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Re: Traditional Polytarp Sail

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  • simonfbroad
    Dave Thanks for the input. I used Jim s method to determine where the CE should be before starting the sail. I had changed David Beede s original layout so
    Message 1 of 16 , May 1, 2013
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      Dave

      Thanks for the input.
      I used Jim's method to determine where the CE should be before starting the sail. I had changed David Beede's original layout so determining the geometry or mast, sail, foil had to be done from scratch. There is a diagram I made on the WB thread mentioned earlier.

      I hadn't thought about taking a tape measure to the actual sail itself and verifying it's CE. I'll do that before I start the Mayfly. First of all I need to remodel the SB back to David's original layout - my changes were unwise (and contrary to David's advice) but it's been a highly educational project and I feel much better equipped to take on the next one.

      Simon.

      --- In Michalak@yahoogroups.com, "PolySail_Dave" <polysail@...> 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 sail
      > 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
      >
      >
      > --- In Michalak@yahoogroups.com, "PolySail_Dave" wrote:
      > >
      > >
      > > Simon,
      > >
      > > I just sent out a sail to Gene Berry who is also converting his Mayfly
      > > to a 60 sq. ft. leg o' mutton (Bolger's name for this sharpie sail.)
      > > Just for your information, the CE of that sail normally falls at about
      > > 35"-36" back of the luff and about 47"-48" up perpendicular from the
      > > foot. However, it's always best to check the CE directly on the sail
      > > using Jim's method. I do it with three 1" wide straight tape measures
      > > from the corners to the midpoints of the opposite sides. You might
      > have
      > > to adjust the mast or leeboard position for best handling if you go
      > > forward with this conversion. Gene assured me that he could simply
      > > adjust the rake of his mast to bring the CE into the right position.
      > > Maybe you can do the same. You want the CE about 2"-3" aft of the
      > > centerline of the board for slight weather helm and best handling.
      > >
      > > Dave
      > >
      > >
      > > --- In Michalak@yahoogroups.com, "simonfbroad" wrote:
      > > >
      > > > Nels
      > > >
      > > > Found a couple of photos of the finished product.
      > > http://woodenboatblog.com/blog/2463
      > > >
      > > >
      > > > Doesn't have quite the polished professional look of one of Poly
      > > Dave's fine productions, but it was a hell of a lot of fun to make it
      > > from scratch, and I learned a lot about sails doing it.
      > > >
      > > > Although only 69 square feet, I'm thinking to put it on my Mayfly 16
      > > (when I get to build it) so that it is underpowered and I can learn to
      > > use her gently, and without messing about reefing. Does that sound
      > > feasible?
      > > >
      > > > Simon.
      > > >
      > >
      >
    • PolySail_Dave
      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
      Message 2 of 16 , May 3, 2013
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        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
        photo.
        >
        > http://woodenboatblog.com/node/594
        >
        > To me it looks like the mast is too far forward but then the
        centerboard
        > is further forward than on the original plans as well? I think you
        could
        > get a good idea of the CE just drawing the lines on a copy of the
        photo?
        >
        > 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
        sail
        > > 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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