Purcell Prusik (an extendable cowstail safety line)

Post Reply
User avatar
bjorniam
Posts: 7
Joined: 27 May 2018, 20:20
Full name: Bjorn S
City: Sydney
State: New South Wales

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.

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

Next you thread the tails through the prusik knot.

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

Image
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.

Image
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.

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

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

Image
Step 6 – complete.

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

Image
Step 7 – tie into your harness.



Flynny
Posts: 49
Joined: 22 May 2018, 16:10
Full name: Craig Flynn
City: The 'Go
State: New South Wales

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.

User avatar
T2
Site Admin
Posts: 219
Joined: 15 May 2018, 10:33
Full name: Tim Vollmer
City: Blaxland
State: New South Wales
Contact:

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.

Post Reply