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If I'm using two prusik's to climb a rope (one as a foot loop and one attached to harness directly) What's the minimum diameter of cord I should be looking for to remain safe?

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I am confused. You seem to be asking for the diameter of the accessory cord used for the Prusik loops themselves, yet AM_Hawk answer, which you Accepted, seems to describe the diameter of the rope you are ascending. Could you please clarify? –  Mr.Wizard Jan 11 at 14:39
    
@Mr.Wizard Thought I would answer the big picture, and the cord diameter is in the answer as "2mm less than rope" –  AM_Hawk Jan 11 at 19:05
    
@AM_Hawk I guess your answer then is 5mm Prusik cord. Nylon or Tech cord? –  Mr.Wizard Jan 11 at 19:09
    
@Mr.Wizard Static,Tech and Nylon can all be used. I suggest to anyone reading this to educate themselves on melting points for example Dyneema may not be suitable for use as friction hitches due to their lower critical melting point (150º Fahrenheit) as compared to polyester or nylon (350º Fahrenheit). –  AM_Hawk Jan 11 at 20:52
    
@AM_Hawk Personally I would not be comfortable on 5mm Nylon unless there was a back-up attachment to the system. Are you supposing that 5mm Nylon is OK as one's only attachment points? –  Mr.Wizard Jan 11 at 21:00

2 Answers 2

up vote 2 down vote accepted

EDIT: When the question was posed I misinterpreted it to be asking the diameter of the "Climbing Rope" not "Cord" However my answer for Cord diameter is also listed as "2mm less than cord diameter and not less than 5mm"

I would not go any lower than 7.5mm*, it is a common diameter used by rescue technicians.

There are several reasons why I recommend 7.5mm, listed below.

  1. You need to remember and account for the diameter of your prussik, which following the general rule of thumb, is at least 2mm less than the rope diameter. A 5mm (Static,Tech or Nylon) prussik supplies a good amount of grip. If you chose a rope of smaller diameter your prussik will be approaching some very small diameter cord.

  2. A 7.5mm such as the Sterling Escape Tech provides good abrasion resistance, and has the ability to withstand falls, something a rope of smaller diameter may not be able to provide.

  3. Any smaller than 7.5mm the rope itself will be difficult to ascend, and if you chose to descend on it the rope will have a tendency to coil up and even knot itself.

*According to numbers a 6mm static cord can hold 1600 lbs however I would not recommend using it for the purpose of ascending, let's keep it for hoisting the bear barrels into the trees.

If you attempt to ascend a rope using only friction knots please use a safety back up such as the system detailed below!

As a safety precaution tie off the slack end of the rope, using a figure eight on a bite, at progressive intervals and clip to your harness. To avoid having a clutter of these knots all hanging off your harness you might choose to untie and release each previous one. Note, however, some climbers prefer the clutter to potentially getting the trailing rope snagged far below.

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I just noticed that you linked to a 7.5mm Twin rope as an example. So you are describing ascending two strands of rope at once? You should be able to use at least a 6mm Prusik loop in that configuration. I was assuming that you were referring to a single strand of Tech rope, like BlueWater's 7.5mm Escape-Tech. –  Mr.Wizard Jan 11 at 21:06
    
@Mr.Wizard No, ascending one rope...Picked a new rope to make it clear. Thanks for pointing that out. –  AM_Hawk Jan 11 at 21:11
    
When I read this I get very confused about whether you are talking about the rope on which you are placing the friction knot or the rope used to do the friction knot. It would be great if you could clarify that. –  imsodin Aug 7 at 13:01
    
@imsodin Edit added at the top to clarify. Thanks! –  AM_Hawk Aug 7 at 13:16

I am going to approach this question differently. Since the question is marked with the tag the use of a 7.5mm Technora escape rope that AM_Hawk's answer focuses on seems unlikely. Also, the method by which he arrives at a minimum Prusik cord size doesn't address certain issues.

Cord diameter

All else being equal a smaller cord for a friction knot, such as a Prusik, will grip better than a large one. I have seen several guidelines, such as use cord no larger than 70% of the diameter of the rope being ascended, but truly this is only a guideline. A stiff and slick cord will have much less purchase than a limp and rough one, therefore maximum diameter is determined pragmatically by what will grip. There are cords made specifically for friction hitches, and these typically limp cords will work better even in larger diameters than will stiff accessory cord not intended for the application. For example, BlueWater 8mm Dynamic Prusik Cord is limper and will grip better than their 6mm Accessory Cord.

As far as minimum size, there really is none from a friction standpoint as the smaller cord grips better; it is entirely possible to use a shoelace, paracord, a string from your chalk bag, etc., so long as it can hold your weight. One would be a fool to have these thin cords be his only attachment to the rope of course. You can protect the ascent by progressively clipping in to loop knots in the rope, as AM_Hawk described, or you could use a non-toothed progress-capture device such as a Petzl Shunt. (Please don't implement a system without a full understanding of the devices, failure modes, and risks involved; this is merely an example.)

Perhaps you are seeking a minimum size of friction hitch that is safe as a primary connection point. That is hard to answer. One possible failure mode is burn-through; that is, if the knot slips the friction will melt through the cord long before you deck. Hypothetically a smaller cord, if the loop is appropriately sized and the knot well dressed, might be less susceptible to this as it should be less likely to slip, but a larger cord stands a better chance of arresting the fall before it melts or tears through. There is also the consideration of fusing. Several test reports describe the friction hitch fusing to the main line after slipping; while this is less immediately terminal than burn-through it could still leave you stranded.

Resources

I was unable to find dynamic (drop) testing of small diameter Prusik cords. However, here are some resources that seem relevant.

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