These consist of climbing along the rope using slings which are tied to the rope with a hitch knot which enables the sling to grip the rope. A person can then sit in such a sling and remain hanging in it. By alternating between moving the sling up along the rope and sitting in it the climber can advance upward along the rope. The quality of the prusik hitch and its loading strength are of course the most important elements. In general it is the case that the thinner the prusik, the better it holds to the rope. The other side of this is the fact that thinner knots have lower breaking strength and are more inclined to failure. We have to have prusik hitches with high breaking strength in the event that we will fall into them. The most likely failure of these slings would be caused by cutting with a falling rock and fraying (burning) during a fall from height. Before cinching the prusik hitch upon loading with the body of the climber, this knot may slide down the rope at a certain instant. This is a very dangerous moment. The slide cannot be too long. Friction between the rope and the hitch creates a great deal of heat, and the hitch can be burned through. This is especially the case for situations of falling into the hitch on the rope from a height.
For example, a 5 mm line will not withstand even a 2 m fall (this can come about even during an imperceptible slack in the line; it is enough for the climber to slide the prusik upward and it will shift above her in such a way that the rope goes slack all the way to her ankles).
In general it is the case that any fall from height into a prusik hitch is always dangerous. If we are prusiking along a rope whose top anchor is not secure, it is necessary to belay using other means as well.
For example, to be belayed by a fellow climber by another rope, either from above or below, as with regular climbing while building intermediate points of protection.
The effectiveness of various types of prusik knots is brought about by inserting a carabiner (sometimes even a metal ring) into the knot. Such a carabiner or ring must have a significant amount of loading strength. Pay close attention especially to the welding of the metal ring.
When advancing along the rope using a prusiking sling the knots can come untied or loosen. While prusiking it is therefore necessary to keep a close watch on the knot to ensure that it will be cinched at the moment of loading!!
Be careful of a conditioned reflex. If a person sits unexpectedly into a prusiking sling (e.g. after losing his balance or slipping), he is likely to become startled and reflexively grasp with his hand at the point where the sling is hitched is tied. If he exerts force from above on the knot, the knot will not hitch and will be pulled down by the fall. And when this happens, the climber will fall as well.
Prusik hitches can be divided into two groups:
- symmetrical
- asymmetrical
Symmetrical prusik knots hold the same way in both directions of loading along the rope. Asymmetric ones hold best only in one direction.
This distinction between symmetrical and asymmetrical knots according to the way they are secured to the rope is not absolute. There are a number of prusik hitches which are asymmetrical and yet hold rather well in both directions of loading (e.g. the Klemheist knot), and on the other hand certain symmetrical prusiks hold only in the direction of loading and in the other direction do not hold at all (e.g. the Vánočka prusik).
Why is it necessary to differentiate between the two directions of loading? Because a situation could occur in which the direction of loading will change. For example, when advancing along a fixed rope in a traverse, where the climber is protected by a prusik. If the traverse is jagged, in certain moments upward and in others downward, it cannot be guessed in advance which direction a fallen climber will strain his prusik. The direction of loading can also change during direct vertical upward progress. If a fixed rope is anchored to the rock somewhere above the climber as well as below the climber, then in the case of snapping or other failure of the top anchor the climber will fall downward to a point under the bottom anchor and her fall will catch the prusik fixed to the rope. It will then be strained in the very opposite direction than it had been during the upward progress. For these sorts of serious situations it is good to use slings with high breaking strength for prusiking (at least 20 kN).
Caution! Even a strong sling is not entirely resistant to burning from friction during a longer slide of the prusik hitch along the rope. It naturally holds longer than a thin sling would, but it cannot be assumed that it will withstand everything.
Several factors influence how well a prusik hitch will hold on a rope and the what the ratio of the diameter of the sling must be to that of the rope. On one hand current weather conditions; a prusik will have different hitching abilities in dry conditions from wet conditions, and different still in snowy conditions. (For speleologists we should also add muddy conditions). A prusik will hold worse with new slings and rope, where the weave is still smooth and coated; it will hold better to a worn sling on a hairy rope. But watch out – a worn sling usually exhibits poorer breaking strength than a new one! Dull, hard slings will “bite” the rope better than soft slings.
In the following list of the most frequently used prusik knots the characteristics of the knots are given in relation to the diameter of the sling (listed in mm). In light of the paragraph above it is necessary to take this data only as approximate. In diverse practice it is possible to encounter partially differing characteristics. The data indicated in the following list has been determined such that a person weighing approx. 80 kg has sat in the prusiking slings, which were installed on mountaineering rope 11 mm in diameter. The weave of the rope was new, smooth and clean. The tests were performed in dry conditions in a roofed room.
Prusik knot
Named after the Austrian mountaineer Karel Prusik (1896 – 1961). In common camping or scouting terminology it is called a double cow hitch. It is the simplest and most basic prusik hitch. It can be tied with one hand. According to the number of turns of the knot around the rope we can identify the simple prusik, double prusik, triple prusik, etc. This prusik hitch is symmetrical, and therefore holds the same in both directions. A disadvantage of this knot is that it only works with slings significantly thinner than the rope. For example, a sling with diameter of no more than 5 mm will work on a rope with a diameter of 11 mm. Thin slings, however, have less strength and are less resistant to failure. When using thicker slings, the rope starts to slide. Only round slings can be used, with flat webbing slings the hitch doesn’t work reliably and therefore cannot be recommended for webbing.
Simple prusik
- 4 mm sling: works
- 5 mm sling: starts to slide, but can still sometimes hold
- 6 mm (and above) sling: doesn’t work
Double prusik
- 5 mm sling: works well
- 6 mm sling: works well
- 7 mm (and above) sling: doesn’t work
Continued in the book >>
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