Tag Archives: Knot failure

Conclusions on the use of the BotB

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So, the footage has circulated, tests have been done and reports and statements have been published. Where are we on BotBs?

I echo the BCA advice that the BotB is safe to use provided that a cowstail is always clipped into both loops. I will continue to use the knot in my range of personal and professional tools. I think that the knowledge of this method of failure has been there since the BotB came into use, I do think that a number of people were shocked as to the extent of the failure when they saw it. This knee-jerk away from the BotB has calmed down as people have looked into the evidence and scenarios of failure.
Hopefully enough information is out there now to allow cavers to make their own choices on knot application.

A concern may crop up for some riggers where they are leading less experienced cavers or those who simply do no know about the single loop issue. The rigger may descend the pitch and those following could inadvertently clip a single loop placing them at risk of this failure. This scenario perhaps would benefit from the use of a knot that won’t fail if a caver clips a single loop – However the advice to clip both loops does not change.

What if you don’t want to use BotBs?

Well, a number of suggestions have been put forward.
The Fig 8 ‘Bunny Ears’ / Double Fig 8 on the Bight is the most common knot but lacks the ease of adjustment and untying that the BotB has.
The Fusion (Karash) Knot is simple enought to tie but is a struggle to dress on stiff rope and has zero history in UK caving prior to a few months ago.
Alpine Butterflys or Caver’s Butterflys can provide a good Y-hang but an overhand knot would need to be introduced to one of the loops to give a central rescue point.
The Double Bowline on the Bight is one extra twist from the BotB. It adjusts, ties, unties and looks like a BotB, meaning it is easier to use and spot mistakes for existing BotB users.

The choice is down to the individual rigger. The evidence is out there for you to see.

Warning – correct use of Bowline on the Bight Knots

BCA’s Training and Equipment & Techniques Committees would like to highlight the importance of ALWAYS clipping a cowstail through BOTH loops of a Bowline on the Bight knot. Although this has been taught for years by BCA instructors, it appears
that many cavers are not aware of the importance.

The problem is that in a fall the knot can slip in such a way that the rope going down the pitch can actually run all the way back through the knot. This means that a caver falling
at a pitch head with their cowstail clipped into only ONE of the two loops could
potentially plummet all the way to the bottom of the pitch. This cannot happen
if they are clipped into BOTH loops.

A useful tip is for the rigger to leave an HMS karabiner clipped between the two loops to make it easier for the rest of the party to clip in and it is always worth remembering that two cowstails are preferable to one.

From here: http://www.british-caving.org.uk/?page=150

The report from drop tests at BPC on the 30th Jan 2013 is here: http://british-caving.org.uk/equipment/Initial%20results%20from%20a%20Preliminary%20Investigation%20into%20Y%20hang%20knots.pdf

UKCaving discussion here: http://ukcaving.com/board/index.php?topic=14602.msg191183;topicseen#new

2nd round of testing on the Bowline-on-the-Bight knot

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Thinking that 2 heads were better than one I set out today with a good friend and colleague, Jez Parr. Jez in another CIC holder with many years of experience. He was also my mentor through the CIC scheme and the ideal person to bounce ideas off.

Our first stop was local gear shop and purveyor of lovely shiny things, Hitch N Hike. We purchased 5 metre lengths of all the most commonly used semi-static rope they had, in both 9 & 10mm. The test this time was set up identically to the ones done previously on the 8th Jan. We had a mock Y-hang arrangement that allowed us to position the test ropes at varying heights and angles as well as a solid bar from which to rig a secure dynamic safety back-up for ourselves.
The aim of the first testing done previously was to identify the main contributing factors that caused the failure in the Bowline-on-the-Bight, or BotB. This was achieved and we could generate failures almost 100% of the time. This test is written up in the last blog post.
Today’s testing was all about trying to get a failure in a normal use environment with normal conditions. All the tests we did were on knots that has been tied, dressed and set as we would and have done thousands of times underground. We were not wanting to tie sloppy knots, we wanted to see if it would fail when it was done well.

Results:

For every test of every rope in every combination of orientations we tried, so long as the cowstail was attached through both loops of the knot there were no failures. This was as expected and ties in with the best practice advice that has been taught for years with the use of the BotB – ALWAYS CLIP BOTH LOOPS.

For tests carried out where we dropped onto a single loop – the one formed from the traverse line or stopper knot, we could also generate no failures. However this is outside the intended use of the knot so little time was spent investigating that scenario.

When connected to the Bowline-on-the-Bight with a cowstail attached to the single loop formed by the pitch rope we were able to generate failures in knots that were tied and dressed correctly that had been hand tightened or on occasion body weight tightened.

Below are a pair of images showing slip through the knot. Note the blue mark.
BotB test 3

BotB test 4

A second set of test images:

BotB test 1OLYMPUS DIGITAL CAMERA

For the previous test film see here:

Conclusions:

The Bowline-on-the-Bight can fail when you are clipped into a single loop and the knot is dressed correctly and tightened. The failures we generated included ones where we both agreed that we had tied a perfectly acceptable BotB before testing.
Although we can identify many modes in which we could not get a failure to occur, the fact that one can occur in normal* use is very worrying.

* normal use if you only use one loop to attach your cowstails as is against the best advice – always clip through both loops!

So now what?

Cavers should draw their own conclusions from these tests or indeed conduct their own perhaps.
I believe that it is still appropriate to teach the Bowline-on-the-Bight as one of the standard knots for caving alongside the information about how to use it safely and the consequence of misuse.
I will copy this information to the British Caving Association for review. It is not my place to advise on caving policy for the UK.

I have been offered the chance to use a rope tester to look at potential replacements for the BotB. The test will give us a chance to see how a replacement compares with the existing methods in terms of strength and durability.
The French’s preferred option – the Fusion Knot, which has also been called the Karash Knot, is high on the list of contenders but I feel that a similar knot to the BotB would be more easily absorbed by the caving community. Both myself and Jez think a closer look at the Double-Bowline tied on the Bight may give a very good alternative. We’re calling it a D-BoB, or Double Bowline-on-the-Bight, until we can establish its correct name.

Testing a method of failure with a Bowline-on-the-Bight knot

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A recent video clip has come to light on UKCaving Forum from the French Caving School which identifies a method of failure for the Bowline-on-the-Bight knot. The School’s film shows a failure that could lead to serious injury or even death when clipped into only one arm of the Y-hang.
I have not seen or heard of any occasions where this failure has happened in the U.K. and have certainly not seen it with my own eyes until today’s tests. It is rare for such a statement to come in regards to such a well used technique so I had to investigate.
We made use of the excellent training facility at Pindale Farm in the Peak District and the not so excellent weather.

Background:
The Bowline-on-the-Bight or ‘BotB’ knot is widely used and taught as a preferred knot for most SRT rigging applications involving Y-hangs and SRT. The knot has certain shock absorbing and self equalising properties that make it a good choice. These traits form one side of a double edged sword, the other side being the knot is easy to undo and therefore easier to loosen in use. It is this property of the Bowline knot family that can lead to slipping failures like the one we are looking into here.
For anyone who does not know the BotB knot, I will not explain it here as there is a wealth of literature and information on them online. Spend 5 mins Googling or pick up a copy of a UK caving manual.

The Failure:
The problem occurs when a caver is clipped into one arm of the Y-hang formed by a BotB. The two arms are formed from different parts of the knot, one coming from the pitch rope up into the knot, twisting round and emerging to form a loop and the other from the traverse line or on occasion a stopper knot.
Being clipped solely into the arm formed from the pitch rope can cause the rope to pull through and effectively untie the knot when loaded by the caver. If on a straight pitch with no other attachment the caver could fall to the floor. Even with a re-belay or end of rope knot present to stop the rope being pulled totally up the slip can still occur. I ruined a length of rope today, wearing through the sheath on a slip of less that 2 metres.

Observations:
We managed to work through a set of differing conditions and factors to arrive at a scenario where we could generate an almost 100% failure rate. We used the same rope throughout the tests, although the knot was tied in different sections to prevent too much damage to the same area.
The rope was a 5 year old length of 10.0mm semi-static rope widely used by cavers. We also tested a length of smaller diameter rope from another brand with similar results. The rope was stiff but could still form and hold a knot well.
We varied the knot for each test in the following ways:

  • Dressed correctly or not
  • Tightened or not
  • Evenly loaded or not
  • Clipped into left or right arm
  • Clipped into both arms
  • 10 & 9mm rope tested
  • Wet and dry

In the outdoor world we use a accident analogy called the ‘Lemons’. If you imagine a slot machine is your work and the tumblers and their ‘fruits’ are the small events that happening, the lemons are the poor practice, bad judgement or bad events.
As a lemon appears on a tumbler it is usually outweighed by a good fruit, i.e. a bad thing/poor practice is safeguarded by the other good practices along with it and the chance of an accident is low.
When all the lemons come up on a line together we have a series of poor decisions or events that collectively form a chain of events and choices which dramatically increase the chance of having an accident. Hence, it is better to follow the adage of prevention being better than cure. It is this analogy that I think fits perfectly with this method of failure in the BotB.

When the following conditions were met we experienced a near 100% failure rate.

  • Wet rope (very high rate of failure on dry too)
  • Knot not fully tightened
  • Y-hang arms not equally loaded – specifically the one formed by the pitch rope
  • Caver only being clipped to the loop formed by the pitch rope
  • A dynamic fall similar to someone slipping on the lip of a pitch
  • Knot recently tied – i.e. first person down after rigging.

It is worth noting that we did have other failures with different test conditions but on a far smaller percentage of tests. The loose arm being the common factor in all of our testing.

Best Practice:
We teach what is known as best practice. This ideal of coaching changes and we are constantly evolving our advice. This is one of the reasons I went and did these tests today, to ensure that what I did was still the best it could be.
Current best practice advice for SRT, rigging and progression where relevant to this article is as follows:

  • Rope used should be of good condition and supple enough to hold a knot well
  • The rigging should be tight and all knots dressed correctly and tightened down before use
  • Y-hangs should be loaded equally
  • The caver should always have their cowstails clipped into both arms of Y-hangs (you’ll notice we use larger krabs than most for this reason)

Conclusions:
We experienced no test where a failure occurred when all of the best practice conditions above were met.
Failures did occur in other tests, the chance was lower unless all of the ‘Lemon’ factors were involved.

Under no circumstances should you use a technique that is unfamiliar to you. Get trained, get experienced, get informed.