Has anyone ~actually~ measured boil times of wide vs. narrow pot?

Yes, I have. It is the most boring, snooze fest I have sat/stood through. LOL! I did it for the article I wrote for BP mag last year when testing out pots. And I have done it for my own personal use.

IMO it really depends on a lot of issues – type of stove, type of pot metal, outdoor temps, blah blah blah. Although to put it this way: I prefer a wide flame and a wide, shallow pot. And that says a lot ;-)

Thanks Sarah. Is there an BPL Article URL out there somewhere I can read up on?

I wonder if I still have my notes? We kept really indepth notes – be fun to publish them on my blog :-) (Which I can do now since enough time has gone by). It was interesting doing the testing and pretty wasteful of fuel (sigh).

Ah, so this was b4 my time on BPL when there were actual newsletters, and not online articles. I missed out. :p

If you get around to it, that'd be great, otherwise no biggies. I get the jist of it. :)

It sounds as though Sarah is referring to Backpacker Magazine, not BPL. Am I correct, Sarah?

Ah, good catch. That would explain it. :p

Yeah…it was a 2 page article I did for Backpacker. I need to figure out where I put all my notes. A lot of it was on paper (I know – so old school!)

It was writing that article that I realized that I test gear so different than others do and that their methods made my eyes twitch at times ;-) Us nutty light packers I think have gear testing down to a science and "get it"! I'd rather read what us crazies come up with..lol!!

…

Bryce: As part of a bigger effort to study painted versus unpainted pots, I also picked up two SS pots, one wider at 6-3/4" (17 cm) and one narrower at 5" (12 cm). Pots were the same height and unpainted. Notice that is a pretty tight range, but right in there with 0.9 to 1.5 liter BPing pots.

I ran them each on identical stoves from cold. 1000 g of 4C water to a rolling boil. And then cooled everything down and repeated with them on the other stove. Stoves were Primus canister stoves, new MSR butane/propane, full throttle.

Results:

Narrow: 5:30 and 5:25. (330 and 325 seconds)
Wide: 4:08 and 4:20 (248 and 260 seconds)

So, as noted by others, wider is better.

Savings: 254/327 = 0.78

Cost: 327/254 = 1.29

Go from narrow to wide: You'll use 22% less fuel.

Go from wide to narrow: you'll use 29% more fuel.

I suspect there is an optimal pot diameter to flame diameter ratio of somewhere between 1.5 to 2. Most people going UL probably have pots on the narrow side. And that's costing them 25% fuel weight – which is a LOT, compared to another few grams for a different pot.

Optimal throttle setting likely varies with pot size. Smaller pots would benefit from lower flame settings. But only with a good lid to minimize wind and evaporative losses.

David, nice test. No, I'll not nitpick about results. By guess and by gosh I figure about 10% per inch over 3-1/4". Of course this, breaks down at around 10". Nice to know I wasn't that far off, but it is conservative by your numbers. I am guessing that for efficiency a 4x or 5x flame diameter/pot size would be more correct.

Actually, all pots benefit from being heated on low with a canister stove. They burn real hot. Even my old stove is to hot and only kicks out 2000-2500BTU on low. A 25% increase in fuel efficiency for some additional wait time seems to be a fair trade off when solo. With a partner, I usually assume a higher flame simply to produce a higher volume of boiled water in about the same time…two people carrying the fuel weight/wind screen/stove system.

It would be interesting to evaluate a weight trade off (lighter/smaller pot) vs fuel savings (say 20%) over about 14 days. This may represent a pardigm shift to larger pots, even if they are more difficult to carry, because of the fuel conservation they represent. !40% fuel savings to go from a 4" dia pot to an 8" dia pot would definatly be worth it over a week trip!

Many people have trouble thinking of a cone in relation to a self contained stove. An alternative methode that works well for me is to drape a larger piece of Aluminum foil over the pot and down to the flame with a few exhaust vents above to keep heat moving. A stone on top keeps the thing faily well in place.

Anyway…Thanks!…a nice test!

Those wacky engineers.

No tests done here. But I suspect a windscreen, particularly a cone that funnels the heat around the sides of the tall pot, may reduce this difference. I suspect that a good windscreen allows a fair amount of convection along the tall sides of a narrow pot.

James:

So your rule of thumb is you save 10% of your fuel by increasing pot diameter an inch? Interesting, I love quick-n-dirties like that.*

So if you'd have need 100 g of fuel for your trip with your 4" pot, would project:
90g with a 5"
81g with a 6"
73g with a 7"
65g with a 8" pot?

Or were you thinking 90%, 80%, 70%, 60%?

That has me wanting to get a few more points on the curve. I can't find exactly the same style pot in many diameters, but maybe I could get something above and below my last two and graph it up.

The plot pot weight plus fuel for 1 or 2 weeks versus pot diameter. Obviously for an overnighter, it's all about pot weight. And for 3 weeks unsupported, it's about fuel usage.

Yes, windscreens and aluminum foil hats help significantly. And the aluminum foil hats can be multipurposed to stop the government from reading your thoughts.

*Water (or gasoline) flow in a properly sized pipe (and very often in natural water courses) moves at 5 feet per second, i.e. walking pace).

Ben: I suspect you're right that a windscreen may narrow the gap a bit because:

1) there's more heat to had along the sides of the smaller pot because less was extracted by the smaller pot's bottom and

2) a narrow pot has water higher up the sides which increases the height and area for potentially extracting more heat from between the pot and windscreen.

Has anyone tried their Backcountry Boiler or Kelly Kettle or Storm Kettle over a small stove? Then your tall heat exchager and windscreen are all in one. Then put a neoprene cozy on the outside and I think that might beat all exterior heat exchange options.

David,
I don't think it really matters. I just figure about…by guess and by gosh. Rule of thumb is typically off by a bit but somewhere close enough. (I actually use 10% in either direction rather than get into all the bleeding descriptions and mathematical type precision…it is just a guestimate.) Just wondering, anyway. I suspect there might be minor influances besides pot diameter and flame size. Heat production along a flame, size of center point "dead" air column, pressure of gas, actual jet spread, angle of jets, angle of wind, pot material and thichness, etc…many of which "SHOULD" cancel out by reversing stoves, but letting off pressure will cool the gas container, which effects gas pressure, which effects…well, by guess and by gosh, close enough.

Ha ha…no. Aluminum hats don't help in my case…too many security checks. They already KNOW my bad thoughts, they just want the good ones.

I remember from my old engineering school days being surprised at how high the convection rates were for a horizontal surface like a tall pot has.

"I remember from my old engineering school days being surprised at how high the convection rates were for a horizontal surface like a tall pot has."

Um, wouldn't those be vertical surfaces? :-)

Yes, vertical. I have a mental glitch that always switches those 2 words up.

"Has anyone tried their Backcountry Boiler or Kelly Kettle or Storm Kettle over a small stove? Then your tall heat exchager and windscreen are all in one. Then put a neoprene cozy on the outside and I think that might beat all exterior heat exchange options."

That was discussed several yers ago on the backpacking light list. We also discussed a wider, donut shaped pot with a wind screen so both sides were heated similtaneously. I think we had discussed a plain jello mould. I did some testing and initial tests looked real good for efficiency. But, it seemed the limiting factor was the surface area exposed to the heat/hot exhaust gasses was more important than the actual design. Turbulence was also a large contributor on taller pots. As heat is transfered, it creates a slower moving, but low friction area for hot gasses to slide over. This is similar to laminar flow charistics of high speed aircraft but is induced by the high energy of the heated air. Exchanging this layer with induced turbulence will also speed up boiling times for no more fuel, or, overall efficiency.

>"Exchanging this layer with induced turbulence will also speed up boiling times for no more fuel, or, overall efficiency."

So we need vortex generators on our pots, like aircraft wings have? I can imagine that helping, especially within a windscreen.

"So we need vortex generators on our pots, like aircraft wings have? I can imagine that helping, especially within a windscreen."
As I remember, one of the members there sugested a helical heat exchanger between the wind screen and pot body.

Kind of a poor picture, bit here is my conception. Outer wind screen, inner pot, connected by one example heat exchanger fin with one on the bottom. Of course you would want 20 or 30.

Some water heaters have a helical ?deflectors? on the inside of the water tank. Like a steep spiral staircase (without the stairs). Maybe 18" vertically per full cycle in a 4-5" HX diameter. They're definitely trying to create turbulance and stir the exhaust gas around in there.

James, I agree that surface area exposed directly to the flame is the biggest factor. Unfortunately increasing it via spirals or ridges would likely cause the price to increase by orders of magnitude. Vortex generators are interesting. That might be easier to apply. I wonder how crude they can be and still have a significant effect.

With alcohol stoves, there are compounding factor. As the pot gets smaller so does the windscreen and therefore the enclosed space around the stove decreases. I have found that the same stove burns hotter and faster with smaller diameter pots. I believe that this is due to trying to push out the same wattage through a smaller air volume. I have found that my 1.3 liter system is way more fuel efficient than my SP 700 system (15 ml to boil 2 cups compared to 17 ml). Additionally, my 1.3 liter has a faster time to boil (6:30 verse 8:00). For my uses, wider pot is far more efficient than a small diameter mug. My 2 cents – Jon