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Best design for bomb proof bivy
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Robert Merrick
(Flyfisherman) - F
Best design for bomb proof bivy on 03/05/2008 20:47:38 MST Print View

I will be making two bivy sacs in the near future. I have already purchased some momentum, cuben, 2 layer gortex, silnylon, tyvek and a polycryo ground cloth.
The first one is to be a sleeping bag cover under a tarp. Originally I was going to go with momentum for the upper and cuben for the lower. Now though, I was thinking about momentum top and a polycryo bottom in an envelope style.
The second one is for emergencies only on the PCT in Northern California. Must be bombproof! I was thinking about using a gortex top and cuben bottom.
The question is what style is the most bomb proof? Envelope, draw cord top and bottom, zipper along the top and sides like the Outdoor Research bivy’s. What do you think?

Mike Hinsley
(ArchNemesis)

Locale: England, UK
Best design for bomb proof bivy? on 03/06/2008 02:39:06 MST Print View

What sort of emergency are you expecting?

You are already carrying two forms of shelter in addition to raingear.

If you really had to you could spend the night outdoors in just raingear provided that you could get out of driving rain and could stay isolated from the ground (by crouching, leaning or sitting on your pack) - and small terrain features will do that.

Your tarp is 100% waterproof so in a bad scenario you could wrap yourself and your kit inside the tarp and lie down on something insulating or just crouch and wear it like a cloak.

The classic emergency shelter is a big polthene bag or a bothy bag.

In an emergency you are not worried about comfort you are worried about staying alive - which means maintaining core body temperature and not being eaten by things like bears. That's all.

If you really want a bomb-proof bivy then the classic design has a 4-6oz PU Nylon base and a goretex upper with a shoulder zip. It also weighs in around 600g - which will be the weight of an entire shelter system.

Sometimes at night the safest course of action might just be to walk to somewhere safe rather than hunker down.

The lightest emergency shelter that you could build would be to do something like a bothy bag in Cuben - this is a bag that you sit in and use as if it were a giant hat - this gives you a good volume for little fabric. Alternatively a cuben sack. Such an item could be stored in a tiny pouch on your waist.

Emergency kit if carried should not be carried with your ordinary kit. If you lost a pack then everything that you are wearing is all you have to survive. It's not going to be a lot in an UL scenario.

Derek Cox
(derekcox) - F

Locale: Southeast
re on 03/06/2008 04:38:10 MST Print View

in most cases...its not that big a deal if you get wet... just deal with it. whatever bivy or shelter you bring with you make sure it keeps you dry enough and you will be fine. no need to bring an emergency bomb proof shelter...

Robert Merrick
(Flyfisherman) - F
What sort of emergency on 03/06/2008 13:36:43 MST Print View

Good question Mike.

I really used the wrong word. Maybe - a just in case the weather forecast turned out to be wrong and I didn't want to carry a tarp along with a bivy for just the weekend.
In the northern California Mountains, it is common to see a thundershower now and then, but largely it doesn't rain in the summer. I guess I could just carry a large trash bag and call it a bothy bag, but I will be planning for a weekend trip and thought a bivy without a tarp would be ideal AND comfortable.

Here is the question put a different way. How long will a Momentum bivy keep you dry in a rainstorm without a tarp?
Which bivy style is less likely to leak; an envelope style or a clamshell style?
Sorry, that I can't seem to express my thoughts in writing very well!

Tim Marshall
(MarshLaw303) - MLife

Locale: Minnesota
Lightest option on 03/06/2008 14:54:46 MST Print View

Don't use the polycro for the bivy bottom unless you plan to remake it every few trips. It will not last at all. If it has to be as light as possible use the cuben, i would use the silnylon though and use the cuben for a tarp. That way you have a bivy that is around 6-8oz and a tarp that should be under 7 (probably under 5)for a total of under 14oz (could really be less i am estimating high) which will most likely be lighter than any "bomber" bivy. Then you don't need 2 bivys, just the 1 and the crazy light cuben tarp.

And your "just in case" kit will have a tiny tarp in it along with a sul bivy, and still pack smaller and be lighter than any soggtex unbreathable bivy you could make.

I hate goretex, it's breatheability is a joke and sleeping under any bivy w/o a tarp you will either get wet from water coming in (at zippers face holes or seams)or from it not going out (exhaled vapor being trapped by less than breathable fabric).

so i suggest you use a UL Bivy & SUL Tarp combo at around 14oz

I would just use a bigger tarp and no bivy for well under 10oz

Mike Hinsley
(ArchNemesis)

Locale: England, UK
Best design for bomb-proof bivy on 03/06/2008 15:53:05 MST Print View

Well,

a DWR bivy will keep you dry for maybe half an hour tops of light rain. A pertex style will last a little longer.

A drawstring bivy will leak quite quickly. One with a zip closure will not leak unless the zip is exposed.

I've spent a night in heavy rain on saturated grass in a bivy that was goretex-wannabe that had a horizontal zip on it. I stayed dry and slept but in the morning the bag was damp from my breathing.

The lightest solution on this that I've worked out (but didn't test on the last trip) is this:

A fully waterproof base with a pertex top. The pertex shouldn't touch the ground. Over the top of this you place and peg large sheet - in Grandma Gatewood style. The sheet can be smaller than the smallest tarp (1.2m tapering to 0.9m for a 2.3m length) and requires at most 6 pegs.

If weather is fine the pertex bag covers most things but if it's looking a bit rough you can deploy half or all the sheet (blanket tarp?)

A Pertex bivy weighs 200g or less. My heavy sheet weighs 200g (I cannot find very light SilNylon).

But this is all over-engineering.

I don't personally feel it's wise to take shelter that is less than what you might reasonably expect unless you have a safe alternative - like walking to a natural shelter or whatever.

On my last trip out when I was expecting a clear night I just slept in a DWR bivy bag - but I did have with me a tarp in the bag.

It's all about what choices you are willing to make. If you only carry a lightweight Bivy bag then you might save 200g over taking a tarp as well but then you might also have a rough night if you are wrong.

I've grown up with weather reports that don't seem to apply to the mountains and to mountain weather that can be rough regardless of what you are expecting.

So, only in the midst of a high-pressure system will you find me out in the hills without a full shelter of some description for overnighting. Only in a high-pressure system will you find me without a waterproof jacket.

The lightest fully waterproof bivy bag that I have is 500g (4oz PU Base + Aquabloc upper + taped seams) . The lightest tarp setup I have is also about 500g. A more typical setup is around 7-800g.

If you want to be comfortable then take more shelter than you think you need.

There are no hard and fast rules. A lot comes down to how you feel about weather and what you are comfortable with and so on.

It's well worth testing kit out and seeing what you are comfortable with.

When I'm trying something new I will take two lots of kit with mne - the new stuff and an alternative that I know will work.

When I took the blanket tarp out for testing I also took with me a fully enclosed tarp. That way if it was a total disaster I would still be safe and warm.

.....

For your scenario where you are looking at high ambient temperatures and very low risk of rain (something I can only dream of!!!) then the lightest option would be to use a DWR or Pertex bivy with a SilNylon base (don't bother with a groundsheet underneath, just use a better grade of SilNylon - it will weigh less overall) and then use a cuben tarp that can be pegged out to give you a waterproof cover - no poles just peg it over the top like a blanket.

That will give you a comfortable night for most nights and a less comfrotable night if it rains - which is improbable anyway.

If you can camp in a forest the trees will stop most of the rain. If you camp in a cave it will stop all of the rain. There is a lot to be said for natural shelter as the ultimate lightweight option.

Sorry I've rambled a bit, there is no best; it's a case of what works for you.

...

From a practical viewpoint rain on your face probably will wake you up before your DWR or pertex starts to leak in any meaningful way and so it's just a case of deploying.

The quickest way I thought of doing this is to peg a blanket tarp at your feet as a 'roll' wrapped with velcro to keep it in one place.

If it rained you could reach down, unvelcro the loops and then peg the top two pegs above your head - but I've not field tested this yet.

I really should watch more TV ;-)

Edited by ArchNemesis on 03/06/2008 16:14:01 MST.

Robert Merrick
(Flyfisherman) - F
Great Ideas on bivys and a "Gatewood blanket" on 03/06/2008 18:54:31 MST Print View

Tim, your idea of just a larger tarp is probably how I will ultimately do it. I made an 8'x10' cuben tarp a few months ago and it weighs right at 8oz. After it was made, it sure seemed awfully large for just me. I do like the bivy setup with a quilt; especially when the mosquitoes are out.
Mike, thank you for the idea of the “Grandma Gatewood blanket”. I will try it. Do you think there is any danger of suffocating with a plastic sheet over my head? Keep me informed on your progress with the Velcro. This seems like a great idea and light too. I wonder why I haven't heard of it before. Kind of like a double walled flat tent.

Tim Marshall
(MarshLaw303) - MLife

Locale: Minnesota
blanket tarp? on 03/06/2008 20:16:48 MST Print View

I think this idea would be about the least comfortable thing i could ever dream up.

A tarp lying on top of you just pegged out, you'll feel every drop of rain, your escaping vapor will be trapped right on you (worse than the worst condensation you've ever seen in a tent or tarp)Head room, nope, read a book at night, nope, breath comfortably, nope, exit fast for late night pees, um nope.

This concept is far more extreme than i would ever be willing to go, the 8oz cuben tarp is money. Sew a few cinch straps to your quilt to bring it into lock down and leave the bivy at home, or go with a top bag next time and forget about crawlies. If the bivy's job is too keep bugs out and splash away i'd replace it with a larger tarp to fix the splash issue and some wedding vial mesh to keep bugs off your face. I just don't like or see the need for bivys. You build a great tarp with good head and elbow room then crawl into a claustrophobic waterproof envelope and seal yourself in. It's not for me.

But if you really want a bivy make it with silnylon floor to forget about ground sheets, momentum top for ultimate breathability and put it under a tarp for real weather protection.

I know that tarp-less camping can be done, but we could also attach arms to our butts. Just because we can doesn't mean we should.

Obviously i have an opinion on this, please don't take it personally. Everyone must find the kit that works best for them, I know that, it's just that my way IS THE BEST!

Sorry couldn't resist

Mike Hinsley
(ArchNemesis)

Locale: England, UK
Best design for bomb-proof bivy on 03/07/2008 01:21:17 MST Print View

Tim,
it would still be used OVER a pertex bivy bag and pegged out with short guys.

There would be airflow underneath it and so the condensation [I think] will be within what the pertex can manage.

It is a little 'extreme' but it is a minimal solution for minimal weight.

Not everyone carries trekking poles and for some reason trekking poles seem to be given an amnesty in pack weights.

As I said, it's not been tested yet. It's not designed to be the best night you've ever had in the hills, merely a surivable one...

Mike Hinsley
(ArchNemesis)

Locale: England, UK
Best design for bomb proof bivy" on 03/07/2008 01:31:41 MST Print View

Robert,

At the head end I was planning to use the remnants of my pack (which has a 'frame') to provide the headroom so that it's not claustrophobic (which is not an issue for me BTW).

The sides would not be pegged flush to the ground but pegged on very short 30cm guys.

It's important that there is good airflow underneath so that the pertex can do its job.

WPB fabrics are heavy, pertex is light and some WP such as cuben are exceedingly light.

Thre is a contact point on your body/pertex where tarp goes over you but any sort of airflow will cause this to vary as will your natural movements in the night so I don't think it will be a failure-point.

To visualise it imagine what a tarp would look like pitched very, very low to the ground.

If you didn't mind an extra few grammes in weight then the head-end of the blanket-tarp could be pitched with a small 40cm pole (buy an arrow shaft and convert it).

My blanket tarp has been made by cutting up a tarp design that wasn't very efficient.

Don't bother cutting up decent fabric on something experimental - use the cheapest PU that you can find.

Remember, I've not yet field tested this. My last test trip was called off.

OBTW this is not a new idea - it's a more civvy friendly version of a couple of shelter-building techniques in US Army Field manuals and of course Grandma Gatewood.

Edited by ArchNemesis on 03/07/2008 05:27:39 MST.

Jaiden .
(jaiden) - F
ultra low tarps? on 03/07/2008 05:46:45 MST Print View

Why not use a "stick" or a nearby "tree"

No need to over engineer anything.

Joshua Mitchell
(jdmitch) - F

Locale: Kansas
Re: ultra low tarps? on 03/07/2008 07:37:30 MST Print View

Or a bush...

Anyhow, I, more or less, did this once when I was in scouts we were 'sleeping under the stars'. I had one of those, relatively heavy, brown crinkly tarps that I was using for my ground sheet. It was overbig... Well, it started raining so I just flopped half the tarp over me in such a way that it overlapped the ends of the half I was still laying on... slept like a baby (and it was raining pretty hard, lightening and all). Now, I was in knee-high grass which helped cut wind (as I and the tarp was sunk below the top of the grass), and it also helped keep the tarp from falling down on my face (as did my normal movements).

Man I was ticked when they came by, woke me up, and told be we were all leaving because the other boys couldn't handle it. I was prepared, apparently they weren't. I got more wet as we left than when I was sleeping...

scott rebello
(scottrebello) - F
Alternate bivy top material on 04/02/2008 14:51:02 MDT Print View

Maybe a bit off topic but, has anyone thought of making their bivy top out of no seeum? Its lighter than other fabrics and more breathable. Your tarp keeps water directly of the top of the bivy. Planning to make a double bivy for my girlfriend and I. We use ponchos doubled up for our tarp.

David Wills
(willspower3) - F
Re: Best design for bomb proof bivy on 04/02/2008 18:06:28 MDT Print View

Have you considered a fully WPB bivy for your weekend kit? Some WPB fabrics are lighter than 4-6oz nylon (paclite for example). Look on the montbell website for theirs (proprietary and goretex). You could drape your rain jacket or umbrella over your head and/or sleep on your stomach b/c of the drawstring closure. If you made a clamshell out of full goretex, you might have what you are looking for.

Jan Rezac
(zkoumal) - MLife

Locale: Prague, CZ
Backpack as a shelter support on 04/03/2008 08:00:00 MDT Print View

I can't resist to add my experience with using my pack as a shelter support. I was sleeping under the stars as the weather was nice. When light rain had started, I wanted to build a poncho shelter, bu we slept on flat rock where it wasn't possible and I was too lazy to put my shoes on and move all my belongings to nearby forest. I put my backpack (quite big, with internal frame) to my head and supported it by some stick so that it stood well. Then I attached the poncho to it's top and tucked the other end under my sleeping pad, so that the shelter was touching only my feet. It worked nicely, but the morning was cold and I couldn't gather up my legs for more warmth because it would have broken the shelter.