quarta-feira, 30 de março de 2011

MYOG: 3mil Plastic Tarps

Great article how to bild a tarp from www.backpackinglight.com MYOG: 3mil Plastic Tarps: "
by Jerry Adams | 2011-02-08 00:00:00-07
I may get beat up for talking about using plastic for a backpacking tarp on Backpacking Light, because it’s so cheap and flimsy, but here goes!
I have spent many nights sleeping under polyethylene tarps. They’re cheap, fairly lightweight, as waterproof as any other material, and possible to make robust enough to survive fairly bad weather. I’ve done about eight different designs and usually just use them on one trip of up to four nights.
I mainly use polyethylene tarps to prototype a design I want to verify before doing it in more expensive material. Such tarps would also be good for backpacking on the cheap, as you'd be able to equip four people for about $20 and some labor. In these economic times, frugal is popular.


I have always used 3 mil (0.003 inch thick) polyethylene, which is commonly used for protecting stuff while you’re painting. You can get a 10 x 25 ft piece from Home Depot for $10, which is enough for two or more tarps, depending on the tarps' size. The 3 mil weighs 2.0 oz/yd2, which is fairly light.
Just to experiment, I made a tarp out of 2 mil polyethylene, which weighs 1.4 oz/yd2. It held up in my backyard, through rain and wind, for more than a week before the duct tape reinforcing came off, which wasn’t even a polyethylene failure. Based on this test, I may use this lower weight sheeting instead.
As a comparison, silnylon, a common lightweight tarp material, is about 1.4 oz/yd2 and Cuben, the state of the art material, is 0.75 oz/yd2 for the thickness most commonly used for tarps.
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For guylines, I use Mason’s twine. (Shown: Mason’s twine, some of the slippery pale stuff that doesn’t work very well, and 16-oz coffee cup for scale.) I use this for all my tarps, food hanging line, ditty bag ties, etc. It’s fluorescent red to minimize running into or losing it. Home Depot sells it for $5 for 250 yards, which is enough for 30 tarps. The braided version works best for using a taut line hitch. It’s #18 thickness (0.0625 inch diameter). It weighs about an ounce for 25 feet, which is enough for most tarps.
Several other common guyline materials and weight for 25 feet:
  • Triptease: 0.7 ounce
  • Aircore 2: 0.25 ounce
  • Aircore 1: 0.1 ounce
I tried some similar line from Lowes, and it was thinner and slipperier so it didn’t work very well. The Home Depot line was a brighter fluorescent color. This could just be a particular lot and a different store, or a completely different product. I’ve also used twisted twine, but it comes untwisted too easily.
I use #0 size grommets on my tarps (0.25-inch diameter hole). Outdoor Wilderness Fabrics sells a grommet setter, hole punch, and 24 grommets for $15. I have a grommet setter similar to the OWF version, though mine is cast iron. You have to hammer on the die to squish the grommet down, and it’s a bit of a trick to get it right. You have to hammer the grommet enough so that it doesn’t rotate around in the plastic, but if you hammer it too much, the grommet's edges will cut the plastic. You should experiment first - hammer it too much so it cuts the edge to see what to avoid. I sometimes have to pry the grommet from the die with a screw driver.
I use readily available duct tape for reinforcement for guyline tieouts. One roll is enough for many tarps, and you probably don't even need to go buy some because you've got it lying around.
I have used two trees to hang a tarp, but it’s hard to find a location with a flat area between two trees. I like camping in alpine areas without many trees anyway, so I finally got a pole which works much better for a one-pole tarp configuration. It’s easier to find one tree next to a flat area. I don’t normally use trekking poles but those make excellent tent poles.
I use some Easton Aluminum tent poles from TentPole Technologies. Quest Outfitters is another good source. I get the 26-inch lengths and cut them down to about 21 inches for each section - 41-inch length as assembled. This is pretty flimsy and could collapse in strong winds, or could collapse if you run into it, but is quite lightweight - 1.5 ounces for the two-section pole. I’ve used this in 20 MPH winds a number of times and have gotten away with it. The tip fits into the #0 grommets. I whittled down the tip a little because it’s a tight fit.

Guyline and Pole Reinforcement

The first method I’ve used to attach guylines is to put a small pebble inside the corner of the plastic and tie around it with the guyline. If you’re not familiar with this, it can be handy in repairing a tarp that rips out in the field. I object to carrying pebbles, so I use 0.75-inch diameter styrofoam balls from the craft store.
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First put the ball in the corner and wrap around it with guyline.
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Tighten and tie a knot.
This is a strong connection and distributes the load to the plastic evenly. The only bad thing about this method is that it doesn’t pull on the precise corner of the tarp, but at a point a couple inches in from the corner, so there will be a few inches loose all around the tarp. Because it's not tensioned, it will flap in the wind. Also, this method doesn’t work very well as a connection to the sides of the tarp, only in the corners.
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Loose edges.
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The method I have used most often is duct tape for reinforcement with a grommet to run the guyline or pole tip through.
For example, last June I used a tarp in windy, rainy conditions for three nights at Burnt Lake and East Zigzag Mountain on Mount Hood in north central Oregon. Winds got up to about 20 MPH, and while the tarp held up fine, I noticed the duct tape slipped about 1/8 inch relative to the plastic. The adhesive on duct tape isn’t that great, and this wouldn’t last many more nights. Any suggestions on a better tape to use?
I wanted to find a way to attach guylines that would last a little longer than duct tape, so I constructed a simple test - 3 mil plastic, duct tape, and grommet on one side, guyline around foam ball on the other side. I then used it to hold 8 pounds (gallon milk jug of water).
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Test setup.
After about six hours, the duct tape slipped a little (which is what happened after three nights of backpacking in wet windy weather), and after twelve hours, the duct tape slipped off completely. This was my control... then I used the test setup to evaluate a few other methods.
First I put five staples through the duct tape/plastic. It held up fine for a week, so I think this would work well for a tarp on multiple trips.
Second, I tried something not requiring a grommet.
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First put the duct tape on one side of plastic and place knotted twine on the very edge of the plastic.
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Fold the duct tape over onto the other side of plastic, enclosing the knotted twine.
This held up for about a week in my test setup - the twine eventually slipped out of the duct tape. I also used it on a 2 mil plastic tarp that held up in rain and wind for a week before the duct tape slipped off, so this would make a reasonable connection for one trip. I might make the loose end longer and tie it to the main line with a taut line hitch so it won’t slip out.
I also used the test setup with 2 mil plastic. Foam ball worked best, staples in duct tape worked pretty well, no staple slipped off after about a day, knotted line without grommet held up for three days.
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I use a taut line hitch to tighten guylines. I don’t want to insult anyone by assuming they don’t know this knot, but just in case, here’s a taut line hitch loose.
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Taut line hitch tightened up. You have to really pull on the two loose ends (marked with red arrows) to tighten it up enough to hold on thin twine like this. After the tarp is set up and the lines are under tension, go over the taut line hitches one more time, really tightening them up. Also push the loops together on the two tight ends (not marked with arrows) to get the knot so it won’t slip.
This thin line is marginally thick enough to use a taut line hitch. You can always use tensioners instead of taut line hitch. For example, BPL's Aircore Nano Dyneema Spectra Guyline weighs 0.8 ounces for 50 feet of cord plus 12 tensioners, all for $15.99.
I use 0.34-inch Easton Aluminum poles, and the tips fit in a #0 (0.25-inch) grommet. This pole is a little light and might collapse, especially if you accidentally run your foot into it, but I’ve used it for years, with a 40-inch length, and it has never collapsed on me.
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If you don’t want to use a grommet to hold the pole, then use a guyline and make a clove hitch around the tip of the pole. This would work with a bigger pole, trekking pole, or stick.

Design Examples

Classic Pup Tent

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I used this style tent for my first backpack when I was 12 years old, and it is probably the most common tarp method. Basically, you just string a line between two trees or poles, put the tarp over it, and guy down the four corners
It works better if you have a guyline tieout at the middle of the tarp on each side at the ridgeline. Have a short guyline and tie it to the ridgeline line with a taut line hitch. This keeps the tarp stretched out along the ridgeline. Alternately, you can omit the ridgeline line that goes under the tarp, and instead go from both sides of the tarp to tree or pole, but this puts more stress on the guyline tieout, and if it fails, the tarp will collapse on you (this isn't a problem with fabrics so much as it can be with plastic).
I don’t really care for this design because it requires two poles, which is heavier, and if the wind blows into either end, then rain will blow in and get you wet. You can try variations, like lowering one end to reduce rain blowing in or making the tarp longer or wider. Adding a catenary ridgeline curve is out of the scope of a plastic tarp, but improves performance dramatically.

One Pole Tarp

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One pole tarp, in plastic.
This is what I have used more than anything else. On one end it’s like a pup tent, and on the other end, the two corners are about 40 inches apart and guylined to the ground. I added a “beak” at the pup tent end to reduce rain blowing in - folded the plastic and used duct tape/grommet to pull out with guyline. There’s duct tape with a grommet at the peak to put the pole through.
This uses less fabric than a pup tent because it’s narrower on one end. Only one end is open, which can allow rain in, which is bad, but only half as bad as a pup tent's two ends. It has only one pole which weighs less than a two-pole pup tent configuration. It is also easy to find a spot where you can use a simple tree instead of a pole, making it possibly a no pole design.
I’ve used various versions of this on about six trips of three or four nights each over the last couple years. I stayed fairly dry, and it held up pretty well to the wind. Obviously, you want to point the foot end into the wind to minimize rain blowing in, but the wind can always shift on you and defeat this idea.
A good size for one person is 40 inches wide at one end, 76 inches wide at the other end, and 9 feet long. Five square yards of fabric weighs about 10 ounces in 3 mil or 7 ounces in 2 mil or silnylon. Pole and stakes are maybe 4 ounces, so this makes for a pretty lightweight package.
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After successful polyethylene versions, I made one with silnylon to be lighter and more durable. I lengthened the beak and put in a zipper. I made catenary curves from the pole peak to each of the two corners on the low end.
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Head end view with zipper open. This weighs 11 ounces, plus 2 ounces for the pole and 1.25 ounces for the stakes.

Minimalist Tarp

I wanted to have an even lighter tarp that provides a better view from my sleeping bag for when there is only a small chance of a little rain or dew. I’ve slept without a tarp many nights when there was dew, and I slept fine and stayed dry inside, but then in the morning I have to dry things off and any gear also has to be dried off.
My idea is to have a tarp 44 inches wide guyed to the ground and 76 inches wide with two poles on the other end, 6 feet long. This only covers the top half of my sleeping bag, but my bag is water resistant - good enough for dew or slight rain. If it’s continuous hard rain, I’ll get wet.
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In 3 mil polyethylene it weighs 7 ounces. The poles weigh 3 ounces. The stakes weigh 1 ounce. I used this on one trip, and I was okay with it. I really like how I can see out of it almost as well as no tarp at all, but it isn't wind resistant.
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I liked it enough to make a version with polyester spinnaker cloth from Seattle Fabrics. I lengthened it a little to 7.5 feet.
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I tried it one night. When the winds got more than 20 MPH it was really noisy so I just took it down. It would help to have catenary curves on the two ridges.
The fabric is advertised as 0.75 oz/yd2, but I weighed it as 1.1 oz/yd2. It weighs 7 ounces, plus 3 ounces for poles and 1 ounce for stakes. I’ll play with this on a few trips to see if I like it, and probably find some modification to keep me busy.


If you enjoy making your own gear but hate making mistakes on expensive performance fabrics, get familiar with polyethylene. You can try several variations before making your first cut on the pricey stuff, and it's even sturdy enough for field testing.


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