So Richard, Do you think this makes Epic is more rain resistant then cuben fiber?

Also did you happen to work on the calculations on the hypothetical hydrotstatic head of 3mm rain traveling at 60mph and why the psi wasn't strong enough to cause the cuben fiber to mist even though it would of hypothectically been 25+ times more then what the material should be able to handle.

I look forward to your thoughts.

I am sending Richard samples of 0.33oz and 1.26oz cuben to get HH numbers for these materials.

I suspect that the reason that 0.7oz cuben seems to be testing low, yet performing excellently in the real world is because the HH test holds the pressure on the fabric for a long period of time, whereas a real rain drop impact lasts for just a split second. Due to capillary action or something else, cuben may be capable of holding a high HH value for split seconds, but if you sustain pressure over a long time then it seeps through... And this doesn't happen in the real world...just some pure speculation on my part.

Edited by dandydan on 03/24/2011 19:19:08 MDT.

"It's been pretty well known for quite awhile that these tents are considered a compromise. No big secret there. "

Yup. Worked so well for Black Diamond that they discontinued using the fabric and made a point of marketing their new 'more waterproof' fabric.

; )

So I am wondering what the difference is between the mylar used in space suits and
helium balloons vs mylar used in the cuben fiber? What IS the hydrostatic head of
helium at sea level?

When I first call cubic tech to find out about their product, one of their customers
was using it to make sail kites with the structure to keep the sail open made up of
air filled sections that were inflated with a pump. I wonder how that worked out?

Do the heavier duty space blankets remain waterproof?

Will Aussie wool ever be a good shelter fabric?

Sorry, long day.

Hi all

There have been some very valid requests for more information about the pressure exerted on a fabric by high-speed rain drops. Would you believe that this is an area where we have very limited information, either theoretical or practical? Yes, I have been scouring the research literature for some time.

One reason for the limited knowledge is that 'rain on a tent fly' is not an area which can attract a huge amount of research funding. Very sad. Some allied areas exist, but they too get little funding, and their test conditions are not the same.

Another reason we have a problem is that the dynamics of what happens when a drop of water hits a rigid surface turn out to be extraordinarily complex. Raindrops are not round for a start, and their shape depends on their size and speed. The bouncing around inside the raindrop of the impact shockwave, and the reflections off the surface inside, are horrible. Validating the assumptions made in creating such a model is also extremely difficult. Supercomputer stuff for sure.

Testing what happens in the real world in a reproducible manner is equally complex. How do you measure the pressure at the surface of a raindrop as it hits that rigid surface? Bear in mind that the dynamics mean that you have to be able to measure the pressure over an area of (say) 0.1 mm x 0.1 mm or smaller, with a time resolution of about one microsecond. We don't have affordable sensors which can do that, as far as I know. And yet, this pressure is what seems to matter.

Now, having thought about all that in relation to a fixed rigid surface, what about the dynamics of the fabric surface? How will it deform? Over what area? and what about the impact angle? How will this affect the behaviour of the raindrop? Basically, dunno.

The final problem is that BPL does not exactly have a NASA-sized research budget. Sigh!

But we will try to get some answers. It just may take a little while.

Cheers
Roger

Beyond the samples and financing there is the hidden world of "dumping it." When people think they have a piece of gear that has failed they tend to get rid of it even if it means quietly absorbing the loss. This is at least the case in America. When you extrapolate what this means for expensive, used gear , well ...

Edited by Meander on 03/24/2011 18:50:29 MDT.

Lawson,

I might come as a surprise but I have far more questions than I do answers. We may find that aging creates very unique degradation curves for each fabric/coating combination. If that happens, we will need to answer "what's best" questions in the context of being qualified by where each material/coating is in its unique degradation curve. Maybe .7oz Cuben does all of its aging the first time it set up and then never changes. Maybe all other .7oz Cuben virgin samples test at >3,515 mm H2O and they don't degrade with aging.

Protocol B was created so that everybody could participate in some fashion; everyone can contribute samples that they have access to, a tester can't give a dishonest test result that couldn’t be easily challenged by any forum member which results in the sample being retested in another lab, by another tester, with the exact same sample, and the same test protocol.

If forum members don't contribute samples from a wide range of sources, the testers won’t be able to provide enough data to answer many questions. For example, if only one sample set of Skylite is sent in we can plot its aging curve. If multiple samples sets of Skylite are sent in, we may find that there are various manufacturing sources, a broad range of quality, and a possible identification method to make sure you can identify the good stuff.

You asked, "Do you think this makes Epic is more rain resistant then cuben fiber?" With the strong qualifier, that I only tested one sample of each, yes. If we get enough samples contributed we may find out that all of the other .7 oz. Cuben samples test better. I think we can all agree that without multiple samples and something like Protocol B there is a much larger chance of having lengthy heated arguments that never lead to common understanding and shared conclusions.

You asked, “Also did you happen to work on the calculations on the hypothetical hydrostatic head of 3mm rain traveling at 60mph?” A 3mm raindrop has a maximum speed of only 19.5 mph (its unique terminal velocity). Its kinetic energy just before it splats is 5.35E-04 joules. If it SPLATS perpendicular to a firm surface its force is equal to 3,639 mm H2O. I did my part of the calculations (smile), now you can look at any source that explains the Impulse-Momentum form of Newton's Second Law to see how different variables affect the net result. That simple exercise is left to the reader. Don’t forget to add the 60 mph winds into your calculations. After you go though this exercise you also need to have the tarp you were using tested by a recently calibrated Suter. At that point you finally have the relevant information so that you can discuss your results with Dan M. and others. For me personally, I am far more interested in studying this problem at a higher level of abstraction.

Edited by richard295 on 03/25/2011 12:13:58 MDT.

(smile)

Hey, Michael.....Michael...yeah, up here! I did not want to make another post so yeah, it's too much time. See you in the real world. Thank God for that forever edit feature they have here! I can't wait to start blasting stuff with my garden hose......girl friend has a power washer.......Later.

Edited by wildlife on 03/24/2011 20:54:05 MDT.

This is the thread to end all threads. You guys take something simple, like backpacking gear, and make things wayyyy too complicated.

I say use the Cuben Tarp, and if you get wet in it, then ask for a new one or a refund. I have gotten some very poor sil-nlyon gear from the same person as well though. (crap yellow rain cover)

Also, Dan you really don't have time for all this, now get cracking on you know what! :):)

Edited by mfog1 on 03/25/2011 08:30:37 MDT.

Hey Richard,

Terminal velocity is the constant maximum velocity reached by an object falling under the pull of gravity so I agree that a 3mm rain drop might have a terminal velocity of 19.5mph but when rain is being propelled by the wind, it can reach the max wind speed. Its the same reason a .30 caliper bullet has a terminal velocity of 300 feet per second but when its shot out of a gun it can travel at 1,990 feet per second. Make sense???

I just did the math and it looks like a 3mm raindrop traveling at 60mph would be hypothetically equal to 11,196mm or 15.87psi.. So this would mean that the rain was 26.5 times more then the material should of been able to handle.

Also when you get a chance you should take some epic and some cuben and do a spray test with your hose. All the lightweight epic I have seen will allow streams of water to pass through the material after it wets out but I am willing to bet the cuben doesn't do a thing. Give it a try..

Best Regards,
Lawson

Richard,

Ya, I had noticed those when I read the thread for the first time and did not realize that they were at a pretty high magnification. That's cool. -So, if all the samples were micrographed as new, and then HH tested, and then micrographed again, especially in and around the test zone, this may provide some clues.

"Only when we have these results can we then have the debate as to whether they correlate with experience and if not then why not."

I agree with the above very much. But not so much with the second part of your statement. Most of us are not equipped to do science. I know I'm not. But keeping the conversation civil and asking well-thought out questions does help allot.

This makes for interesting Background Reading.

In Another Thread I've laid out a bare bones approach, but thus far have had no responses.

Comments?

@Richard and Roger - do either of you have any samples of the Brooks Range Rocket CT3 fabric yet? If not, let me know and I will see what I can muster. It has a HH of 5,000 mm, according to the Brooks Range website.

Perhaps the brightest future ahead is in shelters, etc. using a PTFE laminated Cuben, such as the CTB1B3-1.0/NF that Steve Evans used in his rain jacket. His specs state that it has a claimed HH of 9,000 mm. It would be great to get some samples of these versions of Cubic Tech's fabric as well. As I recall, Javan has also used a version of this.

Thread reference: Waterproof/Breathable Cuben Fiber Jacket


As for BD's Epic Tents, keep in mind that BD switched over to the Nanoshield fabric over a year ago now, which is stated (per BD) to have about 50% better HH figures.

@Lawson - Thanks for participating in this conversation.

Let me vouch for Lawson's objectivity by calling to remembrance the hammock-tarp combo that he was working on but decided to pull the plug on because of unacceptable failure characteristics of the Cuben fabric, which appeared too static and insufficiently strong under dynamic tarp loads. Yes, he has Cuben products to sell, but I believe out of careful consideration of their benefits he and others will continue to do so, due to the relatively strong, albeit anecdotal, performance records of the various products on the UL market.

A case in point would be Joe Valesko, who has used the .5 oz. Cuben fabric for his shelter for presumably several testing weather events:

In 2009 I tested the cuben fiber Hexamid on a full Continental Divide Trail thru-hike. The trip took 154 days traveling roughly 2,651 miles through the rocky mountains. Over the course of the trip the tent went through just about every type of weather, from gusting wind, to rain storms, snow storms, sub freezing temperatures, etc.

Source: Zpacks.com Hexamid Page

Does wind increase the speed of a rain drop or slow it? Does wind change the shape of a rain drop?

Changing the shape might actually slow a rain drop. Doesn't wind actually place resistence on a falling object?

Just some questions and I do not know the answer. I do know that when you jump out of an airplane, changing the shape of your body (moving it in difference positions can speed or slow you), and wind generally slows you (before chute deployment).

"This makes me think of Black Diamond's Epic tents. Wonder what the HH is for this?

David et al: Last year Roger tested some unused Epic Malibu for me, the earlier version fuzzy on both sides, and the later version shiny on one side. They both were dry to around 15 kPa, or very roughly 1500 mm HH, and then quickly developed many running leaks - a showerhead might be a good analogy. This was totally unlike the silnylons he tested for me, that developed some wet spots and/or transmitted a few drops somewhere between 1200-1600 mm, but continued to resist water penetration to varying degrees until the pressure became much higher. Note that I only sent Roger silnylon that, from squeeze and other primitive tests, appeared to be the most above average in water resistance.

When stretched in a circular 9" plastic embroidery loop and placed over pails on my back deck, the same Epic treated fabric survived numerous rainstorms without wetting out, until some really long storms lasting most of a day and really pouring all night.
They then became quite wet on the inside, but did not transmit measurable amounts of water into the pails. Some samples of silnylon that tested best on Roger's device, were stretched right alongside the Epic ones, and also remained dry underneath until long and heavy storms, then developed some wetted out spots and a few drops of water on the inside of the fabric and in the pails. After each rain, all the pails were dried out inside, to insure they contributed no condensation during later rains. I also think it is important that these samples were set horizontally, exposed to the full impact of vertically falling rain. To keep them from blowing away, the pails were weighed down by a brick sealed in a Ziploc bag, and the loops were held down by wooden slats that were in turn held down by bricks (not over the fabric).

After finally wetting out, and drying out, the same Epic samples were tested in further rainstorms (it rained a lot here last Fall), and along with the silnylon, behaved much the same as originally; in that they remained dry on the inside during shorter rains, but became wet on the underside after extended heavy rains. Other Epic samples were also treated with Atsko and Scotchguard silicone based sprays with no noticeably different results, except I could not tell if the sprays affected vapor permeability. I'm not sure if the Epic actually "wetted out," in the sense of being totally saturated, after these longer rains; but it made no difference for my purposes, as the wet exterior and interior would limit vapor transmission, and be uncomfortable for an occupant in a tent.

Why go to all this trouble for the Epic Malibu? Because it is a strong, quiet, drapable polyester, making an excellent tent fabric, with some elasticity but much less sagging with temperature changes than nylon; is vapor permeable even under lower vapor pressure (as in a tent wall- note the horizontal line for the Nextec on the graph in Alan Dixon's article), and will certainly resist water well for long enough to get a tent up in the pouring rain. Then, as Dan McHale suggests, it can be covered with a very light Cuben fly if the storm is expected to be prolonged.

The problem with this approach was weight. The Malibu weighs close to 1.9 osy, and with the Cuben cover added, is in the area of 2.5 osy. For me, that was too much of an increase over a single wall of 1.3-1.4 osy of silnylon, in terms of the weight difference expected for the whole tent. That, along with an unrelated consideration, the instabilty of dome tents with one pole hubbed at its ends, or two poles crossed once overhead, even with elbows to allow stiffer pole material, led me to drop the whole concept; and go back to dome designs with the poles crossed twice, and with elbows and stiffer poles as well, using a single wall of silnylon and the greatest amount of ventilation and netting between the occupant and the tent wall that I could devise. That work is still in progress.

So the length of the exposure to water pressure is an important factor perhaps not well addressed by hose and shower tests of limited duration. Another really important factor, also alluded to here, is the effect of crumpling, folding, stuffing and the like on water resistance, and is the one that gives me the most pause; but from Richard's posts, it doesn't sound like his new shelter had been used to that extent. Intuitively, flexible silicone coated nylon would be more resistant than Mylar film to such treatment; but that is only guessing. The testing protocols being developed by Roger and Richard should provide more insight about that.

Was going to comment about sensationalizing, personalizing and ridiculing; but what the heck, it's spring in North America and the sap is running. Interesting though that the sanest voice on this thread comes from Oz, where Fall is arriving. All the same, the many contributions are very helpful to my passion for tent design and much appreciated; so I don't mind the flak and yes, no denying that I said it in the earlier post, hot air. Sorry to have offended anyone.

Hi Lawson

> I just did the math and it looks like a 3mm raindrop traveling at 60mph would be
> hypothetically equal to 11,196mm or 15.87psi.
Since academics have been using supercomputers to model what a raindrop does as it impacts, and getting very unreliable results to boot, I am interested in how you did the calculations.

Cheers

=>Terminal velocity is the constant maximum velocity reached by an object falling under the pull of gravity so I agree that a 3mm rain drop might have a terminal velocity of 19.5mph but when rain is being propelled by the wind, it can reach the max wind speed. Its the same reason a .30 caliper bullet has a terminal velocity of 300 feet per second but when its shot out of a gun it can travel at 1,990 feet per second. Make sense???

I don't think I agree with this. I may be wrong, but this is how I understand these things. Terminal velocity of a falling object is reached because a falling object (in a simplified world) has two forces on it (the drag force and gravity). Gravity is a constant (at least in a Newtonian sense). The drag force is the difference in the pressure forces on the bottom and top of the falling object. Since the object is falling downwards, the pressure on the bottom of the object is greater than the pressure above, causing an imbalance and ultimately a net force upwards. This pressure difference is usually modeled to go as the velocity squared (and the shape of the object is very important). This indicates that an object will accelerate downward for as long as gravity is stronger than the drag force. The drag force will increase, however, because the object is speeding up. Once the two forces equal out, the object will stop accelerating and be at terminal velocity.

This is totally different than what is happening with a bullet. With a bullet, when it is in the gun, it has 3 forces on it: the explosion of the gun (I don't understand guns, so forgive me), the drag force due to the air, and the drag due to the barrel. My guess is that the explosion on the gun is many orders of magnitude larger than the resisting drag forces: this is why the bullet goes 0 to 2000 fps in the space of a few inches. Once the bullet has left the gun, however, it will behave like the falling object from above, only flipped on its side (and without a propelling force to help it maintain its velocity). The bullet has one force on it: drag (or the difference in the pressure between the front and the back). Since the bullet is traveling so quickly, the drag force will be very high, slowing it down until the bullet ultimately hits something. As an analogy, if a falling bullet had a terminal velocity of 300 fps and you shot that same bullet out of your gun at 2000 fps, it would slow down until it reached 300 fps.

The raindrop in the wind has a terminal velocity, just like a falling bullet or a falling raindrop. It's true that it's shape might be affected (which would matter greatly), but the pressure difference driving the drag would be in the direction of travel, i.e. not straight down. If the shape of the drop changes, then the drag force would change. If the shape change is not taken into account, then the drag on a raindrop traveling 20 fps nearly sideways is the same as the drag on a raindrop traveling 20 fps vertically. At the moment that this drag forces equals he driving force, the drop will stop accelerating. In the nearly sideways case, that force is gravity and wind (in some ratio determined by the strength of the wind).

Hmmmm...I mind rescind some of this. I will leave it up there because I do want to know if I have been thinking incorrectly, but I just figured out how you get that the drops can reach the speed of the wind. If the drop is being pushed by the wind, then the drag force is driving the drop. The drag force, in this case (and in the other), is a function of the difference in the velocity of the drop and the velocity of the air, squared. This will drive to accelerate until it is traveling with the wind. Ha. Thanks much. I'll hang up and listen.

Ha. Nice. Thanks for making me think this through. I still don't understand the bullet thing though.

Hi biointegra

> @Richard and Roger - do either of you have any samples of the Brooks Range Rocket CT3 fabric yet?
Nope. Interested.

Cheers