Wingmast - how much kick is needed?

Discussion in 'Sailboats' started by sigurd, May 20, 2016.

  1. gggGuest
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    gggGuest ...

    Not in my experience. Loads coming from battens and sailcloth are orders of magnitude smaller and not a problem.

    Test ship for a scaled rig shouldn't be a big problem any cheap beach cat will do, and when you've finished you can pass it on without much use, so shouldn't depreciate too much. Just don't make any permanent changes. As such things are fashionable you might even be able to sell the test rig if not too snagged.
     
  2. sigurd
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    sigurd Pompuous Pangolin

    Catsketcher, one of the C-class guys mentioned that the difficulty of controlling high AR soft sails was one main reason hard wings came out better for them.
    I think this rig I sketched is about the AR of a normal A-class rig.

    Gary, why didn't Phillips just tie some string from one half of the 'bone to the other, through holes in the sail, making the 'bone superstiff?
    "split the main into two sails just above the single STRAIGHT boom attached on the heavily reinforced position where the sail track would have originally run. "
    That sounds workable too, similar to a 49'er in principle except 49'er has upside down vang (and ofcourse they'd have a sheet track to control twist when beating)?
    Thanks for the pics.

    ggg, interesting, do you have any pics or example boats I could google? I think I might see what you mean after all. If you rotate the boom or the mast then, you have to fiddle with the outhaul, right?
     
  3. Skyak
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    Skyak Senior Member

    Lots of great experience and advice here but it appears to me that this is a higher aspect sail on a shorter boom than any of us have tried -it's extrapolation not interpolation.

    I don't have experience of others here, but since nobody has gone to this extream I think we need to quantify the forces with a model to relate to any existing solution -back to engineering.

    The way I would model your rig is panel by panel loads on the battens applying force to the fiber (actually the full sail material properties) running from the clue to the halyard attachment. This can be done on a spreadsheet for your rig geometry and compared to the predicted and actual results of common geometries. The importance is capturing the increased leverage of your extreme high aspect and getting the material properties right.

    My opinion is the best option is the wishbone at some angle (to be calculated) and connecting the arms will greatly stiffen the wishbone (catenary again). But I also think you might find that extending the foot lowers the stress and weight of the rig greatly. Rectangular sail might just be inferior to a trapezoid.

    Wild idea -could you do a junk rig style sheet to some battens?
     
  4. sigurd
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    sigurd Pompuous Pangolin

    I just checked this again, was a long time since I first drew this.

    A-class:
    14m2
    9m mast
    8.5m luff?
    ergo AR = 5.15

    This here rig we are discussing:
    21 m2
    10.8m luff
    AR = 5.55

    So you are right, it is a fair bit more radical than the A, when considering the relatively longer mast chord and the more rectangular shape. The boom length was sacrificed to not get in the way of something, but I could be more accommodating (probably will).
    Outside of that little issue I'm pretty sure the tapered planform you suggest is better.
    Possibly the A-class guys knows the sail just gets ugly if they make it any taller, and is thus already at the limit of feasibility. They have a bendy mast which changes things in unpredictable ways (for me) too.

    EDIT: Junk sheets: Sure, why not. Be neater without it.

    EDIT: Gary, could you provide the approximate measurements of that sail in the pic?
     

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  5. gggGuest
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    gggGuest ...

    I haven't got any photos from when I ran a wing mast on a stump I'm afraid (it didn't last long - the tech available to me in the early 80s was *not* good enough to make a stump work. Needed carbon). The stump setup with gooseneck on the stump is standard in modern Moths, and also gets used in 12 ft skiffs and some Cherubs, but not usually with wing masts. The Moths have a rotating mast, but not an over-rotating one as the wing spar needs to be.

    The most recent shot at it I can recall was Alex Vallings on an R class (~ 12ft skiff). He writes about it here. http://www.c-tech.co.nz/ArticlesVideos/C-Tech Wing Masts.html . In practice I don't think he's carried on with the concept.

    I don't recall ever noticing the outhaul as an issue. The mast boom relationship tends to stay the same (or a mirror image!) The effect will be the same on each tack, so you only need to alter it if you change the amount of rotation, which is likely to be only upwind/downwind transition or if the weather changes significantly, when you may well be tweaking the outhaul anyway. But in practice I think the mast rotation will only be changed in quite a small range of degrees, which, depending on your pivot point, may well mean the effect is minimal. TBH I'd say its the very least of your worries.
     
  6. sigurd
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    sigurd Pompuous Pangolin

    Thanks a lot ggg, investigating...

    I forgot to mention, this rig can spin as many times as it wants as long as the two sheets are uncleated.
     
  7. Gary Baigent
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    Gary Baigent Senior Member

    Connecting string between the wishbones for stiffness? What happens when you want to reef? More stuff to adjust/tangle when Sod's Law hits the fan.
    Check out the 49er lower double luff and reverse vang setup; works for them ... but they never reef. And you're carrying a tall. tall rig that will have to be reduced in a blow.
    FWIW, I'd think about the true double luff main with a D shaped cross section mast - where you have two mast tracks on the outside of the D and the boom internal - and that could be set up on a short middle track (Like sliding spinnaker boom mast setup) so you could tighten or loosen the clew for varying wind strengths. On multihull with high aspect ratio rigs and high apparent wind, in my experience, the foot is mostly kept near flat; only in drifting stuff will you be easing the outhaul pressure. Problem with double luff is weight/double batten set - but then the double sail can be built light with modern fabrics and design.
    What many people forget is that the wing mast HAS to be over rotated for correct mast/sail flow. And that means room and adjustments for the spanner to swivel to around 80 degrees either side.
    And this means the hounds have to be set forward on a beak at the mast leading edge - otherwise, if stays are mounted on mast sides, it will lock up. I see this all the time on many other rotating designs. So all they get, at most, is only 40 degrees rotation either side.
    The sail rig on Groucho is 15.5 metres x 3m, and I think, 1.5m or thereabouts at sail head - and the mast is 520mm chord.
    Just realized you're talking of freestanding mast so disregard my rigging comments.
    I built freestanding wing masts for the Cox's Bay Skimmer, but changed over to lightweight stayed rigging. So I'll keep my mouth shut.
     

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  8. OzFred
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    OzFred Senior Member

    No it isn't. For all current production Moths, the boom is bolted to the mast, there is no gooseneck.

    Bolting the boom to the stump means the stump must rotate. The only recent production Moth that did that was the Assassin, and most owners chopped the top off the stump and bolted the boom to the mast to lower the mast and sail.

    [​IMG]
    Hayama Moth Worlds photo gallery.
     
  9. gggGuest
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    gggGuest ...

    Oh gosh yes, I hadn't spotted that properly either. That's a mighty big ask with that aspect ratio. I guess it's done in every modern airliner, it must be possible, but the structural implications and calculations would scare me off.
     
  10. sigurd
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    sigurd Pompuous Pangolin

    I scaled and rotated the three rigs, A-class (blue), Groucho (red) and my sketch (yellow), the comparison looks a bit worse for mine than I thought. Ok, so I'll extend the boom a bit.

    corrected sketch
     

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    Last edited: May 28, 2016
  11. CT249
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    CT249 Senior Member

    A few points of observation;

    * In wing masted cats when going upwind we routinely change rotation over a range of about 45 degrees, at a guess from tuning guides that are calibrated by referring to the part of the boat that the spanner points to. That may change three or four times up a gusty beat. The top cat sailors (which excludes me) may change more often. So whatever rotation system you use will have to work efficiently.

    * Some top sailors in competitive classes de-rotate completely in a breeze, to obtain the correct gust response, so you may not always be using over-rotation at all. At other times you'll be close to 90 degrees. So in (for example) fairly fast cats you have to have a range of rotation of about 85 degrees each side, as noted already.

    * Nope, the outhaul is not normally adjusted as rotation changes.

    * As noted already, connecting strings between wishbones are extremely problematic because the relationship between the points you connect the string to the booms and the location of the hole through which the string passes will change according to rotation, outhaul, downhaul and twist settings. So you would end up with a big hole in the sail, which would surely cause significant issues.

    * I've raced against a world champ and boatbuilder who had a D mast with double surface sail. With that rig he was slower than a wet weekend in Dapto, and after much effort he ditched it. Windsurfers have been using double luffs since 1984 and after an enormous amount of costly dedicated professional experimentation have settled down to small double luffs that are only worth having if you are a dedicated racer or speed merchant. That seems to indicate that the scale of the benefits that some wind tunnels have shown do not occur in reality.

    * Even with 6m sails, some windsurfer booms at 90 degrees to the mast bend and affect sail shape. As others have noted, higher angles won't work.

    * I still remember a regatta when we had Glen Ashby and Steve Brewin on their As. Downwind in the light stuff they were getting smoked by Mosquito cats with kites. The Mossie is sort of like a baby (16') Tornado from about 1972, with a very small low-aspect rig on it.

    Theoretically, Gashby and Stalky with their exquisite carbon As and high aspect rigs would have been flying. In reality, they were getting killed by the Mossies and the Hobie 16s were doing very well downwind in the light stuff. The Taipan 4.9 in cat rigged version was (according to Glen) faster in very light winds than the As, yet they were running low and square (EDIT- to use the term 'low and square' as our fast cat sailors use it in such a context, which doesn't mean dead downwind) and finding the Hobie 16s hard to beat.

    That was just one example that indicates that in real life, the theoretical benefits of very high aspect ratios are not borne out. I wish they were; high AR sails are really pretty. But a lot of the time they don't actually work as some people claim they do - so are they really worth the hassle?
     
  12. sigurd
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    sigurd Pompuous Pangolin

    The sails made for frequently falling in the water have a small luff pocket. The sails that go faster have a longer luff pocket. Some of the fast sails have a medium size luff pocket.

    High AR rigs? In general? That's out of my area of expertise. They can have lower parasite drag for a given tallness.
     
  13. Gary Baigent
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    Gary Baigent Senior Member

    Sigurd, Your art work for the Groucho main, I think does not take into account the half metre wing mast chord.
    Here's a shot of Sid's rig - which is maybe a better comparison.
    I'm just guessing, CT, that the double luff rigs you have observed or heard about, did not rotate the masts correctly, (check out correct mast over rotation in 2nd and 3rd photographs of Miss Lancia) were too flat, leading edge pointing directly to windward ... and that is always slow. In fact I've found that setups like so, flattened mast rotation to near fore and aft, is what i do to slow boat in too strong wind conditions.
    Although the double luff Italian C Class challengers Signor G and Miss Lancia were beaten by more sophisticated C Class hard sail rigs ... they performed well enough to provide good competition - and one of them cleaned up a Lake Garda race against some decent and numerous and much larger competition. And I wouldn't write off double luff sails with the developments that are being pursued today.
     

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  14. Skyak
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    Skyak Senior Member

    Down wind AR of 1 is theoretically highest performance. The theory didn't fail, the conditions just predictably favored the old boats downwind.
     

  15. sigurd
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    sigurd Pompuous Pangolin

    Ah, I misunderstood about Groucho: Correcting my sketch (above) now (after the plan in 'alternative to marvellous Buccaneer' thread). So the sail is exactly the proportion of the A-class one I found. Megapretty Sid sail, please some approx numbers? Thanks for your help.
     
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