The design of soft wing sails for cruising

Discussion in 'Boat Design' started by David Tyler, Jan 19, 2014.

  1. David Tyler
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    David Tyler J. R. A. Committee Member

    I’m now absolutely certain that there cannot be a second, horizontal, axis of articulation in the battens. There can be a malfunction, with the noses tending to rotate about their axis so that they are almost “on edge”, as much as the mast diameter inside them will permit.

    What happens is this, as far as I can observe. The problem has only occurred on a dead run in a strengthening breeze, when I needed to haul down reefs with the sail full of wind. The batten, the hinge and the aft end of the nose all drop readily, but the forward end of the nose is pressed against the mast, and does not drop. Thus, it is at a large angle to the batten, perhaps 30 degrees. Then, it cannot help but be rotated, under the pressure of the strong breeze. I have to use the downhaul to bring it down, but by then, it’s too late. The nose will not return to its correct orientation until I round up into the wind. A photo is attached.

    Three days ago, I ran up Juan de Fuca Strait in the usual summer conditions of fog and near gale in the afternoon. I was happy to be under two panels, which was a good snug rig for the conditions, and much better than with my low AR fanned junk sail, which had larger top panels. This time, the battens rotated the opposite way to the photo, that is, the higher side was to leeward. I retired into Sooke Inlet to spend a day thinking about the problem.

    Yesterday I sailed from Sooke to Victoria, and when coming through Race Passage, again with a rapidly strengthening breeze needing some quick reefs. This time, I heard noises of something breaking. The top two sheeted noses have broken, and they have broken away from their hinges. To be fair, I had drilled through to attach some rigid bridges behind the mast, found that it was not a good thing to mix rigid bridges and cord/tube parrels, and taken them off again. The noses broke at this point. However, I tend to the view that they would not perhaps have broken if they had not been unfairly loaded by being capsized. Whatever the reason for breakage, I have to repair two noses, or make new ones.

    So while in Sooke, I did some thinking and drawing. It seems to me that there must be as much integrity in the hinge area as possible; the extra axis was a weak point. Also, there does need to be a rigid bridge bearing against the back of the mast, on all battens, not just on the two top sheeted battens, which are the heaviest loaded. This brings me right back to my original GRP wing sail battens. These were a poor shape, aerodynamically; did not articulate readily in light airs; and took a long time to assemble into the sail and around the mast.

    I now think that the planform of the sail was the main culprit in the poor articulation, as the current battens articulate well enough in light airs. The present configuration of the head of the sail, with the yard greatly reducing the compression on the top batten, works well. The articulation starts at the bottom and proceeds upwards, like a mexican wave, showing that the top battens still have a slight reluctance to articulate, but not to any great extent.

    I found it quite difficult to assemble this rig, with no bridge to resist the tendency of the heavy sail bundle to swing forwards, until I had rigged the parrels.

    So all in all, I want to return to having the mast enter the nose from the forward end, via a removable piece, as I had before, with the rest of the nose being all in one, bonded together, with better strength and reliability. Attached is what I have drawn for Shearwater’s rig [a 28ft cat ketch trailer-sailer]:

    You will see that there are:
    two simply made side pieces;
    a hinge box that is bonded to their aft ends, with its centre cut away to reduce friction;
    a bridge that is made by cladding 6mm HDPE sheet with carbon cloth, and bonded to their forward ends (this principle is working well on my yard);
    a nose made by cladding 6mm HDPE with carbon cloth, that is held on by two bolts, only one of which needs to be removed for the mast to enter. This shape can also be used for intermediate riblets.
    There would be a zipper in the sail, not at the centre of the luff, but around to one side somewhat, at the mast position.

    Clearly, these components cannot be entered into pockets in the sail, and would be laced or sewn in place; but once in, they do not need to be removed in order to remove the rig from the mast. This is important in the case of Shearwater, a trailer-sailer.

    I believe that this design answers the requirement for Shearwater, and can be scaled up for the larger boats.

    I hope that I will be able to make progress on rebuilding my noses/hinges during August. I think it would be as well to hold off from working on other rigs until I have got my rig absolutely sound and secure.

    I must say that I like sailing with this rig, problems with the battens apart. The performance is all that I wanted, and the handling - hoisting, reefing, gybing, heavy weather running - are easier than with a pure junk sail of this size. I am still going to persevere to get it right.
     

    Attached Files:

  2. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    I don't remember you mentioning the problem of the wishbones canting with your first wing sails. Why is it happening with this sail? With your previous sails the mast was more centrally located (longitudinally) within the wishbones. Did that configuration better stabilize the wishbones?

    At this point, if you were free to relocate your mast anywhere you wanted on your boat, where would you locate the mast within the wishbones?

    How do you adhere carbon fiber to HDPE?

    With your current sail did you zipper the sail at the batten hinge point? What advantage is there to instead zipper it at the mast?

    Are you at a point where you can give general design or shape ratios for your wing sail, assuming that the configuration of a given boat doesn't require a specific compromise?
     
  3. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Maybe you can prevent the wishbones from canting by installing light, cross-bracing lines between the wishbones at their widest point. One problem that might result is if the loose lines foul when reefed. It might be possible to solve that problem by sewing the cross-bracing lines to a panel of light-weight cloth, so there are no loose loops. It may not be necessary to install the cross-bracing lines at every sail panel; maybe only every other panel, or every third panel?

    [​IMG]
     
  4. Yobarnacle
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    Yobarnacle Senior Member

    One of the things I admire about the designers of ancient Chinese junks, was their freedom from the constraint of symmetry. Thinking outside the box.
    While western craft for centuries place their masts on the centerline, not the Chinese.
    They installed their masts at the gunnels where the bulwarks formed one side of the tabernacle.
    The masts were staggered port and starboard, so not blanketing each other, giving better clear air to the sails.
    And the total arrangement provided for an unencumbered main deck for work deck and deck cargo and numerous cargo hatches, which WERE aligned down the centerline.

    Just in case you plan to relocate mast. :)
     
  5. David Tyler
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    David Tyler J. R. A. Committee Member

    1) I believe it's purely a function of having introduced the second axis of articulation.

    2) I don't believe that was the case.

    3) Currently, the mast is at 6% of chord, and this gives enough room forward for the sail to flake down without creasing it too much. I wouldn't want it further forward. I've tried a mast position at about 40% of the distance between luff and hinge, and the articulation becomes rather hesitant. I would put the mast anywhere from 6% to 15% of chord.

    4) With toughened epoxy, WEST G-flex or similar. Preparation is important - 80 grit sanding, isopropyl alcohol wash, and flame treatment to oxidise the surface. While Gougeon give figures of about 1 ton/sq in, I get rather variable results, and would like to get away from total reliance on this bond. However, it will be fine where there is a large contact area.

    5) Yes.

    6) Aiming for easier assembly, but if the nose assemblies are laced in, not slid into pockets, there may be less reason to go this way.

    7) I can only give the proportions I'm working with, with no guarantee that they are optimal:
    luff length/chord = 2.1
    eight sheeted battens, horizontal or at 5 degrees
    yard length a little more than half the chord, to get its after end aft of the lifts, and yard angle 15 or 20 degrees
    top, unsheeted batten shorter than the others and midway in angle between other battens and yard.
    batten pitch = 26% of chord

    I've thought about adding a stabilising panel of cloth between the sides of the sail. It might be possible, and it might work, but only if the noses are very tightly attached to the sail, and mine aren't, they are in slack-fitting pockets. This allows them to tip, independent of what the sail is doing.
     
  6. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Thanks for the information. What is the chord of the wishbone batten vs the chord of the tail battens? What is the thickness of the wishbone camber vs the chord of the wishbone?

    In case I didn't make it clear, I intended the cross-bracing lines to be attached to the battens, not the sail. Thus, they might prevent the wishbones from canting even if the wishbones were loosely attached to the sail. It might be easy to add them to your current sail without any permanent modifications.

    [​IMG] [​IMG]
    note added cross-bracing lines in second photo
     
    Last edited: Aug 2, 2014
  7. David Tyler
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    David Tyler J. R. A. Committee Member

    The hinge is at 40% of the chord. The wishbone has a thickness/chord ratio of .33
     
  8. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Thank you for the added information. I've added photos to clarify my last post.
     
  9. David Tyler
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    David Tyler J. R. A. Committee Member

    I understand what you mean,but if the sail is greatly twisted, the diagonal lines would actually cause the noses to capsize.
     
  10. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    I'm having trouble visualizing how the cross-bracing lines would cause the noses to capsize? (I am assuming that the phrases "noses to capsize" and "noses on edge" and "wishbones canting" each mean the same thing) It seems to me that the addition of these bracing lines would actually work to resist sail twist. Maybe it would be beneficial to install two sets of cross-bracing lines -- one set directly aft of the mast, and another set halfway to the hinge point -- though their presence would conflict with the internal topping lift you use to support the hinges.
     
  11. David Tyler
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    David Tyler J. R. A. Committee Member

    Consider the situation when I've just started to reef, on a dead run in a stiff breeze. I've eased away the halyard, and the top of the luff has come down, but the lower luff is pressed against mast, and won't drop until I apply the downhaul. So the sail panels are slack, the battens are closer together than they should be - and so the diagonal lines would also be slack and inoperative, just at the very time when I need them to work well.
    A good thought, but wouldn't work in practice.
     
  12. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Ah, yes. But if the battens are closer together and the diagonal lines are slack, then it wouldn't be the fault of diagonal lines that the nose battens had capsized. It seems to me that simply by tensioning the downhauls to take the slack out of the panels, the now-taut diagonals would do their duty and right the capsized nose battens. Of course I understand your preference that the battens not capsize in the first place...
     
  13. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    In again looking at your capsized sail (below) I note that the top panel appears to be completely taut, and the lowest panel is most loose and most capsized. I'm guessing that the leeward side of the sail is completely taut, and possibly is not capsized at all. (Is the top nose batten completely horizontal transverse to the chord, or is it tipped too, just not as much as the lower battens?) This suggests to me that a nose batten capsizes primarily by lifting one side up.

    I'm now thinking that if diagonal bracing lines were installed the whole sail would collectively have to go slack (all raised panels on both windward and leeward sides) for any of the individual panels to capsize. The exceptions would be the lowest raised panel and any panel(s) already reefed. These two groups of panels would not have a lower cross-braced panel to prevent one side from lifting, and so if capsized would have to be righted by tensioning their respective downhauls. But the upper part of the sail might automatically be prevented from capsizing

    [​IMG]
     
  14. David Tyler
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    David Tyler J. R. A. Committee Member

    I remain 100% sceptical.
    Any views from anyone else on this?
     

  15. Nick_Sinev
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    Nick_Sinev Junior Member

    A quotation from "Flexsys" discussion http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/flexsys-51054.html


    Sails already done:

    http://soft-wing.ch/en
    http://www.omerwingsail.com


    My proposal:

    [​IMG]

    The profiile is based on the combination of a circle (leading edge) and a quadratic parabola. I have reasons to think that cubic parabola for this task could do better, but I was trying to compare my ideas with NACA symmetrical profiles, which are based on quadratic parabolas.
    The circle touches the parabola at the point with the same curvature radius.

    The roller can move to the other side of the wing when tacking.
     
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