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Is it safe to girth-hitch nylon webbing through my harness tie-in points and then shorten the webbing via an overhand loop (assume it is too long) and clipping the non-bight part to the bolt in the rock? I am wondering if the knot can come undone in this case. I understand the knot would not come undone if the bight was clipped in, but my question is about when the non-bight part is clipped in.

Picture below: example

I know that a Purcell Prussik is a better option if I have a cordelette, but let's assume that I don't.

  • Why use a sling at all if it's the wrong size? Use your rope and a clove hitch and viola you have a anchor exactly the correct length. – user2766 Jan 8 at 13:15
  • A tip: do not use an overhand, but a bowline on a bight or an alpine butterfly. Those are easy to untie. – QuantumBrick Jan 8 at 15:45
  • @Liam I am not using a clove hitch because I am preparing to rappel (single pitch sport climb) and thus the rope needs to simply be run through the anchor rings so that, once I have rappelled, I can just pull the rope through the rings. – Chris Morris Jan 8 at 18:14
  • An overhand is perfectly fine in this scenario and actually probably the safest knot to use @QuantumBrick . For example a figure eight can roll, a bowline is easy to tie incorrectly and a alpine butterfly is only safe if the loop is clipped. "Those are easy to untie" ermm..no not really. – user2766 Jan 8 at 22:29
  • @Liam I disagree. The bowline on a bight is one of the easiest knots to both tie and untie, and the butterfly would do the job too. I use the bowline on a bight (BB) for fixing both hauline and lead rope; when rope-soloing I trust a BB as main and butterfly as second; I attach haulbags to the swivel with a BB; I create on-spot belays with a butterfly; in self-rescue I attach each element in a tandem with a butterfly; since BBs can serve as bunny ears I often create rope belays with them; I abseil with huge weights by attaching haulbags to my Petzl dual evolve with BBs. The list is infinite. – QuantumBrick Jan 8 at 22:41
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You should worry about the opposite: An overhand knot on nylon webbing will get really tight and hard to undo once no longer in use. That's why I rather use a figure-eight in this scenario (double bowline would be even better for undoing, but a bit more cumbersome to tie). This setup is somewhat standard practice for abseiling here in Switzerland, where you tie a 120cm sling to your harness, make a knot at about 2/3 length and attach your backup (prusik) to your harness, the braking device (e.g. tuber, eight) to the lower loop of the sling (same position as in your image) and the biner for self-securing in the outmost loop of the sling (undone when abseiling, attached when at belay-station). So plenty of anecdotal evidence that they definitely don't slip.

And if you don't trust that: Would a failure be catastrophic? No! The loop would just extend to it's full length, you are still clipped into the "main" loop so you wouldn't get disattached. That's also why this setup is ok even with dyneema slings. Again anecdotal evidence says by not shockloading, they just pull tight (and in case of the narrow slings very annoyingly, potentially almost irreversibly tight). Tests have shown that under high stress, they do creep.

tldr:
It won't come undone, it will get really tight. And even if it did (it does not), that wouldn't be a catastrophic failure.

As to breaking of the nylon webbing (not in the question, but just to be thorough):
The rule of thumb is to assume a knot weakens the nylon by 50% (common climbing knots lie below that). A nylon sling as a loop typically has 22kN breaking strength, i.e. 11kN on each strand (as a ring is always equally loaded, as long as you don't fix your biner with a clove hitch or something). Now one strand has a knot in it, meaning now the breaking strength of the knotted strand is 5.5kN and the weakest strand breaks first, so 11kN for the entire sling.

  • If the knot slipped, wouldn't the webbing extend and thus be shockloaded, which could be catastrophic? – Chris Morris Jan 6 at 5:47
  • If the slipping were frictionless, aka the knot become undone at once, then yes. I can't imagine that, but I know my lack of imagination isn't a good argument. However I mean it doesn't slip at all, so why should it slip without friction. And even if it would, it doesn't come undone (kind of cyclic here, but really the point of discussion is moot). – imsodin Jan 6 at 7:55
  • Your breaking strengths seem off by a factor of 10 - climbing equipment is typically rated to 22 kn. Also, I'm not sure if your calculation of the breaking strength is correct (i.e. if they add up like this), or whether the weakest strand would break first. That would result in a breaking strength just 550kn (which should still be enough here). – anderas Jan 7 at 6:54
  • @anderas Thanks for noticing, my mind was thinking in tons (bad me - mass isn't force). As to the weakest strand: You are right that I was wrong, but I am quite sure you aren't quite there too: The weakest strand will break (at 5.5kN), but the load will still be shared by the two stands, so to reach 5.5kN in the weakest strand you need 11kN overall. Or am I wrong again? :) – imsodin Jan 7 at 19:28
  • @imsodin First: A slipping knot generates friction ie heat, which is a very bad thing for webbing. Do not use loose knots as a means of shock absorber! Secondly: A figure eight rolls much more easily than an overhand, making it a terrible choice for this application. (For the same reason that we dont use a fig8 for an EDK.) – Guran Jan 8 at 6:48
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This is perfectly fine. Not ideal perhaps, but perfectly fine.

One caveat though: Never climb above the anchor when clipped in with a static sling. Especially a knotted sling. And if you do - do not fall.

There is a famous video from dmm demonstrating what a dropping weight can do with a knotted sling. https://vimeo.com/27293337

But take that with a grain of salt. Your body is not a steel ball. Still, a fall onto a static sling hurts.

So TL;DR version: Shortening the sling as in your pic is fine. Climbers do this every day. In any situation where the knotted sling is bad, an unshortened sling is at least equally bad.

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