Building a Woodgas Backpacking Stove

In previous posts I've mentioned that I generally use an alcohol stove or Esbit tablets to boil water for rehydrating meals while backpacking. However the concept of using a wood stove is beginning to catch on among backpackers. The reason is that the fuel is available in the forest so you don't have to carry any fuel. I've been experimenting with this for a few months and I now have a design I like and am confident enough with to use on backpacking trips. In this post I'll show you how to make it.

Now in general I am opposed to collecting firewood in the wilderness because dead wood plays an important part in the forest ecosystem and if every hiker made a fire then the forest would suffer. However I am going to show you how to build a stove that can cook a pretty big meal with literally a handful of twigs (what most people would use as kindling to start a regular fire) so the impact is very small.

There are two keys to this efficiency. First we will cook with flames rather than coals so we don't need to wait around until coals have formed. Second we will design the stove for more efficient combustion. In fact what we will burn is woodgas. Woodgas is the gas that is formed when wood is heated. It's an excellent fuel. In fact during WWII there were tractors with internal combustion engines that used woodgas for fuel. Our stove will be a downdraft gasifier, meaning that the wood will burn from the top down. The stove design I will be following is basically the Garlington woodgas stove with a few modifications in the windscreen and potstand.

To begin with take a normal soup can and cut the top and bottom off. It's nicest to do this with something like a Good Cook can opener which leaves no sharp edges.
Next decide which end of the can will be the bottom of the stove. Cut some tabs in the side of the can and bend them toward the middle. Now cut a circle of hardware cloth (or wire mesh) to fit inside the can. The tabs will hold this up.
What you are looking at in the picture above is how we get primary air to the stove. Air comes in the holes at the bottom that we cut the tabs out of and then flows up through the hardware cloth to the wood.

Now we need to make some holes for secondary air. This is the air that will mix with the woodgas to produce the flame we will cook on. Flip the can over and cut some slits in the side of the can near the top. Then insert a scredriver into each hole and bend the can so that the slits turn into vents leading from the outside of the can up and into the can.

At this point the stove itself is done. We fill the stove with twigs about the diameter of a pencil broken into pieces. I've found that it works best if I make the pieces about 3 inches long and put them in the stove the long way. The other approach is the make shorter pieces and just pour them into the stove. I find standing the twigs upright seems to help with airflow.

Now we can put a little kindling on top and start a fire. This is counter-intuitive because we are used to starting a fire the other way, with kindling on the bottom igniting wood above. But in this case it is very important that the fire burn from the top down. What happens is that the top layer quickly forms into a layer of charcoal. The wood below is heated and woodgas and air rise through this charcoal and a reaction called pyrolysis takes place. A main product of this reaction is hydrogen gas and that is what will burn when it meets the secondary air coming in from the holes at the top. In fact the way you know your stove is working correctly is that a few minutes after starting you will see that the only flames appear to be shooting into the stove from the secondary air holes. The flame will be quite steady and fairly blue in color (perfect combustion would yield a blue flame, yellow indicates some lack of efficiency).

Now in order to make this a good system for heating water I needed to rig some way to suspend a pot above the stove. I also needed some way to block the wind so it doesn't carry all the heat of the flame away before it can heat the pot. Actually a wind screen serves a secondary purpose in this case. The air around the stove inside the windscreen will become quite warm and so the secondary air entering the stove will be preheated, contributing to more efficient burning.

I built my windscreen/pot-stand out of a sheet of aluminum roof flashing. I drilled two holes and put wing nuts in them to hold the flashing in a cylindrical shape. Then I cut triangular holes in the bottom for air to enter. Last I drilled four holes about 1 1/4 inches from the top into which I can insert two tent stakes. The pot will sit on these stakes about 1 1/2 inches above the top of the stove. Using tent stakes is a perfect example of a dual use item. My total pack weight is reduced by carrying items I can use for more than one purpose. I always have to carry tent stakes for my tarp. So I might as well put them to good use.

The pot I use is an AntiGravityGear 3-cup aluminum pot. It's a great little pot. About the right size, very light, inexpensive, and the outside is already black so the black residue from the fire won't make it look bad. I sized the windscreen to allow a gap of less than an inch all the way around the pot for fumes to escape.

In the pot is 16 ounces of cold water (I kept it outside overnight and the temperature was in the high 30s). After a few minutes I have a nice rolling boil. That's not smoke in the picture, it's steam. A good woodgas stove produces almost no smoke. After a few minutes of boiling the flame begins to die down. Inside the stove is a pile of glowing charoal. After a few minutes this has been reduces to a fine ash, probably about 1 tablespoon full. So in this way also the impact on the forest is very small.

The downside of a downdraft woodgas stove is that all the fuel has to be in before you start the fire. You can't add wood later. Another way to build a backpacking wood stove would be to make a very similar system to the one I have built but also allow a space for wood to be fed in. You also get fairly efficient burning (better than a camp fire) because of the secondary air supply but it's not a true woodgas stove. However in some instances that might be better because it would allow you more control over how long a burn you need. You could keep such a fire going indefinitely if needed. However I like this system better. Mostly just because it's so cool!

Review of the GoLite Ultra 20 down quilt

I picked up a new piece of gear this week. My homemade quilts are synthetic and quite thin, perfect for spring or fall camping but not good for winter. So I bought a new quilt which is down-filled. It's an Ultra 20 from GoLite. It's rated at 20 degrees, meaning that it's supposed to keep you comfortable at temperatures of 20 degrees or higher.
Down is the warmest insulation you can get for it's weight (for those interested in the down vs synthetic debate scroll down to the bottom of this post for my thoughts on the subject). GoLite uses very high quality down in this quilt (800 fill power) so it's amazingly light. In addition to being light weight down is also highly compressible. Amazingly this quilt can be put into this tiny stuff sack.
The quilt and stuff sack together weigh only 20.1 ounces on my scale.

I had heard mixed reviews about this quilt. Some people feel that the company was too optimistic in calling it a 20 degree quilt. It's always been reasonably priced ($225) but I found an amazing sale and got it for $168.75. I figured that for that price it couldn't be too bad. So on Friday night I took it outside for a test. It was 22 degrees, just perfect to see if this bag would really live up to it's temperature rating. The short answer (for those who don't like to read all the way to the bottom of an article) is that it passed with flying colors. I love this quilt!

Temperature ratings in general are nonsense. There is no amount of insulation that is right for every person to sleep comfortably at a given temperature (if you don't believe that then get married). Fortunately I only needed to know if this bag would keep ME comfortable in the cold. I tend to be a warm sleeper anyway (I use very few blankets at home). And like most lightweight hikers I sleep in my hiking clothes. So I wore the clothes I would be wearing on a hike in this weather (two layers of wool on my upper body, thick polypropylene thermals on my lower body, thin liner socks and wool socks over them). So this makes it more likely that this quilt would be adequate for me personally. On the other hand most people understand a 20 degree temperature rating to mean you would be comfortable in a tent. I'm not a tent person anyway so I was conducting this test under the stars: no tent or bivy.

I mentioned in a previous post that I had been cold while sleeping on top of Taum Sauk mountain when using two summer quilts. Looking back on it I think that was not all the fault of the quilts. I think I made a mistake in how I set up my sleeping arrangements. On that trip I used an air mattress for comfort. It is a good quality air mattress but air mattresses are problematic from a warmth perspective. The bottom side of the mattress becomes cold against the cold ground while the top is warmed by your body. Convection currents within the mattress bring the cold air up to your underside and cools you off from underneath. My Big Agnes Insulated AirCore mattress has some Primaloft insulation inside to try and cut down on these convection currents but it can't eliminate them. Last night it was much colder and I noticed it right away when I lay on the air mattress alone.

I had read that the warmest way to sleep in the winter is with 3 pads. You put one closed-cell foam pad on the ground, the air mattress on top of that, and another closed-cell foam pad on top of that. So I tried it. I put my Walmart CCF pad on the bottom and my RidgeRest on top. That's a lot of pads and a lot of weight but since it was just in the backyard I didn't mind experimenting.


One interesting thing about the Ultra 20 is that it has detachable straps which can be used to hold the quilt down to the pad as you can see in this photo from the manufacturer.

However in the picture they have a short RidgeRest and I own a full length one. The footbox of the quilt is too narrow to allow the pad to lay flat inside it. I think the idea is that if you have a full-length pad you should let the pad be outside the footbox. But I bent the edges of the pad up into a half-pipe and shoved it inside. I think that was a great idea actually. It held the bag away from my feet. My feet were wonderfully warm. In fact after a few minutes I had to take off my wool socks and just leave my liner socks on. But I noticed that if I reached up with my foot and touched the corner of the footbox it felt cold. So I think I will try this strategy again in the future on very cold trips.

I was toasty warm, even without the wool socks. In fact I probably would have been fine with only one layer of wool. I was very impressed. Of course part of this may have been that I was also very careful to keep my head warm. I wore a down hood that I zipped off my parka. Preventing heat loss from your head goes a long way toward keeping your whole body warm.

Many people make a serious mistake with their down bags. They store them in their stuff sacks so the down is fully compressed. This can cause the down to lose it's lofting ability over time. To maximize the life of your down bag or quilt you should store it fully lofted hanging in a large bag.


The Down vs Synthetic Debate

Warning: verbose explanation of obscure branch of knowledge follows.

Backpackers are divided about whether down or synthetic insulation is better. It is well known that down is warmer per unit weight and packs down smaller. Since backpackers are so concerned about weight (and, to a lesser extent, bulk) you might think that everyone would prefer down. However there is a "downside" to down if you will pardon the pun. Synthetic insulation is not much affected by getting wet. Both synthetic insulation and down will survive a wetting of course, that's not the concern. What I mean is that the insulating properties of synthetic insulation are much less affected by whether the insulation is wet or dry. Dry is better of course but if you and your gear got soaked on a cold day (say you fell in a stream or got caught in a winter rainstorm) and you wrapped yourself in your wet synthetic sleeping bag it would keep you warm and could save your life -- a down bag won't under the same conditions.

To understand why you must understand why insulation (natural or synthetic) works. The purpose of insulation is to trap tiny pockets of air. Your body produces heat in your muscles and this warms whatever is next to you. Convection will carry that heat away from you as the warm air next to your skin rises away from you and mixes with the colder air around you. Insulation prevents this by trapping that warm air. Now a balloon traps air but surrounding yourself with filled balloons won't keep you very warm. That's because inside the balloon convection currents would occur within each balloon which would exchange heat from the warm side of the balloon (your side) and the cold side of the balloon (the side away from you).

Insulation is something that traps air in such small spaces that no convection currents can be established. So most of any insulation is air pockets. They aren't sealed pockets like a balloon though. But they do make it hard for air to move around. Now if you squeeze the insulation the air will escape and the insulation will no longer function. So the insulation must be allowed to maintain it's loft in order to keep you warm.

Now go back to your wet sleeping bag. When a down bag or quilt gets wet the down clumps together and you have no loft. Synthetic insulation doesn't lose it's loft when it gets wet. So if you have a down bag you must be careful to keep it dry.

That doesn't sound like much of a concern does it? You can keep the down quilt in a waterproof bag in your pack so that rain or an accidental drop into a stream won't wet it. And presumably your shelter will keep water from getting on you while you sleep (it's not much of a shelter if it doesn't). Proponents of synthetic insulation will say that "the best laid plans ...". Of course it's the unforeseen accidents that could end up being dangerous. Hypothermia is nothing to mess with.

Fair enough, if you go with down you have to be extra cautious. However there is another problem. Our bodies are constantly giving off water, quite a bit over the course of a night. Our breath is the biggest source but not the only source. Our pores are constantly giving off moisture to keep our skin from getting dry. This moisture gets trapped in the insulation with the warm air. If it stays warm then there's no real problem. But on a cold night the moisture will condense in the outer layers of the insulation. In fact it may even form ice crystals. So in the morning you ought to air out your sleeping bag and let it get warmed by the sun if possible. If it's too cold then this won't be possible. Climbers on Mount Everest report that their sleeping bags get heavier and heavier each night as the ice accumulates. If the bag is a down bag then each night you lose a little insulation value too. Not a good thing.

What can you do to minimize the moisture in your down bag? First of all don't breath in the bag. There's a temptation to cover your head and keep your face warm. But then all the moisture from your breath enters the bag. Instead I like to drape a jacket over my face when it's cold at night with a little space for my breath to escape. Second, in extreme cold weather you need to keep the moisture from your skin from entering the down by using a vapor barrier. A vapor barrier is a waterproof layer than keeps the moisture from your skin from leaving. A thin plastic bag is the simplest type. Very soon after you put it on the humidity next to your skin will rise to a level at which the body no longer worries about the skin drying out so it stops releasing moisture through your pores. This cuts down on your heat loss as well as protecting your bag.

So where do I come "down" in this debate? Well I feel that you should always use the right tool for the right job. The greatest danger from hypothermia is actually when temperatures are above freezing, like 40 degrees. Think about this: compare how cold you get when you get rained on compared to when snow falls on you. Or compare falling into a snowbank to falling into a puddle on a cool day in terms of which is more unpleasant. When the water around you is solid there is little danger of getting yourself or your bag wet so down is fine (provided you take proper precautions). In the spring and fall when cold rain is likely to fall I want to have synthetic insulation in my bag as an insurance policy.

Heavy backpacking hall of shame

Just before my recent trip with the scouts I saw a post by a friend on a backpacking board who goes by Phat entitled "Anti-Light Gear" in which he was asking for nominations for the worst weight offenders that we commonly see backpackers bring. He suggested the following quite insightful, though humorous, categories.

1) Most often oversold (they needed a volkswagen, but got sold a lambhorgini)
2) Best Newbie trap (market to the clueless! it works!)
3) Most Overbuilt/Overweight (How much heavy crap can they put on it)
4) High Maintenance Prince/Princess Magnet (Attracts those who have to make the woods just like the suburbs)
5) Machismo Magnet (being a lawyer/stockbroker/computer geek isn't that manly, but carrying *this* in the woods is)
6) Gets taken for a ride (it always goes, but never gets used)

I had to laugh at this categorization but I think he has really hit it on the head in some areas. I decided to do a little research while on the scout trip. Many of the scouts were burdened down with heavy packs so I tried to figure out why. Here's what I found.

Backpacks
Interestingly the worst offenders seemed to be the backpacks themselves. Here is one that I saw.

It's a Kelty pack called the Yukon 3000 and one of retailers online says it is "popular with scouts". I think that ought to be a warning, not a recommendation. This monster is 4 lbs 9 ounces empty. That's the youth size. Several of the adult leaders have the large size and it's several ounces heavier. Want to talk about useless features? It has a separate metal bar called a "hold-open bar" which is needed for what exactly? Perhaps it's to hold the pack open so this beast doesn't bite off your arm while you are reaching in.

The irony is that this pack isn't even that inexpensive. It sells for $110 (although I saw a sale price of $89 somewhere).



By comparison the larger of my two packs (the Mariposa) weighs 16.8 ounces and has more internal space (I guess the Kelty people expect you to strap your sleeping bag to the outside so it can get rained on). I think it cost about the same when I bought it. It's been discontinued now but there is a boy-sized one on sale right now for $50. Some parent should grab that quick rather than dooming their scout to being a beast of burden. But even if that one is gone it is always possible to get a very nice, lightweight, but still high capacity pack for under $150.

Interestingly this wasn't the worst pack I saw. Others had multitudinous compartments and straps everywhere. Some were very nearly as big as the kids carrying them. I don't think any of them were under 4 pounds.

Interestingly Gossamer Gear was started by an engineer who got involved with his sons scout troop and then began thinking about the problem of pack weight.

Extra Clothes
Okay, time for truth here. A scout's worst enemy is his mother. It's a case of loving your kids to death. Moms worry so much that their little boy won't have something he needs that they load his pack for him and cram it full of "just in case" stuff. The funny part is that the boys don't know it's there and wouldn't use it even if they did. So all that stuff is just "along for the ride".

I suppose it wouldn't be so bad if they didn't pack things like jeans and other things made of cotton. Cotton is a terrible thing to wear while camping. Try this sometime. Get your jeans wet. Hang them outside and come back occasionally to check if they are dry. How many hours does it take? A pair of nylon pants weighs much less and dries very quickly. But mom's dress their boys in jeans and what do they do in case they get wet? The pack more jeans.

I only carry the clothes I'm wearing. My "just in case" clothes are layers I would put on top. All my stuff is either nylon or wool. Nylon dries fast and wool will still keep you warm even if it is wet, unlike cotton.

News flash moms. Boys don't change their clothes anyway. So don't burden junior with extra weight.

Knives
I don't now what it is about scouts and big knives. I guess it's the Rambo spirit in every boy. He wants a knife that could be used to kill a bad guy or field dress a moose. In reality the most you will use a knife for is to cut cord or open a package of food. Many backpackers carry no knife at all.

But what about other tools? We've all seen those swiss army knives with 15 different things.

A corkscrew, what are they thinking? good thing the swiss are neutral in every war if they think a bottle of Chardonnay is battle fare.

Granted my Leatherman Micra P4 has a few things on it but they are the things I need (mainly the blade and the pliers) and the total weight is only 2 ounces.


Nalgene Water Bottles
These are nearly universal with scouts. They are made of Lexan which is amazingly tough, you just about can't break them. Recently the government of Canada banned them because of a chemical used in their manufacture. Now they are made of something different. But the story is still the same. HEAVY. A bottle that carries 32 ounces of water weighs over 6 ounces.



By comparison my water bottle carries 2 liters and weighs only 1.5 ounces. More than twice the capacity for less than 1/4 the weight. Platypus makes one that is very similar and probably more common. I like the Evernew brand because you can't lose the cap.


Sleeping Bags
Actually this one is hard to get very upset about. A good, lightweight pack is cheap but a good, lightweight bag is expensive. But still I think parents only look at the temperature rating of the bag and ask themselves "will my poor boy be warm enough?" when they should be asking "do I really expect my boy to carry this for miles and miles?"

The lightest bags are quilts as I've stated in previous posts. But also the material makes a difference. Down is the lightest option but there are different qualities of down. The best weighs very little but insulates very well. But it's expensive of course. So people need to do the best they can. That's why I'm not too critical of bag choice.

Often money isn't the issue. Sometimes people get a bag that is too much for them because they let themselves be oversold. I was in an REI this summer and I met a girl who was just graduating from law school and wanted to do a long distance hike before selling herself into the white collar wage slavery we call a career. She planned to go somewhere close like the Ozark trail and was planning to go sometime in August. The saleslady was enthusiastically showing her the top of the line bags. A sleeping bag in August? in Missouri? During my August hikes I slept in my underwear in my hammock. No bag is what is needed.

I haven't talked very much about shelters (like tents) or camp stoves. That's because on this trip the scouts were told to sleep under the stars and to bring food that didn't need to be cooked. I'll talk more about lightweight options for stoves and shelter in another post.

Eastern Taum Sauk Trail (with boy scouts)

This weekend I went with our scouts on an overnight backpacking trip over Taum Sauk mountain. It's the highest mountain in Missouri but of course that doesn't count for much. I think it's something like 1775 feet. It's part of the same St Francois mountain range that I've hiked before. It turns out that the St Francois mountains are among the oldest mountains in the United States. They are much older than the Appalacians and of course the Rockies are just babies in comparison. They are made of an igneous rock called rholite. In the past I've mistakenly called this red granite.

The trail is very rocky and in places you have to scramble up large boulders because that's where the trail goes. In my mind this is another argument for lightweight hiking. It was no trouble for me but I wouldn't want to do it while carrying 50 pounds.




The weather wasn't optimal and we were about 1 week late for the fall colors. On the previous weekend we had hiked the Green Rock Trail in St. Louis at the very height of the fall color season and during perfect weather. That was just a day hike (although it's a pretty streuous 10 miles, I've done it with Kimberly and Brittany before and also done a bit of it with Hyrum). This was an overnighter and we planned to camp on top of the mountain. The colors were still pretty nice as you can see but it had turned cold.






We camped at the top. There is a lookout tower there and nearby there is a glade with a nice view of the surrounding Ozark mountains.

I didn't know what the campsite would look like at the top of the mountain and so I didn't bring my hammock and opted for my bivy instead. On a previous trip with the scouts to fish at Montauk State Park Hyrum and I ended up sleeping in the car because the campground that the scouts picked had no trees. So this time I went to ground to be on the safe side.



Here you can see my setup. Because of the cold I brought both my 40 degree quilts (one is red and one is blue, both are visible in the picture). Inside the bivy you also see my Big Agnes insulated aircore pad. It's 2.5 inches thick and that makes it possible to even sleep on my side like I do in my own bed. It's heavy at 24 ounces but it's worth it.

The small white thing is my FlexAir pillow. It's quite an ingenious device. You insert a drinking straw into a slot on the side of the pillow to either inflate or deflate. When you take the straw out the air can't escape. Your first tendency is to fill it up all the way. The problem there is that it is so small that your head feels unstable, like it's about roll off the pillow. So you actually only fill it part way.

My bivy is an REI minimalist bivy. The term "bivy" is short for bivouac sack. It's basically a one-man tent that is only big enough for you and your sleeping bag. You can zip it shut and you are completely enclosed, although the material over your face is only bug netting so you can still breath. Many, like mine, have no supporting poles. Others have one hoop to keep the material off your face. But a bivy is only water resistant rather than waterproof like a tent. So I brought my poncho/tarp which I could set up over my bivy if it were to rain. That's really the right way to use a bivy. The bivy has enough water resistance so that any wind-blown rain that gets in under the tarp won't get you or your bag wet. The reason that the bivy isn't fully waterproof is because if that were the case there would be severe condensation problems inside the bivy. Even with a breathable but water resistant fabric there is still some condensation. The bug netting over the face helps minimize this by allowing your moist breath to escape but it doesn't eliminate the problem because moisture comes out of your skin all the time anyway. When I wake up in the morning the shell of my quilt is noticeably damp. But nothing serious.

Now I wasn't expecting rain so why bother with a bivy? Well the bivy also protects you from wind and keeps you warmer than you would be otherwise. That was useful on this trip! It was quite windy and we were really cold on that mountain top.

The bivy has another advantage over a tent that came up on this trip. We had hoped to see the Leonid meteor shower which was near it's peak. But when we arrived on the mountain top it was cloudy as it had been all day. Then, about 8pm it cleared up and we had a wonderful view of the stars until about 5am. In a bivy you can lie protected from the cold wind and still have a perfect view of the stars.

Underneath the bivy I tried a space blanket for a groundsheet. Not sure it helped. In the picture I'm using it incorrectly. Any ground sheet should be tucked away under the tent or bivy it's used with. That way if it rains the sheet won't direct water under you. In this case there was no rain forecast so I didn't bother.

Even with the bivy and two quilts I was still cold. I woke up at one point while sleeping on my side because the shoulder that was up was cold. It had compressed the quilt against the top of the bivy and compressed insulation doesn't insulate as well. My feet were a bit cold as well although not as bad as last time. I tried vapor barriers on my feet this time to see if my feet would stay warmer. I used those thing little bags you get at the grocery story to put your vegetables or fruit in. I put the wool socks over that and after a bit I didn't notice the plastic (much). I think they may have helped some but my feet still felt cold. My upper body was almost too warm at times. I'm going to ask my wife to make some insulated booties for me to wear while I sleep. Since they won't be as tight as socks they may help by allowing for more circulation.

The next morning we hiked the loop you can see in the map. Since it was just a loop we left our packs at camp (that helped some of the smaller scouts a lot). We went to Mina Sauk falls which is the biggest waterfall in Missouri. The creek wasn't running that day but it's still a really pretty rock formation. But like a dummy I left my camera at camp and so I don't have a picture.

Past the falls a mile or so you come to the Devil's Tollgate. The scouts had a great time climbing all over these massive rocks.



Here is a slightly better picture I stole from someone's site. It was also taken on a November day but the colors turned out better.



The Ozark trail at this point follows an old wagon trail that passed through the Devil's Tollgate. Unfortunately it was too narrow for wagons so people had to disassemble their wagons, taken the pieces through, and then reassemble on the other side. Unfortunately the hill rises steeply on one side of the tollgate and there is a creek on the other side so there was little choice. Now it's a fun place for the scouts to climb on.