Does a silpoly tarp need cat cuts?

**yenemy** · 02-13-2015, 16:02

Since silpoly is (presumably) lower stretch than silnylon, can I get away without cat cuts? Just a 12x10 (roughly) rectangular tarp?

I'm liking what I'm reading about silpoly so far, so I had some interest in putting together a proper tarp for my hammock in silpoly. Just wondering, since I'm not nearly patient enough to put together cat cuts

**geneaut** · 02-13-2015, 16:25

I figure it is easier to make it square, and then add cats if needed.

Everything I read about silpoly is that it is extremely low stretch, but I have no first hand knowledge.

**ksbcrocks** · 02-13-2015, 17:19

I contacted someone on here that made a straight cut 10x12 tarp out of silpoly and they said that they didn't miss the cat cuts. I just ordered enough fabric to make 3 tarps for me and a couple friends and they will all be 10x12 rectangular tarps.

**yenemy** · 02-13-2015, 20:05

Thanks for the replies! Looks like I will be making a straight cut tarp too.

**Violent Green** · 02-13-2015, 20:21

I imagine that it's like cuben fiber which also is very low stretch. Cat cuts will make for the most drum tight pitch, but not as necessary as with silnylon.

Ryan

**Boston** · 02-14-2015, 10:23

The floppy edge of a straight cut tarp isn't necessarily a result of stretch, but due to the way the lines of tension run through the fabric when it's pulled taut. A catenary curve eliminate's the material that would normally flap, and help'es ensure the tension lines run along the edge of the tarp. Think of it like pulled on the ends of a curved spring - you're basically trying to flatten the curve out, and thus directing the force more along the curve.

Silpoly will also stretch - it's a light weight fabric. The difference is the nylon is hydrophilic (absorbs water), while polyester is hydrophobic (does't absorb water). When nylon absorbs water it's physical properties change and it becomes more elastic, and thus stretches MORE when wet. Polyester doesn't absorb water, and so once tensioned, it wont stretch MORE when wet.

**Brian** · 02-14-2015, 11:53

Boston is exactly right - it's still a lightweight *woven* fabric. One big performance difference between the silpoly and cuben is that the fibers in the cuben fiber are much, much stiffer than that of polyester, and are bonded together in a pliable but not distortable fashion. If you try gripping a woven fabric at two different spots at an angle to the weave (see red arrows in photo), it will stretch and distort the entire fabric because the vertical and horizontal fibers aren't really attached to each other, they're just woven together and take each other along for the ride when they get pulled in weird directions. Cuben has much, much less spread here due to the bonded nature of the fibers, and barely distorts at all.

So a cat cut in a cuben tarp can't take advantage of that same type of spring effect that any woven fabric can. The exception to this might be having a cat cut in a ridgeline and that spring-loads the entire tarp when it's setup, but doing so on an edge doesn't have nearly the same effect.

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