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Re: Digest Number 4048 (Spar Stiffness.)

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  • PolySail_Dave
    Scott, Your experience with polytarp sails is not all that unusual. I mentioned in an earlier post that polytarp sails are probably more forgiving than sails
    Message 1 of 16 , Apr 30, 2013
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      Scott,

      Your experience with polytarp sails is not all that unusual. I mentioned
      in an earlier post that polytarp sails are probably more "forgiving"
      than sails made with other polymer laminates or woven materials. The
      reason, as I see it, is their stretch and resiliency--the ability to
      fully recover from any stretching caused by pressures up to an amazing
      amount of lb. per sq. ft. (In one very simple demo, I hauled my 190 lbs.
      up into a small Bermuda sail suspended off the ground by ropes at each
      corner. Afterwards, the sail was not visibly stretched in any way and
      their were no failures even around the brass grommets.) It is also the
      way that polytarp stretches that makes it best used as a single panel
      rather than being divided into panels as with other "stiff" sail
      materials which are normally paneled by sailmakers perhaps because most
      woven sail materials normally can't be purchased in large widths like
      tarps and other laminates. With polytarp, if you want to control stretch
      more, you go up in weight from 3.1 oz. to 4.5 oz. to 5.2 oz. to 6.0 oz.
      to 8.0 oz. etc. I posted a file from one of the largest Chinese
      manufacturing companies of polytarp that is interestingly, owned by a
      woman and "green." It explains the nature of the material in both
      simple and chemical terms and I've added a few notes from my own
      research over the years about the strength and stretch of this unique
      material. I listed the file under "Polytarp Manufacturing" in the Files
      section if anyone is interested in the details of its manufacturing
      process. I found it interesting that the scrim was described as nylon in
      this piece which might account for the stretch or it might have been an
      interpreter's error. I think the scrim is actually polyethylene strips
      woven together.

      Dave Gray


      --- In Michalak@yahoogroups.com, "tsgosnell" wrote:
      >
      > I agree with Nels, my latest sail was made in a bit of a hurry and the
      layout was done by myself. I was worried the the rounds were a bit off
      and I think I had too much and or too little, just dint look as fair as
      it should. But time being an issue I pressed on, thinking I would only
      be out a small amount if I had to redo the poly sail. I also thought my
      new spars were too flexible, but to my surprise it sails just as well if
      not better than the set I took 2 weeks to complete. Go figure? Better
      yet, just go sail!
      > Scott
      >
      > --- In Michalak@yahoogroups.com, "prairiedog2332" nelsarv@ wrote:
      > >
      > > Simon,
      > > I don't think there is any need to go into it too deeply. Same with
      how
      > > much "round" and "hollow" in the sail, and how much luff tension is
      > > required in a lug sail. Same with yard height and mast canting. A
      > > little bit goes a long way and can be beneficial. But not a
      substitution
      > > to just getting out and sailing!
      > > Some flex is useful in the yard to spill wind in gusts but the yard
      > > should never be allowed to flex ahead of the mast when broad
      reaching
      > > for example. Some flex in the boom makes up for a sail without
      enough
      > > rounding on the foot. Some cant in the mast can help keep the boom
      up
      > > above the water when sailing off the wind. The luff should be dead
      > > straight as should be the trailing edge on the leeboard. These are
      > > things that stuck in my mind.
      > >
      > > Nels
      > >
      > >
      > > --- In Michalak@yahoogroups.com, "simonfbroad" wrote:
      > > >
      > > > ah Nels, I love posts the get lots of people involved, especially
      when
      > > there are so many experts, all with a different opinion :o)
      > > >
      > > > Todd didn't mention the tape method that I recall.
      > > >
      > > > He did say that a lot of the measurements were based on experience
      and
      > > judgement rather than pure maths (to me that makes it an art form
      rather
      > > than a science - bet that gets some responses), not only the amount
      of
      > > bend in the spar needs to be taken into account, but where in the
      spar
      > > the 'centre' of the bend is, it might not be in the centre of the
      spar,
      > > and does it bend more at one end than the other, etc.
      > > >
      > > > I will try and find the thread on WB again and extract the advice
      on
      > > allowing for spar bend. Funnily enough, I got a great deal of help
      and
      > > learned a lot, but the original question I posted was never
      answered.
      > > >
      > > > Simon.
      > >
      > >
      > >
      > >
      > > [Non-text portions of this message have been removed]
      > >
      >
    • simonfbroad
      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
      Message 2 of 16 , Apr 30, 2013
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        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.
      • prairiedog2332
        Sounds like a great plan to me for reasons you give. It might even do better than the designed sail when reefed. Traditional sharpies apparently often carried
        Message 3 of 16 , Apr 30, 2013
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          Sounds like a great plan to me for reasons you give. It might even do
          better than the designed sail when reefed. Traditional sharpies
          apparently often carried a 2nd smaller sail and you can use it while
          making a larger one to original specs.

          Nels


          --- 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.
          >



          [Non-text portions of this message have been removed]
        • PolySail_Dave
          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
          Message 4 of 16 , May 1, 2013
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            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
            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
            Message 5 of 16 , May 1, 2013
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              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.
              > >
              >
            • prairiedog2332
              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
              Message 6 of 16 , May 1, 2013
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                I think the sail referred to is also a lug sail? The one in this photo.

                http://woodenboatblog.com/node/594 <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]
              • simonfbroad
                Nels Yes, the mast is a long way forward, and that is because of the centreboard. The original plans had a leeboard and the mast behind the forward bulkhead.
                Message 7 of 16 , May 1, 2013
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                  Nels

                  Yes, the mast is a long way forward, and that is because of the centreboard. The original plans had a leeboard and the mast behind the forward bulkhead.
                  There is a diagram on the WB thread that gives the actual size of the sail and shows the CE lining up with the trailing edge of the centreboard.

                  Mathematically that all worked out.

                  When I sailed it first time the centreboard jammed, I jibed it too sharply without moving my bulk to the other side of the boat, and the mast step (the only part I didn't use thickened epoxy to glue) gave way.
                  The boat is now at home being re-modeled (back to David Beede's original design).

                  Simon.

                  --- In Michalak@yahoogroups.com, "prairiedog2332" <nelsarv@...> wrote:
                  >
                  > I think the sail referred to is also a lug sail? The one in this photo.
                  >
                  > http://woodenboatblog.com/node/594 <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
                  >
                • 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 8 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 9 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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