so i threw this together reall quick, simple sheet, two sets of holes, stakes through the holes to hold the can. i test it out, lite the esbit tab and...nothing. it barely warmed it, a whole tab! what did i do wrong? are the air vents too small? is the pot to high? the esbit burned for 15 minutes, my alcy stove boils this same can in 6 minutes.

I'm not sure why it wouldn't even warm the can but maybe the esbit is bad or something...

But just looking at the setup, my guess is that you don't have enough air flow. I've built a bunch of enclosed esbit stoves and some of them would give terrible results if there wasn't enough air inflow (holes around the bottom) and air outflow (allow air to escape at the top).

I remember Ayce at thruhiker did a test to see what the optimal distance from the bottom of the pot to the esbit tab and I believe it was in the 1.25" - 2.25" range. Any higher or lower and your "time to boil" would increase.

Edited by Steve_Evans on 02/05/2010 10:05:36 MST.

Yes, I think the problem is lack of air flow. Try using a hole punch and put more holes at the bottom. Also punch some exhaust holes slightly lower than the bottom of your pot.

I think the airflow is the issue too, but would suggest instead of adding out holes just bellow the pot i would increase the size of the cylinder allowing air to escape out the top drawing the hot gasses up the side of the pot not out just bellow it. Increased cylinder size will also give increased combustion air at the base, but i don't think you need to add more holes (if there are 3 of those triangle slits like i am picturing)

Also, if you can get the esbit off the ground it may burn a bit better (can't tell by the pic, but looks like it sits on the ground) The gram cracker style lets air get to all sides of the esbit and keeps it off cold or wet surfaces.

-Tim

Edited by MarshLaw303 on 02/05/2010 11:11:50 MST.

I agree with Tim about the diameter of the windscreen and location of air holes. I have made this same mistake myself. Remember that any surface has a boundary layer of dead air about 1/16" (under dead air conditions at 70degreesF - technical bushway (d'wuryaboutit); if the gap between the pot and the windscreen is smaller than 1/8", there will be no significant air flow. And if the products of combustion have nowhere to go anywhere near the pot, they will not heat the pot.

The air holes should be below the point at which the Heinie can bells out so the heated gasses contact a lot of pot. But just making the windscreen wider will not give the best efficiency. I would start over with a cone, the top of which is not much larger than the pot - either below the bottom ring where the pot bells out or - for a looser fit that won't give problems if (or when) it is bent up, make the top a little larger than the diameter of the largest can diameter. AND I would notch the top of the cone for better air flow than holes can give. (Think of notches with their bottoms ending with an arc, made by intersecting cuts with holes. But just plain notches will work.) The slope of the cone should make enough space for the heated air to flow up and out but doesn't have to be as extreme as -say- the Caldera Cone.

Edited by vickrhines on 02/05/2010 11:36:46 MST.

I'll adjust height, I think it's just over 2.25 inches now so I'll bring it down a bit. and i like the idea of airflow around the tablet, I'll make a wire mesh stand for it. The airflow problem is clear to me now, thanks everyone. I'll experiment with holes, slots, and increased gap. I'll follow up with the results.

You have no hot air flow around the base of the can. No wonder it didn't work. The air inside would be completely stalled, and insulating.

You could use at least 1/4" clearance around the can - 1/2" would not be too much,
Or
You could have a great ring of holes in the wall at the base of the can.

Try both?

Cheers