Here's another style that allows different pole options and can be retrofitted to a finished tarp:



More info from previous forum thread here.

Hope this is useful.

Edited by Lancem on 01/26/2013 17:32:31 MST.

Very useful. Thanks!

Any good info on how to make a catenary curve? Not sure if I need one, but I know it will add strength to the ridgeline.

If you're a member (which I see you're not), there is a great article that includes a downloadable tool for calculating catenary curves. Honestly, for those new to UL, the membership fee is more than worth it. But if that's too much for you, I bet you could find something for free with a simple internet search.

I'd say a big plus one on doing the cat curve though. It was a bit of work on my tarp, but it was totally worth the extra time. I learned a lot more doing it too.

Also, on the beaks, I pitch my tarp high enough that it hasn't been too much of a problem, but I still have Newton's mod in mind in case I get tired of the lower entry some day.

I wrote article that includes Excel spreadsheet for calculating catenary curve

http://www.backpackinglight.com/cgi-bin/backpackinglight/myog_silnylon_floorless_2-person_tent.html

Which is stolen from an article Roger did with minor improvements

But you have to be a member, sorry

Several people used this and reported back with success

Basically, you make a straight line. Make a mark every 2 inches. The spreadsheet tells how far to measure off the straightline for each mark for where the cat curve line is.

You can either measure the fabric directly or make a template out of stiff material and then trace using it.

Another way that hasn't worked quite as well for me, is to take a heavy yet flexible line, suspend from two points so it droops the amount you wish, and then trace along that line.

Maybe 2 inches of droop for an 8 foot length.

That article had some other info that people said was useful.

Tyler,

Info on making a catenary curve can be found at...

http://www.wvi.com/~ulmyog/

...by clicking on the third menu item. It'sthe best and easiest to use that I have found.

Plug in the length of your ridge line, then plug in the degree of severity, hit enter and the largest degree of deflection is displayed along with a table of defections at points along the ridge line and a profile view.

If you are using Chrome as a browser you may have to click enable editing to be able to insert your values.

I used brown paper to make a pattern of the profile and then transferred it to my two panels of material. I cut out the profile and then sewed a flat felled seam on my catenary curved ridge line.

A bigger number plugged into the severity block results in a smaller deflection.

Party On,

Newton

Edited by Newton on 01/26/2013 22:32:30 MST.

That's it - good John

One thing about using brown paper for a template, is it tends to bend a little which will make the amount of catenary deflection more or less. If it bends differently for the two sides, they won't match.

I've done a few recently and just don't bother with the template, just measure directly on the fabric.

I would agree with Jerry on the 2" deflection. I did three on my tarp, and that feels like too much. If I did it again, I'd probably go with something like 2" or so.

Tyler, I'm about 20 miles outside of Raleigh, I'm a Hokie, but living near NC State lets me route for the Pack so I was happy with last nights win.

As Newton said, he credited the first pic to me. That's my fence in the back ground.

I like where you are going with the design. Here is a link to my basic trapezoid tarp that uses a cat ridgeline...

LytW8 Cuben Tarp

click the instructions link for details on the cat cut. I use a 3" deflection which is a lot. I'm real happy with how tight it sets up, but 3" does take out 6" of width from the middle of the tarp... 2" would be a good compromise.

Jamie

+1 for "I would agree with Jerry on the 2" deflection."

We all seem to be in full agreement on the amount of deflection.

My amount of deflection was 2 9/16" over a 120" ridge line.

FWIW using the Catenary Curve Spreadsheet 2 9/16" of deflection over a 120" ridge line works out to a degree of severity factor of 700.

Use the spreadsheet and play around with the numbers. Trig is not my strong suit by any stretch of the imagination. After doing some quick and crude math my "rule of thumb" deflection factor works out to .25625" per foot of ridge line.

FWIW 9/16" equals .5625". 2.5625" / 10' = .25625" of deflection per foot of total ridge line length. Apply the deflection to the center and work out to "0" at the ends. Use the spreadsheet to get the deflection along the other points of the curve.

Sorry for the math lesson so early in the morning. ;-)

Party On,

Newton

Jerry said;

"One thing about using brown paper for a template, is it tends to bend a little which will make the amount of catenary deflection more or less. If it bends differently for the two sides, they won't match."

The way that I solved this problem was to vertically (approximately 90 degrees in relation to the seam) pin my brown paper template in place on each piece of material. Making sure to keep everything as flat as possible, I used a sharpie to trace the "cut" line along the edge of the template/pattern onto the material. All that is left then is to cut carefully down the line from one end to the other on both pieces of material.

Due to the curved nature of the seam some bunching of the material is inevitable but my flat felled seam turned out to be just that when finished, flat! ;-)

Party On,

Newton

Or, you could attach the brown paper template to something stiffer, like a board that's maybe 3/4" x 1.5"

What worked best for me was having a fairly large work area. I used the kitchen table with the center leaf added. There was a little slipping and sliding that needed to be done to accomplish the task because I don't have a 10' kitchen table. ;-)

I could have resorted to using the floor but my back and knees won't put up with that for too long.

Another option for a work surface is a Ping Pong table which is 5' x 9'. Does that size sound familiar? LOL

Party On,

Newton

Ironing your paper will reduce its tendency to roll up. I'd recommend weighting down, rather than pinning it. Not only does that avoid poking holes in something you want water proof, it does a better job of keeping it nice and flat.

What's used for patterns in industry is oaktag, which is essentially the same stuff that's used for manilla folders. It's much, much nicer to work with than kraft paper. It doesn't tear, is stiff enough it doesn't roll up, and has a nice hard edge for tracing against. Its hard to get in pieces bigger than than a folder and less than a 30 pound roll, unfortunately. a piece of cardboard from a refrigerator box or something would like work well enough for making a one-off tarp.

David,

To clarify my pinning process I don't pin through the portion of the fabric that I intend to use as the roof panels for the tarp.

I only pin the portion that will actually be cut off and unused scraps.

I guess I should have been clearer on where I pinned the fabric.

Party On,

Newton

Edited by Newton on 02/04/2013 11:28:09 MST.

Jamie,

I'm moving ahead using your design. Thanks! I'll post back with questions and updates along the way.


Oh and since I'm limited with the amount of raw fabric I have (should be plenty for the plans you listed), I might just adjust the size of my bivy to make it fit, rather than make the tarp fit over the bivy.

I'm in a bind. I cut 120" of raw fabric, but realized the end I measured from wasn't 90*. What's the solution here? If I was going to make a flat tarp, I figure I could hem the ends at 90*, but I'm planning on making a cat-cut tarp from Jamie's design (Lytw8).

Thankfully I started with more length than necessary.

Nevermind - I put on my headlamp and cut along the ripstop line. Problem solved.

A cat-cut seemed too much for right now. I opted for a flat tarp after all. Edges are hemmed and it weighs ~8.7 oz before the tie-outs.

Thanks for all the help!