# Rotating Wing Mast – theoretical discussion

Discussion in 'Sailboats' started by Man Overboard, Nov 15, 2006.

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### Man OverboardTom Fugate

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.

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### nationalJunior Member

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.

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### DGreenwoodSenior Member

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.

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### Man OverboardTom Fugate

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.

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### Eric SponbergSenior Member

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.

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

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### 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.

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### TimothySenior Member

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

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### CT 249Senior Member

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.

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### gggGuest...

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.

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### Eric SponbergSenior Member

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

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### nationalJunior Member

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?

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### CT 249Senior Member

"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.

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### 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.

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### Man OverboardTom Fugate

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.

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### Eric SponbergSenior Member

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

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