I've been doing a bit of thinking about tarp taughtness... I've noticed that no matter how taught it is when I first pitch it, by morning (backyard tests- haven't taken it out in the field yet) it's really saggy. I tried tensioning the guylines further, but it made no difference. I even have mid panel pull outs but that still wouldn't pull the tarp back to how taught it was when I first pitched it. After playing around with it, I realized the perimeter hem and flat felled seam on the ridge-line were still very taught. So- I believe this has to do with the polyester thread and using the straight stitch to sew it all up. I've read on this forum that polyester thread has very little stretch compared to the silnylon fabric, and that when you pull on it the stitches pop before the fabric rips. Now, what is you used a zig zag stitch instead of a straight stitch? There was an old post I found where someone tested using silnylon instead of grossgrain for the guyouts, and found that the zig zag stitch greatly improved the strength, since it allowed the fabric to take the load, and not the thread.

So, what do you think? Would zig zag stitching the hem and ridge-line help to keep the tarp more taught?

BM

So in other words, if I understand you correctly, the ENTIRE tarp stretches, not just the fabric stretched between the ridgeline and hem?

Just out of curiosity, what other fabric is Not between the ridgeline and a hem?

BM,

"So, what do you think? Would zig zag stitching the hem and ridge-line help to keep the tarp more taught"?

Hmmmmmmmmmmm!? It sounds plausible to me. Now it is up to someone who has done it, or is willing to do it, to post on this thread and share their experience or results.

I wouldn't think that the zig zag stitch would have to be too wide to be effective.

A relatively small piece of fabric, a linear yard of 60" wide silnylon, should be sufficient for a test.

After the experiment and tests are complete the mini tarp could become a shaped doggy tarp for a backpacking pooch. We gotta keep this kind of stuff dual use you know. ;-)

Party On,

Newton

Edited by Newton on 05/14/2011 12:47:43 MDT.

Nylon expands when it gets cold. Wierd, but that's how it is. Polyester does not.

So ... two things to do. Keep the thread tension low - much lower than for ordinary fabrics, and yes, consider using a zigzag stitch as well. I suspect that a low thread tension might solve most of your problems. You will also need to stretch the fabric as you sew, to prevent puckering. There have been several Forum threads on this recently.

Cheers

If you are using low thread tension, and stretch the fabric as you sew, then when you stop stretching, the fabric will return to its unstetched condition, and the stitches will have loose loops.

What we need is a high quality light nylon thread that will expand and contract with the fabric when the temp. changes. All the ones I have found were too heavy to do this effectively. Theoretically, as nylon is stronger than polyester, it should be possible to have finer nylon thread that is also stronger than the polyester.

In the meantime, I think all we can do with silnylon is to sew with low thread tension, as low as you can go without creating the loose loops, and sew with the fabric at normal tension. If the fabric is puckering after sewing, the thread tension is too high.

Haven't tried the zig-zag stitch, although the sailmakers seem to feel it is worth doing. But they are using basting tape and just a couple layers of fabric. If the seam is lap felled, with reinforcements and maybe some pole sleeves or netting as well, that adds up to many layers of material at the seam vs just one layer outside the seam. Not sure that a zig-zag stitch at the seam is going to help that much with such a thick seam. But would be interested to hear otherwise.

Design also helps. If the tension can be kept uniform over the canopy surface, as with a hooped tunnel tent like yours or the Warmlite, for example; then it is easier to reduce the sag by tightening the tension on the canopy.

I think you and some other folks have posted about using shock cord or other elastic material on the guylines with some success. But it won't work if the design is such that the seams, being less elastic than the canopy surface, prevent the guys from pulling the canopy taut.

Would love to know what experiences owners of the silnylon mids have had with this problem, with the triangular and catenary shaped panels with seams that do not run parallel to the grain of the fabric.

Wish I knew more about this subject and could deal with the sag issue better.

If the fabric was wanting to stretch, and the polyester thread didn't, the thread would just break.

This is what happens on the flat felled seams on my pyramid tent ridge lines at the 4 corners where there is the most tension

The silnylon stretches and the polyester thread breaks

Some day I am going to put in a zigzag stitch, but so far it hasn't made any difference - there's no lateral tension there - and I'm too lazy to fix it.

The next time I make a tarp I'll give it a whirl. My wife's sewing machine has a small zig zag stitch mode that I think would work pretty well. My current tarp is a 8x10 model, and I might later go ahead and make a smaller one like a 5x8 or 5x9... I don't know for sure, but in the end my sewing practice is leading up for a large pyramid shelter project... I have many kids (too many) and as they get older I'd like to be able to take them on overnight hiking trips and a larger 4+ man deal is needed.

Maybe it'd be worth it to get some cheap 2nds and just make some 4' or so square pieces, hem them one with a straight stitch, and a 2nd with a zig zag stitch, add guy outs on the corners and pull it taught. Let them both sit overnight, on a night when some dew is expected and see what they both look like in the morning.

BM

You might try to find some T-30 or B33 nylon thread. Has a breaking strength of about
4 lbs.

Edited by oware on 05/16/2011 14:10:41 MDT.

Looked at your site, but no reference to these threads.

Could you provide source data or other info for us'ns?

Thanks.

> Nylon expands when it gets cold

Probably more significantly in this case, nylon expands when it gets wet or damp. Quite significantly. That's why static-pitched nylon flys are saggy in the morning dew, and stiff as a board in the sun. And why many nylon flys use bungy-cord pegouts (e.g. TN Quasar).

Polyester absorbs much less water & stretches less, which is why polyester flys are less affected by this issue.

Edited by captain_paranoia on 05/20/2011 09:09:29 MDT.

Google is your friend.

http://www.google.com/search?q=bonded+33+nylon+thread&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

Nylon threads should solve the problem. I have many spools of this I bought several years ago for fly tying. That might be a bit thin for sewing, though...
an example

Nylon thread is slipperier than polyester so is more likely to unravel

I use shock cord tensioners all the time with really good success. Seam orientation doesn't matter so much if you are willing to pitch the tarp a couple inches off the ground (as most do).The tensioners pull up the slack as the fabric changes during the night.

Seams with Poly thread have very little stretch, but it still takes up the slack everywhere else.

Here is a little link we have on tying tensioners (you can do it a million ways, but this is fast and simple)

http://seekoutside.com/instructions/how-to-make-tent-tensioners/

"Nylon thread is slipperier than polyester so is more likely to unravel"

These threads are bonded. (Hence the B for Bond in B33). This reduces fraying due to abrasion.

You will note that nylon thread is commonly used in climbing harnesses and runners.
If there were an issue with unraveling, ----.

for the search link. Got the B-33 in light green from The Thread Exchange. When I get it, will test on a large swatch of silnylon and if it sews all right, see if it will expand and contract with the fabric when placed outside. Back to the 9" plastic embroidery loops on the back deck.