Silicone Polyester Ripstop - Cat Cuts?

**daverr** · 09-29-2015, 09:24

Does anyone have any experience making hex tarps without cat cuts with silicone polyester ripstop fabric(RBTR SilPoly or Dutch Xenon)? I'm looking to make a Fronkey Hex tarp and am considering doing it without the cat cuts. Some things I am particularly interested in:

Are the sides taut?
Does the middle of the tarp sag on either side?
Do you notice it performing less optimally after being soaked by rain?

Thanks in advance for your thoughts!

**Dutch** · 09-29-2015, 09:31

I have been using very minimal cat cuts with success with the Xenon fabric. Just 2 inches over about 4 foot distance. It makes it so much easier to sew.

Dutch

**ksbcrocks** · 09-29-2015, 09:56

RBTR MEMBRANE silpoly definitely doesn't need any cat cuts at all.

**ShellHammock** · 09-29-2015, 10:01

Heres a video of my rig at the Grayson Highlands Hang last weekend... constant rain, pitched the tarp once, and never re adjusted. Square 8'x10' tarp. Look at that tight pitch! That's the SilPoly from RBTR

**daverr** · 09-29-2015, 11:06

Thanks, guys.

Sounds to me like the challenge will be ensuring that I have a laser straight cut on the hex so as to not introduce any irregularities that would cause floppy sides. That should be relatively easy on the bottom of the tarp (follow the ripstop grid), but the sides could be my challenge...

**ripstopbytheroll** · 09-29-2015, 11:23

Originally Posted by daverr

Thanks, guys.

Sounds to me like the challenge will be ensuring that I have a laser straight cut on the hex so as to not introduce any irregularities that would cause floppy sides. That should be relatively easy on the bottom of the tarp (follow the ripstop grid), but the sides could be my challenge...

The next Membrane silpoly tarp I sew will be without cat cuts. I've had the original one I built out in extended rain a handful of times now and the pitch just doesn't change.

I would however advise against using the ripstop grid as a guide if you're looking for laser straight cuts. It can be hit or miss on how straight the grid lines are.

Speaking of making your life easier, if you don't have one, I would suggest investing in a 6 ft straightedge from Lowe's or Home Depot. I got this one: http://www.homedepot.com/p/Empire-72...4006/205694615.

**Boston** · 09-29-2015, 21:50

Originally Posted by Dutch

I have been using very minimal cat cuts with success with the Xenon fabric. Just 2 inches over about 4 foot distance. It makes it so much easier to sew.

Dutch

One thing I've realized over the years is a lot of the "recommended" shapes for cat cut's are a lot deeper than needed, regardless of material. My standard depth is about 1"-1.5" per yard.

One other misconception is that "polyester doesn't stretch". It does, but to a lesser degree (technically speaker polyester generally has a larger modulus of elasticity than Nylon, if you're interested in that sort of thing). And it still deforms along the bias. The main benefit of polyester is it wont sag when wet. The main problem with nylon is it's material properties actually change based on moisture the material absorbs, it becomes more elastic when wet. This is why nylon tarps tend to sag when wet.

The purpose of a cat cut is to provide tension through the middle of the tarp fabric, because of the way tension lines will run through the material when trying to stretch it taut. There are two forms of compliance being addressed here: natural sag (which will be less with a less elastic fabric), and stretch from pulling on the bias (which poly woven fabrics are still susceptible to). For these reasons I still recommend adding some form of centenary curve to your tarps.

Because the tarp doesn't sag MORE from weather conditions means it's impact is less noticeable, and many get away without it, but honestly adding a catenary isn't that hard, and I think more people shy away from it due to some intimidating perception, than anything else.

**daverr** · 10-30-2015, 08:52

I went ahead and went with small cat cuts - 3 inches over 10 feet. They are so small that you really do not even see them. But I do think that they help significantly with the pitch, especially since its impossible (for me at least) to get a laser straight cut on the fabric or, even harder, to sew the edges perfectly. I'm quite happy with the design - thanks to everyone here for their input.IMG_20151029_175639.jpg IMG_20151029_175654.jpg