Rotating Wing Mast – theoretical discussion

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

  1. national
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    national Junior Member

    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.
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    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
     
  3. national
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    national Junior Member

    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
     
  4. Man Overboard
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    Man Overboard Tom Fugate

    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?
     
  5. national
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    national Junior Member

    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.
     
  6. Timothy
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    Timothy Senior Member

    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.
     

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  7. Eric Sponberg
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    Eric Sponberg Senior Member

    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
     
  8. Timothy
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    Timothy Senior Member

    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?
     
  9. Man Overboard
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    Man Overboard Tom Fugate

    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).
     
  10. PI Design
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    PI Design Senior Member

    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.
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    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
     
  12. CT 249
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    CT 249 Senior Member

    "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.
     
  13. PI Design
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    PI Design Senior Member

    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.
     
  14. BOATMIK
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    BOATMIK Deeply flawed human being

    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
     

  15. BOATMIK
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    BOATMIK Deeply flawed human being

    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
     
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