Bowline on the Bight - A Method of Failure

[Pete K]

@Cave_Troll playlist sorted, thanks for spotting that.

As mike says above, the rig has to be set that way. The apparent missalignment of the drop weight as the tests progress with each knot is not a fault of the rig, it is that the bowline variants all acted like a slip knot for the loop formed by the active end and essentially 'unequalised' themselves.

I'm nowhere near as experienced to answer some of these questions as Bob so hopefully he will pop along soon.

@ianball11

The Peak Force is the ammount of force transmitted to the other components in the system (anchors/caver etc...). This figure is relative in these tests, so a lower peak force shown is a greater amount of shock absorbtion through slip etc of the knot. The higher the peak force, the less slip and absorbtion in the knot.
10kN is equivilant to 1000Kg of force. 0.5kN is equivilant to 50Kg of force.

Drops survived give an indicator of the knot's (and rope for that matter) ability to repeatedly absorb shock. The failiure comes when the rope can give no more elasticity to absorb or heat has damaged it enought to make it weak and break.
Essentially a knot with a higher number of drops survived would be a good choice where a shock load risk could occur, i.e. cave bolt failing etc...

@jopo

Clip your cowstail carabiner into a single strand of each loop, thus connecting you to both loops, not entirely around one. Very easy to do on Bowlines, harder on Fig8's.

@stu

I see your point about the Fig8 loop / tied on the bight. I'll pass that on to Bob who wrote the report.

Can I just remind everyone that this was one set of testing only and cannot statictically prove a trend. Yes the DBoB survived mentionably more drops than the others in this test but even Bob thinks we'd need dozens and dozens of tests to be able to make a solid statement.

The BCA have made comment on the BotB. The French have chosen one route. We have provided film and testing. Cavers will choose to continue to use it or not, it is up to them. I hope that the research we have managed to do recently will just allow everyone to make an informed choice on knot selection.
If in doubt speak to an experienced club rigger or a CIC.

 

[mikem]

Has any testing been done on normal bowline versus double bowline to see what effect the second wrap has?

Mike

 

[Jopo]

No one has suggested clipping in to both strands of a single bight, they are using the strands from separate loops.

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.

Perhaps it would be much clearer if it the term 'into the loop of each bight' was used.

Jopo

 

[Pete K]

Has any testing been done on normal bowline versus double bowline to see what effect the second wrap has?

Mike

Not by us. The testing is a very long winded and time consuming process. The guys like Bob and Roy are working on their own time in sheds. I think the list of testing that we would have to do alone to give a statistically sound set of data would take months. The number of testable variables is mind boggling. If anyone wants to help they could perhaps convince Lyon to do some testing. DMM are producing caving kit now, they may be up for some fun too.

My gut feeling was correct about the Double Bowline on the Bight and I suspect a Double Bowline would survive more drops than the Standard Bowline. However, in Bob's world of scientific rope testing, I am very much a beginner.

 

[ianball11]

Thanks Pete, lower the better.

What do you think is indicated if anything on a small sample size, of the fusion knot breaking at the knot, when all the other broke at the other end?

 

[mikem]

My gut feeling was correct about the Double Bowline on the Bight and I suspect a Double Bowline would survive more drops than the Standard Bowline. However, in Bob's world of scientific rope testing, I am very much a beginner.

It should survive more drops, as the wrap provides additional shock absorption, but I'm wondering whether this alone could account for doubling the number of drops survived in the tests? It's more likely that the fig 8 knot was tied in an abnormally strong section of rope.

What do you think is indicated if anything on a small sample size, of the fusion knot breaking at the knot, when all the other broke at the other end?

On a single test it could be a weakness in that section of rope or maybe the fusion knot creates a narrow radius turn in the knot when loaded, either way, all the knots are strong enough to stop a falling person...

Mike

 

[Stu]

I think you may have proved your own point Stu.

I read it as:

Fig 8 on bight (AB#1085 Fig8 Y-hang, bunnyears)

Fusion  (Pete Knight Blog-Fusion )

Double Bowline on the Bight ( Pete Knights Blog-DBotB)

Standard Bowline on Bight (AB#1080, the knot in question)

Fig 8 loop (AB#1047,  Flemish Loop)

Now I read it like that it makes much more sense. The "standard" of standard BoB is redundant really.

 

[Stu]

Has any testing been done on normal bowline versus double bowline to see what effect the second wrap has?

Mike

In terms of overall strength/slippage? Most searches throw up stronger (or less weak) results but figures aren't referenced so 

 

[Stu]

The testing is a very long winded and time consuming process. The guys like Bob and Roy are working on their own time in sheds. I think the list of testing that we would have to do alone to give a statistically sound set of data would take months.

Not wishing to be critical; is there anything new being found out in these tests? There is plenty of referenced tests on knots such as the Fo8 loop, though I concede not being a statistician it might be that duplication of tests has to be done every time to provide some comparable control 

My feeling is that testing in abnormal ways i.e. the abusive ways people use knots (out of the box testing) might be of more real world use. We know that Fo8 loops can roll when used to join ropes, so they aren't used.  How about testing single loops of double loop knots such as if an anchor fails.

Add to the mix: what effect does water have on ropes? Didn't Sterling ropes in USA show ropes are increasingly weak when wet?

 

[Stu]

Geek alert.

What is being termed Double Bowline on the Bight (Pete's reference http://www.peakinstruction.com/blog/knots/how-to-tie-a-double-bowline-on-the-bight). That name is already taken ABoK#1083. You end up with four loops.

 

[Chocolate fireguard]

In the real world the bights are fixed to the anchor points not the falling component. I am curious as to why the test was done upside down.
If you watch carefully the knot becomes unequal which must mean that the strands are moving within the knot, I suspect this causing a increasing unevenness of the load as the test progresses. It may be insignificant but I do wonder why this method was chosen.

The load cell is at the top of the rig, recessed up in the centre and well protected. The signal lead to our instrumentation is therefore stationary and can also be well protected.
It would have been possible to have attached a cell (we have 2 of them, nominally identical) to the falling mass and had the rope as a Y but the cell would have been falling with the mass with all the problems that would have brought for the cell and the lead. Also there is a lack of a suitable pair of strong points at the top.

We are happy that our method made no difference to the forces involved for 2 reasons, one practical, the other theoretical.

Several times over the past couple of years we have used both load cells, one at the top and one on the falling mass. Usually with about 1m of rope with a Fig 8 knot top and bottom.
It was always a bit nervy, as we were worried about the bottom cell and its lead.
The only times we saw any sensible difference between the forces top and bottom were during knot-slip events (which show up as a momentary fall and rise in the force as one or other of the knots tightens - often more obvious on stiff old rope) and just after the rope broke. These last for a couple of milliseconds, whereas the whole force/time trace is typically a couple of hundred milliseconds. These are expected, because during knot slip and after the break the rope is certainly experiencing high acceleration for short periods, so there must be a net force in one direction or other. But apart from these very short events the forces at each end of the rope will be the same to as close as we can measure.

The second reason is that if there was even a 1% difference between the forces top and bottom, over the whole event (where the forces  typically rise from zero to 10000N in a tenth of a second) then the acceleration of the 0.1kg of rope would take it about 2.5m up or down. Not sensible.

So although I agree that it might look as though the upside-down method will affect the results, the forces on each end of the rope will be the same as they are in the normal orientation.

That being the case, I think that movement of the strands within the knot (which certainly happened) would have happened whichever way up it was tested.

 

[Pete K]

My feeling is that testing in abnormal ways i.e. the abusive ways people use knots (out of the box testing) might be of more real world use. We know that Fo8 loops can roll when used to join ropes, so they aren't used.  How about testing single loops of double loop knots such as if an anchor fails.

Add to the mix: what effect does water have on ropes? Didn't Sterling ropes in USA show ropes are increasingly weak when wet?

We did drops across the loop of the DBotB, Fusion and BotB. Slipping occured but no fail on the VERY small series of tests we did.

Ropes are weaker when wet. For testing, sections cut from a dry rope can be regarded as identical, whereas sections from a wet rope may have differing moisture content.

Good spot on the name. Any suggested correction?

 

[Stu]

We did drops across the loop of the DBotB, Fusion and BotB. Slipping occured but no fail on the VERY small series of tests we did.

Ah. Is that the Active/Inactive results on the table?
Where loops C & D are mentioned are they just the left and right loops (tested in tandem)?

Ropes are weaker when wet. For testing, sections cut from a dry rope can be regarded as identical, whereas sections from a wet rope may have differing moisture content.

http://www.blackdiamondequipment.com/uploads/black-diamond/files/Pages%20from%20wet%20rope%20article.pdf%20-%20Adobe%20Acrobat%20Pro.pdf Blimey...

Good spot on the name. Any suggested correction?

Not a fricking clue. You invented it (and ABoK isn't showing anything similar and neither does Google), think you get the honour  Double Loop Double Bowline or Pete's Knot 

 

[Pete K]

Ah. Is that the Active/Inactive results on the table?
Where loops C & D are mentioned are they just the left and right loops (tested in tandem)?

Thats it. We had a play and any real testing would need to be done more thoroughly. I'd be interested to drop some more stuff simulating bolt failures and cross loads etc, but again the work load would be huge.
C&D lengths are the measured arms of the y-hang, so yes, left and right loops.

Thanks for the ego massage. I've spoken to other cavers who have seen the DBoB before so I can't claim invention. Is ABOK a cited reference book of knots that is generally accepted as gospel? I only ask as the DBotB name does seem to fit correctly within the Bowline name structure.

 

[Stu]

Is ABoK the bible? Ask this lot http://www.igkt.net/ they love knots!! Even more than we do 

Ashley's reference is pretty universal and after that Geoff Budworth took up the mantel. None of it's fixed in stone though.

 

[ianball11]

I wonder if Scouts use AB numbers, they are known for their knots.

 

[Pete K]

Geek alert.

What is being termed Double Bowline on the Bight (Pete's reference http://www.peakinstruction.com/blog/knots/how-to-tie-a-double-bowline-on-the-bight). That name is already taken ABoK#1083. You end up with four loops.

Having Googled and spent some time reading I think what we are calling the Double Bowline on the Bight is correct. The ABoK#1083 knot seems to be some form of French Bowline and does not fit with the way the other knots are named.
For instance:
Take a Bowline, tie it with 2 internal turns and you get a Double Bowline.
Take a Bowline on the Bight, tie it with 2 internal turns and you should get a Double Bowline on the Bight.
Look at a Double French Bowline - that appears to be the ABoK#1083.

I'm content to go with the current naming as I think it more correctly reflects the Bowline naming pattern that everyone is familure with here. Of course a name is just a name. People have been mixing up BotBs, Double Bowlines and Triple Bowlines for years.
Perhaps I can finally apply for my Cub Scout knots badge.

 

[Stu]

Perhaps I can finally apply for my Cub Scout knots badge.

Submit it to the Uber-knot geeks! You might even get a medal!

 

[ianball11]

Abridged Ashley Book of Knots


Although interesting, the section of interest is removed in the free preview here.

 

[Stu]

Now I have the book to hand and not a PDF on my phone I can see it better. Pete is right and his knot is still nameless. DBotB fits.