Cat cut advise wanted for solo trapezoid tarp

[Monte Masterson]

I know the diagram looks crude (laughable), but you get the picture. View of what the tarp would look like laid out flat. Those are finished dimensions. Tarp will feature a ridgeline seam adjoining 2 pieces and I want to know what the optimum cat cut would be? Also, should I do a cat cut on the perimeter( long sides) and if so how much? The foot end will be cut in a swallow tail 2″ inward at ridge. I’ve made a couple of cheap mock ups (without cat cuts) and I’ve found that makes the pitch better. Input would be greatly appreciated. Thank You.

[Mike B]

https://backpackinglight.com/myog_tech_note_catenary_curves/

This is a good primer on cat curves. The link to the spreadsheet in Rogers article is broken but 2/3 of the way down the first page of comments Nick links to a spreadsheet on his google drive that I presume is the one referenced in the article.

[Roger Caffin]

Broken link: noted. I will try to get this fixed.

In the meantime, if anyone wants to contact my via [email protected] I can send it as an attachment.

Cheers

[Ivo Vanmontfort]

http://angrysparrow.com/hammock_info/CatenaryCurveGenerator.xls

[Ivo Vanmontfort]

https://ivovanmontfort.blogspot.com/2012/06/silnylon-tarp.html

This tarp is 293 cm long. 5 cm cat curve on the ridgeline

also on the perimeter but the effect is minimal. maybe you can skipp it.

[Matt]

Do I understand correctly that cat curves on the sides (ground edges) do not help that much?

[Diane “Piper” Soini]

I found this web page for a cat cut poncho tarp. If you ignore the poncho, and adjust for the length you need (he is really tall), the measurements are still useful:

Cat-cut poncho tarp

https://www.longdistancehiker.com/diy-ponchotarp/

Weight = 5oz (without guylines and stakes)

Length = 116″ (8″ longer than most tarps, as I am 6′ 5″)

Width @ Front = 84″

Width @Rear = 60″

Catenary Cut on Ridgeline = 3.5″

Catenary Cut on Long Edges = 3″

Catenary Cut on Short Edges = 2″

[Monte Masterson]

Thank you all for the input. Glad I posed the question because I was going to only do about an inch cat cut (at most) on the ridgeline. And yes, like Matt I’d like to know more about perimeter cuts and how much more it would really help to create a taught pitch. I’m going to use 7d silnylon and it’s expensive so I want everything to be right.

Ivo, you gave me some very good how to instructions on bonding reinforcements back in 2016 and that really helped with my projects.

Regarding a cat cut poncho Diane, I know Ron at MLD puts a ridgeline cat cut on his Pro Ponchos. I wish I knew how much. He says it’s only “slight.” I don’t think he wants to overdo it so the poncho will still wear comfortably.

[Michael B]

Just looking at your diagram, you could reduce the width at each end to 60” and 52”, you could get your whole tarp out of 3LY of most standard widths of Silpoly/Silnylon, as they are usually 58” wide with the selvedge. If you are buying Silpoly XL from RBTR, then you can get it wider.

I think for the tent I am designing, I went with 1.5” cat cuts on all ridges (all 6ft or less in length), and similar for the perimeter. I think it is helpful to keep the fabric tight to have cat cut on any edge which will be subject to tension. I think the effect is less drastic around the perimeter though, and the pros need to be weighted with the cons to determine on a personal basis what is most important. Some value the ability to have the entire perimeter on the ground, but many others would value that space for ventilation and the aforementioned tighter fabric surfaces.

[Sam Farrington]

Monte,

There was a thread a while back in which math gurus maintained that the formula for a catenary curve is almost identical to that for a parabolic curve.  The latter being what you get if you flex a dome tent pole by securing it at each end of the curve and moving the center of the pole to the amount of deflection you want.  In practice, the ends of the pole are joined with cord, the ends of which can be held by hooks used for hanging curtains, and the amount of deflection set by using your preferred slip knot.

How much you flex the pole depends on how much deflection from a straight line cut you want and how much your cat-cut should cave in the surface of the tarp.  That one is a variable I’m going to have to work a little harder on.  Because when the curve is traced on each side of the line on the fabric, the distance between halves at the center line will be close to double the deflection amount traced on each half of the tarp before they are sewn together.  On a pyramid tent, that amount would be less if the walls are at right angles to each other.  And on a pull out vestibule with walls at even less than right angles to each other, with a sharper leading edge, even less still.

Beardoh’s link that covers long and short edges is interesting; but having traced my desired deflection, I’d not hesitate to change it and pin up a different stitch line before hemming if that seemed better.  For that reason, I’d want to stitch the ridge line first, then set up the tarp before deciding on the amount of deflection for the long and short edges.

But the parabolic curve is simple to replicate and trace and transfer to fabric.

[Dave @ Oware]

Old school from the 70’s method for curves for tents (Gerry I believe) used a long wood ruler to get the bend. Alternatively a real chain hung from the ends and the curve traced on fabric or paper behind on the wall. Depends on the stretch of the fabric and how the bias lies on the pattern for how deep the curve should be. Since a curve in the ridgeline limits the number of ways a tarp can be set up, if you are going to put a curve in it you might as well go big and really take the sag out of the shelter.

[Diane “Piper” Soini]

Really, Michael, you can really make a usable tarp from 3 yards of silnylon? I bought a 3 yard remnant. It could be good practice before I ruin any DCF.

[Michael B]

Diane, it depends on how much coverage you want. But yes. 3LY of fabric, cut on a diagonal –  for the size I suggested above you’d cut off the selvedge, leaving  about 57” of usable material. Then you’d measure 30.5” from opposite corners, connect those points with a straight line and it along that line. That would leave you with two trapezoidal shapes, 9 feet long, with 30.5” width at one end and  25.5 to 26.6” width at the other end, depending on how much of the selvedge you cut off. Then you just match the like widths with each other and sew the outside edges together to form the ridge line. Or, I thought it might be nifty to flip one of the pieces and sew along the diagonal cut, forming a “fish-shaped” tarp with a slightly longer ridge line and 9’ sides. The second option may present a weird bias stretch, but it may prove advantageous with the addition of cat cut ridge line and perimeter.

sorry for the off topic, just thought it might be an idea worth trying since OPs tarp size is just outside a near optimal use of 3LY, which with any of the standard 1.3OSY fabrics would yield a 6oz tarp.

[Author]

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