Regarding offsetting the flat felled seam at the top:

I use a 1/2 inch seam allowance, folded over twice, so the flat felled seam is 1/4 inch.

I try to center the pole and tie-outs on the center of the flat felled seam, so theoretically, one side of the tarp is 1/4 inch wider than the other side, but you'll never notice it.

Regarding twisting and contorting under load:

One problem is if you don't lay out both pieces perfectly parallel to the grain of the fabric (edge of the fabric as woven). You can have either the edge of the tarp or the ridgeline (before catenary cut) parallel to the grain.

Another problem is if you don't make both pieces a perfectly smooth curve. Sometimes I'll have a place that's a little inside or outside of the perfect catenary curve and I can see it when I pitch it.

You have to measure as perfectly as possible.

Jared –

When creating a catenary-cut flat-felled seam, did you have any problem with the finished seam not stretching flat under tension?

Sometimes yes, but not to the extent that it interferes with seam sealing. I center the seam like you do as described in a previous post.

There are a couple of things you can try to help keep the seam flat.
1. After the first stitch line, pin the folded seam down. You don’t have to pin in advance from end to end, but at least several pins and a couple of feet ahead of the presser foot. If the fabric does stretch and bunch, you’ll see it and can make adjustments.
2. Use a wide presser foot on the sewing machine and adjust it side-to-side for best coverage of the seam.
3. Sew the seam under a bit of tension.
4. Use a narrower seam like Jerry does.

When attaching the netting skirt to the underside of the tarp, what was your technique to mark a symetric seam line (the white markers) that fit perfectly with the perimeter length of the mesh?

I’ve been using CAD software and have full sized patterns printed at the (expensive) local print shop. The patterns have alignment marks on the corresponding pieces.

Here is an alignment mark on the mesh wall pattern. Chalk doesn’t work well on mesh, so I snip the fabric to mark it


You can see an alignment snip mark through the mesh hem here:


Fly pattern cut to expose alignment template:


Alignment marks transferred to fly fabric:


Hope this helps,

-Lance

Edited by Lancem on 06/28/2011 13:23:38 MDT.

I've been following this thread and just made a mock-up of the tarp described by Jerry and the OP. I like the simplicity of it versus the more shaped, side-entry shelter I've been designing on another thread.

I'd like to pitch it supported by two hiking poles in an a-frame at the peak, pole tips down. Looking for a good, lightweight method to attach poles together, as it seems somewhat unstable otherwise. A poster on my other thread had made an aluminum widget that looked interesting, but a bit heavier and more complex than I had in mind:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=47007&skip_to_post=420512#420512

Here's pics so far (still need to sew a cat. curve in the ridge and beak):

This won't work for you because I use Easton poles rather than trekking pole, but anyway:

Made a loop out of aluminum flashing. Holes for pole tips to fit into. Holes to use 1/4 inch bolt to attach to tent. About 1/2 ounce.

Finished two shaped tarps more or less designed like those discussed in this thread. Cat cuts on the ridges, and the edge of the beak. Using poles in a-frame for support.

Just got back from three night in Hells Canyon/Seven Devils, and they performed well--saw a little rain and wind, plus pitched one night in the dark. Need to add zipper for easier access, but weight for tarp + guylines in 30d silnylon 2ds (from Backwoods Daydreamer) is only 9.5 oz.

Here's a pic from a site near Bernard Creek, overlooking the Snake:

The OP initially asked for an all-purpose tarp. That means a rectangular tarp with maybe a catenary cut doen the middle.

Take a look at Don Ladiogin's book "Lighten Up" with illustrations by Mike Clelland.
On P. 13 there's a great drawing of what looks like a 9' X 10' tarp set up in a real bomb-proof configuration. As you can see it has ties down the middle as well as on the perimeter. I've use this setup back in my tarping days even in winter and it works well.

BTW, the example shown of a 24 oz. tarp reminds me that my former TT Contrail only weighed 24 oz. Why make a tarp when you can get an even better setup at the same weight?
(Disclaimer: I sold the Contrail and now own a 28 oz. TT Moment for it's great design advantages.)