Feb 6, 2010 at 1:07 pm #1254928
So in the last couple weeks I have finished my first few projects, a large Sil Tarp and a Sil tub/Momentum bivy, also sewed a removeable stove jack into a BD Mid. I couldnt be more pleased with all of it. Doesnt fall under the SUL category but it is going to do exactly what I need it to.
Weights are 14.5oz for the two man tarp and 5.1oz for the bivy.
I didnt make a catenary cut to the ridge and there is some sag there after its been pitched for awhile, im going to add a couple tie outs for this one but am planning to make another tarp here soon. I cant seem to find any info on figuring a catenary ridge for the uneven pole heights.
The excess sags more near the lower pole. I dont have the room in the house to hang a chain. I saw LMarshalls spreadsheet, is there anything out there for uneven heights??Feb 6, 2010 at 2:18 pm #1570537
Jamie ShorttBPL Member
@jshorttLocale: North Carolina
Ryan, Congrats on the projects, please post pics if possible. I've only done one project with a cat cut ridge line. Mine had uneven heights. I used a standard formula/spreadsheet and it worked great. But you bring up a great question…should it be different if the heights are different? It would seem like it should now that you point it out, but maybe the difference is so small you dont need to worry about it.
JamieFeb 6, 2010 at 3:34 pm #1570575
Thanks, you're probably right. The height difference isnt likely to be enough to bother with it.Feb 6, 2010 at 8:59 pm #1570673
You don't have to worry about it. Just down load the spreadsheet on cat curves from this website and it will calculate the curve for you. Just assume the straight line is between the tops of your fabric at the poles. I used the 2.5" max deflection over 99" and had more than plenty of curve: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=27609 for pictures.Feb 6, 2010 at 10:30 pm #1570690
Thanks guys, thats what I did. Just finished another tight flat felled ridgeline, max deflection 3.12 over 94. Couldnt wait for more replies, them feed dogs was barkin'. :)Feb 7, 2010 at 6:14 pm #1570943
@herman666Locale: Northern Virginia
A chain hangs in a catenery and and if the endpoints aren't at the same height, the curve isn't symetric like it is in the spreadsheet. I just proved this by experiment. The nadir moves toward the lower pole. That said, if the heights don't differ too much, the difference will probably be negligible. It may even be advantageous to keep the nadir centered. I don't know that much about the physics of tarps.
I sweated over constructing a catenary until I checked out the spreadsheet. It turned out that the error from using a circle instead of a catenary was about a 64th of an inch for my project.
Anyway, if you have a piece of bead chain the length of your ridge, you can hang it between you poles with the desired amount of dip and it will give you a catenary section.Feb 7, 2010 at 7:11 pm #1570968
I knew that the deflection should drift towards the rear pole but decided that just sticking with the standard midpoint would probably get me close enough.
Just finished the last of the GG tie outs. Dumping snow out but I'll likely get out and pitch it to have a look. If the pics turn out, I'll share.
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