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Re: Report on Directional Stretch and Resiliency of Polytarp Sail Materials

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  • Andrew
    Bolger group: I have been having a short PM correspondence with Dave on his test results and he has agreed we should put the conversation up for the benefit of
    Message 1 of 2 , Feb 10, 2010
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      Bolger group: I have been having a short PM correspondence with Dave on his test results and he has agreed we should put the conversation up for the benefit of the whole group. Here it is with some edits.

      I caught your message on the bolger group and was interested to have a look at your results. Your test method is great and you have obtained some very interesting and useful results.
      However (and I think you were asking for constructive criticism) your conclusions/calculations for loads are incorrect. I studied and practiced materials engineering for 25 years and the wisdom from this experience and training is:
      Your 16 ½ pounds of force are NOT supported by 27 square inches of material, this face of the material runs parallel to the lines of force in the material. The actual area supporting the weight is 1 1/2 inch by the thickness of the material. So if you wanted to extrapolate your test results you would do something like this:
      Example of a square sail (because it is easy): Yard length = 5 feet = 60 inches, Therefore upward force on yard that could be supported by the polytarp would be 16 ½ multiplied by 40 = 660 pounds. That is still almost a third of a ton. Now the halyard through a 2:1 purchase could pull a maximum of say 400 pounds. So you still have a big margin of safety.
      The other thing that, as an engineer, I would do would be to plot the results on a graph. The engineering convention would be load on the vertical axis and % stretch (ie (loaded length-original length) divided by (original length) times 100%) on the horizontal axis. But that is really just presentation and up to you.
      Anyway, I hope that helps. Thanks for your input into the world of economical sailing!



      Thanks for the insights from a materials engineer. I see your reasoning but suspect because the load was supported at the top and bottom by grommets that even the full 1 1/2" width did not come into play. Similarly, if the tarp was supported by a yard, the support for the material, if I am understanding you, would have to be all along the head of the sail in order to reach the theoretical 600 lb. Then there would also be the thickness that is measured in mils. I'm not even sure what that measurement is, metric or imperial, so I'm still not clear how thickness is factored in in your example. It looks like you just arbitrarily used a 1 for that number. Is that correct? I think that the 5.5 oz. material is considered to be 11 to 12 mils thick. What would that do to the numbers?

      I would appreciate your further help in making sense of the data--if there is, in fact, enough to tell us something. I would also appreciate your help in understanding what kind of a test could be done to give us data on loads perpendicular to the surface.

      Thanks for any help you can offer. I'm a novice when it comes to extrapolating results from the data. I appreciate your contacting me directly, but in the interest of accuracy, I would hope that you would publicly suggest these corrections.



      When it comes to the thickness I glossed over the fact that it becomes irrelevant. Because the thickness is the same in your test sample and the sail, only the width changes, it cancels out in the calculations. For instance you could say if 1.5 inches x thickness will hold 16.5 pounds then 15 inches x thickness will hold 165 pounds. This is the same as saying if 1.5 inches will hold 16.5 pounds then 15 inches will hold 165 pounds. If the thickness changed then you couldn't make this extrapolation.

      I just went outside and measured the thickness of a fairly lightweight tarp that is draped over some firewood (no idea about the weight). It measured at 20/1000 inch. Hmmm. Not sure what is the right answer here.

      You are right about the eyes – they will be the weak point, even though you have doubled over. So all you can say is that the material is stronger than whatever result you get, but you cant say how much stronger.

      Having thought about the results for a day I wonder if the stretch is actually the more important factor. When sails fail do they tear or do they stretch out of shape. I think the most common is stretch. So you have the right sort of test method to be evaluating this failure mode.

      The other, much more complicated failure mode that is probably important in sails is tearing. It is easier to tear a sheet of paper than to grab each end and pull them apart til it breaks. Same with sails I suspect.

      To test tear strength materials engineers use a "trouser leg" test. It is the same as your bucket test but instead of a strip sample you use a wider strip with a single scissor cut in it. It looks something like a pair of jeans with two legs. You then pull down on one leg with the other leg fixed at the top. You don't bother measuring deflection, just the load at which it tears. This load is unrelated to the width of the strip or of the width of each leg (if you don't believe me then imagine tearing a large sheet of newspaper versus tearing a small sheet – no difference in difficulty). The tear strength will be different in different directions.

      But how does that help you? You want to know what is enough tear strength, what is enough resistance to stretching, what is enough tensile strength? I don't know the answers here. If it was me I would probably start by testing some known products that work and compare them against polysail material. Then there must be a theoretical calculation somewhere to work out how much stress (load) there is on the sail material of a certain size in a given wind condition – but I don't know that one.



      My reason for conducting this test in the first place was in response to one of my sailing buddies' concerns about cutting a sail so that the leech was across the width of the sail rather than along the length. His concern was based on Michael Storer's directions for this sail that said it had to be laid out so the leech was with the long side of the tarp because that was the stronger side. I said that while that might be the case with many sail fabrics that had a distinguishable warp and weft to the sail material, that I didn't think there was much difference in the stretch between the wide and long sides in polytarp. Turns out we were both right I think, depending upon the weight of the tarp. All of us might have been surprised, however, by the amount of stretch along the diagonal relative to the stretch in the long and wide sides. However, I think this diagonal stretch and the material's resiliency go a long way in explaining why most tarp sails, even those made with minimal darting, seem to work pretty well.

      Your measurement of the thickness of the firewood tarp seems strange based upon what I know about various tarps.

      With respect to tearing, I know polytarp is very susceptible to being sliced, but resists punctures well.

      Wind loads on sails are determined by formula according to the US Sailing Association. The formula is: Load in Pounds = Sail Area * (Wind Speed )2 * 0.00431 (Wind speed is expressed in knots per hour.)

      Please go ahead and post your comments. Thanks again for your help.

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