The term Prusik knot or Autoblock is often used to generalise a number of different friction hitches that we use regularly in climbing.
Below are three friction hitches that I use a lot, their pros and cons and when or where I might use them.
The Prusik Knot (aka Original Prusik, Classic Prusik)
Developed by Austrian mountaineer Dr Karl Prusik, this hitch works by threading a cord around a rope and back through itself, usually 2 or 3 times, to provide a locking friction hitch that is difficult to release under load.
It’s useful as its quick and easy to tie and can be used in both pull directions. It uses the least amount of cord to make, which can be handy if you want to connect the Prusik to your harness without having to extend it with a sling.
Its less effective on wet or icy ropes and you should never use a sling for a Prusik Knot.
Perhaps its best use is for a scenario like escaping the system, where you don’t want the hitch to release under load, but don’t intend to slide it or move it along the rope much.
This hitch works by wrapping the cord around a rope multiple times and passing the bight from one end of the cord through the bight of the other end of the cord.
It locks really well, in fact, it can be practically impossible to slide when weighted if used properly. It can be released easily and a sling can be used as well as a cord (see below).
However, it only works in one pull direction and its too catchy to use as a back up for abseiling.
Because it grips so hard, it works really well when used in an unassisted haul set up, for escaping the system or when ascending or descending a fixed rope.
French Prusik (aka The Autoblock)
This knot is tied by wrapping the cord around the rope and clipping the bight at both ends of the cord together with a carabiner.
Its advantage and disadvantage is the same thing, it can be easily released, even under pressure.
As this is often used as a “fast-moving” hitch a sling isn’t advisable here either as it could slip/slide more easily and melt.
This makes it perfect for using as a back up when abseiling when lowering someone on an Italian hitch or as a clutch when using a haul system. It would not be an ideal friction hitch to use as a primary safety when ascending a rope, however.
Why do I need to know this? (couldn’t I just use a mechanical device?)
While there are purpose made devices out there like the Tibloc, there are a whole host of reasons as to why knowing how to make/use a friction hitch is a good idea. Below are just a few:
- Cord is cheaper and weighs less than metal
- you can wrap a friction hitch around two ropes
- you can use a sling for some setups (please see below)
- It can be untied and used for tat to abseil off
- it causes less wear to your ropes
- it can be used easily in descent
- it could be used to make a runner with two crabs if you run out of quick-draws
How do I tie a cord for a friction hitch?
- Use rated cord somewhere between 60 – 80 % of the diameter of the rope. 6mm cord covers most scenarios. If the cord is too thick, it won’t grip the rope, but too thin and it could melt or deform quicker.
- Approx 1.2/1.3 metres of cord is needed.
- Tie both ends of the cord together using a double fisherman’s knot and make sure there are no twists in the strands before tie-ing.
- Make sure there are sufficiently long tails, so the cord doesn’t pull through the knot when weighted.
Can I use a sling for a Prusik?
It is absolutely best practice to use a rated cord for friction hitches, but a sling can work too, if used right, let’s look at when or why.
A Dyneema sling (least resistant to heat) or a Nylon sling (slightly more resistant to heat) are poor choices for Original Prusiks or French Prusiks, but if you were stuck, could be used as a Klemheist, especially in a scenario where they are butting up against a larger rope knot and are prevented from slipping, like when built behind the master point.
Aramid slings have a high melting point and I have successfully used them as a back up French Prusik when abseiling, without any damage or melting. This makes them suitable for all types of friction hitch.
Advanced uses of friction hitches
In this article I’ve spoken bout some advanced techniques liked hauling, escaping the system or ascending fixed lines and about knots which may be unfamiliar to some climbers.
If you don’t understand these techniques and terms used above then consider doing a training course and up-skilling your climbing knowledge.
Id strongly recommends using an AMI qualified instructor to show you too, as they will have been trained and assessed on what to teach and how to teach it.
I hope the above info is useful. If you would like to discuss any aspect further, please get in touch and Id be happy to chat.