Cat cut/flat cut tarp?

[Monte Masterson]

Regarding a ridgeline cat cut on an A frame shelter with disparate heights. For example a 38″ head and 26″ foot. In an above post engineer Paul Johnson says “both end points do not have to be a set height. If they are the curve will be symmetrical. If not it will be asymmetrical when viewed in respect to y axis. The cat cut allows a structure to assume the lowest potential energy state.”

So does this mean that if I hang a cord to simulate the shelter, gravity will show me where to put the cat cut. In other words, the proper cut will be hyperbolic instead of parabolic (with different head and foot end heights)? You can tell I’m not an engineer, but I hope I made the question clear. Thanks

[Jerry Adams]

maybe most any reasonable smooth curve would work – cat, hyperbolic, parabolic,…

the mechanics of a hung cord is different than if you’re pulling on fabric.  For one thing, fabric stretches different along the grain than on the bias.

I am like a dog that won’t let go of his bone.  Everything is about stretching on the grain vs on the bias : )

sometimes I’ll make the tent with a straight seam, pitch it, then find where the seam needs to be to remove all the slack without pulling the seam down and reducing headroom, then make a line where a smooth curve is that goes through those pins, then finish the seam along that line.

For example, if I want to use a cat curve I’ll just use this method to find what deflection is good, then lay it out using that cat curve deflection

[Geoff Caplan]

Vick

Can I check that I’ve understood your approach to cat seams? What I think you’re saying is:

1) Secure the end points

2) Mark the lowest point of the curve

3) Sew a straight line from each end point to the lowest point

4) When pitched, the fabric will form a nice curve.

Or have I got it wrong?

[Lance M]

Eliza,

“… does anyone know of a pattern readily available for making a cat-cut tarp?”

A few years back I worked on a tarp pattern for distribution but never posted it.  I posted it here today.  There are no instructions but I think it’s possible to figure it out.  The ridgeline is a true flat felled seam than will be perfectly centered.  The pattern is in two parts for printing full sized on 36″ wide continuous paper.

Hope this is helpful,

-Lance

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