Wednesday, June 20, 2012

The Purcell Prusik

As a guide and climbing instructor I frequently get asked how climbers should attach themselves to the anchor at belays and between rappels during descents. There are lots of options for anchoring, from daisy chains and slings to Purcell Prusiks and premade rope lanyards. Not all of them are good, and many are misused by uninformed climbers.

Reading these three articles and watching the associated videos will go a long way in explaining why tether choice is important. Here are the links to look at:
While climbing, the simplest and best answer is to use the climbing rope. At belays I tie a clove hitch using the climbing rope The clove hitch is easy to adjust and testing has shown it doesn't slip, even under high loads. For tethering during rappels and random anchoring while building anchors I use a Purcell Prusik.


Overview of Tether Options


The Purcell Prusik is both strong and capable of absorbing loads well. Most climber tethering configurations, excluding the climbing rope, are either strong enough, but could hurt you in a factor 2 fall scenario at an anchor, or incapable of holding high loads at all. Let's look at a list of tools climbers commonly use for anchoring during rappels, and also at belays if they choose not to tie in with the climbing rope.
  • The Daisy Chain -  Daisy chains gained popularity because aid climbers used them and enjoyed how adjustable they were. Unfortunately, they are easy to misuse and poorly understood by most climbers who use them. If you inadvertently clip the loops incorrectly (this is easy to do) and the bar-tacking between loops fails under load there's a chance you won't be clipped into the daisy at all. For this reason they're a poor choice for tethering.
  • Adjustable Daisy Chains - Most adjustable daisy chains are convenient but are only meant for body weight. They lack the strength necessary for any real anchoring purposes
  • 120 cm (4') dyneema runners - Dyneema is strong, lightweight and has exceptional abrasion resistance. This makes it ideal for many climbing applications. It does not stretch very much meaning that any fall onto a dyneema runner attached directly to your harness can have disastrous consequences. The loads generated are enough to cause bodily harm (greater than 10kn). Place a knot in the runner and the same sorts of falls have caused dyneema runners to break.
  • 120 cm (4') nylon runners - Nylon runners fared better than dyneema in DMM's drop tests, but still place very large loads on a climber's body and still fail when knotted and exposed to fall factor 2 scenarios
  • Metolius PAS - A good, strong anchoring option. However, a fall onto a PAS could hurt you, as large loads in short falls can cause internal injuries.
  • Purcell Prusik - A surprisingly strong and absorbent lanyard - it dissipates loads well. Drop testing has helped convince me that this adjustable tether is a good option.

THE ADVANTAGES OF USING A PURCELL PRUSIK and how to make one


I began using a Purcell Prusik last fall, and have been using it most days now for random anchoring. It stays neatly wrapped in a knot with a locking carabiner on one of my harness gear loops until I need it. Only then will I attach it to my harness for anchoring. Too many things around the belay loop bother me.

When tied correctly the Purcell prusik can be shortened to as little as 60cm (shoulder length runner) or extended to 120 cm (double length runner). One can easily adjust it's length.

The Purcell prusik next to a 120 cm Dyneema runner

In the event of a fall while anchored, the prusik will slip, absorbing load. This means that the force exerted on the anchor is less, and the force transmitted to your body is less. These are both good things. Obviously your anchor should be able to hold large loads, but human bodies don't seem capable of tolerating much more than 10kN in a fall scenario. Ask anyone who's taken an unexpected, jarring fall and they'll tell you a day later that it feels like their insides were shaken violently.

The process of making a Purcell prusik is pretty simple. You'll need about 11' (3.5m) of 6mm cord. The first step is situating the knot near, but not at the end of the loop. Many diagrams show the eight knot joining the ends overlapping (like a flemish bend, but with a third strand in the knot). Alternately, tie the cord into a loop using a double fisherman's bend. At the end without the knot, tie a 3-wrap prusik around your finger. Remove your finger and place the other end of the looped cord through the prusik. This should give you the approximate set up. Attach it to your harness and make micro-adjustments accordingly.

Here is a great guide on how to make the Purcell Prusik