Sail Making: Broadseam versus Roach

Discussion in 'Sailboats' started by AlanX, Mar 23, 2022.

  1. AlanX
    Joined: Mar 2022
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    AlanX Senior Member

    Can roach on the leach and foot of a sail still work to shape modern sail cloth (i.e. Dacron).
    See this example:
    http://www.tackingoutrigger.com/malibu_outrigger_2.GIF

    Note: The Malibu sail cloth is probably "duck" or sailing canvas (cotton), which would not be considered a low stretch material today.

    Or is broadseaming (sowing the shape into the sail) required?
    If I was to design a broadseam for a 10% chamber at 40% of the cord,
    how much should I expect it to stretch (order of magnitude)?

    Okay, I am not talking about high performance sails used in the America's Cup,
    but your typical DIY daysailer (<10m LOA).

    Regards AlanX

    Postscript:
    I just noticed I am a Senior Member? How did that happen? My innocence has been lost!
     
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  2. gonzo
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    gonzo Senior Member

    You need to know the forces involved, the deformation of rig under those forces, and the physical characteristics of the fabric to calculate the stretch. However, the stretch of the fabric will affect the deformation of the rig and the distribution of forces. It is a really complex calculation. For the boat you show, a flat cut sail would be fine.
     
  3. Tops
    Joined: Aug 2021
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    Tops Senior Member

    I am a sucker for those old Popular Mechanics build-a-boat-at-home articles...
    Seems like a person has more rounding/roach information than the straight line luff, foot, leech.
    Scientific Wild A-- Guessing at the straight line lenths and putting it into Sailcut CAD I get the following.
    Mind you that I don't know what all the buttons do in the software but it looks cool anyways.
    It does look like the rounding is doing most of the heavy lifting in the sail shape and that one's ability to trim the sail are probably more important than the actual shape. malibu_dev0.png malibu_dev3.png malibu_dev1.png malibu_dev2.png
     
  4. AlanX
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    AlanX Senior Member

    Hi Tops,

    Sailcut uses broadseaming for sail shaping.
    I thought I was losing my mind for a moment when I could not see the luff roach as the yard is bent.
    Not a good example of what I wanted to show.
    Actually when I go back and look for examples it was hard to find one.
    But I found one in (16-ft-sailing-scow.pdf - attached file).

    I think however, no-one used roach on the luff (or foot) any more as no-one uses "duck" (cotton) sailcloth any more.
    So really my question has answered.
    You can expect a sail to hold its shape if made of Dacron.
    Old sail designs with roach on the luff and foot should be reworked if using Dacron.

    Regards AlanX
     

    Attached Files:

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  5. Tops
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    Tops Senior Member

    The development from Sailcut CAD does not appear to have broadseams. The panel edges are curved and the seam width is uniform. Some coffee break action from work:
    20220324_100226_resized.jpg 20220324_101634_resized.jpg 20220324_101855_resized.jpg 20220324_101910_resized.jpg
     

    Attached Files:

  6. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    Luff round will put shape in a terylene sail, albeit well forward. If that wasn't the case mast bend wouldn't be as powerful a tool for racers to control sail shape as it is. So ideally you would use both.
     
  7. AlanX
    Joined: Mar 2022
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    AlanX Senior Member

    Hi Tops,
    By broadseaming I mean using a seam to make a curve.
    So cutting a curve and sowing a "straight" seam has the same effect of sowing a straight edge with a curved seam.
    Forgive me if my terminology is not exact.

    Hi gggGuest, this is my conclusion, don't do it for real sail cloth but it works well for "stretchy" materials.

    I have found a paper on the sail cloth numbers I need and will have a go at estimating the sail cloth stretch shortly.
    But today I go on a road trip with my partner.

    AlanX
     

    Attached Files:

  8. Tops
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    Tops Senior Member

    AlanX, my apologies. My mind was stuck on 'broadseaming' equals 'tapering' and not ALL cutting and sewing in shape outside of rounding. There is nothing wrong with your terminology and is better used than mine. Thanks again for the old plans and documents and looking forward to what you conclude about your project.
     
  9. messabout
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    messabout Senior Member

    Get a copy of the Sailmakers Apprentice book. The book will resolve all the questions and will also guide the DIY person to produce a sail that works.
     
  10. AlanX
    Joined: Mar 2022
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    AlanX Senior Member

    Okay I am back from my road-trip.
    I did do some order of magnitude calcs on sail stretch.
    I wont go into the details here as I suspect no one is interested in the details.
    Using the data on the Dacron TNF 240 (~8 oz), a strong breeze (44km/hr) and a 6m sail cord, I estimate 8% increase in sail depth for a 10% sail depth.
    I think my estimate would be +/-50% (say 4% to 12%).
    So in normal use, the stretch in modern sail cloth is pretty minimal.

    Regards AlanX
     
  11. messabout
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    messabout Senior Member

    Don't sweat the stretch chrematistics of modern sail cloth. If you want to build a good sail, get modern fabric and go from there. A good dacron sail cloth will last for years if you treat it with reasonable respect. If you plan to sail in the wildest of extreme conditions then maybe consider mylar.
     

  12. AlanX
    Joined: Mar 2022
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    AlanX Senior Member

    Hi Messabout,

    You are correct.

    The investigation into sail stretch was due to comments (on the WoodenBoat forum) that basically designing a sail shape (mathematically) was a waste of time due to sail stretch.
    I landed here (from WoodenBoat) as the forum comments are usually pretty reasonable.
    Although there is little information on the Internet with regard to real world data for sail/sailcloth performance, I can now satisfy myself that sail stretch IS minimal.
    Sure, Dacron can stretch ~12% before failing, but sails operate at a small percentage of the ultimate strength (~3% in my example).

    Now don't get me wrong here, if I was making a sail I would almost certainly use SailCut.
    But here is an example of what I was playing with (10% chamber at 40% cord gunter rig):
    SailDesign4Seam2.png

    and a paper cutout:
    Gunter.jpg

    I still have a couple of open questions:
    1. What safety factor do sail designer use?
    2. What gust factors do they use?
    3. What strength factor do they use for seams?
    4. What factor for UV degradation during service life.
    When I consider these issues I suspect if may be as low as 15% of the ultimate strength.

    Regards AlanX
     
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