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7816Re: To Cat or not to Cat, was re:Tyvek

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  • Dave Womble
    Jan 5, 2005
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      "1) Do you use grossgrain or webbing on your tarps around the
      perimiter seams? I can't tell from the pictures."

      No, I've never understood why folks do that, I just hem the edges.
      On the 130"x126" tarp that I configure as a hammock tarp tent I put
      velcro on the ends so that I can close the ends off when I want to
      and that shows up as a black edging in the photos.

      "2) How do you determine the "amount of sag" over a given distance
      for best results. I always thought it was about 1 inch of sag for
      every 1 foot linear, but it looks like you use lesss than that. It
      looks to me like Brian uses a lot more sag in his tarps."

      I determined it by guesstimation. I looked at photos, also set up a
      rectangular tarp in the backyard, stretched it taut into a catenary
      ridgeline as a tarptent and came up with 5" for 10'. I then just
      generated the curve on a spreadsheet and plotted it on templates of
      up to 12 feet. I wouldn't recommend comparing Brian's curves to
      mine, although it appears we are doing the same thing, we are doing
      it in a little different way since I use the catenary curve on the
      ridgeline and he doesn't. I'm kind of using surgery and he is kind
      of using brute force. There are some specific stretch
      characteristics of the orthogonal weave silnylon that comes in to
      play along with the amount of tension you apply to get a taut tarp.
      Anyway, here are some points along the curve I use on the templates:

      For 12 foot template
      0' (center): -7.22"
      +/- 0.5': ...-7.14"
      +/- 1.0': ...-7.02"
      +/- 1.5': ...-6.77"
      +/- 2.0': ...-6.42"
      +/- 2.5': ...-5.97"
      +/- 3.0': ...-5.42"
      +/- 3.5': ...-4.77"
      +/- 4.0': ...-4.02"
      +/- 4.5': ...-3.17"
      +/- 5.0': ...-2.21"
      +/- 5.5': ...-1.18"
      +/- 6.0': ... 0.00"

      For 6 foot template
      0' (center): -1.80"
      +/- 0.5': ...-1.75"
      +/- 1.0': ...-1.60"
      +/- 1.5': ...-1.35"
      +/- 2.0': ...-1.00"
      +/- 2.5': ...-0.55"
      +/- 3.0': ... 0.00"

      I actually use two templates because the longest one is so difficult
      to manuver, one for up to 6' long and then I tape two six foot
      sections together to have one for up to 12'. Each of the templates
      are 12" wide. I just plotted mine on the cardboard sewing board
      (comes as 3'x6' with 1" grids) and cut the templates in straight
      line segments (I think it was 6" segments), figuring the hems
      sufficiently take out the straight line segments. You just use the
      center portion of the templates to match the length that you are
      dealing with.

      "I really like your idea of the tarp tent and that is what I am going
      to build although I would like to make it 120 X 96, any thoughts on
      that (is that too small, or what would you do different)? I also
      want to use a straight ridgeline, not cat cut. I think this would
      allow me use it better as a tarp only shelter (no hammock). I also
      see you have a third tie out in the long side of the tarp, would
      this be needed if the cat cut (sag) was deeper?"

      The size tarp you need will depend on your hammock, especially the
      length. It is a three dimensional problem and if you go back and
      look at my hammock tarp tent photos this will be apparent when you
      look at the length of the ridgeline and compare how far that exceeds
      the length on the tarp tent near the ground. I can tell you that 96"
      is not enough width.

      I recently saw where another person used plastic drop cloth to model
      a tent design... I thought that was very clever. Why don't you get a
      9'x12' piece of 2 or 3 mil plastic drop cloth, use reinforced
      strapping tape and make a model to see what dimensions you want and
      where you want to put your tieouts and panel pullouts? That might
      also give you a feel for what catenary curves might do to help the
      tautness. On my 96" wide tarp I used 3 tieouts on the long side of
      the tarp, I think I need them. On the 126" wide tarp I used four
      tieouts on the side and panel pullouts at the same spacing, the tarp
      tent configuration dictated the placement of those.

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