Classic Prusik Knot

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Drifter
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Classic Prusik Knot

Post by Drifter » Wed Oct 29, 2008 10:21 am

Can a Classic Prusik knot (5mm cord) slip on a dry dynamic rope (11mm) or a dry static rope (11mm)? I have heard different opinions on this.

I have read on the internet that a French Prusik knot can slip and it is better to use the Classic Prusik knot, that a 5mm prusik cord must be used on a rope with a diameter of 9.1 to 11mm when ascending a rope and that you will have a problem if you use a 6mm cord on a 9.1mm rope. Does a 6mm cord slip on a 9.1mm rope?

Can a Classic Prusik knot slip on a wet rope?

Heinrich
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Re: Classic Prusik Knot

Post by Heinrich » Wed Oct 29, 2008 5:38 pm

well, yes, it can slip, but if you do it right, it won't. I usually wind the chord about 4 times around the rope and it never failed on me so far. But the winding is only one factor, the other factor is how tight it's wound - generally, the tighter the chord's wound, the better it will bite, but also the more difficult it is to slip it up the rope.

I'd always go for the thinner chord, i use 5mm and it works well. at least with this thickness i know that i can make a prusik on a very thin rope (< 10mm) and it will not fail.

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Nic Le Maitre
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Re: Classic Prusik Knot

Post by Nic Le Maitre » Thu Oct 30, 2008 8:44 am

Hi

If you dress the knot properly i.e. make sure that the turns don't cross over one another and that each turn makes as much contact with the rope as possible then it shouldn't slip.

The greater the difference in diameter between your prussik and the rope the fewer turns you will need to make the prussik work, but in general the more the better. It is easier to release and move thicker prussicks than thinner prussicks.

The real difference between the classic prussik (http://www.animatedknots.com/prusik/ind ... dknots.com) or the Kleimheist (http://www.animatedknots.com/klemheist/ ... dknots.com) and the French prussik (which is like the Kleimheist except BOTH ends of the prussik are clipped into a biner) is that it is possible to release the French prussik under load while the classic and Kleimheist will not release.

Prussiks actually work better on wet ropes than most mechanical rope grabs. I guess that the cohesive forces in the water help.
Happy climbing
Nic

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noki
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Re: Classic Prusik Knot

Post by noki » Thu Oct 30, 2008 9:02 am

Prussiks can be made incredibly complex, but the principles are very simple (which is why you may say "Duh!" to some of the things I mention below).
Here are the basics:
1. Prussiks work on friction. Different variations have more or less friction, and should be chosen based on the context in which they are being used. More friction means better bite, but also a harder knot to manipulate. So, for example, classic prussiks are often the best choice for prussiking, because they bite so well, and french prussiks are a good choice as an abseil safety. A minor note here is that different prussik knots consume different amounts of cord, so when you need a long cord, you may have to go for the classic, as an example.
2. Despite the variations in friction produced by the various knots, the amount of friction also depends on two other factors:
2a. The ratio of cord to rope size. The bigger this ratio (i.e., the greater the difference between the diameter of the cord and the diameter of the rope), the more friction. So that's why a 6mm can slip on a 9mm, because the ratio is so small (1:1.5). Some people suggest that a maximum for this ratio is that the diameter of cord be no more than two thirds of the diameter of the rope.
2b. The number of loops. More loops=more friction. so if your cord-to-rope ratio is less favourable (and your prussik slips), you can increase the friction with more loops and get the thing working right again. Or, if you have a very thin cord, and it keeps jamming when it shouldn't, you can decrease the friction by taking out a loop.
3. All prussiks knots will slip, given enough weight. Add more friction (which, thinking form physics, is a mechanism to dissipate energy), and more mass (a form of energy) is required before the knot will slip. So, alternatively, if your prussik is slipping, you could go on a diet! :wink: Very little proper testing exists to give you an exact guide (I have seen some informal test results), and the problem is the many factors involved--different diameters of cord and rope, condition of cord, condition or rope, number of loops, etc. But there simply is no prussik loop that will hold indefinitely, and, as a general rule, the knot will slip well before the cord will break. I will discuss this point in two ways below.
4. There are a lot of external factors involved as well, alluded to already. A shiny new teflon-coated (dry treated) rope will require more friction from the prussik. A wet rope will need more friction. An old and stiff rope may also require more friciton than when it was supple and soft. And the list just goes on and on...

In the end, it's a case of use what you have, and if it's slipping, add another loop. You will eventually either run out of cord, or have a knot that works, and its normally the latter.

Finally, a word about strength. The thicker your prussik cord, the stronger it is. It is instructive to look at the ratings for cord, like these:
http://www.bealplanet.com/portail-2006/ ... es&lang=us
Of course, if you choose a 5.5mm dyneema cord (note, not the pure dyneema advertised in the above link, but dyneema core with nylon/perlon sheath), you are looking at a super-strong, super-light prussik. And super expensive, too, b.t.w.
Some people go overboard here. For example, rope rescue guys can be found using 8mm prussik, but then they generally also use 12/13mm ropes and carry two people and a stretcher...
Generally, I prefer 6mm, as it is a good combination between thin enough and strong enough. I actually carry, all the time, one short, and one long 6mm prussik, and a third 4mm prussik for emergencies where I need a third.

Drifter
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Re: Classic Prusik Knot

Post by Drifter » Tue Nov 04, 2008 7:10 am

Thanks for the information everyone!

According to this information I read on the internet (please see link below), a prusik knot will slip further down a rope on a wet rope than on a dry rope when you put a heavy weight on it. I don't know if I have read and understood this report correctly as and I am not familiar with the system they set up. They are talking about a rescue load on a prusik knot. Maybe someone can shed more light on this?


http://cmru.peak.org/Rigging/july97_drop_test.htm

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noki
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Re: Classic Prusik Knot

Post by noki » Tue Nov 04, 2008 8:30 am

Drifter
The site you referred to is a rescue site. The context in which they are te4sting prussiks is very different from the normal use of prussiks in climbing. Specifically, the high line systems they use have to be tensioned a lot (understatement) and this tensioning puts a huge load on the anchors. Their (i.e., rescuers) reasoning is that a sudden achor failure would be catastrophic (no kidding), and so they don't tie the rope directly to the anchor. Rather, they attach two prussiks (hence the "tandem prussik") to the rope, and then clip these into the anchor. Note the 8mm prussiks on the 11mm rope, making it quite difficult to get the prussik to bite. So the rope is not effectively tied to anything--of course they do tie down the end so that if all else fails, it doesn't just go slipping over the edge, with the rescuer and patient still attached to the other end..., but the tension in the rope starts at the prussiks, not at an attachment knot like we would do in climbing. Generally, they *want* the prussiks to start slipping (but not fail completely) as a kind of warning signal when the load on the system starts getting too high. But to cut it short, these prussik are used to build a belay system, something climbers *never* do. Climbers generally use prussik for abseil safety and prussiking/self rescue.
Now, having said all that, the tests shown there (which are still informal tests) just confirm the general principles of prussiks which I mentioned above (e.g., the wet rope needing more friction for the prussik to bite, or, conversely, given the same amount of friction, slipping earlier than the dry rope). So don't let the different context confuse you. Those tests don't add anything new, just confirm what has alreadt been said. If you want to get into rescue, contact your local MCSA branch.

ciao

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Nic Le Maitre
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Re: Classic Prusik Knot

Post by Nic Le Maitre » Tue Nov 04, 2008 8:51 am

Hi Noki

Its not actually a highline system (aka Tyrolean traverse) that they are testing. What they are using is a standard system for raising a litter with a patient and a medic up a vertical cliff. The load is attached to 2 lines: the load line and the belay line.

They are testing to see what happens if the loaded line fails :shock: and there is a shockload on their belay line as it catches the weight. They are using double pussiks and a prussick minding pulley as their rope grab on the belay line.

You are quite correct in saying that prussiks will slip under load, but we have found by doing some informal testing (12:1 advantage system with 10 people pulling) that the load required to make the prussik slip is so great that some other part of the system will fail (probably the anchor) before the prussik will slip.
Happy climbing
Nic

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fanta
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Re: Classic Prusik Knot

Post by fanta » Tue Nov 04, 2008 3:43 pm

Prusik knots are with out a doubt the most fantastic knots around. They will grab just about ANYTHING. I've 'heard' that guys on oil rigs etc. use Prusiks to pull steel cables that are caked up with grease :shock:
Another point on Prusiks is the double fisherman’s knot used to make up the Prusiks. On pull tests done to test the strength of the knot they had allot of issues; the knot kept pulling through i.e. the tails would just slip right through the whole knot. Now these were rather large loads so don’t worry yourselves too much. Eventually they tied stopper knots in the 'tails' to stop this and to get the knot to break. For this reason some pieces of ‘string’ require a triple fisherman’s knot. The point I'm trying to get to is: make sure the tails on your Prusik knots DON'T look like the tip of a mole's snout sticking out the knot, leave a tail of a few centimetres... at least this way you'll have a chance to see why your arse is about to hit the deck before it happens.

Hey Nic I have heard it many times... "Prusik knots don’t slip, we used this and this and this and still nothing..."
The "IDEAL" Prusik knot in the correct situation in rescue will always slip, usually at around 400kg. Ideal being an 8mm cord, three wraps to get the classic Prusik on an 11.5mm static rope (yes there are other factors that play a role but we wont go into those). A theoretical 12:1 will in practise only give you a 10:1 due to friction losses. It has also been recorded that the 'average person' (how you get such a thing I don’t know) pulling on a haul line will only infact pull 30-40kg :shock: This wasn't my study so don't shoot the messenger... Once you do the math you'll see you could have been very close to making it slip. What I'm trying to say is: Unless you put a load cell into the system and you can prove what forces you are exerting you'll never know for sure.
I hope to some time in the very near future take some of the rope and Prusiks we use for rescue here in JHB and see what results I get, more than anything to put my own mind at rest as I'm too somewhat doubtful at times.

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Nic Le Maitre
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Re: Classic Prusik Knot

Post by Nic Le Maitre » Wed Nov 05, 2008 8:38 am

It was an 8mm 3-wrap/classic prussik knot on a (fluffy old) 11mm rope. It did actually slip but it took the rope sheath with it... Image Image :shock:

A 10:1 with 10 people pulling 30 kgs each = 3000 kgs... Even if we were only putting 400kgs on the rope it is still almost twice the accepted rescue load. If you have that much load on your system I think that something else (ie rock gear) will fail first.
Happy climbing
Nic

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fanta
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Re: Classic Prusik Knot

Post by fanta » Wed Nov 05, 2008 10:10 am

Yeah I must have been having a bad math day... I was missing a zero somewhere :oops:
As you know we use the Prusik as a load limiter and indicator of things about to go pear shaped and I know I always will, I'm sure you will too? I don't know of any other piece of gear available on the market to do the same thing. In a worst case scenario even though the sheath broke I'd rather loose that and a pair of jocks than have the rope break :lol: . How old and fluffy was your "fluffy old rope" do you remember what brand it was?

In the same way you made your hauling system's anchors strong enough to put 10 people on a 10:1 I'm sure you would do the same on a rescue, you'd make sure you build a system strong enough to do what you have to do. When and if you have doubts you'll start putting more gear in. We try to stick to a general rule of a ratio of 12 i.e. 12 people to do a straight haul no Mechanical Advantage in the system. 6 people on a MA of 2:1 then 4 people on a MA of 3:1 and 3 people pulling a MA of 4:1. If you cant move your load with that you should check why before you put more people on your haul team.

The answer you looking for is still unknown: 'what load did it take to tear the sheath'. To the best of my knowledge a jumar can tear the sheath of a rope at around 400 to 450kg althogh this is a mechanical device and different forces are applied by it. Like I said in my first post. I want to do some testing of my own WITH a load cell!! but even so at the end of those test the results will be specific to the conditions of everything present then and there...

Comming over 200kg and closer to 400kg can easier than you think on a Tyrolean for instance and friction over an edge can add considerable loads.

Another question: When you did your 10 people on a 10:1 did you just get everyone that was there to grab the rope and pull like mad or did you try a methodical approach of adding one person at a time until you could get something to break?

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