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Prusiks
Submitted by An Outdoor Idiots Team Member.
History of Prusiks
The original Prusik knot, as demonstrated on the previous page, was invented by a Dr. Karl Prusik, who gave the knot his name. Karl Prusik was an Austrian mountaineer. He served as president of the Austrian Alpine Club, and is known to have pioneered a great many climbing routes. He lived from 1896 - 1961, and the best data I can find suggests that he came up with his remarkable knot in the year 1931. It is important not to underestimate the significance of the knot, or even to think with hindsight that this type of knot is in any way "obvious." Indeed, even much later than 1931, there are accounts of some climbers that have died while dangling on the end of the rope, where it is believed, had they been aware of the Prusik knot, they might have been able to rescue themselves.It seems that the original concept of the Prusik knot was as an "ascending" knot, i.e. a climber could use a Prusik (or more precisely, two Prusiks) to ascend a fixed rope. Indeed, sometimes "prusik" is used as a verb, as in, "prusiking up a rope". These days, things have changed a bit. First, the original Prusik knot now has many common uses in climbing, and is not just thought of as an ascending knot. Second, there are now a whole class of knots that are called "Prusik" knots, and serve the same sort of purpose as the original. Third, a Prusik knot should not be thought of as something just to be used by climbers, or something that is only suitable for being tied around a climbing rope. I hope this article will go a long way in demonstrating this last point.
Understanding Prusiks
The principle behind a Prusik is fairly clear: When the knot is pulled at the right point, this causes the knot to contract, so instead of sliding down the rope, it grips it. However, when this force is removed, the knot can slide freely along the rope.A really good way to get a feel for the forces going on inside any Prusik is to tie one, using thin cord such as a guy line from a tent, around your finger. When you pull the Prusik, you can feel exactly how and where it contracts.
Before you even do that, how about this, to get a feeling for the basic principles:
First, simply loop a thin piece of cord around your finger a couple of times, as in Fig. 9:
If you clamp the end of the cord against your finger with your thumb, even very gently, you should find that you can pull (downwards, as oriented in the photograph) on the other end very hard, and you will be able to stop it unravelling simply with a small amount of pressure from your thumb. Very little of the force of the pull actually reaches the bit of the cord that you are clamping with your thumb. Most of it is applied to compressing your finger, due to the friction between the cord and your skin. That doesn't mean your thumb isn't playing a vital role, however - if you stop clamping the cord while still pulling on the other end, it will unravel easily.
The principle may seem obvious, but what might be surprising is just how little cord needs to be wrapped around your finger in order to generate huge amounts of friction. On a different topic, note that most belay devices used by climbers simply involve twisting the rope through 180 degrees in one direction, then through 180 degrees in the opposite direction, and the rope only makes contact with a small amount of the metal of the belay device. However, that is all that is required to enable one climber to control the entire weight of another climber using only one hand. The point is: a huge amount of friction can be generated over a very small length of cord when it is twisted around an object.
Why not try adding more turns? You will quickly get to the stage where you realise that the first turn or two are compressing your finger very tightly, while further turns seem quite loose. Depending on the length of your finger and the type of cord, you may get to the stage where you don't even need to clamp the end with your thumb.
But the arrangement in Fig. 9 still has a long way to go before it becomes a useful Prusik. For a start, if, instead of pulling down on the cord, you pull in the direction that your finger is pointing, then the force generated will clearly no longer cause the turns to compress, but instead will cause the turns to unravel. One simple addition is required to make the beginnings of a Prusik. If you tie a loop in the end by the thumb, as in Fig. 10...
... and then feed the free end through that loop, as in Fig. 11...
... then you have the makings of a Prusik. Now, you can pull the cord in any direction, and it will result in the turns gripping your finger more tightly (Fig. 12). Also, there is no tendency for the turns to unravel - the forces that would cause this now balance out where cord passes through the loop. Further, if you try this with quite a few turns, you will notice that it is no longer the case that just the first few turns apply almost all the friction - now, the last few turns also apply a fair bit of the friction.
That is probably enough to get a good feeling for how any Prusik works, though if you want to compare the various types, it is worth trying them around your finger.
This article will now cover some of the more popular types of Prusik, other than the original Prusik knot (though this is still very popular) which was covered on the first page. In all cases, note that the actual number of turns used can vary - there is no precise formula. Generally, 3 turns is a minimum. It is nice to get a balance between something that will lock solidly when loaded, but will then slide easily when unloaded - sometimes the number of turns can affect this balance. In practice, though, often a Prusik will have to be manually "loosened" a bit after it has been heavily loaded.
Note, the Prusik loop seen in most of the photographs in this article is made from cord that is 6mm in diameter and 1.5m in length before made into a loop. The rope seen in most of the photographs is 10.5mm climbing rope. This is a good arrangement for demonstrating Prusiks, but remember that most of these Prusiks will work with different materials and different thicknesses - it is always worth experimenting if you want to make a Prusik work in a particular scenario.
Page 2 of 10 Previous Next [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
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