Cat cut/flat cut tarp?

I recomend if you are making a tarp is to make one of each from a cheap material and decide which will work better for you. but I would recomend a flat tarp because of the versitility in having many different set up options when I think catenery cut tarps can only be set up as A frames

There is more than one Cat pattern or spreadsheet on line, I stumbled across them **after** I built my tarp tent. It just takes a bit of searching. Ask in the DIY forum on http://www.backpacking.net.

Eliza,
Complicating matters, there are 2 kinds of catenary cut, the basic flat tarp with the ridgeline cut to a catenary, and the radical catenary tarp – so called parawing or hyperbolic parabaloid, in which the ridge is on the diagonal, but is still aligned square to the weave of the fabric. Each as advantages and disadvantages.

The purpose of the catenary cut is 1) to pull out the wrinkles that form because square-woven fabric stretches on the bias and 2) to compensate for the natural sag in all suspended lines. Properly speaking, that natural sag is the catenary. Despite our use of the term, tent makers do not usually calculate and compensate for the catenary. It is a very messy formula. They address the bias sag first and let the catenary take care of itself.

Flat tarps sag. One or two pull-outs on the side panels will take out the sag, AND give more room under the tarp at the same time. This is the strategy used in Ray Jardine’s tarps. See BEYOND BACKPACKING for details. The pullouts protect the tarp against side winds. In a Jardine tarp, I have sat out blows that put down all other tents in the vicinity. Tarps with side pullouts are a little more trouble to erect and require 2 more stakes.

Catenary tarps are supposed to set up tautly without additional pullouts on the sides. They do that at a cost — the catenary cut reduces the head space under the tarp, all other things being equal. The taut fabric sheds wind and distributes the stress better than a flat tarp that is erected WITHOUT properly tensioned pullouts. But not better than a tarp with one or 2 pullouts.

The radical catenary tarps take the catenary cut as far as it can go. They have lots less space underneath than either the flat or the catenary cut tarp. However, they are VERY strong because the cat cut distributes stress over the entire surface of the tarp. They are great for hammocks since their diamond shape fits the shape of a hammock, and they give a tight, wind-shedding setup. In the 8X10, 10X10 or 12X10 diagonal size, you can make them by piecing 8, 10 or 12 feet of 60-inch fabric. Wider than 10 feet, they tend to waste a lot of fabric because they are cut on the bias. Remember, because they are measured on the diagonal, a 10X10 parawing is only 7 feet (finished size) on the sides.

Regardless of what type of tarp I make, I use some catenary to compensate for my inneptitude in sewing long, straight seams. If I boggle a seam, a little catenary will pull the wrinkle out. Mostly.

The easy, foolproof, technically accurate way to make the proper catenary for any particular fabric on any seam is to lay the fabric flat and stretch it along the edge to be seamed. For example, for a 2-panel tarp, stretch one panel of a tarp along the edge that will be the ridge seam; a wrinkle will form, higher in the center and tapering to nothing at the ends from which the stretch originates. Mark along the base of the wrinkle (on the panel side – away from the edge)from end to end, then fold the fabric double along the ridge and use pins or bright light locate the lines so you can adjust the two folded-together segments of the line until they are symetrical. This isn’t as hard as it sounds. Practice on some scrap; you will see immediately how it works. Cut the extra fabric away and use it as a pattern to mark and cut the catenary for the other panel.

You can also put a little curve in the side and end hems – if you can afford to give up some surface area.

Catenary cut will not keep a tarp from starting to sag as the nylon absorbs moisture overnight. Elastic tensioners at all pullouts will take care of that. The easy way is to make bungie loops. Just pull the guts out of some parachute cord, cut it into 5-6 inch lengths. Cut 6 inch pieces of round elastic (3/32 is a good size)and slip 1/2 inch of elastic into each end of empty casing to make a loop. Bar tack the elastic in the casing with your sewing machine. As dab of Superglue will stop any raveling.

Thanks Vick for some great tips.
-Mark

Yeah, Vick, thank you for all that detail. Sounds like I’ll be making two tarps soon out of coated nylon and selling them or giving them away to friends! Next question is, how much could I charge…? :)
Eliza

Six Moon Designs has an excellent set of instructions and patterns for a catenary cut tarp under their Make Your Own Gear section that are free.

How about pinning a rope up to a wall, and tracing the curve? That’s how I made my cat-tarp, using a length of 11mm climbing rope.

It’s not perfect when you think about it in 3d, but it’s pretty close. And the curve looks amazing!

String – good idea. You could do that.

You could “eyeball” it and probably do well since the following mathematical formula actually describes what occurs when a chain is suspended by its ends between two points.

IIRC, y = a * cosh(x/a) (if interested, you might want to verify this, as often my “old-timers” acts up and I forget things, or get them mixed up or exactly backwards)

A catenary curve is often called a “chain curve” (latin, catena = chain – i dont’ remember who gave this curve shape its name, viz. catenary curve). It is the shape/curvature that a chain will take when suspended b/t its two end points and only acted upon by a uniform force (viz. gravity), i.e. it is supporting only its own weight – nothing else hanging from it. Both end points of the chain do NOT have to be at the same height. If they are the curve will be symmetrical. If not, it will be assymetrical when viewed with respect to a vertical y-axis. The cat curve allows a structure to assume the lowest potential energy state. Perfect for that single rigdeline cat curve.

Famous story (in mathematics, at least): Galileo thought that the curve’s equation would define a parabola, but he was wrong, and together Bernoulli, Leibnitz, and Huygens, derived the equation later on. Note in the aforementioned equation that the hyperbolic-cosine function is used. The cat curve shape is therefore, hyperbolic not parabolic.

[Note: Dont’ be fooled by the curvature of cables in suspension bridges. Until the large diameter suspended cables are tied down to the bridge using the smaller vertical cables and begin bearing a segment of the bridge’s load (i.e. the driving surface), the large suspended cables actually hang in a hyperbolic shape per the above equation. Only as the vertical cables are attachd and the suspended cable bears load does it assume the parabolic shape we see in the completed bridge. Another classic school math homework problem is: Can you make square (or any regular polygon) wheels roll? Ans: yes, with the proper repetitive catenary curve in the road surface!!! The properly spaced repeated cat curves form peaks and valleys that allow a square or any regualar polygon wheel to roll smoothly. Pretty neat, huh?!! Like we were told by the Professor: “The proof is left to the student.” Somehow, I don’t think Detroit is going to be jumping onto this anytime soon.]

Paul,
As usual, you are a wealth of information. But… but… I just lay a wooden or aluminum batton along the ridgeline, anchor the ends, then anchor the center with the degree of offset bend I want, mark it and call it good. This gives good curves that do what they are supposed to. It may be distinguishable from the classic catenary on the micro level, but I can’t tell.

Vick, I’m sure that if I could see your handiwork, I wouldn’t be able to tell the difference either, nor could I do a better job than you; mine would be far worse (one needs to know one’s limitations). Theory is only good if it affects the practical. The practical is where it’s at, IMHO.

I’ve worked with a fair number of different curves in the past, and wouldn’t get too hung up on a precision catenary curve for any particular gear implementation: Wind, uneven ground, fabric tension, seams, material differences and a less than perfect connection system are going to distort perfect to practical.
I’d also be willing to bet a cupie doll that for most any catenary curve segment, you can find a virtually identical (within nominal manufacturing tolerances) parabolic or hyperbolic segment that is virtually indistinguishable from a catenary curve in the field.

Just a thought,
MikeB

The white tarp is a Mac Cat Tarp made out of 0.5 spinnaker material (really about 1oz + per sq yard). The green tarp is a computer designed Cat Tarp by Moss. The curve is not hard to do and a Cat Tarp is really nice.

I use a Hammock now so pitching a tarp lots of different ways isn’t something I worry much about. In the old days I did use the Moss Cat on the ground a lot.

I’ve read back over this thread with great interest – some excellent info from PJ and VH here. As a fairly novice sewer, I’m wondering if any of you folks have tips/tricks for sewing felled seams on a curved ridgeline? I’m working on a 2-person tarptent for my wife and me and in practicing with scrap, I’m running into some trouble. I know that Jay Ham will publish more tips in his article, but as always, I seem to need his info a month or two before he is finished!

Thanks,
Ben

Ben,
As you have discovered, the catenary curve means that the raw edge – the edge outside of the first stitch line – is shorter than the stitch line. When you try to turn the edge to fold it under to make the felled seam, the longer stitch line tends to bunch and pucker.
There are a couple of ways to deal with it. Some folks tape the seam. This eliminates the problem but produces a seam that is not as strong when pulled from the sides. The tape, however, is stronger when pulled along the ridge line.
The felled seam is strong in every direction- if it is tightly done. If not, it is as weak from the sides as the taped seam. To make the seam lie flat and tight as you fell it, first, trim one raw edge. You will tuck the other edge under it to produce the felled seam. Trim this short edge to whatever seam allowance you are using. Make sure the other raw edge is wide enough to fold under with enough extra that it will be stitched securely under the second stitch line.
Then give the remaining, wider raw edge a good but careful sustained stretch. Finally, fold it under where you will start your stitch. I prefer to start stitching from the middle of the ridge line, stretching the folded-under raw edge as I go to ensure that the first stitch line does not pucker. Nylon is pretty elastic, so this will not be much of a problem. Then I go back to the middle and stitch in the other direction. The advantage of the two step process is that if I fail to keep the raw edge as taut as necessary in one direction, the mistake does not compound itself over the entire seam length.
I have not found pinning or glueing to be useful for this job, so I just free-hand it.

Vick,

Thanks for the excellent instructions. I took the day off work today, and spent some time measuring, planning, and cutting. I used your technique for drawing the cat curve – as you predicted, it made perfect sense once I actually pulled the fabric, and the resulting curve looks great!

Your instructions above pretty much confirm my suspicions – the trick is to go slowly and stretch the fabric as much as possible. For much of the ridgeline, the curve is so gentle that I don’t foresee any problems, but the curves towards the apex of each end are a bit steeper – this seems like a place where your recommendation to start in the middle will be well worth following.

Thanks again,
Ben

Ben,
Good luck and have fun with it. Sounds like you are on your way.
vrh

Sorry Ben…I’m cranking them out as fast as I have time.:) And thanks to those who have answered your question. I’ll add that I don’t rely on the sewing machine’s feeder dogs to drag the fabric through without quite a bit of my own assistence. I usually use my left hand to pull the fabric through from behind, while pulling the fabric taught in front with my right hand. I’m constantly feeding the fabric through at the rate/stitch length set on the machine, but maintain oposing pressure to keep the seam tight. Hope this helps.

Jay
MYOG

Hi Jay,

That does help quite a bit – that’s what I usually do, so it’s nice to know I’m in good company! Your previous MYOG articles have been fantastic – can’t wait to read more.

Ben

Jay and Vick (or anyone else)

Do you feel that a felled seam is necessary for a ridgeline? I’m actually building a two-man tarp/tent with a double ridgeline similar to the Squall 2. For the ridge seams, should I go to the trouble of felling? Or would a reinforced flat seam or one of the other “cheater” seams work as well?

Thanks,
Ben

> Do you feel that a felled seam is necessary for a ridgeline?
I have made many tents (and sold them too), but I never bother with a felled seam on the ridge. There just isn’t the stress there. (That’s using silnylon – with PU-proofed nylon I might do different.)
What I do is to sew the seam once, fold the two edges back, one to each side, then oversew right next to the first stitch line. The folded back fabric adds reinforcing to the seam, and I use a moderately strong thread here (Rasant 80) on the second stitch line. Then I pitch the tent and seam-seal the seam from the outside with a THIN line of silicone sealant. Works fine, never leaks.

Roger,

That’s good to hear – I think I’ll hold off on the felled curved seam until I have a bit more experience under my belt.

I’m having a bit of trouble understanding the seam you describe – You sew the two pieces together, then open the fabric up and run two more lines of stitching, sewing side A’s seam allowance to the main body of side A and side B’s seam allowance to the main body of side B? I don’t see how that reinforces the seam. I must be missing something…

I was thinking about sewing a basic seam, then folding both edges to the same side, then sewing once through all three layers. Would there be any point in doing this?

Ben

What Roger describes can replace the taped seam, and it is lighter than taping it. But I still like to tape when making a catenary seam because the folded seam Riger describes is stretchier than the taped seam. It may bind and pucker some, but that will not compromise its performance.

Regardless, you will lose nothing by trying it. You may like the result. If it doesn’t come out neat enough, then just cut the folded part off and tape it with 1″ grossgrain. Stitch one side, fold the tape over the raw edges and stitch over it. The tape will stand up like the folded seam described earlier. You will need to seal it on the inside because the tape cannot be waterproofed effectively.

To visualize the folded seam, take two pieces of scrap. Put them together with the edges on one side parallel and flush. Stitch a simple seam 1/4 or 3/8 inch from the edge. Then turn the seam inside out so the raw edges of the side you just stitched are “inside” and the fold is along the stitch line. Press the seam flat, then stitch it again, 3/8 from the folded edge. The original seam will be hidden inside the “bead” this creates. After making a tarp this way, do what Roger says: set it up and seal the seam with a thin layer of silicone sealant.

Got it! That sounds like it would be pretty easy… Thanks Vick, and Roger for the initial explanation.

Ben

Well, I just went for the felled seams, and they were surprisingly easy. The canopy of the tent is now finished – I need to sew on the tieouts and pitch it so I can evaluate the design a bit.

Thanks again for the excellent help.