Uncoated tarp?

I think that an uncoated tarp would absorb a lot more water?

1. Ask yourself why misters can drop the temperature below them up to 30F? 2. Ask yourself why the "give" of a toll booth barrier dramatically reduces the damage from a car collision (an umbrella's frame also provides "give")?

Justin: Maybe. Most coated fabrics have a light coating on the top, but the heavy coating is on the bottom, so they're still subject to water absorbtion. But wet is wet and all this stuff dries out pretty quick, so I'm not too worried about it. I'm more concerned about conditions under the tarp when pitched and it's raining. Richard: 1. Misters rely on evaporative cooling, and relative humidity is around 100% when it's raining. I've been in a 'mid with enough condensation to get shaken loose by rain with no noticeable drop in temperature. 2. Kinetic energy absorbers reduce impact damage by allowing momentum transfer to happen over a greater period of time, thus reducing impact forces. A thinner, stretchier, uncoated fabric has more give than a higher denier coated fabric, so the dynamic HH of impact is lower with an uncoated fabric. It feels like you're presenting 2 arguments against, but I'm finding one point moot and the other in favor. Maybe you can answer with answers?

There are several different coatings for tarps. DWR, Silicone, Rubber, Polyeurathane, etc. Perhaps the most basic understanding of why they work at all is key to understanding the coatings. (Cuben is different, think of a thin plastic membrain, or, a non-woven material.) When the threads "wet out" we usually mean that they become saturated with water. This increases the weight, increases permeability to water, decreases impact resistance, and causes temperature(heat) loss (as Richard pointed out for the last two.) Water has weight. Usually as much as plastic or more. More importantly it has a mass. A raindrop is a drop of water that has reached it's greatest speed in the air or terminal velocity. It has an impact to it. Even thin fabrics are fairly rigid in terms of the water droplets. Again microscopically, they have a large impact force. Mass X Velocity / surface area. If a fabric is already wet, it will force a small leak through the woven square. If it is very thick(like canvas) it will simply be absorbed and form more surface tension. If it is thin, it will leak. Think of a large hose. Water put into it is not the same water you get out of it in the same time period. The water will have a transit time. A damp cloth will soak up water better than a dry one. Water molecules "like" each other. Even on cloth, they do not loose this property. Lets start with a basic cloth: synthetic, canvas, etc. Under enlargement, as Richard does, fibers form small boxes. The finer the weave (denier of the threads used in the weaving process) the smaller the boxes. But, you will always have a certain amount of air space between the threads. As an example, this is usually covered by surface tension of water with canvas. This is why you never touch the inside of a canvas tent during a rainstorm. You break the surface tension and the fabric leaks. Leaks usually form droplets that can get flicked off by the impact of rain droplets or they "mist." DWR is not designed to fill in the holes in the fabric. A DWR will cause the fabric to become hydrophobic. (It needs the air space to provide the breath-ability.) That is, it will force the water to create a surface tension because water does not like the fabric. This will delay wetting, but not stop it. Like a dry towel, eventually, it will wet out and leak. Silicone, rubber, poly coatings will fill in the woven boxes with a membrane. But, this filling will degrade with time depending on the thickness. Anyway, to begin with they are very water proof. As they age, the become increasingly damaged and they leak. Silicone and rubberized fabrics can be restored with silicone caulk and a solvent (mineral spirits.) I do not know of any way to restore poly coatings. If you get misting under a new tarp, it is likely from condensation. DWR and Water Proof coatings are two different animals. They each have their place. But, a DWR should not be used for a tarp. A night of rain will convince you of that. Neither should you make a bivvy sack out of, say, siliconized nylon. It won't breath and you will wake up wet. There is a time and place for the materials in use. Violate the rules imposed by the physics of materials at your peril.

#2 is Newton's second law impulse form, Richard has mentioned that before. If you can prolong the time it takes to slow down the water drop, there'll be less pressure pushing it through the fabric. If you have lighter fabric, it'll be pushed more by the water drop, so it lengthens the time it has to slow it down, so the pressure will be less? Mist droplets are much smaller, so their terminal velocity is much less so you don't need waterproof fabric, DWR is fine. or rain splash. Often, there's condensation on the inside of the fabric which will get knocked off by the rain drops so if there's a little misting through the fabric it doesn't matter if it's less than the condensation misting. I've tried this before, lighter weight DWR tarp and jacket. If there's no rain or just a little mist, it works. It rains too much here though, so I just carry silnylon tent. There's some silnylons less than one osy which makes it less of an issue. Don't let your sleeping bag, or whatever, touch the inside of the DWR fabric, water will wick through it like a little stream. #1 – yeah, in order to rain it has to be 100% humidity, but I think sometimes the rain drops can be cold, so some of the humidity in the air condenses onto the rain drops, so humidity at ground level can be less than 100%. Still fairly high, so probably not that much air cooling. And air flow tends to carry it away if you have a well ventilated tarp. More than air temperature, if any of the misting gets onto your sleeping bag, then your body heat will evaporate which causes cooling. Probably a lot bigger effect than cooling of air temperature. try it out as an experiment and report back

As I understand it (someone please correct me if I'm wrong…I'm in my late 20s so the following precedes my lifetime), fabric tarps were around for a long time before waterproof ones and they worked fine with cotton. However, the difference is that cotton threads would swell tremendously when wet, which then created a membrane to keep water from getting in…this also made for very heavy material when wet. I'm not sure how well that will work with an UL fabric since the threads will be so small but maybe a really tight weave would work. Following that line of thought further, polyester not work well but nylon has potential since it swells/stretches when wet. My education in physics is mostly self-learned so this is very hypothetical. Regarding Newton's 2nd Law…I completely accept that this works for a single droplet of water this principle holds up…However, wouldn't Newton's 3rd Law plays into this as well: Droplet hits fabric, fabric gives way to the force and absorbs the impact…now what? Now the fabric is stretched and wants to revert to it's original shape so it pulled into oncoming the next droplet that follows…hopefully someone who's highly educated in physical can elaborate on whether or not this theory is reasonable. I guess my expectation for this to play out is as follows: 1) Rain comes 2) Droplets hit tarp and eventually wet it out, making it more waterproof 3) Newton's 2nd law absorbs impacts for some droplets 4) These droplets roll off the tarp 5) Newton's 3rd law increases impacts for other droplets 6) These droplets create misting by either pushing the absorbed water out of the fabric or pushing through itself. I think the most important aspect of this will be picking the correct fabric material; Polyester will suck but nylon would be better and something with strong wicking properties has potential. I think my stance is to be a skeptic until I see it done. I'm very curious to see the end result but I'm not confident enough to spend the time and money to see it through myself.

I think fabric will quickly rebound before another drop hits Most of the water clings to the outside of the fibers and in the spaces between fibers. Relatively little absorbed. I don't know that it makes that much difference whether it's nylon or polyester. Nylon does stretch when wet though which polyester doesn't. I'm 60. When I was 20 we had… nylon and polyester. Heavier than today. Polyester used to make leisure suits : ) When I was 20, I could have talked about the 60 year olds that only had cotton. Nylon was 1935. I was 20 in 1974, so a 60 year old would have been 20 in 1934… We had computers too, but you had to punch your program into a deck of cards and good luck not dropping them all over the floor : )

You could just go with Cuben. Of course fabric weight effects stretch/pitching issues and durability too. This is a good example of "stupid light" thinking, placing weight reduction before performance on a critical item. I don't mind taking a 5 ounce hit (for example) to have a major item in my kit that performs well. If I'm going to haul something up the side of a mountain, it needs to work 100%. A light item can look good on the spreadsheet, but not so much in a storm at 3AM.

I have 1.5 oz/yd2 finished weight silnylon mid I wonder if 1.0 oz/yd2 sil is strong enough? I want things to hold up in the worst possible conditions I'd experience, but it doesn't need to be more than that. I will not be pitching it on an exposed mountain ridge. I like to find more sheltered spot in bad weather. If it lasts for a few years, that's probably enough. Vendors often make equipment with heavier fabric. Some customers will abuse it. Vendors want to avoid returns, easier to just use heavier fabric. Most customers won't worry about 5 extra ounces.

I'm well aware that a thin, uncoated, synthetic tarp will mist in a rain storm. No need to elaborate on that. What I'm proposing is to leverage the protection of a splash bivy to a greater extent so you can relax the requirements of your tarp. Instead of asking the tarp to absorb impact forces AND prevent water penetration, just ask that it absorb the impact and let your bivy handle the penetration. To re-phrase my question, would you accept a heavy mist under your tarp in a rain storm if it let you drop a quarter pound off your base weight? We have one clear 'no' from Dale. I've recently shifted my thinking from, "my gear has to be enjoyable in all conditions," to, "my gear should be enjoyable in the usual conditions and only needs to keep me safe in the worst conditions (which very rarely happen)." This is an exploration along that line of thinking. Comfort and safety are two separate things. If it rained all the time, I might get tired of being stuck in a bivy to stay dry, but I live in California and we're having a historic drought, so heavy rain is not the usual conditions (plus, I check the forecasts). Something no one has touched on yet is strength. 15d is considered acceptable – is 10d adequate? Should larger, tiered re-enforcement patches be used? Should the hems be double rolled?

The uncoated parachute cloth the Air Force survival instructors use for shelters does a fair job in rain. Especially at steep angles, doubled up, or under existing foliage to slow the rain. Some cold dry places, you find campers using uncoated fabric for shelters too. The old Penberthy MSR tunnel tents with two layers of breathable fabric separated by foam barriers was an attempt at this. They are no longer around, but I don't know how they worked out. http://www.cascadedesigns.com/msr/timeline

What about using an uncoated shelter in (well) below freezing conditions? Could an uncoated fabric cut down on condensation while being less likely to absorb water given the temps? Perhaps the latter would be particularly so for polyester or polypropylene, which absorb less water than nylon. Is the idea that e.g. a winter-specific pyramid made out of a windproof, uncoated fabric could actually enhance the moisture management properties of a winter shelter simply misguided? If not, I wonder what an ideal fabric would be, in terms of CFM, HH, stretch, tear resistance, etc. Is anyone aware of sub-40 CFM, sub 1 osy uncoated poly or pp fabrics, ideally for sale by the yard?

that makes sense, unless it unexpectedly rains : ) even then, most of the water will be diverted to the ground ripstopbytheroll has some 0.7 oz/yd2 nylon polyester isn't as stretchy which is good I don't think water absorption matters that much. Maybe most of the water absorbed by fabric is around and between the fibers?

Edward, i like your thinking. It's too bad that it's hard to source light and tightly woven polypropylene fabrics. Woven polypropylene is durable and strong enough for such use, though it lacks UV resistance (less of a problem in winter, especially if in the woods and in any case, can be remedied such as adding carbon to the plastic in it's melted/liquid state). They would be ideal for such use as you're mentioning. If you had a tightly woven polypropylene fabric with a CFM of about 5 or so, it should really decrease condensation, while at the same time protect against snow and wind. It should even handle light to moderate rain fairly well for some time. It has a bit more stretch than polyester, but less than nylon btw. I'm hoping that eventually UHMW-PE material and fibers (same material as spectra and dyneema are made out of), will be used in a similar manner.