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JAK
2007-07-17, 01:11
Getting back to simple hobbo twig stoves.
Some things I want to investigate.

Using a very simple design, like a cylinder with a hole at the bottom:

1. How much would it help to insulate the sides and bottom?
2. What is the best strategy for minimizing all the soot?
3. How small might a wood stove be, and still be practical?
4. Should the pot sit on top, or nest into the stove?
5. What is the best way to support a pot over a taller stove?
6. How do you deal with boiling hot water and pots?

Perhaps the best starting point for designing a twig stove is to decide on the best approach for flasks, mugs, pots, cozies etc. This might involve 1 or 2 or 3 containers which together include such functions as carrying the water, heating, boiling, cooking, hot drinks/food in camp, hot drinks/soup on trail, hot water bottle while sleeping, hot water bottle on trail, reheating frozen water, melting snow, easy of cleanup.

One fairly versatile system for winter might be a wineskin, a wide mouthed Nalgene Bottle, and a 8oz mug that the Nalgene Bottle nests into. A Nalgene bottle is fairly heavy, but can hold up reasonably well to boiling hot water. It more practical to heat water to boiling when the fuel is free, so with a wood stove things have to be able to handle higher temperatures. The mug/pot handle/holder needs to be both insulating and flame proof also. I like those stainless mugs where the handles wrap around the sides. I think they are 8oz. 12oz might be better, but 4" diameter seems ideal for most things. Anyhow, for solo winter or near winter conditions, or year round, its not a bad system for use with a wood stove. Not a bad starting point to more on to stove design anyway. The stove for such a system might still be able to handle other mugs/flasks/pots and you can always iterate back and change the choice of mug handle or something like that, to fit the stove design as it develops.

So what we have at this point for heating/boiling the water is a mug roughly 4" in diameter and 4-6" high. If it sits on top of the stove the handle doesn't matter too much. If it fits down into the stove, with the stove as a windscreen, then the handle might need to be attached at the rim only, and perhaps act as the suspension system also. It might also need to be more insulating and more flame proof. So that seems to be what gets worked out second. The best way to suspend the mug on or into the stove and attach a handle. I think I am leaning towards suspending the mug from the bottom. That leads directly into stove design considerations though.

One problem with a mug platform that pierces the side of the stove is that it acts as a heat sink that conducts heat outside of the stove, and potentially does more heat damage at this point also. A better option might be an approach more similar to an alcohol stove, with a small stove that the mug rests on, and a separate wind screen than skirts up the sides, supported by the stove from below, or perhaps by the pot from above. But there are many other options. The stove and windscreen could be simple and continuos, with the mug suspend from above, or from below with a simple stand intergrated with the grate.

I think that is where I am at now. Beginning now at the end, I think I will develop a simple grate / pot stand, on which a 4" diameter mug will sit, and around which an insulated wind screen will be. For making char or charcoal a small tin with lid 4" diameter and 1-2" tall might serve as a retort. It might be possible to make the charcoal while heating water by resting the mug directly on the retort, which is itself on the potstand and venting to the bottom, heated from below by twigs, and/or charcoal from a previous burn. Might reduce soot on the pot if done this way. Alternatively the retort might go under the potstand and grate, or not be used. Seems versatile enough.

So now I need to design a simple grate / pot stand. Eventually titanium perhaps, but initially out of can steel I thing. The retort for making charcoal I will mess with later. The insulated wind screen will initially be of ceramic wool and muffler tape. See how that holds up. I am thinking 0.25" thick, 4" diameter, and perhaps 8-12" tall. I think I will start with 12" and work my way down. That might be a bit heavy. Similarly I think the pot stand will be 6" high to begin with, and I will try and make it shorter and lighter over time. Should be a good enough start to see how it might perform though. Time to build something. This weekend maybe.

CoyoteWhips
2007-07-17, 01:21
I was recently looking at fiberglass cloth and wondering if it's be a good insulator. Wrap it around a tube and hold it in place with stainless steel wire clamps.

JAK
2007-07-17, 03:44
I've got some ceramic wool insulation, like they use to insulate kilns. It should be a better insulator than fibreglass, and handle the temperatures better also. It is also fairly light, and you can peel it in layers to the thickness you want. The thing is though I need to use something like muffler tape on the inside and outside to protect the cloth and keep the dust contained and make it dimensionally a little more stable, which is important to get the gap with the pot right and reduce drag that might reduce the draft unproductively. The muffler tape won't add much weight, but I am not sure yet if it will take the heat. The other option would be insulating the outside of a light steel coffee can. They seem to available in lighter weights than the old ones, and taller and narrower shapes. The might be too heavy though, and conduct heat to the wrong places. Not sure. I will try just the muffler tape and ceramic insulation first.

dropkick
2007-07-17, 07:33
Jak,

I think you're overbuilding.
If you have a good supply of free fuel (wood) why insulate the cooking container? The reason for insulating is to conserve fuel, and if you have a good supply you don't need to do that.

Also with a good supply of fuel why make or carry charcoal?

With wood you can't have a windscreen close in around the pot unless the fire has some other way to get outside air. You would put your pot on top and smother the fire. Wood needs a lot more oxygen than an alcohol stove.

I cooked on open fires for a lot of years and I never worried much about soot. It's something you just learn to work around. I had a canvas bag that I carried my pots and pans in to keep any soot off my other gear.

But if it bothers you, just give it a light scrub when you do your dishes - if you don't insist on a pristine bottom on your pots it doesn't take much time or effort to knock any lose soot off, and the bottoms of the pans, even though they are still blackened, normally won't get anything else sooty.

The only Hobo type wood stove I've ever used was a coffee can with 1/2 inch holes punched in it from top to bottom. The holes were spaced 1 to 2 inches apart. After the fire gets going good the cook pot is set directly on top of the can.
The reasoning behind using this (instead of using an open fire) is that it is a smaller fire and so is quicker to make, and easier to put out after breakfast.

As it was normally quenched with water (often by carrying it to the creek with a pair of pliers and dumping the whole thing in) the can normally isn't warm when it was time to put it away, so it was kept in a large plastic bread sack. - Due to rust, the fire eating the metal, and neglect these stoves usually didn't last more than a season or two.

I haven't made or used one of these stoves for several years though.
In part because I now use different cooking methods and stoves. And in part because I no longer feel the need to be the first person fishing or hunting in the morning and either leave from home or take longer getting up.

CoyoteWhips
2007-07-17, 09:43
I'm a fan of insulation on wood stoves. With a good insulated burn chamber, you get a hotter, more efficient flame. Maybe not important when you're surrounded by twigs, but if you've had to snowshoe around looking for fuel, or you're in a desert, that might be a factor.

The problem with insulation is that its bulkier and heavier to pack. For home experiments, I've used perlite mixed with refractory cement. Not as heavy as you'd think. Sadly, too fragile for packing, in my opinion. I've also heard of a light, fire-proof wall board, but haven't seen any around here.

A short insulated chimney above the flame helps give you a good draft and more complete combustion.

If the cross section area between your windscreen and your pot is roughly the same as the cross section area of your chimney, the draft will remain similar and you'll get the efficiency of keeping the heat on the pot longer and faster boil times.

If you can elevate the twigs so the draft draws air underneath the tips, you get a hotter burn.

With abundant fuel, the easiest camp stove is three rocks, or that coffee can. However, a good camp stove would have the benefit of burning less fuel, cook faster, create less smoke; drawing air with a passive design, rather than a battery powered fan. It that could be designed break down into a flat, light-weight, durable packing form, it'd be a wicked cool trail cooking system.

Maybe make an efficient stove that fits in a coffee can. :-)

I'd encourage JAK to keep fiddling with his ideas. Could be next year everybody will be talking about how neat those JAK stoves are.

JAK
2007-07-26, 12:36
Thanks guys. Good points. I am probably overthinking it. The simple hobbo stove is hard to beat. Here is what I am going to try this weekend.

Pot (Corked Flask).
Something portable while carrying hot beverage, but still fit where a 1 litre Nalgene Bottle would go, but leaving room for insulation. It would need to be straight sided, and have a plastic or cork lid, secure enough for travel. The pot would be a bit taller than the stove and the wind screen, and the part that sticks out would be insulated with kelvar whipping twine so you can handle it when hot.

Stove:
The stove itself would be like a beer cozy that would snugly fit over the pot, but made of ceramic insulation wrapped with a layer of muffler tape. The walls might be 3/8" thick, or 1/4" thick when compressed with the pot stored.

Wind Screen:
The wind screen would be very thin and just fit over the stove to provide protection when stored in standard bottle carrier. So 4" diameter, and same height as stove. It would slide up to become the wind screen with a gap of 1/4" around the pot. A pot stand would be needed to provide a gap above the stove and below the pot.

Pot stand:
I think this could be made from thin steel can material, three curved pieces 3/8" high that store around the pot when disasseembled, but clip together to form a three pointed star shaped pot stand that fits over the stove and inside the wind screen and under the pot.

Stove stand and Ground Pad:
Perhaps something similar to the pot stand, only a bit bigger. Stove might need some reinforcing in places to sit on such a stand, and take the pot stand and the weight of the pot. Some screen also to rest on the stove stand inside the stove, and an insulated pad to go under the stove stand. Stove stand and screen should disassemble and nest between the stove and pot. The insulated pad would be the same diameter as the stove and bottle carrier so it would store underneath and provide bottom insulation.

Dimensions:
Corked Flask: 3.5" diameter, 8-10" high. Aluminum Jolt Can?
Stove: 4" diameter, 6-7" high, 3/4" thick. Ceramic Insulation & Muffler Tape.
Wind Screen: 4" diameter, 6-7" high. (slides up). Muffler Tape is nothing else.
Pot Stand: 3/8" high raises pot about 1/4" to 3/8". Can Steel.
Stove Stand: 1/2" high raises stove about 3/8" to 1/2". Can Steel.
Mug: 4" diameter nests over everything and fits inside bottle carrier to hold everything together when stored. Corked flask can still slide out for drinking while hiking. Mug can sit on top of stove with wind screen lowered. Melting snow, simmering. Cooking, Baking? Maybe with retort under it.

My retort tin for making char would fit on stove or even in the stove, but it would store away elsewhere with flint and steel and char cloth and lighters and candles and stuff inside. So it would be about 3" diameter and 2-4" tall, so it might also fit a small tealight lantern. Separate project.