Dacron Sail Stretching

Discussion in 'Boat Design' started by Inquisitor, Dec 4, 2005.

  1. Inquisitor
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Here’s what I know (or think I do) about my premise. I’m sure you’ll tell me its already been done or why it can’t. I just want to learn the reasons why it didn’t work. Its set up like a math proof:

    1. I want to extend the optimum aerodynamic life of Dacron sails. I think it is hard to ditch a set of structurally safe sails just because they are stretched beyond their most efficient form.
    2. Dacron sails are cheaper than carbon/Kevlar reinforced Mylar (CKM) sails. Dah!
    3. From what I gather the main benefit of CKM sails is that they hold their proper aerodynamic shape better/longer than Dacron sails.
    4. If its not already known, it would be fairly easy to plot an elastic/plastic strain curve for Dacron.
    5. With closed form (and/or finite element methods) loads could be calculated on the Dacron sailcloth.
    6. This could be validated with experience by people with old stretched Dacron sails.
    7. Could Dacron sails be designed so that they stretch into the more desired optimum aerodynamic shape. Instead of starting with the proper shape and spending most of their life out of shape?
     
  2. RHough
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    RHough Retro Dude

    You're house bound aren't you ... :)

    What you want is cloth stability. The tight weave and finish of Dacron sail cloth is an effort to make the cloth stable.

    It's like FG cloth, not stable at all until the resin locks the fibers in place.

    Dacron is most stable in all dimensions when new. As soon as the finish starts to break down it becomes less stable. A Dacron sail can be just as fast as a composite sail until the first tack, then it gets slower each time the cloth flexes. It gets slower every hour of UV exposure. It get slower each time it's folded.

    Stretch is progressive it doesn't go from stable when new, then stretch to a new stable when old condition. There are two changes in sail shape. Elastic elongation, that is recovered when the load is removed and non-elastic deformation as the finish breaks down.

    Sail designers try to align the most stable dimension of the cloth along the load lines. As the sail ages the non-elastic stretch is minimized, but it's there from day one.

    If what you want was possible within the elastic limits of Dacron, a surface finish would not be required and the sail's shape would change predictably with load for years.
     
  3. nero
    Joined: Aug 2003
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    nero Senior Member

    I like where you are going with #7 and the rest of this post. Kinda like selling shoes that will feel great and last a long time ... after you break them in. Not great feeling, spring running shoes that go flat after a few months. smile

    From the Sailmakers's Apprentice, sails get recut several times thru their lives. Perhaps the material never stabilizes?
     
  4. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    Inquisitor,

    Yes they can, and sail-deformation under aerodynamic load is something I'm looking at in considerable detail, by taking results from CFD, putting them into FEA, then back into CFD etc.

    it requires adaptive meshing which is hard, but other than that, it is possible. A full Navier-Stokes solution is needed, so a few machines will be required by the time I've finished. It is quite a difficult problem to solve, and it will need specialist programs to do. It is soluble though.

    Tim B.
     
  5. RHough
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    RHough Retro Dude

    Even if bias load is reduced to 0 on each panel, you still have the nature of any woven fabric to stretch more than the fiber strength would predict.

    Lets assume that you invent a design with no bias loading, and have every yarn aligned with the local load. What keeps the yarns in alignment?

    Lets assume that you solve that problem without doubling the weight needed for the strength you need.

    When the windspeed changes or the sailing angle changes, the angle of attack changes, the sheet position changes and the optimum camber and draft position change. Will you still have all the yarns aligned with the loads?

    Is the load per unit area equal in all panels? If not are, are you going to have a custom fabric made to match the loads in each panel?

    I don't think it can be done with woven fabric. Let alone polyester.

    Each yarn can be compared to a single line. For a known load the stretch per unit load is known from the material's properties. For a design stretch per unit load the volume of the material is known. One of the reasons that polyester is not used for sheets and halyards is that the weight and volume is too high to control the stretch per unit load.

    In polyester, the "perfect" sail would be several times heavier than the same sail made from a high modulus fibre like Vectran. Why consider Dacron at all?
     
  6. Inquisitor
    Joined: Nov 2005
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    From the responses, I gather that my premise is (at least) not common practice. So, I’ll continue adding fuel to the fire…

    It’s called a rectal extraction… :rolleyes: I’m ignorant on the choice of materials for sails. All I know is that whatever is the most common (and probably cheapest) has a problem with stretching. For my little research project, I want cheapest… period. For my next boat, a long-term live-aboard cruising yacht, I will want the life/cost factor to be maximized. I’ll definitely look into the Vectran. But for the case at hand, I thought Dacron was still the most common.

    I don’t want to squelch your argument, but the premise is ONLY for permanent deformation… not dynamic or not elastic. I just wanted to address permanent “baggy” sail type deformation!

    I have access and most of the qualifications to run the analyses you are suggesting. If I really wanted to, I could probably take the code of the two programs and have them feed each other so that the solution converges without me messing with it. Although, I’m prone to complicating things sometimes, I was thinking of a little less rocket science.

    If sailcloth stretches anywhere near linearly (see attached picture) for years till failure, then there’s no merit in my premise. (BTW, how do I get the pictures in the body of text, like some other people do?) However, if sailcloth follows the more logarithmic curve (like most polymers) then we might have a possibility. Someone who has some baggy sails that they think are more up on the Time curve… say above Pick (this is an experience call, not an exact science.) They would measure the amount of length/width at some key points… and calculate the percentage that is permanent/plastic/baggy strain and take that out of the new cloth panel.

    A simple illustration - Say one panel normally starts its life with simple strait sides (in blue). A year later, they’ve grown to be (in red). The percentage (not the actual dimension) of unloaded strain can easily be calculated. This would then be used on the new sail to cut the new panel (in green) so that a year later, it will stretch out to the blue (optimized) cut.

    Or like you suggest… should I get out more!


    The only problem that I’m imagining is if the “year strain” is way too large, then the concaveness of the freshly cut sail will be too drastically different from what a sail needs to be to work properly and thus not get loaded properly and thus not ever strain to the optimized shape. This is where a little real world experience would far outweigh (and avoid) a lot of theoretical self-abuse! This is why I threw out this thread… so I could get out more. :p
     

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  7. RHough
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    RHough Retro Dude

    I think I understand where you are coming from.

    There are three causes of "baggy" that I can think of off hand.

    One is stretch of the fibre past it's elastic limit.

    Another is internal "creep" of the fiber. Some fibers have a property where the internal bond between individual molecular "strings" is lower than the tensile strength of the string. The strings move in sheer. In long term load (halyard) the creep causes the fiber to elongate while retaining its short term elastic properties. Think of a carbon construction in tension where the resin is not solid but has very high viscosity. Over time the structure elongates but the properties of the carbon have not changed.

    The third and most important is the progressive failure of the resin holding the fibers in alignment. From your work with composite lay-ups, you know that woven cloth has little strength on the bias until it is locked in place by the resin. Sail cloth is no different. Bias stability is achieved through very tight weaves and a flexible resin. As the cloth ages the resin breaks down and bias stability is lost.

    Each of these are permanent. Once bias stability is lost no part of the shape that relies on bias stability can be repaired. The cloth would have to be re-manufactured, the weave brought back into alignment and then locked again with new resin.

    For cheap non-stretch sails, use poly-tarp. Home Depot to the rescue again! :)
     
  8. Inquisitor
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Are you serious? :?:

    I would like to try several sail designs... thus cheap (near on disposable) is needed. But I need be able to correlate it to what a Dacron sail might achieve. And I don’t mean to this stretching topic… just in general. If you’re not just yanking my chain...
    How long would you think they would behave as designed? (ballpark: first hour, ten hours, hundred hours...)
    Could I make a relative value judgment about their power relative to Dacron, your Vectran, or Carbon/Kevlar/Mylar sails? (ballpark: 50%, 75%, 100%, 150%)
     
  9. RHough
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    RHough Retro Dude

    Absolutely serious. Ocean passages have been made with poly-tarp sails.

    All the engineering applies directly. You can use duct tape or instead of a sewing machine for seams, hot glue for reinforcement patches.

    http://members.aol.com/polysail/HTML/

    I wouldn't yank your chain, I may need your help with the composite spar design for my new boat :)
     

  10. Inquisitor
    Joined: Nov 2005
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    Location: North Carolina Mountains

    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    Damn! :cool: It's in my "favorites" now! :!: Thanks!!!!!
     
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