As we continue to push limits on weight and waterproofness with tarp/shelter materials, it's inevitable that some design tweaks will need to be made to keep designs reliable in the field.
About a month ago, I was contacted by a customer seeing some stitch elongation in a rectangular tarp he made with our Membrane Silpoly. We worked through the problem and now have what I think is a good path forward for reliable design, so I wanted to share some of that here.
To give a bit of background, here's a pic of the original problem. This was occurring at both ridgeline tie-outs, but not the ground tie-outs:
Essentially what's happening here is that the reinforcement patch is too thick with not enough stretch to be compatible with the ultralight Membrane silpoly. In other words, under tension the tarp stretches and the reinforcement patch doesn't. With something this thin, that's not good. You need a different approach.
Enter the radial reinforcement. After some back and forth and discussion with others, radial reinforcements using the same material emerged as a potential solution. This does two things:
1. Matches the stretch as much as possible between the reinforcement and the tarp material.
2. Spreads the force out radially from the concentrated stress of the tie-out area. This gives a more even distribution than the standard triangular patch.
This turned out to work very well as a retrofit to the rectangular tarp in question. Radial reinforcements were simply bonded with silnet over top of the original triangular reinforcement patches made from HyperD 300. I've kept in contact with the customer over the past month and the report has been excellent. This method of reinforcement appears to have fixed the problem.
Based on this success, I went off and did some testing of my own. For the reinforcement, I used a 4″ radial patch of Membrane Silpoly (same material used for tarp) bonded with sil-net and unsewn around the perimeter. Here are some pics:
I drew up a little template in lieu of using the coffee can, which makes it simple to cut out patches regardless of the tie-out shape. Applied a thin layer of sil-net to the patch, put down, then let cure overnight with some weight on top. Should be noted that I also cut the reinforcement patches in the approximate same direction as the intended force on the base tarp material to match the stretch (pic 3 above).
For testing, I vertically stitched on the 1/2″ grosgrain tie-out and then tested with 50 lbs. For extra stress, I lifted and dropped the whole thing ~2 ft. Inspected everything after and saw no evidence of stitch elongation at the grosgrain stitching or around the tie-out. Repeated a couple more times with the same result.
I then made a horizontal stitch on the grosgrain tie-out and retested. Still looked good. Some pics of the test setup and close-ups of the tie-out after testing:
I also tested without reinforcements (just 1/2″ gg) under the same conditions. No catastrophic failure, but definitely some stitch elongation near the outermost stitch line on the grosgrain. No bueno. Here’s a pic of that:
Overall the silnet bonded radial reinforcement approach seemed to work very well - both for the finished tarp and in my personal tests. I don’t know if there’s an accurate way to convert wind speed to ~force on a tie-out, but I would think the wind would need to be pretty strong to produce a force that would cause this tie-out to snap.
Of course there’s also the force of tension due to the pitch and that’s going to vary depending on the design or shelter type, but hopefully this gives a good point of reference.
I’m going to rig up a way to add more weight and test to failure so that we have those numbers. I'm also planning to test the strength when pulling in the weft and warp directions (ridgeline/side tie-outs) instead of the bias. I’d like to do this same thing with Membrane silpoly PU4000.
In conclusion, my recommendation is that anyone making a tarp/shelter out of Membrane Silpoly use the same method of reinforcement I've detailed here. If you've already completed your tarp/shelter, just keep an eye on the wear around the tie-outs, especially at the ridgeline. You can always retrofit if need be.
To make this process easier for new builds, we're also going to spin off separate UL tarp kits that include 1/2" hardware, the radial template I used in the testing, and full instructions on how to modify the reinforcement areas.
If you have any questions, please don't hesitate. Thanks!
January 22, 2021
Super useful information
Would you recommend this approach for silpoly PU 4000?
And regarding the direction of the patch you cut, are you trying to line up the warp and weft of both pieces of fabric? Or with an angle of 45 degrees?