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Knot strength for accessory-cord-to-biner attachment


Original Post
Serge Smirnov · · Unknown Hometown · Joined Oct 2015 · Points: 235

Is my guess correct that a bowline-on-a-bight makes a stronger accessory cord attachment to a carabiner than a fig. 8 on a bight ?  My reasoning is that, although the Bowline is known to have a lower knots efficiency, in this configuration the weak point with 1 strand is at the carabiner.

(if it matters, I'm interested in a scenario where the load is evenly distributed between the 2 long strands coming out of the knot)

(I'm not including bunny ears in this question because I'm pretty sure it would be at least as strong as the better of these two, but it has the downside of eating more cord length.  However, if somebody knows how much stronger bunny ears would be compared to bowline-on-a-bight, that would be useful)

Jeremy B. · · Unknown Hometown · Joined May 2013 · Points: 0
Serge Smirnov wrote:

Is my guess correct that a bowline-on-a-bight makes a stronger accessory cord attachment to a carabiner than a fig. 8 on a bight ?  My reasoning is that, although the Bowline is known to have a lower knots efficiency, in this configuration the weak point with 1 strand is at the carabiner.

How sure are you that the weak point is at the carabiner?

As the carabiner is clipping a loop, you would count the strength of the loop as double that of the single strand (well, almost).  One example using a carabiner-sized pin to test loop strength showed ~19 kN strength when using 7 mm cord rated at ~12 kN, or about 80% of the doubled cord strength.

Cord usually breaks at the knot, with a reduction in strength relative to the single strand strength.  So, if your double bowline has 60% efficiency and the figure eight has 80%, then I would expect the weakest point in your two systems to be where the single strand of cord enters the double bowline, with a strength of only about 7 kN.

Serge Smirnov · · Unknown Hometown · Joined Oct 2015 · Points: 235
Jeremy B. wrote:

How sure are you that the weak point is at the carabiner?

Not very.  I got this idea from 20kN's post at the top of https://www.mountainproject.com/forum/topic/107900351/figure-8-vs-double-bowline?page=7 , but the videos there use climbing ropes - it's possible the situation is different with accessory cord (which is thinner than a carabiner).

But I'm also not sure what knot efficiency figure is applicable to bowline-on-a-bight..   I have a book with a table that gives something like 60% for normal bowline and 70% for double bowline, but I'm guessing by "double bowline" they mean the kind where the big loop is 1 strand and "double" just refers to the small loop you make at the beginning of the tying process..  I do expect bowline-on-a-bight to be at least somewhat more efficient than a plain single bowline..

Em Cos · · Boulder, CO · Joined Apr 2010 · Points: 5

If the difference in breaking strength between these two knots is going to come into play at all, then I wouldn't use either. In other words, depending on what you're trying to use this for, either these knots are both plenty strong enough or neither is. 

Jeremy B. · · Unknown Hometown · Joined May 2013 · Points: 0
Serge Smirnov wrote:But I'm also not sure what knot efficiency figure is applicable to bowline-on-a-bight..   I have a book with a table that gives something like 60% for normal bowline and 70% for double bowline, but I'm guessing by "double bowline" they mean the kind where the big loop is 1 strand and "double" just refers to the small loop you make at the beginning of the tying process..  I do expect bowline-on-a-bight to be at least somewhat more efficient than a plain single bowline..

In the link in my previous reply, there's mention that the bowline efficiency drops with thinner cord (7-8 mm cord, 5.5 mm Spectra) and webbing (1" and 9/16"), e.g. around 50%, and thus recommends against its use with them.  I didn't have much luck finding good test results for a bowline on a bight and agree with your thoughts on what is meant by double bowline.

Kyle Tarry · · Portland, OR · Joined Mar 2015 · Points: 162

For any reasonable climbing purpose, your safety margins/factors need to be so high that "knot efficiency" shouldn't be a factor.  Just tie whatever is fastest/easiest and make sure the cord is strong enough.  Worrying about relative strength of knots is simply not that valuable.

A little information about the application might allow people to give more useful advice.  Off the cuff, my first thought is that you're way overthinking this, just tie any reasonable knot and be done with it.

Guideline #1: Don't be a jerk.

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