiar with Powerfoil sails?

What do you guys think of the powerfoil sail concept? Anyone familiar with something similar?

http://www.powerfoil.com/works.htm

Quote: Conventional sails generate their aerofoil shape, by pre-designed seam shaping (Camber) and create a thick, turbulent layer of air on the windward surface, due to the presence of the mast. This layer of turbulence forms a cushion to the airflow, creating a "virtual" surface on the windward side. Unfortunately this turbulence spills away downstream and creates large amounts of drag, limiting the speed and efficiency of the sail.

Powerfoil has a unique, pre-designed double surface aerofoil, which actively changes its shape in response to the airflow, always adopting the most efficient aerofoil shape for the conditions it encounters. This autoshaping system is the heart of the Powerfoil sail. Because there is no mast turbulence, the airflow is always smooth and free of mast induced drag.

The sail envelope incorporates an open, self draining leech and foot, making Powerfoil easier to up haul and waterstart than a regular sail. The autoshaping system prevents the sail from deforming near the leech, damping out sheeting forces and giving superb balance and light feel on the water.

The Powerfoil twin clew outhaul permits very precise twist control,(without needing a loose leech) by allowing the twin surfaces to set independently of each other. Twist response may be adjusted by simply varying the outhaul settings. Aerofoil camber can also be increased or decreased in the same way, making possible a very large range of camber settings, for many different wind speeds. Because the twin battens act together near the leech, they form a rigid "flap" like those on aircraft wings. This gives the sail more lift and stability at lower speeds, and improves accelleration when starting or coming out of turns. Inter-batten connectors within the sail, impart a pre-set flap angle, which makes the sail rotate as it changes tack - just like a cam equipped sail, but with the smooth action of a rotational.

 

Think these are similar twin skinned system

http://www.wingsails.com/

and


http://www.dynawing.com/index.html

Hell to put a reef in though

 

BTW welcome to the forums!

 

I think it has potential. On larger yachts I'd like to see it tried in combination with a Carbospars Aerorig or similar (if the sail can be raised/lowered/reefed).

Sails that wrap around the mast or have sleeves (like the Laser) without full battens have not proved as effective as had been hoped because the leeward side ends up pulling too flat just aft of the mast. Because of this and the fact that mainsail roaches put battens in compression, and that force must be absorbed somewhere if the mainsail is roachy, rotating wing masts are currently the preferred solution for multihulls.

 

I met the inventor of the Powerfoil last year and was impressed with his ideas. Independent reports of the windsurf sail are good.

We are in the process of building a 30 foot yacht on which we intend to fit a prototype double-sided wing sail mounted on a swing boom (as per the Aerorig). The intention is to produce a rig that is as cheap to buy as a conventional bermudan rig but easier to reef and hand and with improved performance on all points of sail, especially upwind and running before the wind.

We are on the hunt for funding to accelerate the development of the prototype so if anyone out there is interested in finding out more with a view to becoming involved please get in touch.

Malcolm Henry - mh@qei.co.uk

 

Double Surface Sails

I think the Powerfoil sounds like a good idea, and I believe the Dynafoil folks are finally headed in the right direction. But I'm not sure that the explanations they give are all that accurate.

The typical argument goes something like this, "Aicraft wings are more efficient than sails. Aircraft airfoils are thick, double surface sections with modest camber. If a sail has thickness and a double surface it will be better than a single surface sail." The problem with this is that it doesn't take into account the different requirements for wings and sails. Plus, aerodynamic tests and calculations invariably show that for any given design point, a thinner airfoil will outperform a thick one. So why use a double surface sail?

I can only think of three reasons to have a thick foil:
1) The depth is necessary for structural stiffness,
2) A sharp leading edge loses out on leading edge suction, which results in an increase in drag,
3) The airfoil has to operate over a wide range of angles of attack.
Other than these factors, a thick section will generally have more drag and less maximum lift than a thin section.

An aircraft has to operate at a very low angle of attack in cruise because it can't change its wing area, the lift has to equal the weight, and the only way match the weight at high speed is to use a low lift coefficient. It's the optimization for cruise that drives the airfoils to a semi-symmetrical shape. However, for landing, airplane designers do everything they can to change to a highly cambered shape to produce high lift. And when the wing is configured for high lift, drag is actually helpful because it makes the landing glidepath steeper.

The requirements for a sail are quite different. The sail needs to produce high lift with as little drag as possible. The sail doesn't need to operate at low angles of attack because if you are that overpowered, you'll reduce the area by reefing. It's much better to use a smaller area near its maximum lift than a large area that's lightly loaded, because the smaller area has less parasite drag (less wetted surface). The sail does have to perform well for a certain range below max lift, but that range is nowhere near as great for a sail as for an aircraft.

The ideal shape for high lift would be a thin, highly cambered airfoil. This is provided very well by traditional single surface sails. Such a sail has a narrow groove, however. If the sail is operated at too high an angle of attack, the flow separates on the lee side and the sail stalls. If the sail is operated at too low an angle of attack, the flow will separate on the windward side, just behind the leading edge. This causes drag.

http://www.tspeer.com/temp/wm10m35r10a08.JPG shows the pressure distribution and boundary layer displacement thickness around a wingmast/sail airfoil, as calculated by XFOIL. The angle of attack is 8 degrees, and the section is close to lee side separation (stall onset). The mast and sail shape were created by taking the lee side contour of the Clark Y airfoil and reflecting the leading edge to form a teardrop wingmast comprising 10% of the chord. The white dotted line shows what the pressures would be if there was no boundary layer. The yellow line shows the pressure on the lee side and the blue line shows the pressure on the windward side. The difference between the blue and yellow lines is the normal force on the sail You can see similar flow features in wind tunnel test pictures such as http://www.wb-sails.fi/news/Stallpics.HTML

The pressure on the windward side of the wingmast would have a steep increase approaching the mast-sail junction. But the flow separates there, as you can see by the flat constant pressure segment and the way the boundary layer stops following the contour of the mast. But the flow reattaches to the sail at about 20% chord, as seen by the pressure returning to the inviscid pressure distribution and the boundary layer getting thin again. In between the separation and reattachment points, the flow recirculates forming a separation bubble. Essentially, the flow bypasses the mast-sail junction. But there's drag associated with the separation bubble.

This drag could be reduced if the sail had a pocket luff that was about twice the size of the mast. Then the sail could take up the shape of the blue contour. The lift on the sail wouldn't be affected because the flow outside the boundary layer would have the same shape. But you wouldn't have the losses due to the separation bubble.

If the double surface were extended much farther aft, that would raise the velocities on the windward side and reduce the lift, just as it does nar the mast-sail junction. The lee side pressures are assumed to stay the same, because the lee side pressure distribution is already stressing the boundry layer there as hard as it can without stalling. Cutting the Reynolds number by 70% (light wind condition) doesn't enlarge the size of the separated region very much (http://www.tspeer.com/temp/wm10m35r05a08.JPG). So some double surface is useful, but not a completely double surfaced section.

http://www.tspeer.com/temp/wm10m35r10a04.JPG shows the same section at 4 degrees angle of attack. The same general picture holds for the windward side pressure and boundary layer development. But now the reattachment point at the end of the separation bubble is closer to 30% chord. So a bit bigger double luff region would be in order here. Such a shape wouldn't reduce the maximum lift very much, and it would widen the sail's groove. And a flexible double luff might actually provide the ability to adjust this somewhat, especially if tied to mast rotation.

Although the leading edge of this mast is fairly rounded, it isn't circular. It's still important for the sail to have a leading edge whose shape is dictated by the required lee side pressure distribution. The change in curvature between the round mast and the luff pocket will lead to a spike in the lee side pressure distribution that will promote early separation and stall, as well as a thicker boundary layer for higher skin friction drag. That's why the partial ribs forward of the mast found in some of the double luff concepts are important. Another way to get the same effect would be to have an egg-shaped or D-shaped mast inside the luff pocket.

Finally, for another application of a partial double surface sail, take a look at Spitfire's rig (http://marine.bdg.com.au/spitfire_gallery.html). It was designed by an aeronautical engineer well versed in airfoil design, and sports a pocket that extends less than half way along the chord.

So I think we're seeing a convergence on a sail shape that has a properly formed leading edge, smooth lee side contour, a double luff for 20% to 40% of the chord, and a single surface for the remainder of the mast-sail section. And the purpose of having a double luff is to cut the drag asociated with the windward side separation bubble.

 

Double Surface Sails

I used to own a boat with a junk rig designed by Blondie Hasler, Batwing. I cruised all across the Pacific in that boat. I have been thinking about adapting the junk rig to an aerodynamic rig. For those not familiar with the junk rig, a junk sail is fully battend, is balanced (ie. part of the sail extends foreward of the mast,) and is sheeted to the after end of each batten. It is famed for ease of handling, as it is self-tacking, and is very easy to reef.

After reading and posting quite a bit to the newsgroup at groups.yahoo.com/group/junkrig/ I realized that this would be possible using a new development in the junk rig, the hinge. The hinge is one way to put camber into the junk sail. (Blondie Hasler's sails lacked camber.)

A few months ago, I came upon Tom Speer's website, and was struck by his drawings of wing mast, cloth sail combinations. The version in which the wing mast is equal in length to the cloth sail could be viewed as an overview of a junk rig with a mast sleve, and a hinge half way aft of the luff. The mast outline could be made as a teardrop shape batten section, and the after (almost) straight section could be a conventional stiff junk batten. The two batten parts would be connected by a flexible hinge.

Other posters on the junk rig newsgroup informed me that this idea had been developed and marketed as the "Swing-Wing" rig.
With some difficulty, I found a drawing of the swing wing rig. It was indeed the idea I had come up with, but it did not look as though any knowledge of aerodynamics had gone into its design.
The sleeve was sharply pointed at the luff, and the hinges were well aft of the center of the battens.

I am intrigued by the idea of using Mr. Speer's ideal mast/sail combo as the pattern for developing an aerodynamic rig that is as seaworthy as the junk rig. (Which, in my experience is more seaworthy than the bermuda rig.)

I am not sure how efficient it would be, as in a cruising monohull, you would be constrained to a fairly low aspect ratio, about, 2 to 1, which is a much lower aspect ratio than Mr. Speer visualized.

Tim Dunn
steelsil@cedarcomm.com
www.steelsilhouettes.com

 

I also spoke to the people at powerfoil late last year about using thier sails on a beach cat. They told me that they had no intention of making non windsurfing sails as they did not beleive that there was a market for them, they explained that in thier experience sailors wanted simple cheap and durable rigs.

Tom,
Having just looked at your web site I am interested in your twin rig A-frame concept.
http://www.tspeer.com/landyachts/twin/Concept.html

It is something I have been thinking about myself recently.
www.fourhulls.com

Is it also applicable to sailing catamarans?, could the wing be substitued for a more conventional sail, and what happens near the top of the mast when the sails/wings touch?

All the best

Gareth

 

In principle, the A-frame rig could be done with soft sails. However, I don't think that is a good idea for several reasons:

- The concept depends on being able to feather the leeward surface. A soft sail will luff. This means one cannot really reduce the leeward surface's lift very much, defeating the whole purpose of the A-frame rig - reducing the effective moment arm by transfering the load to the windward surface.

- Down force on a landyacht is beneficial because the sideforce is limited by the tires' traction. Down force on a watercraft just adds more drag.

- The lee surface is heavily loaded in compression, plus it has a side load. The loads can be far higher than on a conventional mast because stability of the A-frame yacht is higher. In fact, if one backwinds the lee surface, the loads increase without bound as the apparent wind speed builds. A wing has more cross sectional area than does a mast + sail, so it can be built stiffer than the mast.

A better approach for catamarans would be to move the mast step to leeward and cant the sail to windward. I believe Chris White has a patent on this concept.

 

Tom,

I think the concept has merit beyond that of a land yacht. If you look at it as a biplane rig with the top of the masts joined you get a big structural advantage. The amount of load the masts can carry (A frame compared to unstayed biplane) increases dramatically and the way that load is transmitted through the boat is also better (No torsion and smaller compressive loads).

If both sails are working equally (i.e. don't feather the leeward sail) would you get anymore downforce? In fact isn't it benificial as the windward sail generates downforce and the leeward sail generates lift counteracting the boats righting moment.

However I am not an aerodynamicist and my concern is how well the sails perform when they are nearly touching at the top of the rig. This is is the aspect I am looking for comment on.

All the best

Gareth

 

Yes, this was a significant player for the landyacht configuration, too. Instead of having the axle and body be highly loaded in bending, stays fore and aft from the apex would suspend the body like a hammock and the only purpose of the axle is to hold the wheels apart.

However, compared to a typical mast, there are no supports to keep the leeward wing in column. Diamond stays could be added but would significantly increase the parasite drag.

Compared to the unstayed biplane rig, the biplane does not carry any compression loads (outside of sheet and halyard tension), so column buckling is not a factor. Bending loads on the A-frame panel are less than the biplane rig because it is simply supported at both ends instead of cantilevered from one end. But when you add in the required stiffness to overcome buckling, it's not obvious to me which one will actually be lighter. I've not done any real structural estimates for the A-frame, so I don't have an answer to this yet.

The vertical forces are small when you trim the wings together - see http://www.tspeer.com/landyachts/twin/dragstudy2.gif. (I can't explain why the single wing prediction also had a vertical component, but the net twin predictions are of the same order of magnitude)

Most of the heeling moment relief comes from the windward panel and its displacement to windward from the centerline. Taking moments about the center of gravity, the windward and leeward panels are both reducing the heeling moment, as you point out. However, taking moments about the lee hull, the heeling moment of the lee panel is essentially the same as if it were vertical and mounted on centerline. The moment arm for the windward panel's vertical force is doubled, however, so you get the same benefit regardless of where you take the moments.

However, it also shows that the heeling moment relief could be obtained on a single sail rig by tilting the rig to leeward. Compared to the windward panel of the A-frame yacht, the tilted single sail would have twice the downforce at half the moment arm when tilted to the same angle. And this is a common practice on landyachts. In high winds, we sail with the stays slacked off such that the mast tilts 10 - 15 degrees, and flops over to the other side when tacking.

That's why I published the data on my website - to make them available to people considering similar designs. The predictions on my website were done using a 3D panel code (CMARC). I used an NACA 0012 section without flaps, as I was primarily interested in the interference effects and induced drag as a function of lift on the two wings.

At the tip, the wings were close but not touching. The baseline joint design was assumed to be a rod extending from each wing panel to a ball joint. The length of the rods was such that the wings could turn 90 degrees in the same direction without physical interference (6% - 12% of the panel length), so the wing tips were approximately one tip chord apart. The rods themselves were not modeled in the aerodynamic predictions.

All other interference aspects of the two wing tips were handled by the 3D geometry of the model. At the base, the wings were assumed to be isolated (no body) with a gap between the base chord and the ground. A reflection plane was used to account for ground effects.

Some of the wing configurations are depicted in this illustration: http://www.tspeer.com/landyachts/twin/yacht.jpg. The wing span, apex angle, taper ratio, and position of maximum chord were systematically varied while keeping the wing area constant. Both single taper and double taper planforms were included. The double taper planforms had straight trailing edges and all taper was applied to the leading edges. The double taper planforms varied the tip and base chords such that leading edge sweep angle was the same above and below the maximum chord station. This ensured that one mold could be used to produce all of the wing skins.

When trimmed together, the A-frame produced a higher drive component than the single wing of the same height(http://www.tspeer.com/landyachts/twin/dragstudy3.gif). As you'd expect the A-frame produced substantially more drive for the same heeling moment (http://www.tspeer.com/landyachts/twin/dragstudy5.gif) compared to a single rig of the same span.

It is essential to compare the two rigs on the basis of the same span, not the fineness ratio of the panels. If the single wing's area is divided into two similarly shaped panels, the span will be 30% less and any performance gains from having twin wings will be lost.

I believe these results will be sufficiently accurate for preliminary design of comparable configurations, including wings with flaps and the interference effects of soft sails. I've also fitted an empirical formula (based in part on the Prandtl biplane relationship) to the data that accounts for all of the geometric relationships tested. The formula is useful for conceptual design and initial sizing studies.

 

I'm surprised that the PowerFoil folks didn't seem interested in non-sailboard rigs--maybe it's more a reflection of their interests and production facilities. I'd have thought that a twin surface sail would be easier to design and handle, and be more effective on a fixed-rig vessel.

Back in the late 80's, when windsurf speed events were very popular (and men were men, etc.) a few of us in Weymouth played with twin-surface windsurf rigs. They were reasonably successful in straight lines and *when upright*, but they were such a problem to waterstart when the void got water in it--we tried soft foam inserts, etc., but... Also, a windsurf rig is being constantly tweaked, flexed and pumped, so the shape is constantly being 'spoiled'. Extreme experiments included very high aspect rigs, and even fitting tip fences, but these were next to useless. The popular solution at the time was to put a huge luff sleeve containing camber inducers onto the sail, but, again, they filled with water and were hard to manage. One of the key benefits of these sails, however, was the very stable shape, which was maintained even in gusts, so you could maintain and increase drive rather than getting 'back-winded' when the CoE rushed aft.

 

After building and sailing a crude prototype, http://www.boatdesign.net/forums/boat-design/experimental-sail-needs-analysis-24894.html based on principles best described here by Tom Speer's "Double Surface Sails", I have evermore questions. The 1976 Princeton Report's comparative study shows superior L/D for full double skin over partial sleeve. All sections, intended for aircraft, have thick leading edge spars, tensioned leech cables, no battens and, unfortunately, no specs for fabric used; AR 8.4, t 11.5, est. Reynold's No. 750,000. My rig has mast taper 21/2" to 11/2", chord taper 7' to 4' on 16' luff. Does report apply to such a thin section sailing and what would be best leading edge form? Full double skin would solve slight discontinuity at double to single skin joint on lee sde in light air, and if report's L/Ds are real and applicable, well worth the weight. I would appreciate any feedback for next sail.