Thursday, December 11, 2008

SHT stove carry weight

My stove of choice will be a Pocket Rocket on a Brunton stove stand (MYOG link), with a modification. (Stove stand linky, at REI.) I have added an upright fin to the stove that connects to a brass block. The block wraps around a brass tube, and essentially operates a a pre-heat. I will be taking MSR canisters, which have a higher propane content than most canisters, and they also use iso-butane, which has a lower boiling point than plain butane. In all, the canisters should work down to about -15F, and lower if I take warm the canister.

Anywho, the idea using the PR in winter is that with the stove stand, I use the canister like a liquid feed stove. Propane has a boiling point of -43.8 (F) and isobutane has boiling point of 10(F). This means that as long as the temp is above 10F, the stove is burning vaporized fuel. However, if temperature is below the boiling point of the isobutane, then I must use the stove in liquid feed mode. In other words, I must turn the canister upside down. This way, the pressure exerted by the boiling propane will push out the liquid isobutane out of the upside down canister. The fuel will then travel through the brass tube, where it will be heated by the fin and block, and then vaporize. The cycle will then continue, much like a generator on a white gas stove works to vaporize the liquid fuel.

I have asked the post office if I could leave the canisters at the PO for pick up on resupply. I was categorically rejected, although I have been finding on the PO's website that such drop-off may be acceptable. The question is how much fuel to bring. After consulting BPL's testing, I am curious if their 12.6 g/qt boiled in cold temperatures will hold up to my needs. The tests were conducted without a stove-stand with 40F water and the stove and cans cooled to 10F overnight in a freezer. BPL's more comprehensive testing is here. One curious note from BPL's testing is that although the stove was running flat-out, it used up less fuel because the stove was running at the equivalent of half-flame.

The only option I see is that I need to carry all of my fuel with me from the trailhead. This is going to be quite inconvenient, but I cannot count on getting fuel up there, or someone bringing it to me. The next question arises as to how much fuel I need.

Based on my past winter experiences, and talking with friends who do a lot of winter camping, that they go through about a gallon of drinking water in a day outside of cooking. To this, I need to have 16 oz of water for supper. Since it takes approximately the same amount of fuel to melt snow as it does to take 40F water to a temp just below boiling (~178 F), I would venture to guess that the figures produced by BPL's testing (40F water to boiling) are accurate if not conservative estimates of how much fuel I will need. This means that I need 4 qts of drinking water, plus fuel for another quart (1/2 quart to melt snow for 16 oz of water for supper, 1/2 quart to boil it).

It is important to note that BPL's figures were obtained NOT using a stove-stand. This may vary the results, but I am going to hold to them for now. To accommodate for windy conditions and cold snow, I will increase the fuel consumption by approximately 20 percent, bringing the total estimated consumption to 15 g/qt (up from estimated 12.6). Thus, my table looks like this:

Days and ratio Extra Cans
14 day carry, 15 g/qt 85 5
14 day, 12.6 g/qt 26 4
17 day 15 g 87 6
17 day, 12.6 g/qt 64 5

As you can see I am estimating for both fuel usages for two trip lengths - 14 days, which is the amount of time I estimate the trip taking (or less), and 17 days, the amount of food I will have. I will be carrying one extra day of food at each of the three segements (trailhead, resupply 1 and resupply 2). As you can see, five 227 g (8 oz) cans accommodates my fuel needs under three of the four conditions. My fuel usage per day is 75g for 15 g/qt, and 63g for 12.6 g/qt. This if I take five cans, I will have at least one day of extra fuel under the three estimates circumstances. Furthermore, this consumption estimation does not take into account obtaining liquid water from the abundant major rivers that I will encounter throughout the trail. This is why I am comfortable taking only five cans and discounting the 17 day, 15 g/qt estimation. Should the trip end up taking that long, I know I can still get liquid water. In an absolute worst-case scenario, I will build a fire and melt snow that way. (I will be taking an emergency fire-starting kit, consisting of dried lint, waxed matches, 1 oz of denatured alcohol. Also, the birch and pine trees are abundant. Pine pitch and birch bark make excellent fire-starting materials). This makes the stove's initial carry weight the following:

Item Oz
PR 3.03
PR Case 0.81
Stove stand 5.64
5x cans (empty) 23.46
5x cans (fuel) 40.04
TOTAL 72.98

This weight will drop by 2.64 ounces per day (fuel weight) and then drop another 4.69 ounces every fourth day. This number is based on 75 g/day; obviously I get more gas mileage if fuel consumption decreases. Bolded numbers represent canisters are dropped. The numbers end up looking like this:

Days Oz
0 72.98
1 70.33
2 67.69
3 65.04
4 57.71
5 55.06
6 52.42
7 45.08
8 42.44
9 39.79
10 32.45
11 29.81
12 27.16
13 19.83
14 17.18
15 14.54

Of course, I do not expect the trip to take 14 days. That estimation is based on the number of miles I need to cover (approximately 215, counting resupplies) divided by my estimated speed (1.5 mph). This speed is derived from taking half my normal hiking pace of 3 mph, and then multiplying that number by the number of hours of estimated daylight (10 hrs/day). This leaves me with 15 miles per day of hiking, and leaves me with one day (last) of 20 miles.

I also anticipate that I should be able to hike for more time that that, up to about 20 miles per day. Skurka did 20 miles per day on the Border Route Trail in 18 inches of snow with about the same amount of daylight. He also dealt with a large amount of blow-downs and a less-than-immaculate trail. I would love to be able to finish the trip in less days than it took my brother and I to finish in May (11.5 days i.e., done before noon on the 12th day).

Therefore, my estimate is probably accurate or conservative. Either way, I get out of the woods in an appropriate amount of time and I finish my trip before I must return to school.

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