Boil Less and Save Fuel *or* Physics, Trangia and Me

My hobby is camping. Recently this has morphed into full-blown backpacking. Lachesis has mocked me, and I got old. That means I can’t schlepp so much stuff. Looking for tips to lighten my load, I perused “Ultralight Hiking” websites. That’s where I picked up this little tid-bit:

You boil less water and you use less fuel.(Duh)

I figured, you boil half as much water and you use half as much fuel. No, no, no. (Or as the French say: Non, non, non.) If you boil half as much water you use one third as much fuel. Several sites spouted this factoid. (One said something along the lines of the inverse square, or some such.) So if I need, say, 1 pint of boiled water, and I boil 1/2 pint of water then another 1/2 pint, I only use 2/3 the fuel I would normally to boil the full pint.

That coupled with my stove getting more efficient as it warms up, if I boil lots of little bitty amounts of water, by the end I should be doing the “loaves and fishes” trick with stove fuel.

***** The question: If I boil 1/2 as much water will I really use 1/3 the fuel? *****

I have no idea where to start looking for verification on this, so I’ll start here.
Thanks.
Rue.

I love this kind of stuff. I know, I’m a geek.

First off, let’s spout some thermo chemistry.
[li]Water is water.[/li][li]The specific heat of water is based on WATER, not the amount thereof.[/li][li]Twice as many Joules of energy are required to heat twice as much water over the same temperature change.[/li]
Using the equation:

Joules of energy requires = Mass water * Temp change * Specific heat of water.

If I recall from the teaching days, the specific heat of water is 8.14 Joules/gram*degree Celsius.

Thus, to raise the temperature of 100 g of water from 0 to boiling (100 C) requires:

Joules = (100g)(100 C - 0 C)(8.14 J/g*C) = 81,400 Joules

As can be seen, twice the water (200 g) would require twice the energy.

So you’re thinking, “Spritle, does this mean the 1/2 means 1/3 thing is crap?” Not exactly. Certainly heating only the water you need (and no more) is important, but we have to bring in efficiencies of heating into the mix.

Lab burners and other open flame methods are incredibly inefficient at heating stuff. For every 100 Joules of energy released, 10 to 25 Joules actually go into heating the water (in this case). Element heaters are more efficient, in terms of less heat loss, but tend to be less efficient if one considers how the electricity is converted to heat in the element. Submerged elements are more efficient than contact elements (like the burner on an electric stove). Back to flames…

Why the range of efficiency in flames? Composition of the pot (e.g.; aluminum vs iron) and surface area (interface between flame and pot) have an effect. It is faster to heat water in a skillet over a large flame than in a tall thin pot (soup can). Faster means more efficiency and less fuel is spent. However, heating water in a skillet with a candle won’t work if you are losing heat from the surface of the water at the same rate as it enters.

So… Bunk? probably not. Heating faster is more efficient and therefore requires less fuel. Heating faster can be accomplished by heating smaller volumes of water.

Are you using 1/3 the fuel to heat 1/2 the amount of water? Not sure. There are many other factors involved; pan shape, composition, etc. There’s also the issue if the same pan is used over and over without cooling. This would increase efficiency in that you wouldn’t have to heat the pan each time.

It just seems unlikely the 1/2 means 1/3 thing. Eventually, if you heated enough very small volumes of water would you be adding fuel to the tank?

remember that the water will evaporate while it is heating thus cooling it and the more water you are heating the longer it will have to evaporate. This is what I guess is the biggest loss. My advice is to use a lid, a wide pot compaired to the flame circle. Use liquid fuel or at least high preformance propain/butain mix (camping-gaz).
When you get back from your hiking, evaluate what you have leftover and what you didn’t use so you can go lighter next time.

look towards lighting your pack. Make a list of items you wish to replace with lighter alternitives. Do a cost analysis see where you can get the biggest bang (lbs off) for the buck.

I usually go with 34-40 lbs for a 4 day hike and have gone with 31 lbs.

If you don’t have one, get an aluminum foil heat reflector to surround the pot and burner. I don’t mean one of those fancy-schmancy corrugated “heat exchangers” - I’ve never used one of those. The reflector I use is just a strip of extra-heavy weight aluminum foil that I place around the stove and pot like a fence around a house. They weigh almost nothing (mine came with my MSR stove) and reflect heat back onto the sides of the pot and keep breezes away from the pot and flame.

Always heat and boil with the lid on.

Consider using benzene instead of water - it boils at 80 deg C. Then again, consuming it would probably kill everyone in your party - ok forget that suggestion.

Use foods that you don’t have to heat - bagels and cheese instead of mac and cheese.

Can you post links to the ultralight backpacking websites of which you speak?

Spritle, thanks for the science lesson. So, on with the Official Mr. Science Lab Coat™ and to the basemant for some experimentation… updates if I ever remember…

k2dave, I have as light a pack as I can comfortably afford. Upgrades are Christmas and Birthday affairs. (A 3 day walk-in-the-park hike pack weighs in about 35 pounds.) Backpacking is alot like golf. How much money you got? That’s about 1/3 as much as you need to get the really bitchin’ stuff.

Kamandi, if you want the 2 sites I bookmarked, e-mail me with your address and I’ll send them out. (Would listing them here be spam?) Or you could just have your e-mail activated in your profile. I don’t think these have the “1/2 water = 1/3 fuel statement”, but I went from link to link and didn’t leave breadcrumbs to see where I’ve been. Sorry.

Heck, here are the links I saved. If the Mods think this is spammy, just nuke this paragraph.

  1. http://www.backpacking.net/
    and
    B. http://www.UltralightBackpacker.com/

-Rue

Setting:
My kitchen. Soupo is upstairs watching Mr. Rodgers. Katcha is still asleep.

Equipment:
aluminum pot
electric stove
measuring cups
pitcher of room temperature water
Timex Indiglo wristwatch with sweep second hand

Hypothesis:
Water takes more than twice as long to boil if you use twice as much. (I could have used Spritle’s post to make the hypothesis: “Water boils at a rate proportional to it’s volume.” But I didn’t.)

Experiment:
While I waited for the burner to heat up, I rinsed the pot in cold tap water a couple of times. I dried it off and added 1/2 cup of room temperature water from the pitcher. Put it on the stove to boil. (90 sec.)

Dumped out the boiling water and added another 1/2 cup of room temperature water from the pitcher. Put it on the stove to boil. (65 sec.)

Dumped out the boiling water and rinsed the pot in cold tap water a couple of times. (This returned the conditions to where they were to start with.) I dried it off and added 1 cup of room temperature water from the pitcher. Put it on the stove to boil. (2 min., 35 sec. or 155 sec.)

Conclusion: It don’t make no difference.

I’m sure I saw the factoid about “1/2 the water 1/3 the fuel”. But I tried to retrace my steps and sure as shootin’ couldn’t find a thing. Maybe I was delusional. Maybe I don’t even have a computer. Maybe I don’t even exist.

-Rue. (I think)

35 lbs for a 3 dayer ain’t bad. May I suggest some low cost weight savers:

$15-$25 - ridge-rest or z-rest - i tried going w/o a pad and paid for it as I could feel the heat leaving me through the floor. Inflatable pads are heavy - these fit the bill and are u.l.

$2-$4 - water purification tablets - not as tasty as filtering and not good for some sources of water but you can easilly loose a pound (and gain space) by switching.

free - plan you hikes along water sources and fill up more often (carry less water). Water is Fu&%en heavy, too bad we need it.

free (it could actaually save you money) - use purified water instead of regular to cook. You won’t have to wait for it to boil 3-10 minutes before you use it - and you don’t have to boil it depending on what you are making. Sometimes all you need it warm or hot water. Here you use less fuel but have to use your filter or tablets.

$1 - If no to little rain is expected get an emergency poncho - but once you use it don’t expect to use it on another hike but you could use it later on this hike

Clothes - try to pick out light weight - forget the jeans or cotton sweatshirts use flease if you can.

and finally I would want to point out
Boots - very important. You want to get good boots but for these weights you could step down a bit and get a lighter boot. There is a saying that 1 lb on the foot is equal to 5 lbs on the back. If you use leather boots consider making gortex boots your next Christmas present.

Now some of the above you may be doing or at least know about but just thought I’d mention some of my fav. weight savers just in case.