Pseudo-Cat Tarp Experiment

[Frawg]

I've been playing with small tarps to find how little I can get away with, depending on the weather -- currently variations on a 5'x10' rectangular tarp.

This idea has probably come up before but it's new to me and I call it a Pseudo-Cat, for lack of imagination. It's basically a 3 segment piecewise linear approximation of a catenary along the bottom edge. The overall tarp gets hexified in the process.

Basically, you cut triangular wings off the ends of the rectangle



and reattach them at the bottom corners, like so:



Here it is pitched:



I was surprised at how much the pitch tightened up compared to a plain rectangle. Also, the corner ripples are interesting:



I'm thinking they point along the lines of force, which would be interesting information for modeling what's happening with the fabric.

Anyway, I'm going to explore the idea a bit and thought I'd toss it out there for anyone interested.

[Pak-Man]

This is a great idea for what I am trying to do. This would make a great small coverage tarp, with no seams down the middle where it is important.

Thank you for posting this, and let us know how it performs.

"Dirt"

[Frawg]

Just got in from a quick test of it. A thunderstorm blew in so I set it up in the rain and got in my latest small (42"x91") experimental hammock.

I was soaked by the time I had things set up, so I soaked the hammock pretty well under my butt when I got in. Ended up hanging about 6" above a 6" deep lake in the yard. Per the weather data I could access, the winds were gusting to about 15 mph and the rain fell at rates up to about 3/4" per hour.

I pitched the tarp with the sides oriented windward / leeward at about 45 degrees each (90 deg to each other) and the ends open, as pictured above. The bottom edges were about 12 to 16 inches above the grass / water level, and the tarp ridge was an inch or two above the hammock ridge line. The tarp stayed fairly taut with 4 guy lines, and flapping was remarkably less than I'd seen in my previous rectangular 5'x10' tarp pitches with 6 guy lines under similar conditions. It was visibly much tighter as viewed from the hammock, and the windward side didn't quite blow against the hammock edge, as I'd encountered with the rectangular one.

The gusting wind blew a steady but very light spray under the sides, which would wet the bottom of the hammock. Other than that, though, things stayed dry. More accurately, I didn't get any wetter in the hammock than I was when I got in, so the tarp seemed to be doing its job. I was able to enjoy a cup of hot coffee while listening to the rain and thunder.

Although it works well in rain with little wind, 5' is a bit narrow for these conditions and it would be hard to keep everything dry, IMO. A bottom weather shield would have helped, but I think the extra fabric would be better allocated to widen the tarp. I have some 42" wide fabric that I'll seam to make a 7' (nominal) wide tarp and see how that affects the side spray. I'm thinking that's the width I'll probably end up with, as I should be able to pretty much close up the sides. It won't be a smooth ridge, but I've not had any problems with seamed ridges before.

Next time I'll try to have things set up before the rain hits so I can start dry and see how dry I stay. We get a lot of storms this time of year, so it shouldn't take too long to find out.

Not much to report, but I hope it's a little useful.

[Mule]

That's a great idea.

[sclittlefield]

Great report Frawg. I'm going to have 11' of 54" wide sil left over after I make my next tarp (Blackcat style). I look forward to hearing about your next test. I may try and copy your design, it's really quite brilliant and a great use of material.

Do you think hanging higher up would help with splash, or would it be worse with windblown rain? I usually hang between 1 and 2.5 feet off the ground (after hammock stretch). Either way, it will definitely be a fair weather tarp.

[pineapplenewton]

i think the ripples form because for the sides to be straight there the edges parallel to the hammock cant actually be as long as they are if that makes seance (i could be completely off) oh and if you would be interested in selling one of the other tarps that you've made and don't use I would be interested.

[Frawg]

Great report Frawg. I'm going to have 11' of 54" wide sil left over after I make my next tarp (Blackcat style). I look forward to hearing about your next test. I may try and copy your design, it's really quite brilliant and a great use of material.

Do you think hanging higher up would help with splash, or would it be worse with windblown rain? I usually hang between 1 and 2.5 feet off the ground (after hammock stretch). Either way, it will definitely be a fair weather tarp.

Thanks for the kind words, scl! I like your blog, btw.

I have hung higher in the past with a straight 5 x 10 and it stayed quite dry underneath as long as the wind was very light. It was also in a somewhat wooded area (my back yard in Virginia), which added a little protection from wind.

For this experiment here in Florida I've been using a hammock stand, which keeps me pretty low. Previous higher hangs (varied, 18" to 24" above ground ) here with a 5 x 10 tarp left me with "wet butt syndrome" when the wind was gusty and swirling. No splash problems, though.

I was out the other day in some very heavy rain that fell about 30 degrees off vertical, thanks to a light but steady wind. An "A frame" pitch at about a 60 degree vertex angle kept me quite dry, along with my camp shoes under the hammock. Was about 8" above ground that time, and finished a whole sudoku with nary a drop on the paper.

Anticipating a heavy storm today, I set up with a hiking pole "awning" pitch to leeward. I ended up frustrated. A solid front crossed the state with us right in the middle of the line but the rain parted, passing north and south of us. Pity, too, because it would have been a real downpour with heavy winds. I did get a fair amount of heavy wind, though, and the tarp held up reasonably well under that. The windward side did blow up against the hammock (104" x 47" single layer, this time) and the "awning" bulged upward quite a bit at times, caused by the low pressure that develops downstream of an obstacle. There was a bit of flapping, but not an inordinate amount and the tarp stayed rather quiet for the conditions. Had an interesting effect with the hammock, though -- the wind was so hard at times that it triggered an audible (and palpably weird) vibration in the longitudinal folds of the hammock between my butt and my feet.

Anyway, more rain is forecast for the next couple of days as the remnants of Ana head this way. We'll see what happens. I may try to capture some video if I can figure out how to keep my camera alive in the rain.

...

FYI, the wings were cut about 8" back from the original corner. I think I'll try 12" on the next one. Wish I were fluent in tensor analysis, because the warping of the ripstop grid pattern provides a lot of info on how the forces are getting distributed. I'd like to be able to calculate the optimum cut point for this design. Oh well...

I'll plan additional variations on this in the future. I've got a couple of DIY rectangular tarps I can recycle for these experiments and will post results as they come in.

Cheers!

Chuck

[PuckerFactor]

Could it be possible the warping is just slight variations in tension from human error when sewing? Not to belittle your pretty awesome thread injecting skills, but it's next to impossible to have a mathematically perfect seam. I'm planning on getting some "spinnaker tape" to make my next tarp with. It'd be interesting to see how much that helps with tension ridges.

Good work!
Acer

[Frawg]

Could it be possible the warping is just slight variations in tension from human error when sewing? Not to belittle your pretty awesome thread injecting skills, but it's next to impossible to have a mathematically perfect seam. I'm planning on getting some "spinnaker tape" to make my next tarp with. It'd be interesting to see how much that helps with tension ridges.

Good work!
Acer

Yeah, I can do a mean rolled seam. End of skill set...

I could be wrong but I think the corner wrinkle pattern appears because the wings' warp/weft grid is oriented slightly on a bias relative to the base, so the fabric elasticity is different on either side of the seam. (anisotropic, hence my previous tensor comment.** ) All four corners show the same pattern.

On the next one I think I'll orient the wings so that their grids align with the base section. If I get a similar wrinkle pattern with that orientation then I'd be inclined to think it's because of how the edge seams constrain the tension field as it distributes from the corners.

It will be interesting to see what you find with the spinnaker tape.

** I'm not really going to go overboard on the math of this stuff, btw. It's just an excuse for me to study something else I should have learned in a past life, long ago. There is some interesting architectural stuff out there, though -- Google "tensioned fabric structures".

[pineapplenewton]

It probably what you said that the fabric isn't quite lined up but it could also be that when you sew a seem you might take more fabric into the seem so that one part might be shorter? I don't know if what i mean makes seance or if what I wrote means what i mean if you know what i mean