Purcell Prusik (an extendable cowstail safety line)

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bjorniam
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Purcell Prusik (an extendable cowstail safety line)

Post by bjorniam » 28 May 2018, 11:38

Shared from https://fatcanyoners.org/2012/04/19/purcell-prusik/

This is a neat way of making an extendable safety line that I picked up off a friend of a friend I was climbing with in Yosemite (and who happened to teach rope and kayak rescue for a living). He gave me a spare one to try out, which I later carefully dismantled to learn how it all went together.

There are several advantages to this setup. Firstly, the prusik knot will slip under a large shock load, thus dampening the impact. Secondly, the material itself is dynamic, which also reduces the shock load. Last but not least, prusik cord is cheap and readily available.

The importance of a dynamic safety line is made clear here and here. “Even a 60 cm fall-factor 1 fall on to an open Dyneema® sling can generate enough impact force (16.7 kN [= deadly])”

Edit: here’s another excellent study of the purcell including cool testing videos.

Now back to the knot. You will need about 4 metres of 6 or 7 mm prusik cord. Double this over into a large bight with one tail end about 60 cm longer than the other (this will be the piece you tie into at the end). Then tie a classic prusik knot at the end of the bight.

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Step 1 – tie a classic prusik knot around your finger.

Next you thread the tails through the prusik knot.

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Step 2 – thread the tail through the classic prusik knot.

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Step 3 – how things should look.

Now we tie another loop in the tail end of the cord which we will girth hitch around our harness. To do this firstly tie a figure of 8 knot near the end of the shorter tail.

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Step 4 – tie a figure of 8 near the end of the shorter tail.

Then re-thread the figure of 8 with the longer tail forming the tie in loop (this is the same principle as when tying in to the end of a rope). Note that to form this loop you could also use a double or triple fishermans knot.

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Step 5 – re-thread the longer tail through the figure of 8.

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Step 5b – re-thread the longer tail through the figure of 8.

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Step 6 – complete.

That’s all folks, now girth hitch that sucker to your harness, and you’re off!

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Step 7 – tie into your harness.

Flynny
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Re: Purcell Prusik (an extendable cowstail safety line)

Post by Flynny » 28 May 2018, 16:15

Thanks. I've seen purcell prusiks mentioned in a tech manual but but their guide on tying it was really vague. Your pictures making very clear as clear.

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T2
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Re: Purcell Prusik (an extendable cowstail safety line)

Post by T2 » 30 May 2018, 21:41

I absolutely love my Purcell Prusik, particularly for areas like Kanangra where there's lots of big, exposed drops. Nothing quite compares if you're after a really cheap, flexible, and effective safety line.

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Re: Purcell Prusik (an extendable cowstail safety line)

Post by Skinner » 13 Jan 2021, 17:58

I know this is an older post, but I was wondering what brand of cord you are using? It looks nice and flexible. I have found that not all cord is the same with some being too stiff and some grabbing too much.
Can you use the purcell prusik as a normal backup prusik ascender without having to re-tie it? With the way that you have it tied it looks like you could as the is the loop at the end that you could tie around the tope? There may be an issue with it that I am not seeing such as slippage.

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Re: Purcell Prusik (an extendable cowstail safety line)

Post by T2 » 14 Jan 2021, 11:53

There's definitely big differences between the handling of different accessory cords, even those produced by the same companies.

BlueWater do a dynamic prusik cord that's pretty perfect for use as a Purcell Prusik. It is produced in the same way as a climbing rope, so has similar dynamic properties. This provides additional energy absorption if you fall on your safety tether. It's also a much more malleable cord, which makes it handle really well. The dynamic nature is also meant to allow prusik knots to hold higher loads, but still release easily.

https://www.canyongear.com.au/product/b ... usik-cord/

As for using it as part of your emergency ascending setup, it should be possible, but would very much depend on the length of your Purcell Prusik. I haven't tried it (so I'd recommend you having a play at home) but if it was long enough, you could use the end that generally girth hitches to your harness to tie onto the rope and use the Purcell Prusik as an adjustable foot loop. If you do try it out, I'd love to hear how you find it.

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Re: Purcell Prusik (an extendable cowstail safety line)

Post by Skinner » 14 Jan 2021, 21:52

I like the idea of the dynamic cord for the safety use of the Purcell Prusik. Not sure how it would go with ascending being dynamic.
I have however been experimenting with differetnt ways of having 2 Connection points when using Prusiks. Using a short Prusik with a crab and a leg loop will allow me to attach the Purcell Prusik as a backup and being adjustable is very useful to get the right length without getting in the way.
This I think is better than other methods I have seen and the Purcell could be the solution I have been looking for.

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