How To Make A "Catenary Cut" On A Tarp?

**headchange4u** · 10-11-2006, 15:35

I found Mirage's site via a link posted by Just Jeff on WB. On Mirage's site he made a Catenary cut on a hex tarp that he made from Tyvek. I happen to have a partial roll of 10 ft wide Tyvek and I am planning on making both a plain,diamond/sqaure 10'x10' tarp and a Catenary cut.

Is there spefic radius or angle that the sides and the bottom need to be cut at? I am assuming that you need the correct cut to get proper tension when you tie the tarp out. Any help would be appreciated.

**Just Jeff** · 10-11-2006, 17:22

There's a catenary curve generator in the "files" section at the Yahoo group.

Or you can just hang the Tyvek on a wall, tack a string to each end, and let it sag until you're happy with the curve. Something along the lines of 1" per foot should work.

**blackbishop351** · 10-11-2006, 18:33

Originally Posted by headchange4u

I found Mirage's site via a link posted by Just Jeff on WB. On Mirage's site he made a Catenary cut on a hex tarp that he made from Tyvek. I happen to have a partial roll of 10 ft wide Tyvek and I am planning on making both a plain,diamond/sqaure 10'x10' tarp and a Catenary cut.

Is there spefic radius or angle that the sides and the bottom need to be cut at? I am assuming that you need the correct cut to get proper tension when you tie the tarp out. Any help would be appreciated.

Unfortunately, Youngblood's catenary generator file on hammockcamping isn't quite right. I discovered this the other day - he makes an approximation which deviates from the "true" catenary form. If anyone cares, I'll elaborate, but it's really not necessary.

To generate your curve, you have to solve a transcendental equation - one that doesn't have an analytic solution. This has to be done numerically, which I wrote my own program to do. I assume that Youngblood made his approximations in an effort to avoid having to do this.

I ordered sil from Ed Speer, made a catenary pattern, designed a symmetric "perfect" hexagon, and cut away. I just finished the hemming/seaming last night. All that's left is adding tie-outs, so I should have it hanging in the next couple of days. I'll post pics!

**blackbishop351** · 10-11-2006, 18:35

Oh - since my program is written for MatLab and I don't know enough Fortran or other language to translate it, I'll be happy to generate measurements for anybody who wants to make a cat tarp...just give me the length of your edge(s) and how many inches per foot of sag...

**headchange4u** · 10-11-2006, 19:15

Wow that all sounds very complicated. I'm really trying to cut back on my transcendental equations as my wife says I spend way to much time on them.

All jokes aside, it would be great if you could help me with this. Like I said I am working with material that is 10' wide. I may have to cut the width down to get the radius right. Just let me know.

**blackbishop351** · 10-11-2006, 19:18

Originally Posted by headchange4u

Wow that all sounds very complicated. I'm really trying to cut back on my transcendental equations as my wife says I spend way to much time on them.

All jokes aside, it would be great if you could help me with this. Like I said I am working with material that is 10' wide. I may have to cut the width down to get the radius right. Just let me know.

I just throw around math and physics terms 'cause it makes me feel important....which in turn keeps me sane through the horrible amount of schoolwork I have to do on a normal basis!

PM me with an email addy and I'll send you dimensions for the cat cut and explanations of what they mean!

**Happynewhanger** · 10-12-2006, 17:18

Check out this site. http://www.backpacking.net/makegear/cat-tarp/index.html
I am not a sewer but this looked interesting.

**Aramis** · 10-24-2006, 18:55

Yeah using a bit of string on a piece of cardboard sounds easy enough. You could stick pins in it and then bend a thin batten around the pins to mark it. I'd be inclined to cut it using the batten as a guide, since I seem incapable of following a line with any known tool

I'm wondering if a batten with a bit of string between each end (strung like a bow) would make an accurate catenary curve. I should know, but first year engineering was a long time ago.

While I think of it, what are people's thoughts on grosgrain tape vs hems for the edges? The MacCat tarps use grosgrain and are still very light, so I doubt that there's much difference in weight.

**Arkwater** · 10-24-2006, 19:05

I would think a hem would be stronger because you have two or three layers of fabric not only being held together by the thread but are also woven together along their entire length. Grosgrain would only be attached by the thread. My 2 cents worth.