The Purcell Prusik is a personal anchor system or tether that supposedly absorbs shock if you were to fall on it by slipping a little. It's never a good idea to fall on any sort of tether or PAS, but are Purcells better???
What Are Prusiks?
Friction hitches, including prusiks, grab ropes like an ascender without teeth. A prusik is basically a loop of cord that goes inside of itself 3x while wrapping around a rope. Because it's symmetrical, you can slide it up or down and it will always grab the host rope when weighted. Ideally, host ropes should be at least 3mm bigger than the cord you use for the prusik itself.
Purcell Prusiks
A Purcell is a special kind of prusik, a loop of cord that is folded in half with a prusik going around itself. You can adjust it to be as short as folded in half or as long as fully extended. The side you attach to yourself with a girth hitch can either be the end of the loop of cord, or you can isolate an eye using a BFK (Big Fucking Knot).
It can be used as a tether, but also as a foot loop and adjustable legs for a rescue litter. Here are some screenshots demonstrating how to tie it. See the video at 2:21 for detail.
Cost
A short one takes 10 feet of 6mm nylon and a longer one is about 15 feet. At $0.59 per foot, this would cost you less than the carabiner you are clipping to it.
Fun Facts
Most adjustable personal anchors are easy to pull yourself tighter with one hand but sometimes difficult to loosen. Purcells are the opposite. You can loosen them while they're weighted, but it takes two hands to shorten them.
They don’t get you close enough sometimes. You can extend it out all the way and clip that smaller loop at the end to a carabiner directly clipped to your belay loop, putting you only 2 carabiners away from whatever you want. Want to be further? Make your loop bigger until you get to the normal working range of your Purcell.
Results
When does it slip? It’s random. How well did you tie it, dress it, and cinch it down? Is it old and fluffy? Does it have sap on it? Is it super slick because it’s new?
When does it break? 6mm broke around 10kn, 7mm around 15kn, and 8mm around 20kn. 6mm didn’t seem super strong for a personal anchor if it’s the only thing holding you and you fall hard onto it. In our drop tests, especially when we used our heavier weight, you can see that our lowest force was 8kn.
Where Does it Break?
Usually, girth hitches weaken a sling enough to be where it breaks. It actually was the BFK each time. So it’s not a risk girth hitching to something, because the knot seems to fail long before the hitch will..
Other Testing
Kyle Pease did a bunch of testing in 2011 and you can read about it in this pdf. In summary, they got much higher forces. They dropped it a few times but they had a bunch of variables, their load cell only went to 4kn and their drop tester wasn’t built for some of the high forces you can get when falling on such a short piece of rope. It’s great to see other folks' tests so we can compare what we got and learn way more. Go check it out.
When Do YOU Break?
I have hurt myself at 2kN in short bounces, but I’ve gotten 5kn in a rope swing and it didn’t hurt at all because it took so long to achieve that force. So it really depends.
If you quickly got to 6-8kn on a static personal anchor, you’d likely be hurting (if not broken). Anything in the 12-15kn range, it’s just a recovery at that point.
The conclusion is... DON’T fall on personal anchors.
Even Purcells. They might slip and absorb a bit of force, but it's not nearly enough for most situations. Tethers should be used for positioning, not catching a fall.
What's Next
What about Cow's Tails? How do they work?
