Rotating Wing Mast – theoretical discussion

[Man Overboard]

The principle subject of this discussion is rotating masts and the effect of the pressure distribution on their supported sail, including a discussion of the positive and negative aspects of round tapered masts verses wing shaped masts. Three figures are attached for reference. They are taken from Sailing Theory and Practice by C.A. Marchaj-published by Dodd, Mead and Co.; also referenced is Aero-Hydrodynamics of Sailing by the same author and publisher.
Fixed round masts (and elliptical masts) are intrinsically inefficient at most angles of apparent wind; do to the interference of the mast on the pressure distribution on the lea side of the sail. Marchaj illustrates this effect in figure 36. Several designers have mitigated this inefficiency by developing rotating masts. There are several advantages to rotating masts, some of which are discussed on Eric Sponberg’s website http://www.sponbergyachtdesign.com/StateoftheArt.htm I am not sure I understand the rational behind masts that are actually the shape of a wing at least when you consider the following: Marchaj illustrates in figure 67 test results of the effects of profiled masts compared to round masts. There is about a 30% increase in the aerodynamic force of the wing profiled mast; not bad. Except test on round mast that have been rotated such to eliminate the negative pressure on the lee side of the sail (see model C of figure 61) show 28 % improvement in lift over traditional fixed mast (see model D of figure 61) with a substantial decrease in drag, about the same as a winged mast section. Winged masts are about 2 times as long as they are wide, sometimes more. This places the track of the sail farther from the center of the mast which increases torsion loads on the mast. Lift generated by the sail produces a force at angles perpendicular to the cord, which for a wing shaped mast is the weaker section of the mast. It seams that a more efficient use of Carbon fiber would be to build a round rotating mast.
I have not considered the leading edge effects of round verse winged mast sections. Comments would be appreciated as well as info on studies that have been undertaken on this subject.

[national]

A round rotating mast would still suffer from large seperation whatever rotation which in my opinion is more of an issue than building a mast that is capable of taking the forces. You can make a mast stonger but you can;t make a circular mast have less effetcs on the leeward pressure or reduce it's drag.
I believe that in c the mast as well as being rotated also has significant tracks, making it a step towards what a wing mast does.

The introduction of a the mast increases the likelyhood of large seperation on the leeward side, the greater this disturbance the more likely the bubble will engrouse the leeward surface.

Flow past a rotated circular mast. http://www.jonpaton.com/images/t_-70_vcs_2.png

[DGreenwood]

The mast on Ocean Planet and (I think) on the Wylie cats are round rotating masts. Bruce Schwab sailed this rig twice arond the world. They were designed by Tom Wylie and built by Ted Van Deusen. I think Ted could contribute much to the topic.
I think Bruce would tell you that, although the round mast was successful, the advantages of the wing mast pay off.

[Man Overboard]

I noticed in Nationals post, the sail is still behind the mast, even though it appears to be rotated; where as in model C of figure 61 the sail would have to be wrapped to some degree around the mast. This produces a true aerodynamic leading edge. Wrapping the sail around the mast raises some problems. On a round tapered mast it would not really be practical. You can’t smoothly wrap cloth around a tapered pole. You would have to turn the mast to raise or lower the sail, and there is certainly more potential for chaffing of the sail, even if designed for it. Even so it seams to me that most wing mast that I have seen are much more elongated than necessary. The mast only has to be teardrop shaped enough to get the sail track a sufficient amount from center to allow it to rotate without wrapping the sail up. The sail track of course would have to be parallel with the centerline of the span. This would make the leading edge tapered; wide at the bottom, narrow at the top. The mast wouldn’t be round, but it would not have a 2:1 or 3:1 thickness to chord ratio either. For the same amount of Kevlar used, the mast could be made stronger perpendicular to the chord.

[Eric Sponberg]

Leading edge shape is very important, and round is bad, elliptical or modified parabola is very good. Also, on smaller masts where the whole wing section is all carbon, engineering is very easy because the moment of inertia and section modulus of an elliptical section can be defined by simple equations, similar to that of a circle. You need these values to determine wall thickness, and therefore, the laminate schedule at every foot (300 mm) along the mast. If you have an aerofoil section for the mast, you may get marginally better (or worse, if you design it poorly) aerodynamic characteristics overall, but the engineering for exact wall thickness is very much harder to do. It involves mathematical integration of the wing shape at every station along the mast, and that gets very complicated and time consuming. That's why I like elliptical wing sections--aerodynamically effective much more so than round, and very easy to engineer.

On larger masts, such as for Project Amazon and Wobegone Daze, both of which had masts built by Ted Van Dusen, we incorporated a central carbon fiber spar onto which were bonded fiberglass leading and trailing edge shapes. On Project Amazon, the structural section was nearly the shape of a square box, and on Wobegone Daze the structural section was round. The purpose of this type of construction was to better balance the sideways and fore-aft bending, making it more uniform, if you will, and less likely to trip sideways. Since the carbon fiber is very stiff and likes to carry the load, and the fiberglass is very stretchy and doesn't like to carry the load, we get the best of all worlds. The mast responds to the strength and stiffness of the carbon and the fiberglass just goes along for the ride. We don't use Kevlar because it is stiffer than glass and it has really poor compression strength and modulus and so is worthless on the trailing edge side. It could be used on the leading edge side, but being stiffer, it is going to want to take more of the load than fiberglass would, and that has to be accounted for in its thickness. And too, by adding Kevlar to an already carbon and glass mix necessarily adds a third material which has little benefit. Better to stick with just carbon and glass.

It is useless to wrap a sail around a round spar. There is a lot of friction between the sail and the mast. You can't put any fittings on the mast as a result. You still have a lousy leading edge shape, and the sail just never sets right.

I have made many of my masts with an entasis taper, both at the leading edge and at the trailing edge. This taper is fatter up higher than a straight taper. Project Amazon's mast were built this way. The sail track followed the taper which, by necessity to the shape and taper, was not parallel to the mast axis. We had no trouble at all setting the sail, reefing it, or trimming it. Wobegone Daze's masts were double tapered--straight in the lower end, and straight tapered on the upper 3/4s or so. Again, the track followed the trailing edge, not the mast rotation centerline. The sails set beautifully.

Eric

[gggGuest]

I pay some attention to small boat rotating masts, I know nothing about big ones. In dinghies they have rarely been successful, and the only ones that appear to have been successful have developed from Bethwaite's work.

Bethwaite designed a mast with a roughly hyperbolic forward section and a square back which worked well and won Championships in at least two classes. One of those classes has abandoned them in favour of pole masts, whilst the other has moved to a section which could be described as a rounded off diamond or alternatively two hyperbolic sections back to back. Bethwaite's book talks a lot about the development of the square back masts, but not about whathas happened since. I wonder what might be in the new book?

The major challenges with wing masts on dinghies seem to be weight and flex/gust response dynamics. The excellent dynamics on modern pole masts, combined with light weight seem to make up for a whole lot of aerodynamic inefficiency. Wind sheer is another consideration too I believe. Once you move up the scale to bigger and heavier boats with more roll inertia it appears to me that the aerodynamics become more important and the mechanical dynamics less so. I also suspect that weight becomes less of an issue because the effective cross section of a wing mast that will stand up and work becomes closer to that of a pole mast that will stand up, but I really don't know much about rigs on such craft, and this is no more than supposition.

[Timothy]

Eric Sponberg stated that "It is useless to wrap a sail around a round spar".Is it then the case that sails with luff pockets on a round spar are not efective or if indeed they are , would they perform better on an eliptical rotating mast? Tim

[CT 249]

I'm strictly a layman, but I've never been able to ally Marchaj's chart with what wins in the fastest classes. Maybe the problem is that Marchaj seems to have used massive and disproportionately large masts in his tests?

Luff pockets on round spars on windsurfers hold the world sailing speed record, and have for most of the past 15 years or so. They replaced wing masts in the role. Luff pockets on round spars are also used on foiler Moths, the most efficient of all sailing dinghies. If they are slow, they seem to be a very fast way to go slow.

The type with a round spar and the sail wrapped around the lee side of the mast (Marchaj's Type C) is a RAF type sail (Rotating Assymetric Foil) in windsurfing parlance. They are used for wavesails, not racing, because sails with large luff pockets are faster and therefore dominate the racing scene. This is (I think) universally accepted in a part of the sport that churns out probably 250,000 sails per year and therefore has vastly more sailmakers, team sailors and users than any other floating testbed.

There are other factors that affect the choice; the RAF type doesn't get a luff pocket full of water and is easier to handle in some repects, and the big-pocket sails can use a better batten setup. But even before the better batten system (CIs) arrived, most of the time the big-pocket luff sails were the fastest of all. Wing masts never really worked apart from one event, although a Lock Crowther one we used on boards was interesting.

I don't know what sort of performance increase a 28-20% improvement in aerodynamics with a Type C/RAF or wing mast would make. However, I do know that when you take the rotation out of something like a Tasar wingmast, you drop back through the fleet, but not dramatically. It's not like the sort of difference you get when you go from say a big-rig Laser to a Laser Radial, which has 20% less sail. In strong winds, Frank Bethwaite used to lock his rotation on centreline. Some top sailors of wing-masted cats do the same thing in strong winds, because that gives them superior gust response and gust response is that really matters to them.

If taking a wingmast out of rotation only provides a minor performance drop, to my mind that's an indication that the real-world difference between the clean and dirty leading edge may not be enormous. However, this is just a dirtbag's view.

ggggGGGG, Frank says he doesn't look at NS14s any more and doesn't think they've improved at all since he dropped out of them (!). In fact the modern NS is minutes faster around the course than the Tasar which was at first faster than the NS. So I don't think there's much about the modern NS rig or wingmasts in the new book.

[gggGuest]

Eric Sponberg stated that "It is useless to wrap a sail around a round spar".Is it then the case that sails with luff pockets on a round spar are not efective or if indeed they are , would they perform better on an eliptical rotating mast?

I think Eric's statement needs qualifying. There are very many thousands of sails wrapped around round spars in the world that work and set just fine, thank you very much, from Lasers and Toppers and so on which just do it for simplicity on an unstayed mast right through to the highly sophisticated camber induced sails on boards and indeed the vast majority of recent Moths. And indeed the Moths do have fittings on the mast, albeit few, and just have a slot in the sleeve to accomodate them.

But yes, the leading shape is no better (and no worse) than a standard pole mast. Naturally none of this is going to work very easily on a 30m mast on a round the world racer, but at the smaller scales its all very routine.

Its also worth noting that, like the bigger masts Eric talks about, Bethwaite's best wing masts separated the structure from the shape by comprising a roughly rectangular structural woden section at the back with a light balsa wood aerodynamic fairing on the leading edge. This enabled far better bend characteristics than are possible with a monolithic mast. The Tasar mast had the bend charactersitics changed significantly to enable a mass produced alloy spar (source Frank Bethwaite post on Tasar mailing list) and I personally believe were never as good for that reason. In the small scale pole masts have superior weight, superior bend characteristics, are less delicate (avoiding damage in transit to the balsa fairing is a major irritation!) and are cheaper. Its not hard to understand why wing masts have never become numerous. Someday I hope someone will figure out a way to bring a wing mast close to the pole masts on weight, cost, dynamics and practicality and then it will be time to look at them again.

Julian Bethwaite has been heard to speculate (but AFAIK has never tried one) that you could sew a foam padded sleeve onto the front of the sleeve of a standard camber induced type sail in order to give it a hyperbolic type section pointing into the apparent wind to reduce the separation problems.

[Eric Sponberg]

I will modify my opinion on round masts with wrap around pocketed sails. My thinking applies to larger sailboat masts where the diameter of the mast is very significant. On a smaller boat or a windsurfer, the mast diameter is so small that the front of the sail comes closer to a sharp leading edge rather than a blunt object. Indeed, small boats and windsurfers are exceedingly fast. And they do gain advantage by playing with mast bend which is easy to control when the mast is round. Now, let's take this one step further--Would small boats and windsurfers be even faster if they had elliptically shaped masts? No doubt some enthusiasts unbeknowndst to me may have tried it, and it would be interesting to see side by side results of, say, a round masted windsurfer sailed against an elliptically masted windsurfer.

My point about round masts being useless was directed to the leading edge shape being round--not good aerodynamics. Generally, on larger boats with large round masts and sleeved sails, no matter what point of sail you are on, the leading edge of the mast and sail is still round, and round is very unforgiving to the airflow which promotes early flow separation at the mast. The sleeve is difficult to set an keep tight. It makes the sail harder to make and more expensive. For these reasons, and for the ease of engineering, I very much favor an elliptical leading edge with a matching trailing edge on a rotating spar.

Eric

[national]

Has anything been published on mast shape and rotation etc? I have bethwaites book but was wondering if anything had been written in any more detail?

[CT 249]

"Would small boats and windsurfers be even faster if they had elliptically shaped masts? No doubt some enthusiasts unbeknowndst to me may have tried it, and it would be interesting to see side by side results of, say, a round masted windsurfer sailed against an elliptically masted windsurfer."

In about '84, Maui Sails and Fred Haywood broke the world speed record with a wing mast on a board. World champ Ken Winner tried a wing-masted raceboard. Many other people did the same; we (friends and I) built 1 1/2 wings designed by the chief draughtsman for top multi designer Lock Crowther. Lock was fully aware of the project, and these guys had done a lot of wing masts and at least one rigid-wing rig. But the RAF and the Camber Induced sails proved superior. The wings were quick at times, but too "twitchy", perhaps due to lack of twist. The weight and handling problems were not worth an occasional possible burst of speed. Perhaps the same applies to wrap-around luffs in big yachts; it can be hard enough to get the mast to slide along a windsurfer luff, I'd hate to try to reef a high-tension wraparound luff yacht main.

Mast fairings have been tried in boards and International Canoes.

Wing masts have also been tried in;

NS14 and MG 14 class dinghies, where they are almost universal. With just 100 sq ft of sail but very free rules, these classes are perfect for wing masts.

Suicide/125 sq ft development class. Herreschoff's Dragonfly c 1930s.

International Canoes - gust response was the problem. There was also Bee McKinnnon solid wing rig.

R Class Dinghy/skiffs (NZ). Peter Mander (Olympic gold medallist, 18 Foot Skiff champ) in the '50s; McIntoshes (multiple NZ champs) in the '80s; Axel Vallings (multiple skiff/R Champ) in the '00s. The R guys are very good, at times this has been clearly the world's most advanced dinghy class, yet they have never been able to extract consistent performance worth the problems.

18 foot Skiffs. Intrigue (50s), Colourbond ('80s).

Moths. Pat Smith's solid wing ('50s), famed radar pioneer and radioastronomer Paddy Bowen (not a wing mast, but a large luff pocket like a board) in the '60s, luff pockets returning in the '90s.

Tasars

Merlin Rockets

Most of my own rigs have wing masts or extended luff pockets, so I'm not biased against them. However, it seems that apart from a class with a small rig (NS14) the problems with gust response, weight, flexibility etc mean that wing masts are not normally all that effective. Some of the smartest guys in small boats have tried these rigs, no one has got them to really work all-round. While real life is certainly not an ideal scientific experiment, surely consistent failure to achieve the theoretical results must mean something.

Gust response is of course not such a problem with cats. I'd imagine the high aspect ratio must also make a clean leading edge much more important than otherwise?????


National - Look up Tom Speer's site. There were some pieces on the Australian website for the Taipan 4.9 (wing-masted cat).

NS14 masts have changed a lot since Frank Bethwaite's time. The square-back has been replaced by a longer section with a carbon tip.

[gggGuest]

Don't forget Nicola Bethwaite's worlds win (and runners up two years before) in the Cherub with one Chris. The wood Bethwaite mast is quite a bit of kit, with very good gust response for the era. Interestingly they gave it such low hounds (and thus long topmast) that the jib had to be fractionally undersize. By contrast an elliptical section one which was tried in the UK on both Moths and Cherubs was a pig and never worked well.

[Man Overboard]

I went back to Eric’s sight and studied in more detail Project Amazon http://www.sponbergyachtdesign.com/ProjectAmazon.htm
and Wobegone Daze
http://www.sponbergyachtdesign.com/Wobegon.htm
I also dug through my pile of Professional Boatbuilder mags and found number 55 in which Erick has written an article about Project Amazon and the Unstayed Rig. This has lead to some observations and also some questions.

It is not necessarily prudent to compare sail plans of small sailing craft with sail and mast arrangements of large sailboats of the magnitude as in Eric’s examples. The two sails operate at vastly different Reynolds numbers; the mast and sail act together as a component that makes up the lifting device. They respond synergistically with one another whether for bad or for good. Understanding this intricate interaction is important if there is any hope of applying proven computational analysis such as that provided by scientist like C A Marchaj. Case in point is the wind surfing sails mentioned by CT 249. The mast diameter is very small compared to the chord of the sail; there is not a lot of friction to allow the sail to bind up. Consequently the force of lift tends to twist the sail on the mast some what eliminating the interference of the mast automatically. In addition the leading edge diameter to cord ratio is small. To get an idea of the effect of leading edge diameter, (mast diameter) compare the difference in the lift to drag ratios between b and d in figure 61 of post 1. All other things being equal, changing the mast diameter to cord ratio greatly effects the lift drag characteristics of the mast sail combination.

I read with great interest Erick’s article in PB #55. There is a good picture of the mast lying down looking from the but end. You can see the fore and aft fairings added and get an idea of the leading edge angle of attack. Before I comment, let me preface my comment with this note. As a young man some 20 years ago I began my independent studies of sailboat design, with great interest in hydro and aero dynamics. At that time I concluded that a wing shaped rotating mast with soft sail may be the best option for optimal sail plan. This was not a new idea, but certainly an idea that had not been pursued with great interests. Recently I have decided to continue my pursuit of building a large sailboat and happened along Erick’s site. I have great respect for his innovative ideas, and the fact that many of them have been tested, with reputable success.

The mast in my opinion is the preeminent element of the lift generating system on larger sail boats. It defines the sail plan, it is the predominant component that defines the pressure distribution of the attached sail, it is the structural bases that supports and directs the lifting components that propel the vessel. In looking at Eric’s designs, and considering the limitations imposed by sail track hardware, lifting mechanisms and the need for the sail to be raised and lowered at acute angles to the mast, I agree that a fairing towards the aft end of the mast is a necessity. But I am not convinced that a frontal fairing is advantageous. One of the great advantages of a soft sail is its ability to adjust its camber and thickness distribution along the chord for best performance in both light and strong winds. But, changing the camber changes the angle of incidence at the leading edge of the mast. (Presence of a headsail will also have an effect on incidence angle) See figure 2.59 & 2.62. Flow will separate when ever velocity vectors do not remain tangent to the leading edge radious.( Theodorsen: On the Theory of Wing Sections With Particular Reference to. the Lift Distribution. NACA Rep. 383,) the velocity vector in the vicinity of the leading edge of the mast is not at the same angle as the apparent wind; it will change with changes in camber, chord wise distribution of camber, the interaction of other sails, and swinging to and fro of the mast.

It is true, that a sharper leading edge can delay separation, but for a narrow range of incident angles. (With a resultant decrees in maximum lift) The narrower the leading edge the more limited is the camber range. The big question for large sail plan- Is a 15 to 30 inch diameter mast all that big? (I will take a guess at a mast to beam proportion here) If we take a mast that is say 20 inches in diameter, with a boom 25 feet long; that would give us a chord of about 26 feet 8 inches. That gives us a chord to leading edge ratio of .03125% a fairly small leading edge considering some NACA sections have leading edge radius of as much as 3 percent. A more rounded leading edge allows for a wider range of adjustments to the optimal incident angle by rotation of the mast. The mast would have to rotate independent of the boom. In the case of Wobegon Daze, the wishbone does not appear to be independent of the mast, therefore a fixed leading edge sized for the anticipated range of camber the sail is capable of is a compromise in consideration of other design factors.

[Eric Sponberg]

Many thanks for CT249s summary of small boat rigs. Very interesting.

Thanks, too, for the observations and comments on my website and articles.

I think maybe I have not explained myself well regarding round section masts. Airflow does not like a round shape because it tends to separate from the surface rather quickly, BEFORE it gets as far back as the mast axis. You cannot depend on airflow to stay readily attached to a round leading edge shape. Even with a sleeve on a round mast, the airflow will more likely detach before it gets to the tangent of the sleeve on the mast. But if you have an elliptical or modified parabolic shape to the leading edge of the mast, the airflow stays attached past the mast axis and continues onto the sail.

One of the things to remember about sail shape is that wingmasted sails have to be cut very flat. Camber is controlled by sail shape a little bit, but more importantly by clew tension and mast articulation. Sailmakers have real trouble understanding this, coming from traditional fixed mast science. They think that they have to build a lot of camber into the sail in order to fool the fixed mast rig that it actually has some decent shape--which it does when sailing on the wind, but lousy shape when sailing off the wind.

Also, the thickness of the mast and the leading edge shape play a part in the "window of attack" for the airflow. Fatter sections are more forgiving than thinner sections. That is, it is easier to maintain attached flow over the mast and sail over a wide range of angle of attack on a fatter section than on a thinner section. With a thin section, the window of attack is narrower, and with the boat pitching and heeling in a seaway, airflow will detach and reattach more often than on fatter section. This is why I always use a fatter section. Structurally, too, fatter too is better than thinner, keeping in mind that this has to be consistent with proper mast bend. You want the right amount of bend, not too much and not too little. And you can't go too big on the mast or wall thickness gets too thin and local buckling of the mast wall becomes a problem. This is why the engineering of the mast quickly distills to a pretty ideal shape and structure for each particular boat.

Both Project Amazon and Wobegone Daze have very forgiving mast sections and they are quite easy to sail. These are relatively slow boats when compared to monohulls or fast dinghies. The faster the boat goes, the narrower the mast can be because the airflow angle of attack does not change as much, so the window can be narrower. This is why larger multihulls will tend to have longer chord, thinner sections masts. The other consideration with multihulls is that stayed masts that rotate are, overall, much better than free-standing masts because they can be built very lightly to handle the loads. A free-standing mast on a multihull has to be built very heavy to handle the loads, and this also makes them expensive. Heavy weight and high cost are anathema to multihull sailors.

You can do make the mast and boom independent of each other, or connected, with care. On Project Amazon, they are independent, and on Wobegone Daze they are connected. The way they are connected poses some handling problems that you have to deal with. On Saint Barbara, which will be launched next summer, the wing and boom are independent. On my Globetrotter 45 cat ketch, I want half wishbones for the wingmasts, and in this instance I have them connected. But here the gooseneck is very close to the mast axis with its side-of-the-wing mount. This makes the wing and the boom quasi-independent, yet very easy to build and sail.

Eric

[national]

Great thread everyone, i think it's really interseting. I am looking into it at the moment and running some models and will let you know what I find at the end of the week!! keep the thread going I'm sure there is still plenty left in it!! Coming from a purely reserach and not practical background designing a wing mast be incredibly difficult to get the mast bend as wanted. Looking at 2d slices it is quite easy to find optimum shapes for aerodynamics etc but can imagine that when the mast is put into real life many of the wanted shapes are not possible?

I suppose this is why the circular sections with appendices are used.

[Eric Sponberg]

It is actually not hard to calculate stress and deflection on a spreadsheet--just a little time consuming because you have to establish a suitable section shape that will lend itself to good aerodynamics AND easy construction, and then iterate laminate thickness to get the desired strength and deflection. I have developed guidelines from all my mast studies as to what is suitable deflection for any given boat design, so this really is not too complicated a problem.

National--what is the nature of your studies, what are you doing specifically?

Eric


Eric

[national]

Thinking about it in more depth I suppose that the mast actually will stay aligned better with the sail that with a fixed mast therefore the luff curavture would be more constant helping with sail design and shape.

I am doing CFD analysis of varying mast profiles.
I am looking at different mast sections to see how rotation affects them. I have 4 sections and 3 differetn rotations for each. Non rotated, aligned to the luff tangient and fully rotated. It will not be spectacular but I am looking forward to seeing the results. They are just generic shapes representing a few types of mast .

Jon

[Man Overboard]

I happened along Tom Spears website, and recognized him as one who frequents the forum. He has completed some analysis of two dimensional flow of teardrop wingmast sail combinations using XFOIL. Unfortunately there is not a comparison to elliptical shapes, which would have been interesting. As he states on his website: “This paper concerns itself with teardrop-shaped wingmasts. These masts can be rotated independent of the sail so that the lee side can be made a smooth contour on each tack. There are other types of wingmasts, using parabolic (Ref. 2) and elliptical (Ref. 4) sections. The flow around the blunt trailing edges of these sections cannot be calculated by XFOIL because the surface contours are too severe.” (http://www.tspeer.com/Wingmasts/teardropPaper.htm)

CT 249 writes “The wings were quick at times, but too "twitchy", perhaps due to lack of twist. The weight and handling problems were not worth an occasional possible burst of speed”

Tom Spears has made some comments about smaller wing masts that may be a contributing factor to the twitchy response mentioned.

I quote Tom: “Small wingmasts have a much shorter distance between the peak near the leading edge and the joint at the sail. So the pressure increase is much steeper for small wingmasts. This means that a small wingmast has a narrower range of usable angle of attack between separation on the windward surface a low angles, and stall at high angles. Of course, the mast can be rotated to help alleviate this. However, the fact remains that a small wingmast will have a narrower “groove” than a large wingmast. This will make it more difficult to trim well, and it will be more affected by changes in the local flow angles along the mast, such as from gusts or wind shear.”

I am still leaning toward teardrop shaped masts, with a more rounded front than an ellipse; but at the same time, Like Eric says, it needs to be a shape that is easily engineered and constructed. I really would like to build some sections, and pull on them until they break. Erick, is there an effective way of testing a mast section? would a section have to be built full scale?

[national]

Thanks for the paper, really useful. I will have a read tonight.

From memory though isn;t XFOIL an advanced panel methos, from that I would question it's ability to model the seperation inevitable with mast sections.

[Timothy]

Do you think that if you were to use a fully battened sail with a large zippered luff pocket on a free standing rotating tapered round section mast with a crane and ran an external halyard from a head board through a shiv at the end of the crane through the luff pocket and then through the ends of the battens (battens extend into the luff pocket and have shivs on the ends), in effect using the halyard as a jack line, that given sufficient mechanical advantage the mast would bend and the jack line (halyard) would induce camber into the sail. The idea is that until the mast starts to bend there is little pressure on the battens so the sail is free to run up the mast. If it works the sail should flatten and twist in gusts while still retaining its shape and the leading edge would maintain its relationship to the rest of the sail even when twisted. Release the halyard and hopefully the sail drops freely.

[Eric Sponberg]

Reply to Jon (National)--it will be interesting to see your results, and perhaps compare them to what CA Marchaj has already published. Your technology is more sophisticated, presumably, and easier to visualize.

Reply to Tom (Man Overboard)--Scale models could be engineered to testing to destruction. Just be sure that the wall thickness also scales, and this includes the amount of fiber in the different orientations. There has to be about 60% fiber in the longitudinal direction in a wingmast, and about 40% fiber in the off-axis directions, split evenly between 90 deg and +/-45 deg orientations. This can be problematic on very small models because one layer of off-axis fiber may use up too much of its allotted percentage. However, it may be worth a try to see what happens.

Also, the structural test need only be a cantilever beam of set-up but the load should be a distributed load. A uniformly distributed load (constant weight along the wingmast) would be the simplest. There is some justification for using a uniformly increasing load from the "deck" support to midheight, and then a constant distributed load from there to the masthead.

Reply to Timothy--You do not have to induce bend in the mast, it will bend all of its own accord. Designed right, it will deflect in a predicable curve. The task is to cut the sail so that it will set well in most wind conditions on the bending mast. A jack line through the batten cars is never stiff or stable enough to handle the batten compression load--the battens will be useless unless they can press against and be supported by a sail track. Also, if you do run the line through the batten cars, it will likely not have enough mechanical advantage to bend the mast. In fact, on some of my designs we ran the halyard through bullseye fairleads next to the sailtrack specifically to remove the bending load from the halyard. If you don't do that, the halyard tension WILL bend the mast, and then as the mast bends more under load, the halyard goes slack and the sail itself slackens and puckers. A sail sleeve never sets well to a bending mast, it will always pucker. Finally, a sail sleeve, particularly on bigger masts, still has LOTS of friction on the mast and it is really hard to put the sail up and set it correctly.

All the conventional ideas about sleeves and wrap-around sails were tried 25-30 years ago on Freedom Yachts and all the other trendy free-standing rigged boats of the time, and these types of rigs were all eventually discarded in favor of single ply sails on sail tracks, for the reasons described above. Freedom, Offshore Yachts (Tanton designs), Cat Ketch Yachts (Herreshoff and Sparhawk) and the Sea Pearls by Marine Concepts (all masts by me except for Tanton's designs) were about the only survivors. Round masts were built because the engineering and tooling was simple. (Cat Ketch Yachts used my elliptical sections.) Single ply sails were the cheapest, easiest to raise, set, and reef, and so were the most successful, all things considered. There is still a place for them on free-standing rigged boats. But if you want to increase efficiency and safety and ease of handling that bit more, then going to a rotating wingmast is the next logical step.

Eric

[Timothy]

Eric thanks for saving me the work of building a model.I see that you are of course correct that with the arangement I propose the halyard itself would go slack with mast bend allowing the luff to slacken,the exact opposite of what I was hoping to achieve ,tight luff loose leech ,and flat sail. I have sailed a freedom 40 with wrap around sails now for over twenty years and it is true that it is hard work to raise and reef the sail because of the friction on the mast. Perhaps because the halyard is internal it behaves like your bullseye fairlead arrangement. The halyard retains tension and increasing mast bend does result in a flatter sail. By adjusting the out haul and the halyard tension I am able to obtain reasonable sail shape in most conditions and can keep the leading edge fair.I can live with the difficulty of raising the sail and reefing but it would be great to have full length battens to provide more roach , better sail shape, and less flogging. If the only way to do this is to go with batt cars on a mast track then I accept that a rotating wing mast is the way to go. Having said that, I was in Thailand last year and while sailing on my formula board I was passed by a large Warram catamaran with a mainsail that had a luff pocket and a small gaff. The sail seemed to set very nicely. It got me thinking that sails like this might work on my boat. In this months wooden boat there is a design that is described as a modern gaff rig that has a luff pocket sail and a long curved carbon gaff that has no peak halyard but instead slides in a track on a round section carbon rotating mast somewhat like a gunter rig. If anyone has seen it what do you think?

[Man Overboard]

I am at some point going to do some testing. I think I will limit testing to round sections, at least at first. I need to build an autoclave so that I can test under the same conditions that a mast would be built. I will probably stick with prepregs, for the same reasons.

Eric, on the mast you designed for the Saint Barbara, how does the headsail attached to the top of the mast. Is there some type of rotating masthead? I saw the rig design and had to take a double take. The double spinnaker control arm is exactly the setup that I had conceptualized for a yacht I am designing for myself. I have hydraulic rams place port and starboard in the same manner. I contacted HDM hydraulics out of New York; they make custom rams up to 4 inches out of aluminum. I wanted to get an idea of the weight for an 8 foot ram that would extend to about 15 feet. As an alternative to the rams being mounted to the outside rail, I thought mounting them in tandem on a stub mast close to the bow might work. The two poles could rotate on the stub mast together. It might make constant adjusting of the sail easier. I would be very interested in hearing how the design does during sea trials. (Or do we call that lake trials in Michigan).

[PI Design]

History seems to show that wing masts only benefit boats with a surfeit of righting moment (cats and under canvassed dinghies). But I don’t fully understand why this is. I appreciate that a modern Bethwaite rig, e.g. 49er, has good automatic gust response but most boats do not have this level of refinement. Bethwaite claims that the 49er was the first widely available boat to have an ‘autogust' mast. Prior to this, masts were tunable via rig tension and vang, cunningham, etc but required manual input to achieve this depowering. Well, a sail set on a rotating wing mast can also be depowered by pulling on lots of Cunningham, so where is the advantage in the ordinary, pre-49er, mast? At any rate, if the rotating mast is locked so as not to rotate, will it not act similarly to an ordinary mast in terms of transverse bend above the hounds?
I’m just curious, because it seems to me that a rotating mast is no worse than any pre-49er mast in terms of gust response, but that is the reason normally given for their lack of popularity/success.

[Eric Sponberg]

Reply to Tom--On Saint Barbara, the owner is handling the design of the fittings on the mast, and I think he is going to attach the headstay to a bail so that the mast can still rotate under headsail tension. There are also going to be running backs to keep the headstay as tight as possible. So when flying the headsails, you always keep the windward runner on to keep the mast from pumping and destroying the shape of the sails.

Reply to PI Design--The reason you have to tune stayed rigs with vang, backstay, checkstay and cunningham is because the stayed mast/sail combination shape is so very poor an aerofoil on any point of sail except beating or close reaching. On points of sail off the wind, you have to bend the mast and shape the sail to fool the wind into thinking the rig is a better aerofoil shape than it really is! On a wingmast that has the proper amount of bend performance built into it, you do not adjust the shape of the wing or the bend of the mast--the mast bends by itself. You adjust only the clew tension to change camber for varying wind speeds and the angle of the mast to the wind and the sail for the proper amount of driving force. You can depower the rig by under-rotating the mast in heavy air, and over-rotating the mast in light air and off the wind--this causes separation of the wind off the leading edge and so destroys part of the lift. It's kind of like reefing without having to reef. You are not necessarily worried about airflow in such adverse conditions, you are just trying to adjust boat speed and comfort in the seaway with the least amount of rig work.

The wingmast is free to rotate under load, you don't lock it, it stays where it is while sailing, but it can still move. Under normal conditions, the wingmast will spill the excess wind of a gust by letting the top of the mast bend off to leeward--the boat will not heel to the gust, it will not round up, it will not suddenly go to weather helm--the boat just stays steady and powers through the gust without change in attitude. Sometimes you can feel a little lurch forward as the mast straightens after the gust, that is, you get a little burst of speed through and following the gust. So a properly designed wingmast has built-in shock absorber response, and it is also, overall, easier to control because there are so few controls to adjust.

The mast bend is always in line with the pull of the mainsail fabric. On a wingmast that is too narrow, imagining it as a yardstick for example, it is very stiff in the chord direction, but very flexible in the transverse direction. The mast wants to twist and trip transversely under load. So what we try to do is develop a section construction that allows for stiffness that is more evenly matched between the fore/aft bend and the transverse bend. As I mentioned earlier, Project Amazon' masts had carbon fiber central box sections with fiberglass leading and trailing edges. Wobegone Daze's masts had carbon fiber round sections with fiberglass LEs and TEs. Saint Barbara's mast has a carbon fiber elliptical section with thicker side walls and thinner LE and TE, all in a single unit--no central spar or core. We developed the thickness distribution to give us the bend characteristics we think we need to keep the bend balanced fore/aft and transversely. Hopefully next summer during sea trials (lake trials) we'll get to study how close to the mark we really are with our bend predictions and ease of handling.

The reasons that wingmasts to date have not been very popular are: They are generally outlawed by handicapping rules that govern racing sailing boat design. These artificial and political restrictions are starting to fall away, finally. Also, wingmasts, as you can see, are complicated to build because they really take some sophisticated laminate engineering and bearing design. It's not like off-the-shelf mast construction where you lay out any of a selection of aluminum tubes and attach lots of standard design little bits to it, stick it in a boat and off you go. Every boat design is different and requires its own mast design (tooling, layup schedule, hardware, etc.), and ALL OF THE PERFORMANCE CHARACTERISTICS ARE BUILT INTO THE SECTION SHAPE WITH THE PRESCRIBED LAMINATE SHEDULE (I cannot emphasize this enough!), whereas on a conventional rig the performance is derived primarily from the selection and placement of the hardware. These are two fundamentally different concepts.

The hope is that in one-design sailing, and we have already seen this come in the smaller classes, the wingmast will be built right along with the boat--standard designs, but this will take a bigger investment in tooling and construction which will probably make the boats more expensive, at least initially.

Eric

[CT 249]

"At any rate, if the rotating mast is locked so as not to rotate, will it not act similarly to an ordinary mast in terms of transverse bend above the hounds?"

Isn't the moment of inertia inherently larger on a practical wing mast, even side to side? The wall section must be similar to a normal mast, to handle the point loads and occasional crash, and isn't there more wall in a wing section than in a normal mast?????????????????? I don't know, I failed every year in maths but surely the lateral bend of a mast say 200 x 50 is more restricted than one 125 x 50 and otherwise similar????

There may also be the additional complication of having to match rotation with other requirements, which simply means more work. The Tasar, the most popular wing masted mono, has a system where rotation is either on or off, basically, so there's less faffing about.

"On a wingmast that has the proper amount of bend performance built into it, you do not adjust the shape of the wing or the bend of the mast--the mast bends by itself. You adjust only the clew tension to change camber for varying wind speeds and the angle of the mast to the wind and the sail for the proper amount of driving force."

I'm not a great wingmasted cat sailor (good at club level, 1/3 of the way back at the nats until last season's disaster) but everything I've ever found out about wingmasts in cats indicates that they are enormously responsive to downhaul tension and diamond tension. Our wing was designed by a guy who has been right at the top in two of the most competitive wingmast classes in the world (F18s and A Class) and the world A Class and F18 champs talk a lot about the fine tuning required with their wingmasts.

Tasars have wingmats, and they need a lot more than outhaul (actually the outhaul is almost irrelevant) or angle (which is set). They use vang, downhaul and mainsheet.


"Under normal conditions, the wingmast will spill the excess wind of a gust by letting the top of the mast bend off to leeward--the boat will not heel to the gust, it will not round up, it will not suddenly go to weather helm--the boat just stays steady and powers through the gust without change in attitude. Sometimes you can feel a little lurch forward as the mast straightens after the gust, that is, you get a little burst of speed through and following the gust. So a properly designed wingmast has built-in shock absorber response, and it is also, overall, easier to control because there are so few controls to adjust."

With respect, not the way it seems to work in the cats or Tasars. The wingmasted cats don't need much playing of the mainsheet in a breeze, but isn't that partly related to their very flat sail which make a tiny change in camber significant?

I'm not saying these things don't happen, merely trying to continue the discussion by raising the point that as far as I can see (and I'm no expert) the gust response etc of wingmasts is not that good or that automatic.

[PI Design]

The transverse second moment of area is directly proportional to the long length of the wing, but to the cube of the width, so a much longer mast need only be slightly narrower to have the same bend charecteristics. As an example, a 200*50 ellipse with a uniform wall thickness of 5mm has virtually the same 2MoA as 125*60*5 ellipse (63 v 62cm^4). A circular mast 73*73*5 has a value of 62cm^4. So, yes, a more 'wingy' mast is stiffer, but it can be made fractionally narrower to keep the same bend. Of course, on a stayed mast a narrower mast will buckle at a lower rig tension - but this isn't an issue on unstayed masts.

I agree that the gust response of a wing mast may not be that good or automatic, but neither were conventional masts before the 49er. A 470 mast, for example, whilst reasonably tunable to strong or light winds, does not flex massively in each gust. Its gust response is not that good either, so there is no advantage over the wing mast.

I think that the dynamics on larger yachts (like the one's Eric designs) mean that different techniques may apply.

[BOATMIK]

I will modify my opinion on round masts with wrap around pocketed sails. My thinking applies to larger sailboat masts where the diameter of the mast is very significant. On a smaller boat or a windsurfer, the mast diameter is so small that the front of the sail comes closer to a sharp leading edge rather than a blunt object. Indeed, small boats and windsurfers are exceedingly fast. And they do gain advantage by playing with mast bend which is easy to control when the mast is round. Now, let's take this one step further--Would small boats and windsurfers be even faster if they had elliptically shaped masts? No doubt some enthusiasts unbeknowndst to me may have tried it, and it would be interesting to see side by side results of, say, a round masted windsurfer sailed against an elliptically masted windsurfer.

My point about round masts being useless was directed to the leading edge shape being round--not good aerodynamics. Generally, on larger boats with large round masts and sleeved sails, no matter what point of sail you are on, the leading edge of the mast and sail is still round, and round is very unforgiving to the airflow which promotes early flow separation at the mast. The sleeve is difficult to set an keep tight. It makes the sail harder to make and more expensive. For these reasons, and for the ease of engineering, I very much favor an elliptical leading edge with a matching trailing edge on a rotating spar.

Eric

Hi Eric,

Can I push a little bit on this?

Because of the scaling effects the bigger the rig the smaller the mast is relative to the chord of the sail?

I'm just trying to visualise it and I "think" I'm right.

The stiffness of the mast is increasing with D^4 - so D only needs to increase slowly. Euler is moving with L^2.

But chord is only increasing linearly.

Seems to stack up in terms of the boats I know too - mast diameter does not need to increase much to carry a much bigger sail.

Or is there something I am not understanding with the scaling of the aerodynamic effects.

I know too from the work of Gentry et al that round is not particularly good, but perhaps we are only talking a very small advantage or disadvantage whereas big gains can be made in structural simplicity and weight saving by having a round stick.

Agreed about the unseamanlike handling of double luffed sails in bigger sizes.

Maybe I'll be able to explain this better in the morning.
MIK

[BOATMIK]

Hi All,

A friend of mine is an East Coast Australian multihull nut.

He spent a lot of time sailing with Lock Crowther. They were doing a delivery on Locks own boat before or after some race or another. It had a narrow chord wing mast - much less than 10% of the main chord.

Lock is off watch. Mainsail was dropped just before he went below.

Boat is surfing above 20 knots in bursts.

Lock comes up the companionway looking a bit bleary.

"Fellas, I don't like doing over 20knots at night - after all this is just a delivery. Can you pull the jib down?

Peter: "we took it down about 40 minutes ago"

Lock: "Oh" goes down companionway without looking forrard and calmly drops the storm boards in position before sliding the hatch shut.

Best Regards to all

MIK

[gggGuest]

I’m just curious, because it seems to me that a rotating mast is no worse than any pre-49er mast in terms of gust response, but that is the reason normally given for their lack of popularity/success.

A dinghy style wing mast, if its just a single fabrication, will be far stiffer fore and aft than a pole. If its un-rotated then the drag is horrible. If it is overrotated then the tip will tend to bend to windward in gusts (because it bends sideways, not fore and aft) So yes, the gust response is both worse and quite different to a conventional pole mast. And this is not good in an overpowered boat.

Bethwaite also contests that the more conventional pear shaped wing sections are very difficult to get in the groove and be faster as opposed to slower. My personal impressions of running with the things also empirically backs this up.

Finally it does seem that the only really successful wing masts in dinghies have been the hyperbolic leading edge square back mast froom Bethwaite, and its successors as used in NS14s etc these days, which could be decribed as hyperbolic front and rear. I am not aware of a "conventional" pear shaped style wing mast that has worked well enough on a monohull dinghy for its builder to persist with it, yet that is what the vast majority of attempts have been with.

Personally I keep thinking that there's unfiinished business with the things, but with all the problems of section, two part construction (part structural, part aerodynamic only) , tuning, weight etc... well its always been s*d that I'll buy a pole and go sailing...

[Eric Sponberg]

My experience with wingmasts is with free-standing designs, which is different from the experiences of CT 249s cat rigs which are undoubtedly stayed. A stayed mast is much slenderer than a free-standing rig, and as a result the stayed rig begs to be "tuned" with the wires. The free-standing rig, of course, has no stays (or only headstay and running backs in the case of a jib-headed sloop) so there is nothing or very little to play with. You have to design the mast to sail on its own and be as forgiving as possible in a wide variety of conditions.

Eric

[tspeer]

Has anything been published on mast shape and rotation etc? I have bethwaites book but was wondering if anything had been written in any more detail?

See Aerodynamics Of Teardrop Wingmasts (http://www.tspeer.com/Wingmasts/teardropPaper.pdf)

Bethwaite sent me some tracings of Tasar mast sections, but I've been unable to analyze them because the backward-facing step is too extreme for a panel code like XFOIL. Maybe some day I'll be able to tackle these types of mast sections with a Navier Stokes code like NS2D.

But the principles should be similar. There's going to be a separation bubble behind each step, like the case of the under-rotated teardrop mast. However, the separation point will be further aft. So the design objective would be to maximize the leading edge suction while minimizing the size of the windward and leeward separation bubbles. The pressure recovery after the leading edge suction peak would have its steepest portion in the attached flow on the mast.

The windward side separation point can be moved aft significantly, which may reduce the size of the windward separation bubble despite the aft facing step. But the separation bubble on the lee side will inevitably be larger than for the corresponding teardrop section. This may be why the Bethwaite sections have evolved from a rounded wedge to more of a boat-tail trailing edge that has a reduced base area.

[tspeer]

From memory though isn;t XFOIL an advanced panel methos, from that I would question it's ability to model the seperation inevitable with mast sections.

True. XFOIL has a inverse boundary layer method that can handle modest amounts of separation. Using it to model the a wingmast is probably at the extreme edge of its validity, if not beyond.

That said, I think it still provides good insight into what's happening around an wingmast, and the trends due to changes in wingmast geometry and trim.

Unfortunately, what published data I've found on wind tunnel tests of wingmasts don't include the coordinates for mast or sail, and they also tend to be 3D models, not 2D. So it's hard to validate XFOIL's prediction of wingmast section characteristics.

It would be interesting to compare the XFOIL results with, say, tose from a Navier Stokes code. Do I smell a MS thesis topic here?

[tspeer]

One factor to consider with wingmasts is the amount of un-reefable area. Every account I've read of a racing multihull that went to sea with a large wingmast had the skipper coming back home saying, "Never again."

[tspeer]

...One of the things to remember about sail shape is that wingmasted sails have to be cut very flat. Camber is controlled by sail shape a little bit, but more importantly by clew tension and mast articulation. ...

This may be an extreme example, but wingmasts of all sizes are de rigueur in landyachts. The sails are cut perfectly flat - no broadseaming at all. The only camber in the sail comes from a bit of luff round and mostly from stretch of the material.

One sailmaker, Charlie O'Leary, uses a wingmast in a pocket, although in his case, the pocket is just bigger than the mast so it's more a case of not having to deal with a luff groove in the mast than any aerodynamic feature. One day, I was in just the right position to sight right down his mast as he came almost right at me on the beat to the finish line. I was amazed to see he had under-rotated the mast to the point that the sail bulged out to leeward of mast. Of course, he was hooked up and moving fast, and he would have gone with more camber for accelerating off the line.

I think a very interesting rig would be an egg-shaped mast, narrow end forward, in an oversized luff pocket. The shape would provide a far better leading edge shape than a round mast, and rounded back to the mast would avoid any crease in the contour where the sail left the mast. The pocket would fill where the windward side separation bubble would be, maintaining attached flow.

The mast may require positive rotation control, depending on where the pivot was located. There would be pro's and con's to terminating the battens at the mast for camber control or at the pocket.

[Man Overboard]

I too think an egg shaped mast might be interesting. What you are really looking for at least aerodynamically is the ability to adjust the leading edge diameter. As you rotate a round mast, the leading edge diameter remains the same. An egg shaped mast with nose pointed forward, however, would permit the leading edge radius to increase as the mast is turned. Although on small sailing craft with small mast diameters this may not have a great effect. On larger masts with tracks, I don’t think it wise to have the sail behind a rounded section. A smooth transition between the mast and the sail is essential is it not? A tear drop shaped mast with an egg shaped nose may be beneficial for masts with tracks. This shape is very similar to elliptical mast sections depending on the cord to width ratio.

[CT 249]

Modern NS14 sticks; must fit inside a 100mm ring.

Although there's no way of isolating mast development from hull, foil and sail development, it must be said that a Tasar just can't compete with a modern NS14 in most conditions. The NS14 is now clearly faster apart from downwind in light to medium winds, so the mast can't be too slow.

http://www.sailinganarchy.com/forums/index.php?act=Attach&type=post&id=31197



http://www.sailinganarchy.com/forums/index.php?showtopic=40114&hl=NS14+mast


http://www.sailinganarchy.com/forums/index.php?act=Attach&type=post&id=31197

[Man Overboard]

That carbon mast section is being encouraged by a gentleman that goes by the name Emo on the NS14 forums (http://forum.ns14.org/) He has posted a couple of pictures of test sections built by C-Tech (http://forum.ns14.org/viewtopic.php?p=865&highlight=#865) It looks like a miniature version of the mast designed by Eric for Wobegon Daze; an elliptical section with almost the same chord to width ratio. Wobegon Daze mast 21 x 10.5, at the deck I presume. MK4 90mm x 44mm (I’m not sure I understand the drawing. It has dimensions listed in ml) Of course the actual construction technique is different.

[tspeer]

I too think an egg shaped mast might be interesting. What you are really looking for at least aerodynamically is the ability to adjust the leading edge diameter. As you rotate a round mast, the leading edge diameter remains the same. An egg shaped mast with nose pointed forward, however, would permit the leading edge radius to increase as the mast is turned. ...

I suspect I'm a little-ender while you're a big-ender. I see the egg-shaped mast as a way to angle the leading edge to control the camber. It also provides a much narrower leading edge, which is more typical of high-performance sections, but has to be oriented correctly to avoid developing a leading edge suction peak.

[Man Overboard]

I can see where you are coming from, if you are in a racing situation, and you are constantly in tune to your vessel, and able to constantly adjust the rotation to stay in the groove, I think a smaller leading edge has performance benefits, especially if the rotation is adjusting camber at the same time. Then the system can be tuned to give proper leading edge diameter/angle of attack for a range of camber settings. I think you alluded to this in post 36

“The mast may require positive rotation control, depending on where the pivot was located. There would be pro's and con's to terminating the battens at the mast for camber control or at the pocket.”

You are right in saying that I am a big-ender, and I will reiterate for the sake of others who are studying this subject, and also so that my thinking may be brought under scrutiny. Keep in mind as far as my purposes; I am studying to design an efficient Blue water cruiser, so my needs are different from the racer. For instance, I am not always going to be attentive to the mast angle of attack, or even the camber, so I need a system with some forgiveness built into it.

Weick and Scudder have shown that sharp leading edges applied to a Clark Y section has
the following effects; a decrees in Cl max of up to 7.5, A decrease in drag, a shift from trailing edge stall to more of a thin airfoil stall (angles of attack of 15 degrees or less), a delay in separation at the leading edge,(within a narrower range of cambers) with more difficulty reattaching flow once separation has occurred. “The Effect on Lift, Drag, and Spinning Characteristics of Sharp Leading Edges on Airplane Wings”, NACA TN 447, February 1933 radius. This isn’t the best study, there are others, I can’t seem to find at the minuet.

In general larger leading edge radius's have the following qualities:
Most wing sections achieve Cl max with leading edge radius's between 1.5% and 2% of the chord (of the wing section, not the mast alone), fatter nose radius's allow for a wider range of cambers without separation toward the leading edge., wing sections with greater leading edge radius are capable of greater angles of attack, delaying stall. Fatter leading edge helps maintain laminar flow within a wider range of wind speeds. The penalty for all of this is higher drag (but not much higher) These are some of the qualities that a cruising yacht needs.

Ideally a mast needs to constantly rotate to keep an ideal angle of attack. We have auto-helms; in the future we will have auto-mast rotation.

[tspeer]

I think leading edge radius is a red herring. The NACA needed a way to close out their 4-digit thickness distribution so they spliced in a small circular arc to round off the leading edge.

But this is not the best way to shape a leading edge for may reasons. First of all, there's an abrupt change in curvature between the leading edge radius arc and the remainder of the contour. The biggest reason not to use a circular arc for the leading edge is because now we can shape the entire leading edge based on the aerodynamic requirements.

For example, this Wortmann FX 74-CL5-140 airfoil has a comparatively narrow leading edge, even though it was specifically designed as a high lift section.
http://www.ae.uiuc.edu/m-selig/ads/afplots/fx74modsm.gif

What really matters is the position of the stagnation point and the curvature in that immediate region. If the stagnation point is located where the curvature is high, the flow can be turned around the leading edge without generating a high suction peak that can promote separation. This is just like taking a turn at low speed with your car - you can turn sharply without excessive lateral acceleration. But if the stagnation point moves away from the high curvature area, the flow accelerates away from the stagnation point and then has to negotiate the leading edge - this leads to trouble.

Modern inverse design methods, like the ones in XFOIL, can properly shape the leading edge for the design operating range of angles of attack. This may result in a narrow leading edge shape like the Wortmann section above, or it may produce a surprisingly blunt section that can actually have a flat spot at the leading edge and rounded corners with higher curvature before settling into the gentler curve of the main part of the secition - you'd never get that kind of shape by specifying a leading edge radius. These methods also take into account the effect of the trailing edg on the leading edge, which is a massive influence.

If you've no idea of what the flow is actually doing at the leading edge, then sure, a large radius will help to avoid producing a pressure peak that can lead to leading edge stall. But it's so much better to specify the angle of attack range and let the leading edge shape fall out from that. With the free availability of tools like XFOIL and the graphical user interfaces that have been developed for it, there's really no need to stick with an approach that was convenient in the 1930's when they were in the early stages of understanding what made different sections tick.

[Man Overboard]

"With the free availability of tools like XFOIL and the graphical user interfaces that have been developed for it, there's really no need to stick with an approach that was convenient in the 1930's when they were in the early stages of understanding what made different sections tick."

This is true, when you read through some of the old reports, often times test data did not always produce results that where consistent with theory. It is evident that there were other design issues that, at the time could not be taken into account, at least not without further testing. Modern design and analysis tools are very helpful in that several design criteria can be considered simultaneously. For instance, as you have pointed out, trailing edge effect and a whole host of other criteria. I don’t mean to discount these other issues; the focus has been on the leading edge because this thread is about rotating wing masts, of which the mast makes up the leading edge (at least for the main sail).

One note of caution; as we deliberate back and forth, you will notice for simplicity sake, we will use a single wing section as an example for discussion. Sails are capable of producing hundreds of different wing sections. Within a single hour a sail may change its span, chord, camber, thickness, thickness distribution, leading edge angle of attack, trailing edge angle, etc. It becomes evident that what ever design criteria we pick for an optimal mast section, of utmost importance is its ability to perform well across a broad range of possible wing sections. It seams a constantly varying radius from the nose aft is at least one desirable design criteria. Egg shaped, teardrop shaped and elliptical all have this characteristic at least to some degree. It would be nice to see some test data. Until then I think I will focus on other aspects of the mast, like sail attachment, construction details, sail plan.

[BOATMIK]

I can see where you are coming from, if you are in a racing situation, and you are constantly in tune to your vessel, and able to constantly adjust the rotation to stay in the groove,

.....

Ideally a mast needs to constantly rotate to keep an ideal angle of attack. We have auto-helms; in the future we will have auto-mast rotation.

I agree with the thrust here MO,

A cruiser is not going to adjust everything contuously. And there are real limits for racers too.

Generally the fastest boats are set up to automatically adjust - to reduce the time the crew spend with "their heads in the boat".

I can't imagine any useful cue that would allow changing of mast trim in this way - how would it be possible to SEE that the flow wasn't quite right.

With catamaran rigs you can get the rotation right as some of the leeward telltales flatten out when the angle hits a certain point - but I suspect this "looks nice" rather than is an optimum solution.

Upwind anyhow most of us try to minimise the conscious changes apart from tiller angle and some mainsheet trim - at least when settled down after tacking. I might extend to adjusting the vang/cunningham through the gust cycle with some boats that have stiffer masts where things happen less automatically.

But to monitor an accurate mast rotation would have my head facing the wrong way altogether for efficient sailing - if steering I should be splitting between the tufts and oncoming wind and waves and tactics and the rest.

A control that takes a lot of concentration is likely to be slow compared to self adjusting systems.

MIK

[tspeer]

...
I can't imagine any useful cue that would allow changing of mast trim in this way - how would it be possible to SEE that the flow wasn't quite right....

There are a number of ways to see the correct mast trim. One way is to put short telltales in a horizontal row on the luff of the sail and on the mast. These alow you to see how far back the windward separation bubble extends and if you're developing a bubble on the lee side. The closer you sail to the wind, the less mast rotation you need, just like flattening the sail using outhaul tension.

Another very useful device popular with landyacht sailors is a short windvane positioned right at the leading edge of the mast such that the tail of the vane just barely clears the mast. This vane is dominated by the flow at the mast, not the freestream wind direction. It will tend to stay fixed on one side, then suddenly pop to the other side and maintain its angle to the mast as the mast rotation is changed.

What's happening is the vane's tail will be on the opposite side from the stagnation point. When the vane is intermittently popping from one side to the other, the mast is rotated such that the stagnation point is right on the nose. A little more rotation, and the vane will move to the lee side and stay there. A little less rotation and the vane will move to the windward side and stay there. If the vane is mostly on the lee side and occaisonally popping to the windward side, the mast rotation is about right.

And then there's Skeeter class iceboat champion Don Clapp's method of trimming his sail from within a closed cockpit ahead of the mast, where he can't see the sail or feel the wind: "I change something, and if my head snaps forward, that's bad. If my head snaps back, that's good."

[BOATMIK]

Good one Tom,

Was aware of the tufts but not aware of the stagnation pointer thingy.

I wonder how such systems would relate to a boat continually changing pitch roll and yaw. I guess it would be try it to see how it works. Maybe someone with an A-cat. And to see whether it led to a real performance difference.

Is anyone using such systems now on As or the square metre classes? If not - wonder why not?

Also you gave one example of a top level iceboat person not using them at all.

So wonder if the advantage of accurately lining up stagnation point with optimised areas of the mast might be a minor effect and not really worth worrying about.

Like in the way that fat conventional masts when compared to slim conventional masts "never seem to be as bad as we expect" as Milgram once put it.

If I can feel my head snap back when I get it right on a sailboat - I'll be very happy!

:-)

MIK

[national]

Decided to do a bit of work on this.

I have rotated various mast sections and have found really incredible results. Reductions in drag of 60% and an increase in lift of 25%. It is amazing how much there is to gain. As soon as i find somewhere to present/publish it i will let you know. Still struggling to find somewhere suitable though. Answers on a postcard please, travelling not an issue.

[PI Design]

If its ground breaking stuff, I would recommend RINA (as you are in England). Either submit a paper for peer review prior to publication in the Transactions, or offer to present a lecture to a local branch meeting, which are held monthly in most areas. Alternatively, why not just put your findings on here?

[gggGuest]

You probably should take a look at work from the Amateur Yacht Research Society in the UK. You migvht well find others have trodden the path before you...

[CT 249]

Decided to do a bit of work on this.

I have rotated various mast sections and have found really incredible results. Reductions in drag of 60% and an increase in lift of 25%. It is amazing how much there is to gain. As soon as i find somewhere to present/publish it i will let you know. Still struggling to find somewhere suitable though. Answers on a postcard please, travelling not an issue.

What sort of boatspeed increase would you get in a dinghy with a drag reduction of 60% and an increase in lift of 25%?

Emperical experience indicates that the wingmast is very rarely worth it.

It has been tried by Herreshoff (Suicide class, '30s), Curry (Renjollen, '30s), International Canoes ('30s by the US champ, top the '80s by a NA), R Class ('50s by a Gold Medallist; '70-'80s by a multiple class champ; 90s by an ID champ); 12 Foot Skiffs ('90s or '00s by a repeat Interdominion champ); 18 Foot skiffs ('50s and '80s); Moths; Merlin Rockets; windsurfers; even Frostbiters.

They work in NS14s, although even there for all the writing about the superiority of the wingmast, the non-wing poles were very competitive for many many years.

While the results are interesting (as they were when other people got them) isn't it also fascinating that the test results never seem to be borne out in sailing? Is the application in sailing, by gold medallists etc, that is at fault or is the testing missing something and therefore creating incorrect results?

[national]

Thanks for the AYRS, it seems like a good place to start.

[Steve Adolph]

Gents,
There appear to be some talented people on this thread. Maybe I could get some help?

I'd like to be able to compare the following, based on a proctor D mast section.

Case 1) Nominal. Non-rotating mast.

Case 2) Rotating mast, angle of rotation set to be such that the sail exits the mast on centerline of the mast. A symmetrical fairing exists on both sides of the sail, with the purpose being to improve the aerodynamics of the sail/mast. shape TBD

Case 3) as per case 2, but with no fairing

Would Case (2) offer significant improvement?

Any opinions?

Since Proctor (Selden) no longer offers the D, the Cumulus section could be substituted. That sectiion profile is available at the proctor selden site.

thanks in advace.

Steve

[Steve Adolph]

Thomas-Interesting paper!
Could an implementation of a teardrop wingmast be accomplished with a standard dinghy tube, like a Proctor Cumulus, and a foam fairing attached to the sail?



See Aerodynamics Of Teardrop Wingmasts (http://www.tspeer.com/Wingmasts/teardropPaper.pdf)

Bethwaite sent me some tracings of Tasar mast sections, but I've been unable to analyze them because the backward-facing step is too extreme for a panel code like XFOIL. Maybe some day I'll be able to tackle these types of mast sections with a Navier Stokes code like NS2D.

But the principles should be similar. There's going to be a separation bubble behind each step, like the case of the under-rotated teardrop mast. However, the separation point will be further aft. So the design objective would be to maximize the leading edge suction while minimizing the size of the windward and leeward separation bubbles. The pressure recovery after the leading edge suction peak would have its steepest portion in the attached flow on the mast.

The windward side separation point can be moved aft significantly, which may reduce the size of the windward separation bubble despite the aft facing step. But the separation bubble on the lee side will inevitably be larger than for the corresponding teardrop section. This may be why the Bethwaite sections have evolved from a rounded wedge to more of a boat-tail trailing edge that has a reduced base area.

[national]

If you find me the geometries in a CAD format, I can probably run them for you.
The rotation would definately help the flow, both in increasing lift and reducing drag. From the results I have had any rotation on pretty much any mast going upwind is beneficial.
Jon

[MalSmith]

I have used over rotating round masts on both a moth and an NS14 in the ealy 80's. While I have no measurements to support the percieved benefits, there is a noticable jump in pressure when moving the mast from the under-rotated to the over-rotated position. This effect was often commented on by otherwise skeptical people who sailed the boats.

I don't know why round masts are not over-rotated more often. Its not that difficult to do. I would guess that the benefits of over-rotating a round mast are almost as good as the benefits of rotating a more streamlined mast section, with regard to minimising the separation bubble, but there seems to be a general perception that if it's a wing mast, it rotates, if it's a round mast, its fixed. Even if the benefits are only a few percent or less, every bit helps. I'm currently planning to rotate the round mast on my IC.

Mal.

[national]

It's probably because the classes that have the flexibilty to rotate masts probably also have the freedom to change the mast shape, and an elongated mast section will produce more lift. So if you are going to go half way why not go the whole way?!

[MalSmith]

It's probably because the classes that have the flexibilty to rotate masts probably also have the freedom to change the mast shape, and an elongated mast section will produce more lift. So if you are going to go half way why not go the whole way?!

By over-rotating a round mast, you are going more than half way. In practical terms, the lee side airflow (and hence lift) is improved as much as for the elongated mast, only the drag will not be improved as much (if at all).

Round masts sections are freely available. They have known bend charactoristics and the tuning methods are the same as for fixed round masts. An existing fixed round mast can be adapted to over-rotating for minimal cost. Elongated mast sections are not so freely available (admittedly due to low demand) and are therfore more expensive or have to be custom made. Elongated mast sections have bend charactoristics that vary with the degree of rotation. Tuning and using an elogated mast section requires learning a new set of skills. The degree of rotation of the mast strongly effects how the sail behaves and therfore adds another level of complication to the operation of the rig. When rotating a round mast section, usually you will just rotate it as far as possible, and that's it.

Yes, over-rotating a round mast is a compromise, but it delivers much of the benefits of using an elongated mast for less cost and complication.

Mal.

[national]

I disagree on your comment upon lift and drag. Round sections do not create as much lift and create more drag than elongated. The actual flow over the sail is pretty similar but the increased area of the mast actually contributes significantly to the lift and drag reduction.

[PI Design]

I disagree on your comment upon lift and drag. Round sections do not create as much lift and create more drag than elongated. The actual flow over the sail is pretty similar but the increased area of the mast actually contributes significantly to the lift and drag reduction.

Can you explain that a bit more please?

[national]

If you take the mast and the sail as seperate entities they both have a drag and lift force assosiated to them. Together they create a total drag a lift force which is the important factor for a design. When a mast is rotated the force on a mast can actually contribute significantly to the Total lift. Thi sis especially true when an elongated mast is used as it creates significant lift which can actually act to drive the mast forward rather than backwards.

With a rotated round section the force on the sail is similar to the elngated section, the main difference is the force on the mast. This change is significant.

I have spent so long thinking about this I have forgotten what I was writing about! But i think this makes things clearer!

[MalSmith]

If you take the mast and the sail as seperate entities they both have a drag and lift force assosiated to them. Together they create a total drag a lift force which is the important factor for a design. When a mast is rotated the force on a mast can actually contribute significantly to the Total lift. Thi sis especially true when an elongated mast is used as it creates significant lift which can actually act to drive the mast forward rather than backwards.

With a rotated round section the force on the sail is similar to the elngated section, the main difference is the force on the mast. This change is significant.

I have spent so long thinking about this I have forgotten what I was writing about! But i think this makes things clearer!

Taken in isolation, it is true that an elongated mast will povide some lift if it is set at an angle if incidence to the airstream. A perfectly round mast will not provide any lift no matter what "angle of incidence" it is set at. But a sail attached to the mast affects the airflow around the mast due to upwash created upstream of the sail. The stagnation point is shifted towards the windward side of the mast and even a round mast will be providing some of the lift. In fact one of the problems of a circular section mast, as Tom Speer mentioned earlier, is that it's less than ideal leading edge geometry develops a sharp suction peak (local high lift) which leads to early flow separation. When using a round section, it should be over-rotated far enough so that the still attached flow is "peeled off" the mast by the sail before it can separate of it's own accord. I maintain that where the total sail area (mast/sail combination) is limited, the benefits of an elongated mast over an over-rotated round mast are not that significant. If axtra sail area can be gained under the rule by using an elongated mast, then it's the extra area that is beneficial over and above any aerodynamic considerations.

Mal.

[national]

It would be interesting to compare the XFOIL results with, say, tose from a Navier Stokes code. Do I smell a MS thesis topic here?

Tspeer - you seem to know a bit about this XFOIL, is it easy to use?
I also think it would make an interesting study to compare viscous CFD with a panel type, has anyon ever done it for a similar flow type?
It seems to be set up to run airfoils, is it easy enough to run sails and or masts?

[brian eiland]

I was searching for some info and photos of the Beirig's camberspar jib arrangement and this subject thread was referenced. However I did not find it. Anyone know of reference sites??

[Erwan]

It is a long time Tom Speer paper is a kind of benchmark for my research.
XFOIL is stressed to its limits and the Max lift and Min drag could be a bit different than in the real world. So the question to the experts is
How much reasonnable discount on the Lift Coef ?
How much reasonnable increase on the Drag coef ?

In 2D of course with about 1 000 000 reynolds

In fact I would like to reconciliate real world an theory: If you consider a A-Cat windward, a 4.5 meter/ second windspeed is enough to be full trapeze, the apparent windspeed is.... the righting moment is ..the average AoA is... and the IMPLIED Max Lift Coef hardly achieves 1.2.

Of course it is a 3 D result, induced drag accounts for a lot in the global drag.

Can we consider that from 2D to 3D induced drag could decrease max lift from 1.7 (theory) and 1.2 (A-Cat observation) ?

Or other factors must be considered as well ?

Thanks to the experts

EK

[tspeer]

...Can we consider that from 2D to 3D induced drag could decrease max lift from 1.7 (theory) and 1.2 (A-Cat observation) ?
Induced drag, per se, does not affect maximum lift. In principle, the angle of attack can be increased to compensate for the induced velocity ("downwash"), restoring the lift coefficient. The span loading for minimum induced drag will produce a uniform induced velocity across the span. So that just amounts to a bias in the local angle of attack compared to the reference angle of attack.
Or other factors must be considered as well ?
Where 3D effects really affect maximum lift is in the distribution of induced velocity across the span, along with camber shape and twist up and down the sail. If some portions of the sail reach their stall angle of attack before other sections, then the lift will not reach its full potential. If you stop sheeting in when the first section starts to separate, then other sections will be operating below their maximum lift. If you continue to sheet in, some portions will be stalled and others still operating below maximum lift. If you sheet in until the last section reaches its maximum lift, other sections will be deeply stalled.

The maximum lift will be a compromise where the loss of lift due to stalled sections starts to exceed the increase in lift of the un-stalled sections. But in any case, it will be lower than having all the sections performing at their maximum. The local section loading will depend on the section shape (zero lift angle of attack and maximum lift angle of attack), twist (which determines the local section incidence angle), chord length (planform shape) and induced velocity from all the other sections. Twist is probably the most useful parameter to vary to harmonize the span loading for maximum lift.

It may be that a lift coefficient of 1.2 isn't maximum lift for the A-cat either. One can really only operate at maximum lift if both hulls are in the water. If the windward hull is flying then the lift is limited by stability, not aerodynamic stall. CL=1.2 may be the best value for performance if the drag increase above that reduces performance more than the increased lift improves it.

If induced drag is the largest contributor to aerodynamic drag, then it's likely the lift coefficient for maximum aerodynamic lift/drag ratio occurs below maximum lift. The lift on the hull depends on the aerodynamic loading from the rig. So it makes sense to operate past the point of best aerodynamic L/D if the increased lift required from the hull improves the hydrodynamic L/D.

Where these break-even points are will depend on the windage, hull form drag, wave drag, and board-foil profile drag as well as the induced drag of the rig and hull.

[Erwan]

Tom,
Thank you very much to have taken time to provide me with this academic approach. My question was a bit stupid as I had the basic equation to translate from 2D to 3D using Aspect Ratio.
Of course, the implied 1.2 Lift Coefficient is an average Lift Coef, and therefore we could find higher 2D lift coef for this wing section.
Your remark about the break-even points is very relevant and prompt me to read again the Norwood book.
I feel so lucky to have this opportunity to communicate with you on these issues, that I cannot resist to ask you another question about your workpaper: "TearDrop Mast:
You warn that XFOIL is stressed to its limits,so even without experimental datas, how much would you discount XFOIL Lift Coef, and/or increase XFOIL Drag Coef, in order to get closer to real life ?

I am looking for this information because I would like to investigate the "theorical" interest to have a thick asymetric wing section, regardless of weight constraint, section shape adjustments, trimming issue ..., just theorical advantage in 2D between Teardrop mast rig and asymetric thick wing section.

I promise it will be the last question, I guess many cat sailors harass you about these issues.

Thanks in adavnce & best regards

EK

[tspeer]

...another question about your workpaper: "TearDrop Mast:
You warn that XFOIL is stressed to its limits,so even without experimental datas, how much would you discount XFOIL Lift Coef, and/or increase XFOIL Drag Coef, in order to get closer to real life ?...

Ah, that's the rub, isn't it? Without empirical data, or a more powerful CFD method (like a Navier Stokes code), there's no way to know.

All I can do is what the cartographers of old did with uncharted waters - place a label that says, "Here be dragons."

[Erwan]

Tom
Thank you for your this remark, in fact my question was quite similar to those of my friends who ask me where will be the Dow Jones 3 years from now.
(I was mutual fund manager)

[PI Design]

Hi National,

I've just read your paper in the RINA Journal of Maritime Engineering. I have a few questions if you are still around.

[petereng]

Does Xfoil include the mast section? Or is it only the sail surface?
Peter S

[tspeer]

Does Xfoil include the mast section? Or is it only the sail surface

When I've used XFOIL, I include both the mast and the sail. I represent the sail as a two-sided surface that is approximately 0.4 % thick. That makes the mast & sail a single section with a kink on the windward side.

When doing inverse design, it's unlikely you'll be able to maintain the required symmetry of the mast. I use a spreadsheet to reflect the lee side of the mast to the windward side. So it's an iterative process - start with a wingmast & sail, design a new section with XFOIL, fix up the coordinates with a spreadsheet, and go back to XFOIL to analyze the result.

[petereng]

So Xfoil predicts the lee and windward side separation bubbles? If so... its a good tool to put mast shape and mast /sail interaction issues to rest?? Peter S

[tspeer]

Yes, XFOIL does predict the separation bubbles on both sides. XFOIL uses an inverse boundary layer code that handles modest amounts of separation.

However, what XFOIL cannot handle the flow through the gap between mast and sail. So if the sail is attached with the bolt rope in a groove, XFOIL probably does a pretty good job. If there's a track with slides or cars, the question still remains.

Another thing XFOIL cannot handle are large backwards facing steps, such as you find with a Bethwaite-style wingmast, ala the Tasar. I've tried fairing the step in with the idea that if the separation still occurs at the shoulder of the step, the results might not depend very much on the shape of what is submerged inside the separation bubble. But I wasn't very successful with that approach. I had to sooth things over too much just to get a solution.

[tspeer]

So Xfoil predicts the lee and windward side separation bubbles? If so... its a good tool to put mast shape and mast /sail interaction issues to rest?? Peter S

Here is an example of a mast rotation study using XFOIL, and you can see the change in the separation bubbles on each side.

http://www.tspeer.com/Wingmasts/teardropPaper_files/image025.gif

http://www.tspeer.com/Wingmasts/teardropPaper_files/image027.gif

http://www.tspeer.com/Wingmasts/teardropPaper_files/image029.gif

http://www.tspeer.com/Wingmasts/teardropPaper_files/image031.gif

http://www.tspeer.com/Wingmasts/teardropPaper_files/image033.gif

[C Jenkinson]

I thought I'd keep my question in this thread, though I'm not (specificallly) following on from where Tom Speer finished off. The issue is to do with the configuration of pivot points for a rotating wing mast. I've attached a very rough sketch below of a proposed design of wing mast and have put the mast pivot at 45mm from the nose, which is 30% of the 150mm chord. Is this an appropriate position? Secondly, since the boom must pivot, in the horizontal plane, relative to the mast, where should that pivot point be for the boom; should it be inline with the luff groove bolt rope where the sail tension acts on the mast (i.e. position B in sketch) or should it be set to the side nearer the mast pivot (position A - it just happens to be in the centre of and underneath the halyard sheave in the sketch). Basically, how is mast rotation angle relative to the sail otherwise controlled - does anyone have any diagrams I can look at? I promise, I can sketch better than this, but hopefully it will do for now.

[petereng]

Hi Charles,
If you put the mast axis near its front it will be difficult to rotate the mast when you tack. The compression from the battons pushes the boom forward, this pushes the mast trailing egde forward and "around". If the mast axis is fwd then the batton moment is large and may not rotate when you want it to. The mast will need some sort of "spanner" to control rotation. If you have the mast axis near the trailing edge this moment is reduced and the mast may auto rotate. I'd make provision for two or three mounts starting as near the trailing edge as possible for you to try and pick the best one. Or have the mast mounted on a plate (that is connected to the spanner) that has the mount so it can be adjustable. On really big rotating masts like the french tris they have the boom go the the deck at the mast base as the boom loads are so big the mast can't rotate. have you seen mast spanners? Some are as simple as a pipe on the mast connected the boom with some rope so the boom pulls the mast around, on bigger boats the spanner is controlled with block and tackle or hydraulics. I did a Google Image search for "mast spanners" and it picked up a few good images of these.

Hope this helps. Peter S

[petereng]

Hi Charles,
I looked at your diagram again and since you are having a round bottom section on the mast I'd make a bearing for the boom end that went around the Dia60mm section that would end at the tack of the sail and become round. At this point I'd make a bearing that goes around the mast and connects to the boom. This will allow the boom to freely rotate independant of the mast rotation. A spanner would then control the mast rotation. This will allow easy control of the boom as it will be independant of the batten forces. You have the sail in tension and yes the cloth is in tension but the dominant mast rotation loads are from the battens. This type of mast and boom arrangment has much more load on it and standard goosenecks fail. So a substantial bearing/gooseneck is needed for this type of mast/boom connection. A simple spanner like a NS14 could be used to syncronise the mast/boom if needed but my guess is that you will need a resonable spanner to control the mast.

Cheers Peter S

[C Jenkinson]

I've got the idea of the mast spanner now, from looking on google - thanks for the suggestion. I'm not so clear about what you're thinking in terms of the location and action of the boom pivot point and the influence that the batten (compression?) forces have on this. Could you possibly do a quick sketch and attach it...? I can see that if the boom pivots under the sail luff groove then the batten 'tension' can't have any effect on the angle between the boom and mast because there is no induced moment - this is perhaps the best configuration to keep things simple.

[petereng]

Hi Charles,
The Battens are under compression and push the mast forward. As the mast has to be over rotated (say 45-50degs) the mast is being held on the lee side of the boat by the batten load. Lets call this the batten torque. It is not a small force to deal with, on my NS14 you cannot straighten the mast by hand using the spanner when its under load (if the mast axis is fwd), so your boat will have much bigger loads. When you want to tack the batten torque must be overcome to get the mast to rotate to the other side. If the boom vertical axis and the mast vertical axis agree there is no batten torque. I'm not sure how to get images into the page so I'll read up on that now.

Peter

[tspeer]

...The issue is to do with the configuration of pivot points for a rotating wing mast. I've attached a very rough sketch below of a proposed design of wing mast and have put the mast pivot at 45mm from the nose, which is 30% of the 150mm chord. Is this an appropriate position?
The pivot location doesn't depend on just the section at the foot. It also depends on where the pivot axis goes up the mast. For example, if you bring all three stays together ahead of the leading edge at the hounds, as is common practice with landyachts, then the mast is effectively raked back with regard to the pivot axis. The pivot at the base can be well aft of the quarter-chord, as you've indicated, because it gets its aerodynamic balance from the top of the mast, whose area lies behind the axis.

The most critical time for the stability of a rotating mast is when it is not being restrained by the sail, such as when tacking. So it's a good idea for the mast to be aerodynamically stable. Although very few masts are made this way, it's also a good idea for the center of gravity of the mast to be on or ahead of the pivot. This is almost impossible to achieve unless there is some ballast on a short boom extending from the leading edge - just above the hounds would be a good location. Mass balancing the mast prevents flutter when tacking. Flutter can cause a very large amount of drag - I've pitchpoled a landyacht backwards that way.
Secondly, since the boom must pivot, in the horizontal plane, relative to the mast, where should that pivot point be for the boom; should it be inline with the luff groove bolt rope where the sail tension acts on the mast (i.e. position B in sketch) or should it be set to the side nearer the mast pivot (position A - it just happens to be in the centre of and underneath the halyard sheave in the sketch). Basically, how is mast rotation angle relative to the sail otherwise controlled - does anyone have any diagrams I can look at? I promise, I can sketch better than this, but hopefully it will do for now.

Besides the sail, the thrust at the gooseneck can have a significant influence on mast rotation, so the position of the main sheet and vang/kicking strap can also be important. I once had a scow whose mast rotation was tuned by moving the mainsheet blocks forward and aft on the boom (but it wasn't a very powerful means of controlling the mast).

If the mast rotation is induced by thrust from the boom, one way to control it is to have a spanner from the mast linked by a line to the boom itself. This allows the spanner to be pulled closer to the boom for less mast rotation or eased off for more mast rotation.

Most sailors find that they need some means of positive rotation - being able to forcibly establish the mast rotation where they want it - instead of depending on the loads from the boom. When the sail is sheeted out, it is hard to get enough rotation naturally, and when the sail is sheeted in, one often wants less. A spanner led either forward or aft from the mast can be used to control the mast with a tackle to the deck. Another approach is an arm on each side of the mast with lines led fore or aft from both sides, but this tends to get in the way of the crew.

As to how to determine the best rotation angle, many landsailors mount a small dinghy wind vane so the tail just brushes past the leading edge of the mast. The vane will typically be on one side or the other, depending on where the stagnation point is on the leading edge. Once the vane is on one side, it doesn't respond to further changes in the angle of attack because the flow is parallel to the mast near the surface. The vane really indicates the sign of the stagnation point location. When the vane is flicking back and forth more or less evenly, the stagnation point is right at the leading edge. Landsailors tend to trim so the vane is flicking a little more to the leeward side than the windward side, but that still positions the stagnation point close to the leading edge. This trim results in minimizing any separation bubbles between the mast and sail. The mast will be rotated with the leading edge past the apparent wind direction because of the upwash induced by the lift.

Cat rigs need more mast rotation than do sloop rigs because the jib turns the flow at the mast to be closer to the centerline of the hull. The mast angle for a sloop will be more aligned with the apparent wind direction.

[rwatson]

From practical experience, using the 3 stays joined at the mast makes it really hard to rotate the mast in light winds, as the mechanical tension almost forces the mast back into fore and aft position.

Since light winds are a prime time to use some mast adjustment, it always seemed a poor way to do it.

[RHough]

As wing masts become a greater percentage of total chord, how is twist handled? It would seem that you need to twist the mast or have a very forgiving LE section/radius?

R

[Gary Baigent]

This is a simple and efficient way to set up a reasonably large 500mm chord wing mast. I have no trouble with light or heavy air rotation, beating or broad reaching.

[tspeer]

As wing masts become a greater percentage of total chord, how is twist handled? It would seem that you need to twist the mast or have a very forgiving LE section/radius?

For Mast3 on USA17, one of the requirements was that the mast section should have a stall angle of attack of 10 degrees, without the sail. This was intended to reduce the drag of the exposed mast above a reefed main, but it also had the effect of creating a certain amount of robustness to mast rotation when combined with a sail, too.

The big driver for twist is the jib. The jib makes the apparent wind angles at the foot much smaller than at the head, so it's really hard to get an untwisted mast to cover the whole range. The flow was attached on both sides of Mast3 for about most of the span, but there was a windward side separation bubble on roughly the bottom third of the mast. You just have to trim for the best compromise.

[Gary Baigent]

Yes, compromises. I built a catenary shaped trailing edge wind mast for my 32 cat Supplejack; and the differing chord shapes from mast base to peak were of course different .. and so there was plenty of compromising going on in getting the wools to fly at there compromised best. A straight luff makes things easier ... but still twist causes a compromise setting.
A 10 degree stall angle seems a very shallow angle to me. How did they do that with a thick section mast? I mean getting a good shape leading from wing mast to sail, a sweet flow line without a step, both sides?

[RHough]

For Mast3 on USA17, one of the requirements was that the mast section should have a stall angle of attack of 10 degrees, without the sail. This was intended to reduce the drag of the exposed mast above a reefed main, but it also had the effect of creating a certain amount of robustness to mast rotation when combined with a sail, too.

The big driver for twist is the jib. The jib makes the apparent wind angles at the foot much smaller than at the head, so it's really hard to get an untwisted mast to cover the whole range. The flow was attached on both sides of Mast3 for about most of the span, but there was a windward side separation bubble on roughly the bottom third of the mast. You just have to trim for the best compromise.

I need to think on this a bit more. With a small chord mast, you can get near linear twist, as the mast chord becomes a larger percentage of the total twist in the mast becomes more important if the wing mast-sail is to have the designed profile? A jib screws this up requiring linear twist below the hounds and a different twist above the hounds?

For a practical wing mast on a smallish multi, where would you put the Leading Edge Flicker? Just above the hounds would let you trim the top 25% of the span, otherwise the stagnation point indicator would be in the slot... I can see that you might have the indicator flicking one way below the hounds and the other above the hounds?

Would changing the laminate schedule above the hounds force the mast to twist in the right direction under load? It would tend to over rotate/twist as a gust response, not sure that is what you want ...

Where is my tin foil hat and runes ... :confused:

[petereng]

Hi RH,
The more telltales and flickers the better to get an understanding of the airflow. Once you have an understanding of the airflow and what you need you can remove the telltales and flickers that are not important. So you should then only have the airflow indicators of interest eventually. The concept of having the structure respond to air pressure is called aeroelastic twist coupling (AETC). Being a mast/rig engineer I spend most of my time solving the structural issues. I think that by the time you make the structure soft enough to acheive AETC it will be too soft to function as a mast. We have to decouple the structural requirements from the aero requirements. This is what is happening with Wing Sails. We use a structural spar for the structural (elastic) requirements (max stiffness/min weight) and put a very light element that is the right shape in place to fulfil the aero requirements (max lift/min drag). The light aero structure is relatively easy to twist using spanners or internal strings etc. Many of the discussions had by sailors over the last 50 years could be simplified if we uncoupled the structural and aero issues... they will never be able to be resolved together as they are in too much opposition. Much of the resistance to change is created by the racing rules which seek to limit innovation and the rules are based on arbitary conditions not based in the physics. Box rules have corrected much of this as we can design using good science vs trying to meet geometric constraints. Our sails probably would look like hang glider wings if the rules permitted two sided soft sails and in the classes that permit wing sails they will be more like USA17 and C class cats. Given time they will become lighter and friendlier as all the great minds like on this forum work through the issues and solve them. Currently we are at the end of a poor development path and hemmed in due to the racing rules and tradition vs good design that uses good science. In the AC33 they were looking for 0.5% differences to get a technical advantage (and spending small fortunes to find and realise them) I think we are going down this path with current rigs. Hopefully the AC34 is the begginning of a new era where sailors and sailing can open up and make great gains like the car and aero industry have done in the past. I was reading a paper the other day and it stated that the theoetical max lift coefficient is about twelve (12!) So we are along way from this at present! High lift aircraft wings are about Cl=4.0 Cheers Peter S

[RHough]

Hi RH,
The more telltales and flickers the better to get an understanding of the airflow. Once you have an understanding of the airflow and what you need you can remove the telltales and flickers that are not important. So you should then only have the airflow indicators of interest eventually. The concept of having the structure respond to air pressure is called aeroelastic twist coupling (AETC). Being a mast/rig engineer I spend most of my time solving the structural issues. I think that by the time you make the structure soft enough to acheive AETC it will be too soft to function as a mast. We have to decouple the structural requirements from the aero requirements. This is what is happening with Wing Sails. We use a structural spar for the structural (elastic) requirements (max stiffness/min weight) and put a very light element that is the right shape in place to fulfil the aero requirements (max lift/min drag). The light aero structure is relatively easy to twist using spanners or internal strings etc. ...

Hopefully the AC34 is the begginning of a new era where sailors and sailing can open up and make great gains like the car and aero industry have done in the past. I was reading a paper the other day and it stated that the theoetical max lift coefficient is about twelve (12!) So we are along way from this at present! High lift aircraft wings are about Cl=4.0 Cheers Peter S

Thanks. Excellent post. I've done wing design for RC Gliders so controlling twist to what was designed in was the goal. I had never looked at a symmetrical profile that needed washout to change when the airframe was inverted. :)

The C-Class wings that twist the first element as well as the flap make perfect sense to me. For a wing mast with jib rotating the mast above the jib independently of the mast below the hounds would be interesting except for the vortex the change on profile would create and the practical requirement for a continuous luff to raise the sail. :(

Getting rid of the damn jib makes the whole thing easier to design as long as you can get the area you need whilst controlling the heeling arm. This leads me to think that for comparable weight and simplicity, a wing mast uni-rig might be a good rig. If you were to have a constant chord mast, the leech taper (reduced tip chord) would have the same effect as aerodynamic washout (I think).

With no rule to limit area, you don't need high CL and the high induced drag penalty that goes with it.

If you could get a CL of 1.6 or so with a wing mast and sail without a jib for upwind, you just add area with a screecher or A-sail off the wind and you shouldn't need the high CL's that require multi-element sail plans?

That leaves the mast area that cannot be reefed and could be a problem in a slip or at a mooring if the mast was not free to rotate through 360 deg.

For the small tri I'm planning to build, a 27 ft span x 1 ft chord wing mast would leave area equal to 50% of the jib up all the time. Having to drop the rig when not sailing does not appeal to me and a mooring is not a viable option.

If I'd be dropping the rig anyway, I might as well just design a wing and call it good.

R

[petereng]

Hi RH - There are many many 27'x12" chord wing masts out there on tris that never come down so I don't think this will be an operational problem. Bethwaithe published data on wingmast/sails and regularly got Cl=1.8 so 1.6 is easy. I think triangular jibs exist because the forestay is there, the sooner we get rid of standing rigging the better! I think the cat rig with schreecher is the go and minimise or remove standing rigging. Peter

[RHough]

Hi RH - There are many many 27'x12" chord wing masts out there on tris that never come down so I don't think this will be an operational problem. Bethwaithe published data on wingmast/sails and regularly got Cl=1.8 so 1.6 is easy. I think triangular jibs exist because the forestay is there, the sooner we get rid of standing rigging the better! I think the cat rig with schreecher is the go and minimise or remove standing rigging. Peter

The W-17 is 17' on deck with a 13'-10" beam. Standing rigging is simple, two shrouds and a forestay. Going with a free standing rig would require re-engineering the basic structure so not an option. It takes a bit of fiddling to keep the CE about right with a wing or a uni-rig. Moving the step forward is not a huge problem. Having rig choices is part of the design to begin with and a feature that I wish to keep. A cut down Hobie 18 rig makes it a relatively tame boat, like the WETA. A 27 x 1 wing mast and downwind sails should make it a hot rod.

From an aerodynamic point of view, sailing biplanes up wind makes little sense. Cat rigs make more sense.

If it turns out that the boat is underpowered as a Uni-rig, I can always move the mast back and add the jib.

I think I just talked myself out of building a wing ... :)

R

[Erwan]

Thank you Peter for your comment. My English is too poor to be able to put that so clearly in words. But as you have paved the road, I feel more confident to go out of the wood.

In the title, I make a reference to René Descartes and his famous "Cogito Ergo Sum" when revisited by Steve Clark should be read "Cogito Ergo Twist"

In addition to this famous addage Descartes also used to say "When you have a big problem to solve, try to slip it into smaller problems". That is exactly the spirit of your recommendations Peter. I am investigating an alternative rig solution I would like to disclose.

Basically, you blend the Cogito twist concept for the frame: (telegraphic main mast lower part + tapered upper part + 1st element structural trailing edge equivalent + twist lever and houd + boombox), with a cambered inducer-like system.

Then considering the performance of a well designed 2D mast+sail arrangement as proposed by Tom Speer at many occasions., I think there is an intermediate way to be explored between soft sail and 3 elet wingsail:The single element wingsail.

The only ancillary difference with the single sail/teardrop mast concept as proposed by Tom Speer is that you need a bit more thickness everywhere.
1-At the leading edge to store your telegraphic tube (around 5%).
2-At the 50% 60 % point chord where the second vertical spar is located (around 3%).

As far as I know it is just a question of parameter in XFOIL.

You wrap the structure with a 25% to 40% cord mast leading edge made of composite skins, with little morphing possibilities including:
1-Twist
2-Asymetrisation of the leading edge
3-Morphing lips around the sail luff/mast connection (in order to control the bubble ramp). It is not mandatory the sail slot on the mast is only 10mm wide thick if we have a double side thick soft sail at the luff.

The "1st elt trailing edge equivalent" will be basically a vertical spar located somewhere aroud 50 to 60% from the leading edge, inside the double-side sail.

This spar could be use to control an inflexion shape on the leeward side (ala LIEBECK) as designed by Tom Speer on some recent Tear drop mast revisitation (I cannot remenber which thread) for the high L/D and high lift sailing range.

But it will hold the horizontal structure (aft part of the cantilever-batten/camber-inducers)

The sail
would be a double side soft sail with double batten arranged together in order to provide inertia and trailing edge shape control for aft loading possibility (max lift sailing range)

The 2 trailing edges of the 2 sides of the sail would have to skip on each other, to adjust to the new tack-shape, so 2 vertical "batten" trailing edges seem necessary.

My cogitations are pending at this point. And because the structure is the same than those used for a 3 elets wingsail, and could be therefore very easily recycled, I think it worth the experience (for A-Cat or a Moth).

Thanks for readings

Regards to everybody

Erwan

[petereng]

Hello Erwan, Is this what you are decribing? http://www.omerwingsail.com I'll add something as well, I'm not an aerodynamicist so may get this wrong but. Solutions lie in the soft sail area (like Omer Sails, double sided soft sails which are generally banned in racing), the rigid sail area (C Class cats) and combinations which have been done in the past. RIGID (target Cl=2.5 to 3.0+) if rigid then we require a slot to create camber. But technically if we are aiming at a multifoil arrangment the elements need to overlap to provide the max lift. If the foils overlap they can't change sides easily due to mechanical difficulties. So they usually are very close so they can swing side to side. This is not aero optimum. The flap is used to close the gap so it behaves like a single element (this is the bit I may have wrong) So using two or three panel rigid wings have compromises. Plus they have many mechanical hurdles to jump. SOFT sails like Omers are single element designs having a Cl target of 2.0 -2.5 and have not really been explored enough. HYBRIDS rigid forward section and soft aft section have been explored but again not enough. So there is much scope to move forward in sail design in general. Plus most sailors are looking for more lift (power) which is good but they should also be looking to reduce DRAG. Any design that reduces drag is good even tidying up the deck is a good thing. This is rarely discussed but needs to be understood better. The drag on a moth for instance is of the same order as its lift so if you halved the drag you would go much faster! Peter

[petereng]

Another thing I'd like to mention is that I watched the USA17 and Alinghi races for every second and I was amazed at the hugh camber in the USA17 wing. We will never be able to camber membranes (soft sails) like this so ultimately rigid solutions for racing are the go. The acceleration and ease at which USA17 beat Alingi is the big signpost for racing sails. In an interview with James Spithall at the C Class champion ships (I think from memoery) he stated that once the hull was up they feathered the top 1 or 2 panels to depower. It was then a balancing act to depower more than power up going through the course, its rare a racing sailboat has to depower more than power up!. Peter

[P Flados]

A while back, I had mentally worked out a mechanism that would generate a reversible cambered wing. It would have have a rigid partial ellipse (or similar shape) for the leading edge beam/spar, a series of hinged internal ribs, control lines to pivot the ribs (similar to C Cat technology) and a "sock" mylar film covering.

Later when reviewing forums discussing C Class wings, I noted that they have figured out that the slot between elements is a big part of the very high lift performance that is so important for downwind runs.

Now just imagine combining both of the above. The attached file combines 2 elements that are each very close to the Clark Y in profile.

Even though C Class wings can generate high lift, I am certain that use of reversible & variable cambered airfoils for a 2 element wing could probably enhance the performance. The front element of the C Class wings have a flap to try to get close to a cambered performance. However flaps just are not as good as a wing section with the right amount of camber for the desired conditions.

The effort for the above is not anything I would ever anticipate being able to justify for any project I would be involved with. However, if a big dollar AC effort really wanted to try for an edge, this would be an interesting thing to explore.

[CT 249]

Plus most sailors are looking for more lift (power) which is good but they should also be looking to reduce DRAG. Any design that reduces drag is good even tidying up the deck is a good thing. This is rarely discussed but needs to be understood better. The drag on a moth for instance is of the same order as its lift so if you halved the drag you would go much faster! Peter

Don't designers, sailors and sailmakers talk drag reduction all the time? For example, the Moth designers have a very, very strong idea of the importance of drag reduction, which is partly why the boats and sails are designed the way they are - ask Thorpey about the time he spent assessing freeboard in reality and on MBD's computer. AMAC is a world-class designer of windsurfing speedsails, which is an area where low drag is vital.

Similarly, even back in 1972/73 IOR designers were doing things like hiding spinnaker poles under the streamlined blister decks to reduce windage (see Ydra and Frigate). Couldn't it be that they completely understood the idea, but that it proved to make no significant difference and it was faster to put the poles back on deck? The 55 footer Quailo III from the UK Admiral's Cup team didn't go to a three=spreader mast because they were concerned about the "excess" windage of the third spreader. The Lexcen boats of the time were flush decked partly to reduce drag. All those concepts were dropped and replaced by higher-drag options that worked better; arguably, drag was not under-rated but over-rated. Tidy decks can be slow.

As a side note, coming from boards (which have few, if any, of the rule-based restrictions you mention) you can also see that those restrictions can also lead to a design that is more user-friendly in many ways. There have always been classes that don't have restrictive rules, but the free choice of sailors normally makes them less popular. The reason we see so many boats and events to restrictive rules is surely because those restrictions make a boat or event that appeals to more sailors.

Is depowering that rare? In boards, cats and skiffs there's a tremendous amount of depowering going on. The top Taipan sailors, for example, are probably effectively depowering from about 8 knots of wind, at a guess.

BTW do you know what's happening with the Omer sail? About three years ago we were told that the test Elan 37 was going to start trialling against a stock sistership to prove the superiority of the Omer sail. No reports of those tests seem to be available.

[RHough]

Don't designers, sailors and sailmakers talk drag reduction all the time? For example, the Moth designers have a very, very strong idea of the importance of drag reduction, which is partly why the boats and sails are designed the way they are - ask Thorpey about the time he spent assessing freeboard in reality and on MBD's computer. AMAC is a world-class designer of windsurfing speedsails, which is an area where low drag is vital.

Similarly, even back in 1972/73 IOR designers were doing things like hiding spinnaker poles under the streamlined blister decks to reduce windage (see Ydra and Frigate). Couldn't it be that they completely understood the idea, but that it proved to make no significant difference and it was faster to put the poles back on deck? The 55 footer Quailo III from the UK Admiral's Cup team didn't go to a three=spreader mast because they were concerned about the "excess" windage of the third spreader. The Lexcen boats of the time were flush decked partly to reduce drag. All those concepts were dropped and replaced by higher-drag options that worked better; arguably, drag was not under-rated but over-rated. Tidy decks can be slow.

As a side note, coming from boards (which have few, if any, of the rule-based restrictions you mention) you can also see that those restrictions can also lead to a design that is more user-friendly in many ways. There have always been classes that don't have restrictive rules, but the free choice of sailors normally makes them less popular. The reason we see so many boats and events to restrictive rules is surely because those restrictions make a boat or event that appeals to more sailors.

Is depowering that rare? In boards, cats and skiffs there's a tremendous amount of depowering going on. The top Taipan sailors, for example, are probably effectively depowering from about 8 knots of wind, at a guess.

BTW do you know what's happening with the Omer sail? About three years ago we were told that the test Elan 37 was going to start trialling against a stock sistership to prove the superiority of the Omer sail. No reports of those tests seem to be available.

I don't agree on drag. For conventional boats like the IOR designs drag reduction is not overrated at all. You have to look at the total drag and see what parts of the whole are the largest contributors. You must also consider that total avail able power is limited by RM. You can't "add lift" past a certain point, the boat tips over. For this limited power the only way to go faster is to reduce drag.

Once "hull speed" is reached the dominate drag producer is wave formation drag. Second is probably induced drag from the rig having to operate at relatively large CL and short rig height to accommodate the RM limit. The keel has to lift equal to the lateral force of the rig so there is induced drag there also. The windage of deck gear is a very small percentage of the total. The difference a "tidy deck" makes might be very hard to measure. It can be measured on multi's that sail faster relative to true wind speed. Shuttleworth claims that once the rig and foils are right, the next thing to focus on is reducing drag of the hulls and deck. This is "free" VMG upwind.

The taller rigs and deeper keels we see on modern keel boats attack one of the major drag producers (induced drag) directly ... simply increasing the span pays off in reduced drag.

I do agree that high performance boats are looking to depower much of the time. This means flatter sails and twisting off the top of the rig. This changes the spanwise lift distribution and has the effect of reducing induced drag. In effect the twisted off portion of the sail acts much like a winglet. As long as it is not flogging, the twisted off main is faster upwind than reefing.

R

[tspeer]

...
With no rule to limit area, you don't need high CL and the high induced drag penalty that goes with it....

Actually, high CL increases the profile drag, but doesn't increase the induced drag if the height of the rig is kept the same.

If there's no limit to sail area, the optimum area will be the size that results in the lift coefficient for the best 2D L/D.

The height of the rig then becomes primarily a tradeoff between induced drag and heeling moment. A taller rig has less drag, but the lift has to be restricted to match the heeling moment to the righting moment. The keel comes into it, too, because more lift also loads up the keel and affects the L/D of the hull+keel.

[RHough]

Actually, high CL increases the profile drag, but doesn't increase the induced drag if the height of the rig is kept the same.

If there's no limit to sail area, the optimum area will be the size that results in the lift coefficient for the best 2D L/D.

The height of the rig then becomes primarily a tradeoff between induced drag and heeling moment. A taller rig has less drag, but the lift has to be restricted to match the heeling moment to the righting moment. The keel comes into it, too, because more lift also loads up the keel and affects the L/D of the hull+keel.

I'm struggling with this concept (not news, I know).

Say my section has best L/D at CL = 1.1 and + 6.5 deg AoA (+10 deg from zero lift)
The available RM is 5000 pound / feet
How do I get to optimum area from there?

Don't I have to know what the sailing angle, boat speed, and wind speed are to set the area?

R

[CT 249]

I don't agree on drag. For conventional boats like the IOR designs drag reduction is not overrated at all. You have to look at the total drag and see what parts of the whole are the largest contributors. You must also consider that total avail able power is limited by RM. You can't "add lift" past a certain point, the boat tips over. For this limited power the only way to go faster is to reduce drag.

Yes, you're right of course - but the point I was trying to make was that that sort of drag is certainly not "rarely discussed"! It's discussed all the time.

Once "hull speed" is reached the dominate drag producer is wave formation drag. Second is probably induced drag from the rig having to operate at relatively large CL and short rig height to accommodate the RM limit. The keel has to lift equal to the lateral force of the rig so there is induced drag there also. The windage of deck gear is a very small percentage of the total. The difference a "tidy deck" makes might be very hard to measure. It can be measured on multi's that sail faster relative to true wind speed. Shuttleworth claims that once the rig and foils are right, the next thing to focus on is reducing drag of the hulls and deck. This is "free" VMG upwind.

The taller rigs and deeper keels we see on modern keel boats attack one of the major drag producers (induced drag) directly ... simply increasing the span pays off in reduced drag.

I do agree that high performance boats are looking to depower much of the time. This means flatter sails and twisting off the top of the rig. This changes the spanwise lift distribution and has the effect of reducing induced drag. In effect the twisted off portion of the sail acts much like a winglet. As long as it is not flogging, the twisted off main is faster upwind than reefing.

R

Maybe I should have been more specific in the part of the post I was replying to.

The points I was trying to make were (1) as we both agree, designers spend a LOT of time working out how to reduce induced drag, wetted surface drag, etc. The reduction of such drag is not "rarely discussed", as was the claim. It was the claim that designers "rarely discussed" drag that I was addressing.

(2) Rather than being "rarely discussed", drag has been an obsession of designers (even of conventional boats) for eons. I was using the windage of deck gear as an example of that obsession. However, practical experience indicates that (as you said) the windage of deck gear isn't that important in "conventional" boats.

Experience in small cats and boards indicates that extra RM from a taller crew is probably more important than the lower drag of a small crew, I s'pose.

BTW, high performance craft will often reduce sail before the gear is flogging, for instance by changing down to smaller rigs in boards and Skiffs. The bigger rigs do feel to have more drag. I know that if the span is longer the induced drag must drop. However, if the upper section of the rig isn't creating lift because it is dead flat and twisted, isn't it therefore irrelevant when calculating the span? You can't just increase span by adding an outboard section that is creating no lift, can you? Wouldn't that go against the ideal of elliptical planform loading.

The windage, in boards anyway (it's been too long since I sailed a Skiff under the small rig for me to recall) certainly feels dramatically higher with a flattened rig rig, even when the head is completely flat and the leach has negative tension.

The practical experience is that the twisted-off section isn't worth it, as demonstrated by the fact that board sails for high-wind sailing are lower in aspect than they could be, given structural and RM constraints. Handling issues also come into play, of course - the R Class guys had even higher-aspect rigs in the past but they were so high aspect that they were too twitchy, so they altered on of their very few rules and increased SA and lowered the rigs for superior speed and handling. Handling is, it seems, one of those things largely ignored by many theoreticians, but vital to high-speed sailing in many conditions.

[PI Design]

Hi folks,
I don't get the time I would like on this forum at the moment, so I apologise if this has been brought up before (and if it is a stupid idea!).
Is there any merit to having the top of the sail (or top element of a wing) set on the other tack to the lower parts. I know this would have horrific drag penalties, but it would potentially produce a zero heeling moment sail, so the sail could be as big as New York. Never mind efficiency, sometimes there is no substitute for size!

[CT 249]

Tom went through it some time ago in a thread with Wardii, I think, and IIRC showed that the drag increases higher than the increase in RM.

[gggGuest]

BTW, high performance craft will often reduce sail before the gear is flogging, for instance by changing down to smaller rigs in boards and Skiffs. The bigger rigs do feel to have more drag. I know that if the span is longer the induced drag must drop. However, if the upper section of the rig isn't creating lift because it is dead flat and twisted, isn't it therefore irrelevant when calculating the span? You can't just increase span by adding an outboard section that is creating no lift, can you? Wouldn't that go against the ideal of elliptical planform loading.

Think its more complicated than that: cue Mr Speer. But I think that a dead flat twisted off section is by no means irrelevant to induced drag. Consider: induced drag is all about the tip generated vortices, and having a big lump of twisted off sail is sure going to affect them and the effective span. I'm not enough of a mathemetician to make sense of the formulae, but as I understand it in the sums for induced drag calculation you have both the span and an efficiency rating for the planform...

I think I'm right in saying too that as you reef a sail - or at least a substantially rectangular sail, then because the chord doesn't change the actual induced drag remains much the same as the area reduces. So if you increase the span and the area increases with it then the induced drag doesn't drop. Interestingly for the cruising folk, now I think of it, that means that in mast reefing should be way superior to in boom reefing in terms of induced drag.

So if you stump a rig down without changing the chord then I reckon:-

induced drag stays much the same if the planform stays much the same
form drag reduces
total lift reduces
drag as a percentage of total lift increases

However if you just blade off the top of the rig - thus changing the planform
induced drag increases (worse planform, but maybe not too much)
form drag reduces a little
total lift reduces
drag as a percentage of total lift increases

But, and its a soddin' great big but, we have to figure in the dynamic behaviour of the rig and wind changes. The bladed out rig unblades out, even to a good extent automatically if the wind drops, and the total lift increases a great deal.

So what I'm guessing is that unless the wind minimums are such that the reefed rig is going to be fully powered up/overpowered, the bladed rig is going to be faster. Or, as Uffa Fox said 80 years ago or something, in a racing boat you should reef for the lulls...

[RHough]

BTW, high performance craft will often reduce sail before the gear is flogging, for instance by changing down to smaller rigs in boards and Skiffs. The bigger rigs do feel to have more drag. I know that if the span is longer the induced drag must drop. However, if the upper section of the rig isn't creating lift because it is dead flat and twisted, isn't it therefore irrelevant when calculating the span? You can't just increase span by adding an outboard section that is creating no lift, can you? Wouldn't that go against the ideal of elliptical planform loading.

Yes you can. This is exactly why winglets can reduce drag on airplanes. They basically add zero lift span. TSpeer's work shows that the ideal elliptical loading is not so ideal when you have to limit heeling force.

The practical experience is that the twisted-off section isn't worth it, as demonstrated by the fact that board sails for high-wind sailing are lower in aspect than they could be, given structural and RM constraints. Handling issues also come into play, of course - the R Class guys had even higher-aspect rigs in the past but they were so high aspect that they were too twitchy, so they altered on of their very few rules and increased SA and lowered the rigs for superior speed and handling. Handling is, it seems, one of those things largely ignored by many theoreticians, but vital to high-speed sailing in many conditions.

I don't think we are at odds here.

TSpeer's research shows that when moment is constrained (heeling moment for sailboats) high L/D planforms can have "negative lift" at the top/tip and positive lift at the bottom/root. Soft sails cannot do this, they flog and the drag goes up before the zero lift angle.

Also agree that theory and practice sometimes appear to show that the theory is wrong. Your comment about handling and the reduced AR rigs for the R class are a great example of what works in a wind tunnel may not work in "real life". This is not so much that the theory is wrong but that "real life" wind and wind tunnel wind are not at all alike. Very high AR wings can indeed be "twitchy" this is true for high AR foils used for daggerboards, rudders and keels also. What the theorists don't seem to get in many cases is that out of design range conditions exist often and a lower performance (in theory) rig that works over a wide range of conditions can be faster on the water. They are more "forgiving". Bethwaite's study of wind was an eye opener for me, the smoothest air is much more turbulent than I would have thought. +/- 5 deg angle and +/- 10-20% velocity plays hell with theory. Rather than a design point, you have a design area, if your design requires adjustment within the normal range of wind conditions in "steady" breeze it becomes labour intensive to sail. Rigs that react and are self trimming should be faster even though the theoretical performance is lower.

My reefing comment does not really apply to rig changes to reduce area. They don't have to deal with the drag of a bare mast above the reefed sail. The twisted off rig should have less drag than the reefed rig when the drag of the mast is figured in.

The use of shorter rigs for fast sailing also fits quite nicely with TSpeer's idea that you set the area so that the rig is loaded at the best L/D of the section. A taller rig with the same area has to have lower average CL to keep the same heeling force. Reducing the height/span of the rig so that the section is working at it's lowest L/D should be faster, this seems proved by what we see working in practice.

On the drag reduction topic, how many sailors change rigs and sails looking for better performance and don't spend equal time on the hull surface, fin and rudder profiles? The time and money spent on the keel and rudder of boat is a semi-permanent drag reduction since those surfaces don't flog, don't degrade with UV, and don't stretch and lose their shape over time.

R

[tspeer]

TSpeer's research shows that when moment is constrained (heeling moment for sailboats) high L/D planforms can have "negative lift" at the top/tip and positive lift at the bottom/root. Soft sails cannot do this, they flog and the drag goes up before the zero lift angle.

I think Day first showed negative lift at the head was fast:
Day, A. H., "Sail Optimisation for High Speed Craft", RINA 1990

...Very high AR wings can indeed be "twitchy" this is true for high AR foils used for daggerboards, rudders and keels also.

I think one reason for this is twisting off the head lowers the center of effort, but it does not lower the aerodynamic center. Also, a high AR rig is trying to get performance through finesse - lowering the drag faster than the lift drops due to restricted heeling moment.

...
The use of shorter rigs for fast sailing also fits quite nicely with TSpeer's idea that you set the area so that the rig is loaded at the best L/D of the section. A taller rig with the same area has to have lower average CL to keep the same heeling force. Reducing the height/span of the rig so that the section is working at it's lowest L/D should be faster, this seems proved by what we see working in practice.

Dave Hubbard had a great spreadsheet concerning the sizing of the wing for USA17. He looked at a range of rig heights and sail areas for different wind speeds. At a given wind speed there would be an optimum combination of rig height and area that produced the most drive for the heeling moment. When compared across different wind speeds, the aspect ratio tended to be quite similar, with the optimum rig getting smaller as the wind increased. Very much like what happens when one reefs a soft sail.

[petereng]

The bottom line is that we have a load matching problem on a sail boat. The load is applied by the wind which is a function of its Velocity squared. The Righting Moment is basically linear in response and has been designed for low to medium air conditions so the Rm max is at the top of the medium range. Adjusting SA very fast is the only way forward to cope with the big increase in heeling forces on the aero side of the equation. All the aero other strategies are cumbersome. We need a Rm method that increases in tune with the Vsquared part. Has anyone seen hydrofoils that are controlled to force down in response to heeling? This is the only way I can see us matching the gust response (or steady higher wind vel) (Vsquared) as then the hydrodynamics can be more effective in resisting due to its greater density? This would be illegal in most classes (limit on foils) and difficult in the classes that would allow it but seems to be logical. We use a wand to keep Moths up why not use a wand to keep boats flat? I think Happy Feet in Thailand (a cat) is doing this? Peter S

[gggGuest]

Has anyone seen hydrofoils that are controlled to force down in response to heeling?
This happens on any wand equipped multihull like the Hobie trifoiler, so its been done plenty of times. I think the lack of popularity of such compared to the Moths suggests that it may not be the right approach...

[P Flados]

The current fastest Class B sailboat is Sailrocket at 47.36 kts (http://www.sailrocket.com/). From their web site:
Vestas SailRocket employs a wholly different concept (first documented by Bernard Smith in the 1960s) in which the sail and keel elements are positioned so that there is virtually no overturning moment and no net vertical lift. When used correctly this concept results in a boat which no longer has obvious stability limits and in which the only significant response to gusts is a change in speed!

The Ketterman brothers were the first to use an active control system to pull down on the windward side. This culminated in the Longshot (current Class A record holder at 43.55 kts) and eventually turned into the Hobie trifoiler (http://www.hobiecat.com/sailboats/trifoiler/).

Given that the two examples above are the current world record holders for their respective classes, there is more than a little merit in these concepts.

The other example of nearly ballanced roll moment configuration is the Kite Boards. The overturning moment arm is very small such that overturning moment is not limiting at maximum speed. Rob Douglass holds the current overall speed record at 55.65 kts using this technology (http://www.youtube.com/watch?v=CtFxstw9Zjo)

Over in the "Monohull Speed: Speed Dream by Vlad Murnikov" thread, we have been discussing righting moment (portion of my post #172 copied below). Use of fully balanced systems (Sailrocket) or active control systems (Longshot / Trifoiler / Rave) are actually less efficient than a hybrid system. To the extent possible, all discretionary weight (crew for example) should be to the windward and combined with adjustable ballast (water tank filled via a scoop tube). This is because ballast to windward creates the required down force with no drag. In the post below, I discussed a hybrid where ballast is combined with float actuated active control surfaces (wings).

This is a comment by Tom Speer( and it is a direct quote, not a paraphrase) :

" I asked Mark Pivac why Spitfire had water ballast instead of using dynamic downforce on the windward foil. His answer was, "So it only has to be lifted once." And he was dead right. The leeward foil has to lift both the weight of the boat and the downforce from the windward foil. If either downforce or ballast are used, the load on the leeward foil is the same. However, there is a drag penalty for creating the downforce. There's no drag penalty for the ballast."

For the above point, Tom Speer got it right where so few others have even noticed.

Any force generated by moving fluid over a surface creates drag. The more the force, the more the drag. The one force we can get without a drag penalty is when we use simple ballast for the downforce.

Longshot is a craft that looked capable of so much more with some additional development.

Imagine if all crossbeams on Longshot were round beams shrouded by twist-able airfoils linked to the forward floats such that on each side the airfoils would be the same angle of attack as the J foils. You have to have the crossbeams anyway for structural reasons, they are producing drag as is, so why not get some lift and active control out of them.

Add in the ability to take on water ballast in a windward tank using a scoop tube that can be lowered into the water. Now for a specific one-tack run, remove the windward J foil and start out with the ballast tanks empty. As the boat powers up, you take on ballast into the windward tank as needed to keep the windward side of the twist-able crossbeam airfoil close to a neutral angle of attack. The movement gives some active control at very low AOA's so drag is very small. This would completely eliminate any drag from a windward J foil in the water. The leeward J foil would probably need to be designed to run deeper to let it deliver the required lateral forces, but this would not be a problem.

[Doug Lord]

The Ketterman brothers were the first to use an active control system to pull down on the windward side. This culminated in the Longshot (current Class A record holder at 43.55 kts) and eventually turned into the Hobie trifoiler (http://www.hobiecat.com/sailboats/trifoiler/).


-----------------------------------
This happens on any wand equipped multihull like the Hobie trifoiler, so its been done plenty of times. I think the lack of popularity of such compared to the Moths suggests that it may not be the right approach...

===================
According to James Grogono in "Icarus The boat that Flies", the first foiler design to use dual "feelers" similar to what Ketterman used(incidence controlled foils-whole foil moves) was Force 8 by the Pattison brothers in the UK about 1979. The first dual "wand" system used to control just a flap on the main foils(like the Rave, Osprey, Skat and F3 RC model) was developed by Philip Hansford on Philfly in the 80's. Both these systems have the "advantage" of developing 100% all the RM required (up to the structural limit of the boat) with the windward foil pulling down automatically when required and the lee foil increasing lift automatically as necessary. Both modern versions of these systems have exceeded the top recorded speed of a Moth in heavy air but the Moth is far superior in light to moderate air. Both the Rave and Trifoiler were build to the requirements of production boats that resulted in fairly heavy boats severely impacting their light air takeoff capability. The new all carbon Osprey by Dr. Sam Bradfield(who invented the modern planing wand) will be very interesting-and very light.

Click on image:

[P Flados]

-----------------------------------


===================
According to James Grogono in "Icarus The boat that Flies", the first foiler design to use dual "feelers" similar to what Ketterman used(incidence controlled foils-whole foil moves) was Force 8 by the Pattison brothers in the UK about 1979. The first dual "wand" system used to control just a flap on the main foils(like the Rave, Osprey, Skat and F3 RC model) was developed by Philip Hansford on Philfly in the 80's. Both these systems have the "advantage" of developing 100% all the RM required (up to the structural limit of the boat) with the windward foil pulling down automatically when required and the lee foil increasing lift automatically as necessary. Both modern versions of these systems have exceeded the top recorded speed of a Moth in heavy air but the Moth is far superior in light to moderate air. Both the Rave and Trifoiler were build to the requirements of production boats that resulted in fairly heavy boats severely impacting their light air takeoff capability. The new all carbon Osprey by Dr. Sam Bradfield(who invented the modern planing wand) will be very interesting-and very light.

Click on image:

You are probably right.

Greg Ketterman (http://members.cox.net/gkettermanb/Home%20TriFoiler.shtml) (http://members.cox.net/gkettermanb/Home%20TriFoiler.shtml) indicates he was just at the model stage in 1981. The part I find neat is that they did their first full scale homebuilt prototype in 1987 and then progressed to a world record by 1991.

[yojamey]

A couple of pages of fotos and video on Longshot coming out of mothballs for a
day of play in the SF Bay at the end of 2010

http://www.pressure-drop.us/forums/content.php?509-Longshot-Rides-Again

http://www.pressure-drop.us/forums/showthread.php?506-Long-Shot-back-in-the-game!


One of the captions reads for the "last time" making it sound like Namibia is
not in the cards... Rumour has it that Longshot was sold to the Makani Power Kiteboat folks http://project.kiteboat.com/page/project


Foils tend to cavitate at high speed
PBS-Scientific American Frontiers Date: 1994-02-16, Episode: Science and Sports: Speed Sailing: Windsurfer vs. Trofoiler vs. Planning Hull...Also shows foil cavitation in an MIT water tunnel
http://vsx.onstreammedia.com/vsx/pbssaf/search/PBSPlayer?assetId=68868&ccstart=2470470&pt=0&vid=pbssaf405&entire=No

[Doug Lord]

A couple of pages of fotos and video on Longshot coming out of mothballs for a
day of play in the SF Bay at the end of 2010

Scientific American....




=============
Good stuff, thanks!