Dirtbagjens did some testing on the purcell prusiks that we’ve been using to attach ourselves to anchors when we’re out rock climbing. Sometimes these skinny bits of cord are the only thing keeping us attached to the rock. Turns out they’re actually pretty strong, which is good news for us! Here’s his write up of the test results:
When attached to an anchor with anything but a dynamic rope, there are some dangers with increased forces arising from the rigid nature of the connection in a ‘static’ fall. I have been using a purcell prusik as my personal anchor system (PAS) for quite a while now and am very fond of it because it is seamlessly adjustable and very cheap compared to commercial systems. Most importantly it is safer because the prusik knot allows slippage which dissipates some of the energy of the fall and thus reduces the maximum arrest force (MAF).
Recently I have been asked about advice on purcell prusiks by fellow climbers and I decided it was time to do some testing so I can base my advice on practical data. In summary I can say that I am now more convinced than ever of the advantages of purcell prusiks over any other personal anchor system based on slings or lanyard
A Purcell prusik is a user-configured tie, usually made out of 6 mm or 7 mm nylon cord. It is tied in such a way that it includes a prusik hitch around two strand of cord creating an adjustable loop.
They can be used for a variety of rope applications such as ascending a rope, used as a release hitch or as a primary attachment for securing oneself to an anchor.
2. Problems with Primary Attachments
When attached to an anchor with anything but a dynamic rope, falls can be quite violent. This is due to two reasons:
- A fall onto an anchor can easily result in a large fall factor (FF).
- The system is very rigid (we don’t want to fall onto a rigid system, that is why we use dynamic ropes for climbing and why some of us are happy to bungee jump (FF=1) onto a dynamic cord but nobody would bungee jump if they had to tie into a steel cable)
For a more detailed explanation have a look at this article published by Rock and Ice.
And if you would like to see what happens to high performance fibres like spectra or dyneema, which are incredibly strong but very rigid, in a drop test, have a look at this cool video produced by DMM.
Three drop tests were conducted, trying to mimic what would happen if a climber were to step up while directly clipped into an anchor (to adjust a knot or clip a dummy runner) and then slipped. All tests were conducted with a fall factor of 1 and 60 kG weight.
The first test was conducted on a knotted 60 cm loop from 6 mm nylon with a strength rating of 8.8 kN. Test no. 2a and 2b were conducted on Purcell prusik made from the same material. Have a look at the video:
|Test||Configuration||MAF [kN]||Slippage [cm]|
As shown in the results table, the purcell prusik reduces the maximum force needed to arrest the weight by about 40 %, compared to a similar fall using the simple nylon loop. Note how the weight bounces back much more violently in Test 1, than in test 2. This is the result of the prusik dissipating some of the fall’s energy by slipping and thereby reducing the maximum arrest force. The thermal imaging shows how the section of cord that the prusik slips on, heats up and dissipates the energy via heat.
The two nylon strands the prusik hitch slipped on showed abrasion in the sheath. The sheath of the inside of the prusik coils was partially melted. This damage appeared to only affect the sheath.
Next to the advantages of convenience and price, the purcell prusik is able to significantly reduce the maximum force needed to arrest a fall by absorbing some of the energy during slippage. It therefore has some significant advantages over more rigid systems that result in much larger forces. These tests have increased my confidence in the concept and I feel much safer using it for myself and recommending it to others after seeing these results.