Boiling 8 oz. 2X vs. 16 oz. 1X

Every alcohol stove is a little different, so it is impossible to say. Some are so tiny that bringing 8 oz. of water to boil is a big task. Some are so hot that 16 oz. or more is not a problem. Some are great when used in still air, but as soon as you have a good stiff breeze, they will get puny despite the windscreen. You have to test with your own stove to figure out what it will do.

Besides the water in the pot, there are trade-offs with the dimensions of the pot within the windscreen. You have to test.

Besides all that, you have to find the best type of alcohol.

–B.G.–

There is a lag/preheat period for the stove to come up to speed plus the energy needed initially to heat the mass of the pot, and the lost heat in removing replacing the wind screen ect. Weather this is measurable in a practical sense??? I would guess you loose some fuel to the process.

I guess the question "boils down" to this: are alcohol stoves typically optimized for 16 oz, and is the efficiency (with less volume) and inefficiency (with more volume) linear from this point? I've tried boiling more than 16 oz with dismal results, and had faster per-ounce boil times with 12-14 oz, so I was wondering–until I can fire up my stove next weekend–if anyone boils smaller volumes multiple times as a matter of course.

Again, it varies from stove to stove. You need to test your own stove to see what you have. Some middle-of-the-road alcohol stoves seem to give good boil time performance for 16 ounces of water, but there are many much weaker than that and many much faster than that.

There are several basic designs of alcohol stove. Some are optimized for fast boil times, but they use a lot of fuel. Some are specifically designed to be very economical on fuel, but they are fairly slow.

You will see lots of variation just based on the type of alcohol you use. In general, denatured alcohol with high percentage of ethanol, or else almost-pure ethanol is what you seek for maximum heat to weight.

–B.G.–

It seems like I get better efficiency boiling one batch of water (to a point). I don't have numbers in front of me, or in my head right now for that matter, but it seems like I save a bit boiling one batch… Ie, if I boil ~28 ounces of water I use ~20-25ml of alcohol, but if I boil ~16oz I use ~15ml.

Also, some stoves have flames that shoot out the side. These can be less efficient with narrower pots and mugs as much of the flame can spread beyond the bottom.

A tight windscreen can help keep the flame in close.

My most efficient stove is also the slowest.

"My most efficient stove is also the slowest."

For many practical stoves, that is correct. A few people have teeny tiny stoves that are so slow and produce so little heat that they can't get a pot of water up to boiling point. So, with all that efficiency they shoot themselves in the foot.

The bottom line is that each person must test and optimize their own stove, fuel, cook pot, and windscreen for their trips.

–B.G.–

My FeatherFire alcohol stove boils 32oz for about 50% more fuel than it takes to boil 16oz, so it's much more efficient at 32oz. I imagine any alcohol stove would be more efficient with larger quantities up to a certain point where it can barely boil the water, and then the efficiency would drop off. I imagine most alcohol stoves can boil 16oz fairly easily, so doing 8oz x 2 would likely be less efficient with almost any alcohol stove.

The nice thing with the FeatherFire is that you can adjust it with the simmer adjustment, so if you want increased efficiency you can turn it down and wait longer or vice versa.

Being practical-minded rather than scientific, I would boil two 8oz batches of water if I were going to eat two different courses for dinner. I would boil one 16 oz batch of water if my dinner was a single course.

Being only intuitively scientific, I would think that for any stove it would be more efficient fuel-wise to boil one 16 oz pot of water because that 2nd 8oz of water starts out warm, while with the 2 8oz pots, both start out cold.

Good discussion.

I ran some tests. Before I go into the results, I spoke to a mechanical engineer, and his feeling was that as long as the surface area and energy was the same, the difference between 2 X 8 oz vs. 1 X 16 oz would be very minor. We'll see…

Conditions/Materials:
-Outdoors.
-Breezy with some strong gusts
-Air temp 90F
-Water temp 77F
-Snow Peak 600 w/homemade lid (for 16 oz), 3.5" dia., ~48 cu in.
-Cup from Snow Peak Solo set w/homemade lid (for 8 oz), 3.75" dia., ~25 cu in.
-Homemade windscreen, 3" high, about 1/2" away from pots.
-Decagon stove
-Sunnyside Denatured Alcohol
-Polder digital meat thermometer
-210F = "boil"

OK, here are the typical results:

16 oz water, 29 ml fuel (about a fl. ounce)
1:20 prime
210F @ 9:56, no leftover fuel.

8 oz water, 29 ml fuel (about a fl. ounce)
1:34 prime
210F @ 4:56, first 8 oz. (Pot was dumped into a Sea to Summit X-Mug, refilled, then placed back on stove–about 15 seconds.)
210F @ 9:41, second 8 oz., no leftover fuel.
When the second 8 oz boiled, the first 8 oz. had dropped down to 167F. After both were combined to make 16 oz (167F & 210F), the net temp was 185F.

Analysis:
It looks like gross boil time is is about the same. With a stove that gives full output for 10 minutes (the Decagon roars for about 4 minutes after priming, then settles down), maybe some time savings would be realized. (This also suggests that the Decagon is optimized to boil 16 oz at once and conserve fuel–sort of.) The first cup's boil time includes priming (when the pot is not on the stove) so actual boil time is only 4:56 – 1:34 = 3:22. But by the time the second pot is added, the stove is about to settle down, and the pot is no longer receiving the same amount of energy as the first, so actual boil time is 9:41 – (4:56 + 0:15) = 5:11. Perhaps if you only need 12 oz for your meal, you could boil 6 oz. then 6 oz and realize more of the higher output in the first 4 minutes.

Is 185F good enough to rehyrdate a meal? Is 8 oz of 210F water followed by another 8 oz of water 5 min later going to ruin the meal? Given the variances of my own field cooking I'd say this is acceptable especially if using a cozy. Besides, I don't like really hot coffee or scalding food.

Recommendation:
Alcohol stoves vary. Fuels vary. Environmental conditions affecting alcohol stoves vary. Windscreens vary. I purposely chose to do these tests outdoors in a worst-case scenario. But most importantly, hikers' needs and wants vary, and admittedly, breaking a single cooking step into two to save a few ounces won't strike a chord with most.

The Decagon isn't the most highly regarded stove; it needs to be filled fully for each boil for best performance. From what I've seen, other stoves don't have this requirement (they'll just burn shorter with less fuel), which may skew the numbers more favorably.

In less windy conditions I can usually boil 16 oz. in my Snow Peak 600 mug using only 25 ml of fuel, but the wind was a real drain for these tests and I kept running out of fuel, thus the odd 29 ml amount. (I dispense fuel with a child's medicine dispenser that looks like a syringe, and is marked in 10 ml increments.) Also, if your windscreen is too short or you dawdle changing out pots of water, you may experience flame-out on the stove in windy conditions. You may (or have to!) experiment with your setup to see if this works for you.

You'll need a place to store the boiled water if not using it right away; I used a Sea to Summit X-Mug, which is a 2.5 oz. collapsible mug made from silicone. Mostly too heavy otherwise, it happens to fit perfectly as a cap to the Snow Peak Solo cup when collapsed, and holds 16 oz. of water. (It also provides a larger cup of coffee than the Solo Cup does.) Inside the Solo cup I can fit my pot lid, a folding spork, a tiny scrub pad, and a tiny bottle of Dr. Bronner's soap.

1.9 – Solo Cup + Homemade Lid (the lid by itself is immeasurable)
1.9 – Total
25 cu in volume

3.2 – Snow Peak 600 Mug + Homemade Lid
48 cu in volume

40% reduction in weight and near 50% reduction in volume. (OK, if you include the X-Mug, add another 2.5 oz., but the volume remains the same.)

So if you're looking to trim pack volume a bit, and you can use boiling water in 8 oz. increments, and you don't mind fiddling with a pot change, and you don't necessarily need boiling water for meals and coffee, then this may work for you. YMMV