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Prusiks
Submitted by An Outdoor Idiots Team Member.
Getting the Most out of Prusiks, and Prusik Safety
The Right Prusik for the Job?
There are many types of Prusik knot, and some people will swear that a particular type is ideal for a certain situation, while other people might say the exact opposite. This includes authors of popular climbing books. For example, one author suggests that a certain type is ideal for wet and icy ropes, while another author makes a point of stating that the same Prusik is useless on wet and icy ropes. So, more than any other type of knot, one would have to say of Prusiks, "your mileage may vary."What is key, is that since their operation depends on the friction between the Prusik cord and the main cord or rope (or whatever else they are tied around), it is important to always test the Prusik you intend to use, immediately prior to depending upon it. Further, if possible, test the Prusik beforehand under the circumstances you may to be using it in, e.g. with a wet rope. A Prusik you have become familiar with, used with cord or rope you are familiar with, might behave differently (i.e. jam more easily, or not lock properly) when the cord becomes wet, or muddy, or old and furry.
It would be wrong to generalise about the right or wrong Prusik for any occasion. It is a good idea to learn a few, and see what works best at the time.
Using a Prusik for Climbing
If using a Prusik in a situation where it will be taking someone's weight, it is better to use more turns than less. It may mean the Prusik locks easily and is hard to un-jam, but this is much safer than a Prusik which doesn't grip when you need it to. It is also important to be prepared for a Prusik to jam, which means you should have the equipment and / or the practiced techniques to be able to take the weight off the Prusik so that you can un-jam it.Similarly, if using a Prusik as a failsafe when abseiling ("rappelling") or belaying, it is tempting to tie it very loosely around the rope so that you can abseil or belay quickly and with minimum risk of an accidental jam. However, since the Prusik won't work at all unless it has a reasonable amount of contact with the rope in the first place, it is important not to have it too loose. Here is an example of how dangerous this urge to keep things flowing smoothly can be: One text book suggests using a karabiner to help keep the original Prusik knot flowing smoothly, as in Fig. 47:
However, I found with a shiny new rope and a shiny new Prusik cord, this approach was too dangerous. It is easy for the Prusik to become so loose that it won't grip at all, even when "loaded." If a karabiner is to be used to keep things flowing smoothly, I would certainly rather use the Bachmann Prusik as detailed on page 4, with plenty of turns. Note: as with all Prusiks, the original Prusik can have more turns than shown in Fig. 47, and this is likely to make a safer system than that shown in the photo.
Prusik Weight Limits - Part 1 - The Prusik Cord
To keep things simple to start off with, let's stick to static weights (i.e. we are not talking about shock-loading - that will be mentioned later), and let's assume that when a Prusik fails under a static weight, it will be because some part of the Prusik cord snaps.In some simplified situations, it can be possible to come up with an estimate for a weight limit. Here is a worked example, designed to show how careful one needs to be, especially if loading a Prusik with a human being:
Consider a Penberthy Prusik (page 6) tied with "550 Paracord", with one simple load pulling vertically downwards. This paracord is rated to hold a maximum static load of 550lbs, hence the name. I prefer to work in kilograms, so we'll translate this to 250kg. But the Penberthy Prusik is of course knotted. The Prusik itself is a knot, plus it has a loop tied at the bottom in order to attach it to the load. Many tests have shown that a rope or cord can lose from 25% to 50% of its strength at a knot. Playing safe, we'll assume the strength is halved. So it is a reasonable estimate that this arrangement will hold approximately 125kg.
It is important to realise that we are talking about static weights. If you plonk yourself into a hammock, or have a fall when climbing, then you can easily be generating several times the force that your static weight generates. So the above estimate of 125kg should in no way be encouraging if you intend to load that arrangement with a human being.
Other than that simplified scenario, it becomes quite difficult to estimate a maximum weight limit. Suffice it to say, that if you will be loading the system with heavy weights, it is better to use a much higher strength cord (the cord in Fig. 47 is rated to hold a maximum static load of around 800kg, before it is knotted), and it is perhaps better to use a knot such as the French Prusik (page 3), where the load is shared between two loops of the cord. Unfortunately, I can find no reference to any tests that compare the strengths of the various Prusik knots. If you have some information, please get in touch.
Prusik Weight Limits - Part 2 - Applied Friction
Now for a particularly interesting question: Prusiks rely entirely on the friction between the Prusik cord and the main line. It is clear that a Prusik grips the outer part of the main line very well indeed. But what exactly is going on inside the main line?Climbing rope, and indeed most good quality cord, consists of an outer sheath surrounding inner strands. The outer sheath serves several purposes. Most notably it protects the important load-bearing inner strands, and it makes the rope pleasant to knot. See Fig. 48 below. So, when a Prusik is tied around a rope and loaded, is the load being transferred well into the inner strands of the rope, or is the sheath taking most or all of the load? It would be a bit worrying if only the sheath was taking the load, because, for example, the sheath of a climbing rope only accounts for something like 30% of the rope's overall strength. I didn't think the answer to this was particularly obvious, so I tried it out.
Note that the sheath is in no way bound to the inner strands - it glides freely and very smoothly up and down. For the test, I tied a Penberthy Prusik around the rope, near the exposed end, and applied a load to it while grabbing the end of the rope, including the sheath. It locked solid, as you would expect.
Next, I did the same thing, but this time I only held on to the inner strands. The sheath was free to slide down the inner strands as the Prusik gripped and pulled it. Reassuringly, the sheath stayed put - it didn't budge in the slightest. There was evidently enough friction between the sheath and the inner strands for the load to be solidly transmitted internally.
An obvious result? Well no, not really. I tried exactly the same test with smaller diameter cordage. This time, some paracord served as the main line, and I tied a Penberthy Prusik around it using some tent guy line. In this case, it became clear that the sheath of the paracord was taking most, if not all, of the load. I tried very hard to get the Prusik as tight as possible before letting go of the sheath - but I just couldn't get the sheath to grip the inner strands. The Prusik locked solidly to the sheath of the paracord, but the sheath of the paracord slid easily down the inner strands.
Two possible explanations come to mind. One is that the smaller diameter means that there is less surface area of the sheath in contact with the inner strands, so it will not be possible to apply as much friction. The other is that this paracord actually has quite a loose, large sheath, surrounding inner strands that have a bit of room for movement. So as the Prusik crushes the sheath, not all of the inner surface of the sheath will be in contact with the inner strands.
Whatever, the conclusion is this: It is possible that, in some cases, a Prusik will apply its load to only the sheath of the main line, and that could mean the system is only capable of supporting around 30% of the weight you think it is, or some other small fraction, depending on how strong the sheath actually is.
Prusik Weight Limits - Part 3 - Shock-Loading
Some cordage, and some knots, have a proven track record of being able to withstand tremendous shock-loading. When a rope arrests the fall of a climber, for example, the force it is subjected to is, for an instant, well out of all proportion to the force generated by the climber simply dangling on the end of a rope. But climbing ropes can manage these forces (largely because they stretch), and some knots have been proven to handle them well, the Figure-of-Eight being a good example.By contrast, it is important to be aware that Prusik knots are, to put it mildly, not ideal for withstanding shock-loading.
For some interesting insight into how a Prusik might behave when shock-loaded, take a look at this experiment carried out by the Corvallis Mountain Rescue Unit in the USA.
In fact, Corvallis Mountain Rescue can have the final word on the whole Prusik thing.
[The Editor speaks: "Wake me up when you've finished. Oh, you have? About time!"]
Page 10 of 10 Previous [Top of Page]
Page 1: Introduction to Prusiks, and the Original Prusik knot
Page 2: History of the Prusik Knot, and Understanding Prusik Knots
Page 3: The French Prusik
Page 4: The Bachmann Prusik
Page 5: The Kreuzklem Prusik and the Kleimheist Prusik
Page 6: The Penberthy Prusik
Page 7: Example Use (1) - An Adjustable Loop made with the Penberthy Prusik
Page 8: Example Use (2) - A Line Tensioner made with two Original Prusiks on the Same Cord
Page 9: Example Use (3) - Tying a Prusik to a Rigid Pole
Page 10: Getting the most from Prusiks, and Prusik Safety
Page 2: History of the Prusik Knot, and Understanding Prusik Knots
Page 3: The French Prusik
Page 4: The Bachmann Prusik
Page 5: The Kreuzklem Prusik and the Kleimheist Prusik
Page 6: The Penberthy Prusik
Page 7: Example Use (1) - An Adjustable Loop made with the Penberthy Prusik
Page 8: Example Use (2) - A Line Tensioner made with two Original Prusiks on the Same Cord
Page 9: Example Use (3) - Tying a Prusik to a Rigid Pole
Page 10: Getting the most from Prusiks, and Prusik Safety
If you found this to be a gripping subject, then you can discuss things further in our forums.
If you can't be bothered to enter the forums, then shame on you! But you can still leave a comment below. Please try to avoid the profane ramblings of a madman. That's our job.
If you can't be bothered to enter the forums, then shame on you! But you can still leave a comment below. Please try to avoid the profane ramblings of a madman. That's our job.
Your Comments
On 3 June 2007, feralgun 47Bn/33FF said:Awesom site. Great info. Where do you get the tarps that have the loops along the center for the ridge pole? Thanks alot, I'm adding this site to my favorites list. fg419
On 28 June 2006, B Lance said:
THIS . IS . GREAT. THANK YOU!
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