Expanding on David Thomas's Pulk-Shelter idea

[Anonymous]

The more i’ve thought about it, the more i like the basic concept of a combined pulk-shelter, especially after reading the article that Alpo shared about Cameron Smith’s mostly successful design and implementation.

The goal of design is a combo of lightness and durability at relatively low cost.

These are some initial material and design concepts.  Overall design will be similar to Smith’s pulk-shelter or sled hut as he calls it.  A base raised up off two skis, with a hinged at one end top that can be raised or lowered according to need, and when raised–held up by two poles.   The sides and back side will be tough fabric.

Main pulk-shelter materials (composites).  A BPL member very generously offered to give me some kevlar fabric, which will help to make the whole project significantly cheaper.

The main part of the pulk-shelter will be a foam core epoxy wrapped in a combo of kevlar, cuben fiber (Dyneema), and woven bamboo matting–the size of bottom and top panels will be roughly 3.5′ wide by 7′ long.   I recently read that kevlar does not bond as well as carbon fiber or glass fiber in an epoxy matrix.  To help that some, i would first cover most of the front/top of foam with horizontal/vertically placed, epoxied kevlar.  Then foot wide .34 oz/yd2 Cuben fiber strips will be placed and bonded strategically over the kevlar and to the non covered part of the foam.  The crinkled mylar should bond fairly well with the epoxy, then repeat the process over the bottom/back of the foam core.

Then woven bamboo mat (can be sourced for 25 dollars for a 4’x8′ sheet) will be epoxy bonded over the foam/kevlar/cuben composite. The woven bamboo matt/epoxy will add some impact resistance and some stiffness–sort of like a weaker, less stiff, but lighter carbon fiber (density of carbon fiber is 1.75 and density of bamboo ranges between .3 to .4 so bamboo is significantly lighter).  Then a thin layer of perhaps some Kevlar with some cuben over same, or some thin fiber glass over the woven bamboo matt.  Since this is not a boat, car, etc, and it will only need moderate impact resistance for occasional possible flips, the epoxy composites over the foam won’t need to be too thick.

Bamboo has been successfully used in epoxy composites in various applications as well as tested in studies.  When combined with the right materials and done the right way, it’s like the poor person’s carbon fiber, but not as stiff and strong.  With kevlar/cuben fiber/epoxy reinforcement, it should be plenty strong.  All of these materials have certain strengths that complement and balance out each other’s weaknesses.  Kevlar’s weakness is compression and sanding, cuben fiber is abrasion, cost, sanding, lack of stiffness, foam is tensile and young’s modulus strengths, bamboo’s is biodegradability (epoxy and then UV coating changes that a lot), and glass fiber is weight–rather weight to strength ratio (that’s why only one or two thin layers on the very top).  While carbon fiber is not being used, it’s main weakness is cost, and to a lesser extent, weight though it’s very all around strong and durable for the weight.

The foam core of course will add lot’s of insulation and make it a lot easier to do, because then waxed molds won’t have to be done, the foam will act as the basic shape.

There will be some holes cut out of the top piece and over the holes will go a combo of breathable, fairly tough nylon (like RBTR’s 2.2 Hex70 for the outer and Robic for the inner with Apex in between).  This is to faciliate venting.

Further facilitating venting will be the fabric sides.  It seems like in Smith’s pulk-shelter, they used a heavy duty, waterproof material for the sides.  Instead of that, it will be the same combination of RBTR’s 2.2 Hex70 (for outer)+Apex+Robic inner.

Carbon fiber will be used for the poles to hold it up in shelter mode.  What i haven’t figured out, is what to use to hinge the top to the bottom.  Stainless steel and especially aluminum alloy’s apparently work very well in extreme cold temps.  Any suggestions along that line are welcomed.

It’s hard to say how much the final weight will be.  A VERY loose guesstimate is somewhere between 15 and 25 lbs, but i really have no idea yet.

[David Thomas]

It’s an intriguing idea to use woven bamboo mat as the fiber in a composite structure.  It reminds more of the woven fiberglass roving used for large projects (like commercial boats) where you give up a little strength-to-weight in return for lower materials and labor costs.  Also, I’d far rather fill my workspace and breathing area with chopped grass than chopped glass, and I’m sure one’s scissors and shears stay sharp far longer.

And while the bamboo mat would conform to slightly curved surfaces about as well as anything, if you have flat expanses, very thin plywood (1mm to 3 mm) provides good penetration resistance to the underlaying foam and can be had for cheap.  I used to get 4′ x 8′ x 1/8 three-ply sheets of mahogany plywood for $9.98 at Home Depot but that was 1995 so maybe twice that now. Mostly, I’ve been scavenging hollow-core interior doors and closet doors if I doing some foam-core project.  For some roles, like a 6-foot long shelf, I just use the whole door as is.

The P-tex / HDPE bases on the skis take almost all of the weight and friction, but a friend who made cedar-strip boats, including a 4-person narrow skiff often put graphite fiber cloth on the bottom.  It was black, so unlike FG, it covered up the beautiful wood grain, but he felt it had less friction if the boat got hung up on a rock and allowed to slip off with less damage.  If so, a similar approach on the bottom of the pulk might help if/when the sled high-centers on a log, rock, etc.  Regardless, I’d put a glide wax on the bottom of the sled – I mean, why not?

Would you attempt to make the fabric on the back and sides insulated?

Another idea:  I’m reminded of combo gun-cases/duffle-bags that have a locked rifle case as the bottom of a piece of luggage with a duffle-bag on top, or more so, of some pick-up truck build-outs where there are drawers or cubby holes under a floor that you sleep on.  You want your cargo weight (liquid, food) to be low, right?  And the weight of the human body is only in the pulk when you’re stopped.  So a false floor (probably foam core) would allow for storage underneath.  Secured even just with velcro or some sort of toggle and all your food stores, cooking gear, etc, wouldn’t spill if/when the sled tips over on a bump.

After pulling some sleds where the whole bottom of the sled is on the snow, I’ve come to realize why dog sleds have runners about ski width and length: unless you’re on ice, you’ll be compressing snow as you go.  That takes work.  Far better to compress two tracks, 2-inches-wide each, than to compress snow over the whole 24-30″ width of the sled.  That said, the bottom of the pulk body should be smooth and waxable in case you get into deep powder.

[Nick Gatel]

In extreme climates it isn’t just the cold, but wind and large quantiles of snow that must be dealt with, along with the fact the explorer may need to spend days in a shelter to ride out bad weather. It seems the best solution is a well constructed tunnel tent.

[Anonymous]

Thank you David, for the as usual good suggestions. That’s interesting about the graphite bottom–i’ve never heard of that before.  How did he bond it to it?

Yes, would like to insulate the back and side fabric with Apex, but i suspect that it will tend to build up ice/frost.  I’m hoping that using black color for the outside fabric, might help to keep the moisture moving through before it freezes.

I’m also thinking of possibly using needle punctured foam instead of Apex, and design it so i could open up the fabric sleeve and knock ice/frost out of it.

Yes, the underneath cargo space is what Cameron Smith did with his pulk shelter–it makes sense in a lot of ways.  It would be harder to do with a foam core though.  His Sled Hut used a plywood for the bottom/base, which make that a lot easier.

Yep, definitely makes sense to have a smooth and waxed bottom, even if it’s up off the ground.

[Anonymous]

Nick, the following is a very interesting read:
http://www.explorersweb.com/polar/news.php?id=16704

It sounds like Cameron had used tunnel tents many a time before deciding there “must be a better way”.  Except for the lack of venting and too much moisture accumulation (issues i plan to address better than he and his friend did), he said it overall worked quite well, especially during particularly strong storms and very nasty weather.  It also allowed him to bring less insulation, since it was partly insulated with foam, plus composite layers.

Particularly advantageous he says, was the ease and quickness of setting it up in strong winds verse a conventional tent set up.

[Nick Gatel]

Interesting article. The one thing, based on Cameron’s experiences, would be more living space. Given that you can use much lighter materials, You might want your’s to be longer/wider? I really liked how gear was stored under the floor with 6 compartments. Sounds like you will need a better bathroom solution ;-)

 

[EJVC]

Any pictures/sketches? I can’t visualise what you’re talking about.

And on an unrelated note — I find the word pulk a bit strange – here in sweden they are called pulka (one) or pulkor (several).  Why take the “a” off?

[Anonymous]

Unfortunately Cameron doesn’t give exact dimensions, but most likely i’m smaller than him at 5′ 7″.  Both the foam and bamboo mats that i’m looking at are 4’x 8′, so i could use most of that, and get a pretty spacious set up.  I would hazard to guess that Cameron’s set up was closer to 3’x 7′ or a bit less, looking at the pics and going by his verbal descriptions.

Yes, the bathroom thing…  obviously i wouldn’t be able to make a full under compartment like he did, but instead i could cut out a poop hole area that goes straight down to the ground.

[Anonymous]

Hi EJVC,

Check out this site/article and the pictures in same:
http://www.explorersweb.com/polar/news.php?id=16704
The design i’m interested in, would be pretty similar looking in basic ways to what Cameron had built by his Icelandic friend.

The differences are, the design i’m thinking about would be a bit bigger, would use lighter composite materials, and the fabric sides and back would be made out of breathable fabric with insulation (either CS Apex or perforated foam), with larger top vents than he had.  And a bathroom hole that goes straight down to the ground (the hole would be plugged when not used).

His “sled hut” as he calls it, is primarily made out of fiberglass and plywood.  What i would use would be a foam/kevlar/woven bamboo mat/cuben fiber epoxy composite (maybe with a little fiber glass).

Basically, i just want a lighter, larger, and more breathable version of his.

Re: Pulk vs Pulka, i have no idea why it got changed to take the a off.  Maybe to make it sound more masculine and less feminine?

 

[DancingBear]

Justin,

This seems like a fascinating idea.  I can’t imagine I would ever do arctic exploration, but I do find the pulk-as-shelter concept very intriguing.  Cameron seemed to like his quite well, and you have already mentioned improving breathability, which seems to have been his biggest problem.  I suspect you’ll need to address it with closable vents rather than breathable fabric given the temperature gradient involved, but I’m prepared to be wrong about that.

On the question of toilet facilities I had a thought.  As a child my family had a cabin with a privy, and at night my mom would use an old coffee can as a chamber pot so she didn’t have to venture outside in the dark. She had two lids for it, one had the entire center cut out leaving just 3/8″ or so around the rim, for comfort and to protect tender skin from potential sharp edges, and the other was a solid lid to make it spill-proof when not in use.  Of course metal coffee cans are hard to find nowadays (and they’re smaller too) but the concept could be adapted.  Any bucket-like receptacle with a waterproof lid should be workable, and be an improvement on the bags he mentions using.

As for the pulk vs. pulka name, I wonder if the A was removed to avoid confusion with “polka”? Some accents might make those words sound pretty close to the same.

Anyway, keep us posted on your design progress, and good luck!

Walt

[Roger Caffin]

You may have a materials technology problem. I don’t think Kevlar will bond too well to any epoxy, and I doubt the crinkled Mylar will either. Wrong surface energy.

Cheers

[David Thomas]

Obviously, I’m a trekkie because these things look like photon torpedoes and TOS shuttle craft.

I think Justin is mostly talking about the upper version with the hinged lid with some manner of insertable/foldable fabric/foam/reflectix trapezoid to fill that gap when erected.

If you were to use as a dog sled, you’d absolutely want something more like the lower version because the top of the sled is where the handles are and you muscle those handles around a lot to hang on, shift your weight, wrestle the sled out of a hole, etc.   (And the skis need to extended past the back so you can ride on the runners.)

There’s more windage in the taller unit, especially from the side.  But, man! mechanical complexity will bite your @$$ in the Arctic!  You’re really counting on this thing for shelter, warmth, hauling your gear, so it’s got to be bomber if you venture more than a day’s travel out.  Also, when it’s blowing 30 mph, (or 70 mph!) in the Arctic, you just hunker down.

One idiot mistake in these quick sketches, are that the skis are backwards.  The front of the lower model is on the left.  The pointy end should go in the BACK.  Most minivans (and car-top cargo pods) in the world have a lower co-efficient of drag when driven BACKWARDS.  The front should be a little bit rounded, but the back should allow the air paths to smoothly rejoin.  And it’s not like you need to parallel park this puppy in San Francisco, the tail might as well taper fully to a thin edge – that’s easier to construct and a little wasted volume is no biggie.

A few other points: there need to be very solid handles / runners all around the upper perimeter.  This is something a lot of sea kayaks get wrong, too.  Yeah, being all sleek is nice and it looks cool in the showroom, but when you have to man-handle a swamped boat over another boat to drain it, or re-enter a boat in the water while your buddy stabilizes it, you need to be able to hang onto it.  With this sled/shelter/pulk, you need to be able to wrestle over a countless snow berms, secure it several different ways (flexibility and redundancy) to your harness, potentially to your trailer sled for early-trip extra supplies, and to trees and snow anchors so it stays where you leave it in very high winds.

Justin: you get really excited about space-age materials and the lowest possibly strength-to-weight options and that’s cool, and this is BPL, and I do it, too.  But if you were to really build one of these, to actually use for winter travel/camping, I strongly advise you go for well-proven combinations of materials. Wood is the original composite material – a branch blows in the wind a million times without breaking, it’s cheap, handles -90F to +180F, is easy to shape and join and over the last few thousand years, we’ve developed tremendous knowledge and skill in working in wood.  Watch a few videos about stitch-and-glue kayak construction.  My wooden kayak is a few pounds lighter then Kevlar ones (and many pounds lighter than FG), far tougher, cheaper by $2000, and I did it all in my garage.

[David Thomas]

And, without the hinged lid / coffin configuration, it is more “monocoque” construction and can be stronger and lighter.

I’m still imagining a false floor (like in an AirStream travel trailer, where removable cushions reveal storage underneath).  And the storage underneath would be subdivided so the false floor panels can be small and light, the cargo organized, less able to shift, and easier to find.  Also, your fuel could be separate from your food, etc.  The “living space” walls could have light mesh “cubbies” for fluffy stuff like clothing, and your big parka and insulated pants – the stuff you start the morning with, but then shed for hauling, redon for rest breaks, and need when stopped for the night, would be the last thing in, sitting in that otherwise empty, high portion.

I’d be tempted to make one to two cargo access doors – one near the pointy end and maybe another along one side or back.  Then items you need on the trail could be retrieved without entering the main sled and more of the flooring could be fixed which adds strength and saves a bit of weight.

[Anonymous]

Hi Walt,

That’s definitely an idea worth thinking about.  There’s a youtube guy who makes a lot of different stuff out of common materials, and one of his designs is a #2 bucket set up, with foam on the rim for comfort.  (The guy’s name is Grant Thompson aka “King of Random”)

Btw, i’m not sure why i want to do such a trip to begin with.  In a lot of ways, it seems a bit crazy, or at the very least type 3 fun.

[Anonymous]

Roger, kevlar is fairly commonly used in epoxy composite applications from boating, to racing, etc.  From what i’ve read, it’s bonding strength is not near as good as carbon fiber or glass fiber–so it’s not ideal, but it still basically works well enough to use.

You may be right about cuben fiber.  Though, mylar/polyester surface energy is pretty similar to nylon 6/6, and actually higher than nylon 12 and from what i remember, kevlar’s surface energy is similar to nylon 6.  I’ve never heard of cuben fiber being used in such an application though, so it’s hard to say.  If one fuzzed it up a little with a little fine grit sanding, it would mechanically help to increase the bond some.  Just as Jen’s previous, well worn cuben tent eventually started to hold a lot of water because it started to fuzz up.

But i’m not married to the idea of using cuben fiber, especially since it’s expensive, and untested in this application (probably not a good place/time to test it, though i would technically test it in nearly similar conditions first).  The idea would be to use strips, rather than whole sections of fabric, to reduce the cost ratio.

[Anonymous]

Interesting about the aerodynamics David.

Point taken about using proven materials etc.  Before i was offered some free kevlar, one of the ideas that occurred to me, was to take some stiff foam (XPS or similar) and sandwich it in between two thinnish pieces of African Mahogany sheets or something similar (actually, looking into it, A.M. is not all that cheap..).

But you’re right, a lot of wood is definitely lighter than a lot of these other composite materials.  I could do the above, and combine it with the woven bamboo.   Bamboo is a lot like wood, except stronger in most areas for less or similar weight.

Then to increase impact resistance and durability, epoxy bond some Evazote or similar foam to the outside of at least the top panel (for the bottom, you’d might want stiffer material?). It would absorb a lot of the impact force, and further increase insulation and lower weight (as compared to pretty much anything else per volume).

Such a combo as the above, would save me some money, some time, and would save weight.  After all, this is not a race car, race boat, or similar, and i probably don’t need the extreme strengths of higher tech composites?

[Author]

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