Bowline on the Bight - A Method of Failure
- Thread starter Pete K
- Start date
Pete K
Well-known member
It appears that members of the forum of the International Guild of Knot Tyers agree that the name of Double Bowline on the Bight is correct, despite the knot of the same name in ABOK. That knot may be the same one with a variation on the loops. I'll stick with DBoB then.
Bob Mehew
Well-known member
First an apology for not responding earlier but I have been away since we did the work and with limited access to the web over the past few days.
I take Stu?s point about the use of ABoK numbers but was unwilling to do so before I have confirmed the method used to tie the knots does actually reflect that cited in ABoK. (I do have the instruction set used to make each knot.) I will also admit to running out of rope to test an Alpine Butterfly (you will be amazed at how much rope we consume). I have a desire to revisit the rig and test it along with some other work.
IanBall?s question about difference in forces is difficult to answer without recourse to showing the peak forces for every drop which is work in hand. Whilst I am confident that the differences seen in the peak forces are real, I fear that if we repeated the work we would find a fair spread of values. But I am not prepared to guess on what that spread might be as we have yet to do much work on repeatedly dropping samples of rope to obtain a feel for the situation. The Drops Survived column are best used to cross compare between knots tested here rather than with other tests on other kit. As stated, the rope in the fusion knot sample broke within the Fusion Knot. The two loops were not ?destroyed? but I have no data on whether either of these loops could hold any load.
I should say that my prediction was that the Fig 8 lknot (fig 8 on bight / ABoK 1047) sample would break around 3 or 4 drops based on previous experience. (Remember we are using 100kg rather than 80kg for Type B 9mm rope.) I anticipated that the Y Hang knot samples would break at the fig 8 end but expected it to do so at somewhat more than 3 or 4 drops survived. I base this on a bit of theory that the degree of strength of a knot is inversely related to the acuteness of the angle of the first bend of the rope from the active end, see http://iopscience.iop.org/1367-2630/3/1/310 . That is the tighter the bend on the active end as it enters into the knot, the lower the force that will break it. So with these Y Hang knots having two ropes in place of one for most of their presence with the knot, my expectation was that the degree of bending would be less than that seen in the simple fig 8 knot. Also I was expecting the Y hank knots to soften the peak forces experienced because they take much more rope to to tie.
One thing which does worry me is the degree of glazing which appeared on the active part of the rope coming out of the standard Bowline on Bight and to a lesser extent, the furriness which similarly appeared on the other Y Hang knots. I think that these knots react to a dynamic drop by pulling some of the rope through from one loop to the other and also from the other loop into the active end of the rope. That degree of movement is rubbing the rope in a deleterious way which may be a cause for the lesser number of drops survived. Pure speculation at this point in time, but one of those things which we could usefully go back and look at in more detail. (We did not evolve a reasonable method of looking at rope movement within the knot and its loops until we got onto testing across loops.) I would like to think that the Y Hang knots were all much stronger than the simple fig 8 knot but repeated experiments might show otherwise. One thing which does stand out is that if one does fall on a Y Hang knot, then retying it if at all possible so as to move the parts of the rope within the knot out of the new knot would be a good thing.
Chocolate Fireguard has answered queries about test set up so I won't comment further.
Mikem makes some good points but I rather do the testing than speculate. I don?t however go with the idea of varying strength of rope along its length for a new rope. Could you however clarify your bit about what type of bowline v double bowling? Am I right in thinking you mean a standard Bowline (ABoK 1010 / 1716 as per Wikipedia). If so, I have not done any testing on such bowlines.
I can see we need an agreed set of terminology. I accept that my use of ?standard? was probably redundant but I felt it conveyed a sense relating to the more common type of bowline on the bight. Active and inactive loop are new and perhaps not immediately understandable. Possibly a diagram or labelled photo are required but pressure of time precluded doing it for the initial comments.
Wet testing might well change things quite dramatically if only because I expect it to reduce glazing within the knot. Experience from fig 8 knots, indicates that on soaking a rope for at least 2 hours will usually reduce the dynamic test drops survived performance by a factor of around 2.
There is still a lot of work to be done in analysing the data obtained, in part because I hold over 600MB of data and images. So it is going to take some time but I feel the key points have been brought out. If you are interested, I have offered to do a lecture on this work at the BCRA Cave Technology Symposium over 20-21 April 2013 in South Wales.
I take Stu?s point about the use of ABoK numbers but was unwilling to do so before I have confirmed the method used to tie the knots does actually reflect that cited in ABoK. (I do have the instruction set used to make each knot.) I will also admit to running out of rope to test an Alpine Butterfly (you will be amazed at how much rope we consume). I have a desire to revisit the rig and test it along with some other work.
IanBall?s question about difference in forces is difficult to answer without recourse to showing the peak forces for every drop which is work in hand. Whilst I am confident that the differences seen in the peak forces are real, I fear that if we repeated the work we would find a fair spread of values. But I am not prepared to guess on what that spread might be as we have yet to do much work on repeatedly dropping samples of rope to obtain a feel for the situation. The Drops Survived column are best used to cross compare between knots tested here rather than with other tests on other kit. As stated, the rope in the fusion knot sample broke within the Fusion Knot. The two loops were not ?destroyed? but I have no data on whether either of these loops could hold any load.
I should say that my prediction was that the Fig 8 lknot (fig 8 on bight / ABoK 1047) sample would break around 3 or 4 drops based on previous experience. (Remember we are using 100kg rather than 80kg for Type B 9mm rope.) I anticipated that the Y Hang knot samples would break at the fig 8 end but expected it to do so at somewhat more than 3 or 4 drops survived. I base this on a bit of theory that the degree of strength of a knot is inversely related to the acuteness of the angle of the first bend of the rope from the active end, see http://iopscience.iop.org/1367-2630/3/1/310 . That is the tighter the bend on the active end as it enters into the knot, the lower the force that will break it. So with these Y Hang knots having two ropes in place of one for most of their presence with the knot, my expectation was that the degree of bending would be less than that seen in the simple fig 8 knot. Also I was expecting the Y hank knots to soften the peak forces experienced because they take much more rope to to tie.
One thing which does worry me is the degree of glazing which appeared on the active part of the rope coming out of the standard Bowline on Bight and to a lesser extent, the furriness which similarly appeared on the other Y Hang knots. I think that these knots react to a dynamic drop by pulling some of the rope through from one loop to the other and also from the other loop into the active end of the rope. That degree of movement is rubbing the rope in a deleterious way which may be a cause for the lesser number of drops survived. Pure speculation at this point in time, but one of those things which we could usefully go back and look at in more detail. (We did not evolve a reasonable method of looking at rope movement within the knot and its loops until we got onto testing across loops.) I would like to think that the Y Hang knots were all much stronger than the simple fig 8 knot but repeated experiments might show otherwise. One thing which does stand out is that if one does fall on a Y Hang knot, then retying it if at all possible so as to move the parts of the rope within the knot out of the new knot would be a good thing.
Chocolate Fireguard has answered queries about test set up so I won't comment further.
Mikem makes some good points but I rather do the testing than speculate. I don?t however go with the idea of varying strength of rope along its length for a new rope. Could you however clarify your bit about what type of bowline v double bowling? Am I right in thinking you mean a standard Bowline (ABoK 1010 / 1716 as per Wikipedia). If so, I have not done any testing on such bowlines.
I can see we need an agreed set of terminology. I accept that my use of ?standard? was probably redundant but I felt it conveyed a sense relating to the more common type of bowline on the bight. Active and inactive loop are new and perhaps not immediately understandable. Possibly a diagram or labelled photo are required but pressure of time precluded doing it for the initial comments.
Wet testing might well change things quite dramatically if only because I expect it to reduce glazing within the knot. Experience from fig 8 knots, indicates that on soaking a rope for at least 2 hours will usually reduce the dynamic test drops survived performance by a factor of around 2.
There is still a lot of work to be done in analysing the data obtained, in part because I hold over 600MB of data and images. So it is going to take some time but I feel the key points have been brought out. If you are interested, I have offered to do a lecture on this work at the BCRA Cave Technology Symposium over 20-21 April 2013 in South Wales.
If it's not due to variation in the rope or the tying of the knot, then it is quite worrying that a double loop (fusion) knot would break before a single loop (fig 8). Yes I do mean comparing a standard bowline to one with an extra wrap (maybe also a water bowline) to see if using the double bowline does double the drops survived...Bob Mehew said:
Mike
The other knots didn't fail, the fig 8 at the opposite end of the rope did, the Fusion/Karash was the only knot that failed within the double loop. This seems to contradict Karash's claims that it is as strong as a "bunny ears" fig 8 knot, but I guess the knot was loaded with both loops pretty much in line whilst the current tests have quite an angle between the loops.
Mike
Mike
r_walklate@hotmail.com
New member
Is it a regular thing that ropes and anchors are exposed to these forces? What are the alternatives? Give up caving? The way that bowline on the bight has been discussed over last few weeks we should expect several deaths from people caving this weekend?!! I am not naive enough not to realise limitations but I really don't see what all of this achieves?
Stu
Active member
mikem said:
Thanks Mike. Finally got to see the videos and hadn't picked up on the Karash itself failing.
r_walklate said:
I was pondering this the other day. The testing conducted has left me puzzling as to how "real world" it is. Effectively what has been tested is quite an unlikely scenario: the load (caver) has fallen (FF1) on the pitch rope. I'm not sure this is an applicable test (apart from it's interesting watching stuff break).
Thinking about when such loads might be applied, got me to caver jugging up pitch, nears the top knot, disengages Croll and then slips and is then held by top jammer (or cowstail - though this is getting into the realms of having to test every possible option for where one clips their cowstail krab) - whichever kicks in first. If the top jammer took the hit I'd say that the sheath probably being stripped is going to adversely affect any system of rope and knots well before the rope and knots become the problem:
(Given that this type of ascender is designed to grip the rope without slippage, the only way
that the energy of the fall can be absorbed is by the stretch of the rope and sliding of the
sheath down the core if, or when, it is severed. It is, therefore, as much a test of the rope
used as of the device itself. All devices of this type will cut the sheath in an impact of this
severity. The fall is only arrested when the sheath bunches and grips the core, usually after
about a metre of slippage - Lyon/HSE testing).
that the energy of the fall can be absorbed is by the stretch of the rope and sliding of the
sheath down the core if, or when, it is severed. It is, therefore, as much a test of the rope
used as of the device itself. All devices of this type will cut the sheath in an impact of this
severity. The fall is only arrested when the sheath bunches and grips the core, usually after
about a metre of slippage - Lyon/HSE testing).
Lyon found that cutting of the sheath occurred at anywhere between 4 - 6 Kn impact force consistently.
The other scenario was lifelining up a climb or ladder where the belayer has had a brain fart and forgot to take in. But then you'd have to factor in the belay device of choice having some slippage which may "soften" the blow.
If what we want is to know how strong a knot is wouldn't a straight-forward pull test be more appropriate (is there enough of this data available elsewhere)? Or maybe considering the scenario that brought all this about; testing of falls on single loops of Y hangs and falls on two loops of a Y hang. This is a more likely scenario surely?
Lessons that could be drawn out are:
- Don't put yourself in situation where FF1 can occur - which was known about anyway.
- If a part of the system of ropes is subjected to a FF1 then release and retie that part - again advice which was always given about cowstails.
I only bring this up as a means of philosophical critique.
Pete K
Well-known member
r_walklate
The French Caving School went so far as to advise cavers against using Bowline on the Bight knots and suggested a replacement. This is on a par with the BCA telling cavers to stop using Crolls etc...
I wanted to investigate for my own personal knowledge. I posted YouTube and stuff up and Blogged some personal opinions. Bob thought it a good idea to look at the French knot along with some others, which we did. It's all in the name of finding out.
At the end of the day nothing has changed. The BCA advice is still the same. My advice is still the same. A load of cavers had a knee jerk and then settled down. I did this stuff initially for me, I just chose to make public my thoughts and investigation.
We now have a well publicised method of failure (which loads of people apparently didn't know about) and some suggested replacements for those who wish to move on from using the BotB.
What does this achieve? Information for people to use to make their own choices. If you're not bothered don't read it.
The French Caving School went so far as to advise cavers against using Bowline on the Bight knots and suggested a replacement. This is on a par with the BCA telling cavers to stop using Crolls etc...
I wanted to investigate for my own personal knowledge. I posted YouTube and stuff up and Blogged some personal opinions. Bob thought it a good idea to look at the French knot along with some others, which we did. It's all in the name of finding out.
At the end of the day nothing has changed. The BCA advice is still the same. My advice is still the same. A load of cavers had a knee jerk and then settled down. I did this stuff initially for me, I just chose to make public my thoughts and investigation.
We now have a well publicised method of failure (which loads of people apparently didn't know about) and some suggested replacements for those who wish to move on from using the BotB.
What does this achieve? Information for people to use to make their own choices. If you're not bothered don't read it.
r_walklate@hotmail.com
New member
Pete K. "if your not bothered don't read it"
It's a risk you take when joining a forum!!! As is the risk of having an opinion and somebody not agreeing with you.
Watching ropes break are good fun though, just as long as your not on it!!!
It's a risk you take when joining a forum!!! As is the risk of having an opinion and somebody not agreeing with you.
Watching ropes break are good fun though, just as long as your not on it!!!
SirKnotcelot
New member
ianball11 said:
Note that in all cases there are two parallel ends
exiting the knot, but it is generally NOT indicated which
end is to be loaded (the "active" / "live" / "long" end)!!
(Ditto for the overhand eye knot as well.)
Does it matter? Well, what's the point of testing
--maybe to answer that question!
-SirK*
Loading of either end has been extensively tested in the past with at least one study finding that the end that passed around the outside of the knot was stronger in an overhand, the end that last passed around the inside of the knot was stronger in a figure 9 knot & made damn all difference in a figure 8 knot. Double loop knots add a whole other level of complexity with the variations of size, angles, tidyness and tightness of the knots...
Mike
Mike
r_walklate@hotmail.com
New member
Volunteers in whatever capacity do deserve a pat on the back.
SirKnotcelot
New member
One might say that even the testing done just now
is a considerable lot --and I think you'd agree. But
if only one knot were tested and the report out read
"double-eye knots ...", readers should rightly want
to know which ... ; and my point is that they should
be no less attentive as to details of how knots were
tied/loaded.
Really?! I'm aware of just two reports that indicate
that the aspect was even noticed : long back, the
Ontario Rock Climbers' Assoc. Safety(?) Manual
asserted that one way (loaded strand bearing upon
its twin) was 6-10 %pt.s stronger; and the Lyon
report --which I surmise is your referent-- came out
as you noted (bearing upon, pulling away from, and
didn't matter (O , 9 , 8 , resp.). The ORCA reference
was for the fig.8, btw.. Dave Merchant's testing led
him to make some similar claim, though he has yet
another, somewhat irregular orientation of the knot
(still symmetric).
But, for the most part, such aspects are not noted,
and I'll wager not noticed. (And the "with at least one..."
suggests this lack of results : otherwise, we must ask
was there any consensus!)
In the case of the (single eye) bowline, tying it with
the tail "inside" --i.e., the usually presented, "right" way--
yields a knot that is vulnerable to failure (slipping out)
on ring-loading; tying it with the tail outside (sometimes
mis-referred to as a "Dutch Naval" or "Cowboy" or, by
Ashley, "Left-handed" --to be taken pejoratively-- bowline)
significantly prevents such slippage. (The effective knot,
in such loading, can be seen as incorrectly / correctly tied
Lapp bends.) Though I just saw that if the usually loaded
("active") part is also loaded, both versions can fail
--this, by manual loading of 8mm, so way less than
any fall force (but this is for a particular positioning
of the knot : active line beside its continuation
into the eye, but not were it beside the tail's side
of the eye).
This consideration of loading applies directly to the current
BotB issue --either the loaded eye feeds into the collar
from one direction, or the other : the vulernability might
exist in both, but be greater in one vs. the other. If one
group tests it one way and the other the other way, we
should expect different results; we should though understand
why the results differ, and not shrug it off as some vagaries
of testing or ropes (though ropes could well have differences!).
In bowline-vs-fig.8 debates, one can see sometimes
people asserting how even if the last tuck of the fig.8
is undone, it is still secure; do they consider that the
different loadings of the knot imply differences on
what knot results from undoing the last tuck?
(In terms of Ashley, it's roughly #1057 & #1058,
though the latter is ambiguously illustrated and not
quite the arrangement that will commonly result).
-SirK*
