Purcell Prusik (an extendable cowstail safety line)

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– Bjorn Sturmberg

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.

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

Next you thread the tails through the prusik knot.

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

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.

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.

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

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

Step 6 – complete.

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

Step 7 – tie into your harness.

A double figure of 8 knot where the one tail is then re-threaded is called a “Frost 8”. It can also be tied in the one fell swoop as follows. (Although I find the method above more instructive because it parallels the single re-threaded figure of 8 used when tying into the end of a rope.)

Step 4-6 alt. – set up for Frost 8.

Step 4-6 alt. 2 – tie a regular figure of 8 with the 3 strands on either side. EDIT – I have indeed messed this up and tied an overhand know in this pic, hopefully the idea evident. See here for a video of the frost 8 knot.

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13 thoughts on “Purcell Prusik (an extendable cowstail safety line)

  1. Hi guys, quick question…. you state: ‘the material itself is dynamic,’ this [purple] cord looks to me like normal static cord? is that the case?
    I have ‘seen’ Bluewaters DYNAMIC PRUSIK CORD but never used it in this configuration. What do you use, Dynamic or static? any pros and cons for each? cheers.

    • I’ve got to check with Bjorn exactly what he uses, but I assume it is just usual static prusik cord. Most static rope still has about 4% stretch, so is more dynamic than many of the slings that are normally used as safety lines.

  2. Sorry Ric, entirely missed your comment… havent gotten a question on the blog before.

    Essentially T2’s answer is correct – nylon is slightly dynamic and this dynamic’ness has significant outcomes as stated in the second linked article;
    “Dyneema stretches about as much as a steel chain. Nylon stretches more.”

    I had not heard of dynamic prussik cord, and dont really like the idea of elasticity in my prussik loops, however such cord would be ideally suited to this type of safety line. I’ll have to see if I can find some in Oz…

    ps I spent over an hour hanging on the purcell prussik on a recent trad climbing trip and am happy to report it held me on its shortest setting the whole time. Also came in very handy when moving around the belays!

    Hope that helps?

  3. I dont think so.

    Firstly the greatest shock load I can envissage the purcell prussik being placed under is if you fell off a small ledge whilst tied into a fixed anchor (or the ledge broke etc). I think for such forces the prussik knot should hold tight, keeping your fall limited to the length you’ve set it to. Failing this (if for instance the prussik knot is tied untidily) and the knot slips then it will still provide a little extra friction as it slides along to full extension. At this point you would feel the full shock load off the maximum length of the cowstail, which would not bring anything to break or burn through and is where the slightly dynamic nature of the nylon prussik cord will be critically important in lessening the shock impact of the fall and saving your back or the anchor from breaking.

    • Late joining the conversation 😛 Just thought I’d share some thoughts and experiences.
      Commonly one of the loops is dropped on the attachment side of the Purcell so you end up with two raps on one side and three wraps on the other.
      With prusiks you always go for 3mm smaller in diametre than the host rope (rope it grabs) so the prusik will grab and hold.

      Two factors increase the dynamic capacity of a purcel prusik
      1 – The rope used. As already stated, 4% stretch is much more than 0%
      2 – Dropping one of the top loops, the prusik cord being the same diametre as the host rope and it being wrapped around two ropes instead of one (less continuous surface area = less friction) (last point is supposedly a factor, not sure I 100% agree on being less surface area to grab) all ad to its dynamic capabilities

      If shock loaded it is designed ‘fail/slide’ What will happen is the prusik will slip (supposed to) and at some stage (depending upon forces) enough heat will build up and the prusik will fuse to itself. It is designed to give at the initial shock loading period and then slowly(over micro seconds rather than instantaneously) take up the rest of the force before coming to a comlete stop, thus dynamic capabilities.
      Was doing some research on exactly how effective this is, but as luck would have it my load cell wasn’t attached properly and went for a fly in a different test. Meh… I’ll finish one day

  4. I found some interesting test reports about the Purcell:

    The first report has 6mm and 7mm data, the second one focusses in depth on 6mm.

    If you have a look at the actual data in the reports the amount of slipping of the prusik knot is also reported. For one of the factor 2 falls it is actually reported as 54cm of slip. The factor 1 fall tends to get about 20cm of slip. So the claim of being dynamic in this sense definitely holds.

  5. The last picture with the caption “Step 4-6 alt. 2 – tie a regular figure of 8 with the 3 strands on either side.” has a three strand overhand knot – not a three strand figure of eight knot!

    I doub’t tying the purcell like this would be dangerous, but If you need to salvage your purcell for use as rap tat, I’m sure you’d rather have tied it with and three strand figure of eight.

    • I noticed the same mistake. It’s sad when the “experts” post these kind of errors online for all to see and learn how to make the same mistakes.

  6. Thanks for the information. For over many years I never used a personal safety line (put it down to ignorance) but my mate Kent insisted on it when I first met him about four years ago so I’ve just been using a piece of tape with overhand loops on the ends with crabs attached. I had no idea it was so dangerous. I’ve now made up some 6mm prussic according to your directions and will use that in the future. Thanks once again.

    • Allan, great to hear you found this useful. It’s a great little tip that I’ve also been using for a while. It’s handy having some climbers around, as they are often better at uncovering handy new bits of ropework.

  7. I learnt a lot and this was really well written – many thanks. I’m still pretty new to climbing and so I had to do a bit of research to understand where this is coming from.

    This [ http://www.climbing.com/skill/anchors-away/ ] helped me to understand it – that using a daisy chain or a sling as a personal anchor tether can really mess you up as it is 0% dynamic, and it makes way more sense to use a purcell-prusik. Which I now know how to do! So thank you – N

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