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27584Re: Traditional Polytarp Sail

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  • PolySail_Dave
    May 3, 2013
      Wow, I got all fouled up on my photos, boats and sails on this thread.
      Seeing the correct boat and sail clarified the situation for me. Simon
      did a good job in paneling and sewing his sail, and I'm sure that this
      lug sail will fill well, but I would caution him about the use of the
      lightweight blue tarp material. Often that material has only a 6 x 8
      scrim and weighs less than 2.7 oz./sq. yd. Of all the polytarp material,
      this is the tarp that will stretch out of shape and disintegrate the
      fastest because it often carries no UV protection whatsoever.

      For future builds, especially if you choose to use a single panel
      method, I would recommend that you find a polytarp that weighs at least
      5.2 oz./sq. yd., has a 12 x 12 scrim, and offers UV protection. I
      sometimes use a UV-protected 3.1 oz. with a closely woven 10 x 10 scrim
      for racing sails, but I generally warn the customer about their tendency
      to stretch. I made a good-sized lug for Brad Hickman for the 2011
      PDRacer Worlds (which he won) but he managed to stretch the sail by
      applying a 6:1 purchase on the downhaul while trying to keep high
      tension on the luff. Now I recommend no more than a 2:1 purchase for
      these lightweight lugs and no more than 4:1 for the 5.2 or 6.0 oz.
      polytarp lugs.

      One additional note about polytarp weights. Polytarp weights are for a
      full 36" x 36" square yard of material and cannot be directly compared
      to the weights of most materials used by traditional sailmakers.
      Traditional sailmaking materials are usually measured and weighed by the
      "sailmaker's yard" which measures 28.5" x 36." So we are talking about
      the weight of a piece of material that is only 79% as large as a sq. yd.
      of polytarp. Thus a 5. 2 oz./sq. yd. piece of polytarp equates to about
      a 4.1 oz. piece of Dacron or other sail material for comparison
      purposes. I think some sailmakers think polytarp sails must be
      excessively heavy to come close to matching the strength of Dacron and
      other sail materials, but that is really not the case. Even the 2.7 oz
      material that Simon apparently used should be as strong or stronger than
      1.5 oz. spinnaker ripstop nylon.

      Dave Gray

      I hope this information helps for anyone planning to build as sail from
      polytarp in the future.

      --- In Michalak@yahoogroups.com, "prairiedog2332" wrote:
      > I think the sail referred to is also a lug sail? The one in this
      > http://woodenboatblog.com/node/594
      > To me it looks like the mast is too far forward but then the
      > is further forward than on the original plans as well? I think you
      > get a good idea of the CE just drawing the lines on a copy of the
      > Nels
      > --- In Michalak@yahoogroups.com, "PolySail_Dave" wrote:
      > >
      > >
      > > Oops! Correction needed, Simon. I misread the area of your sail as
      > 60
      > > sq. ft. rather than 69 sq. ft. I know the Bolger sail well, but I
      > don't
      > > know the dimensions of your sail. The position of the CE on your
      > > will need to be calculated. The CE is probably a little further back
      > and
      > > a little higher than the figures I gave you for the smaller sail.
      > > However, the CE calculation is easily done using the method I
      > mentioned,
      > > and it's always a good idea before doing a conversion.
      > >
      > > Dave
      > >
      > [Non-text portions of this message have been removed]
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