How to arrange 2 Prusiks when ascending a rope?
If I want to ascend a rope with minimal equipment, I use 2 Prusiks: one tied to my harness, and the second one for my foot.
However, there are two ways to arrange the Prusiks on the rope - the foot below the chest, and the opposite way. The first arrangement is most intuitive and described and in all places on the Internet I looked, e.g. here. Also, this site tells that it's the right arrangement:
Notice that the waist Prusik (E) is above the foot Prusik (F)
but doesn't explain why.
When we practiced crevasse self-rescue, I noticed that my friend used the second "counter-intuitive" way; he said that he always used it, while I always used the "normal" arrangement. I tried the "opposite" arrangement, and it worked marginally better: I could ascend a bit higher each time I extended my leg (this might have been by luck). One difference I noticed is that you have to choose the length of the Prusiks carefully (use a long one for the foot) in order to avoid getting stuck. Otherwise, the arrangements seemed pretty similar.
Is there any reason to choose one arrangement or the other? Should my choice be different in different situations? (assuming I have Prusiks of any length)
P.S. I am not talking specifically about crevasse rescue.
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3 answers
I always ascend the other way around (foot-prusik above the chest prusik). This allows for a bigger movement with your foot and hence a bigger climb during your ascend.
Note: I actually use bloqueurs, but basically the methods should remain the same.
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In case of ascending on a rockface it does not really matter which way round you use it. I put the chest-prusik above the foot-prusik, but this is only because of personal taste. By adjusting the lengths of the slings you can do huge steps in this configuration too. Even though this may not be desirable, as shorter steps can be more efficient (also up to personal taste, try it, find what suits you best).
In case of self rescue out of a crevasse, this is different. The main problem here is getting over the edge, where the rope has most likely cut into the snow/ice. To get over it you have to bring your foot-prusik as high as possible.
Put the foot-prusik above the chest-prusik. Then get as high as possible until the prusik is stuck because of the snow. Then try to get the rope loose by pushing away from the side of the crevasse to get the foot-prusik higher. Then step into it, get the chest-prusik higher. Repeat until you can get over the edge.
In reality I do not think this will be needed very often, as may have crampons and/or axes to help you ascend or in most cases your partner can help you get out. But for the worst case scenario its always well to know a way.
EDIT:
Small remark concerning ascending knots: If you have a biner, I always recommend using the "Prohaska" knot, which does not seem to be known in the english-speaking part of the world, at least I do not find any online reference. The advantage is, that it does not get stuck after being loaded, so no fiddling around the prusik to get it sliding along the rope again. And it is very reliable, not influenced by rope diameters.
The Prohaska knot. Image by alpintech.at (http://www.alpintech.at/wichtige_kletterknot.html)
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I just finished a class using both the Prohaska and Bachmann. If you have the hardware, the carabiner does mind the cord well. The Bachmann can slip through a carabiner if used as a catchment hitch in a mechanical advantage system, such as a "Z"
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