Author Topic: Which knots for Y hangs?  (Read 2698 times)

Offline mikem

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Re: Which knots for Y hangs?
« Reply #50 on: February 16, 2019, 07:45:16 am »
The single strand isn't unsafe to clip, as it is coming from a traverse line. It's when it is coming from a single bolt that it is risky (although modern bolts are much less likely to fail than spits were, so maybe rock fall is now the greatest potential problem outside the control of the caver).

There are numerous traverses that come in above the level of the Y hang & having the line go straight there would mean a higher fall factor on your cowstails if you slipped...

Offline Jon

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Re: Which knots for Y hangs?
« Reply #51 on: February 16, 2019, 09:50:16 am »


Although if we wait for 80% usage surely nothing will ever change?

When taking clients they won't know what they are looking at and rigging with minimal rope and loops and one easy to clip central belay loop has its advantages.

On wide y-hangs the saving in rope length over a traditional 2 loop bunny ears is helpful and it's far easier to clip into than the two loops of a bunny ears of almost any variety which also negates the need for a central "master" krab for clients to clip into.

I would have thought if you were taking 'clients' on an SRT course/trip it would be paramount that they have a good understanding of what they are looking at in terms of safe rigging.

Mark

Depends on their experience really. I can imagine various situations where they wouldn't be interested in rigging, either due to training time constraints or what they want out of the trip.

Offline GT

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Re: Which knots for Y hangs?
« Reply #52 on: February 16, 2019, 10:15:39 am »
Another consideration is how the knot performs when loaded; does it cinch up or pull apart when loaded.



(I know my spelling; bite/bight etc. is terrible...)

Online Mark Wright

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Re: Which knots for Y hangs?
« Reply #53 on: February 16, 2019, 04:57:34 pm »
I wouldn't consider the single strand as being unsafe to clip into with a cow's tail. We are all happy enough to be attached to a single rope with a toothed ascender that will probably break the rope sheath at +/- 450kg. This could easily be achieved with a 100kg caver falling just 45cm regardless of the Fall Factor.


I would argue any traverse line approach you have to put an ascender on is awkward. Ideally, traverse line should be fairly tight, which precludes using your descender to approach the pitch head and makes ascending tricky. If the pitch head is that much lower than the traverse bolts, then possibly a short rebelay from the traverse bolt/bolts would be easier anyway, but it is often much easier if the traverse line goes straight to the pitch head.

Leaving the traverse line slack enough so that you can reach means you don't have a tight traverse line and you use extra rope anyway.

The first drop going from the right-hand wall at the top of Aldo's in the Berger is where I have regularly used an ascender just to give me that extra 25cm or so to reach the traverse line on the left-hand wall. I totally agree this is not ideal rigging and can be quite awkward but the traverse is higher than the pitch head so there isn't much choice. We just need to be sure we are being as safe as is reasonably practicable.

I would argue that a tight traverse line is unsafe due to the significant Vector forces which could be applied to the traverse line and its anchors. If a tight traverse line gave a 160 degree angle at the point of suspension then the resulting load at each anchor point would be +/- 288kg with a 100kg mass. If your cow's tail reduced the potential falling forces on you to, e.g. 4.5kN then the traverse lines and their anchor points would be subjected to a force which could be alarmingly close to the breaking strength of a typical 9mm rope with Fig. 8 termination knots!

The forces applied to traverse lines and their anchors rigged tighter than this will increase exponentially. A 100kg mass hanging (not falling) on a traverse line with an angle of, e.g. 179 degrees could result in anchor point loads of +/- 5,747kg!!! This figure would be halved with a 178 degree angle but still way in excess of the rope's breaking strength.

In an industrial environment, and professional caving would come into that category, traverse lines should never allow angles greater than 120 degrees. 10% sag between anchors would usually achieve the 120 degrees. Safety nets should always be rigged with 10% sag for this reason. 

Depends on their experience really. I can imagine various situations where they wouldn't be interested in rigging, either due to training time constraints or what they want out of the trip.

I would argue that time constraints are irrelevant in a professional SRT instructional caving situation and whether 'clients' are interested in rigging or not is no excuse for disregarding best practice. I'm not the one who would be stood in front of the judge trying to explain why the 'client' died, I would more likely be the expert witness for the prosecution. 

I sat as the IRATA representative on the HSE Advisory Committee for Work at Height Training prior to the introduction of the 2005 Work at Height Regulations. When the 2007 amendment was introduced allowing the outdoor pursuits industry to use single ropes, the author of the above regulation made very clear to me that the full force of the law would bear down on the person in charge should that single rope fail. In effect, outdoor pursuits instructors have an obligation to be extra-extra careful. This means the following of 'best practice' to the book.   

Having recently seen an example of the supposedly professional caving 'best practice' documents, I would suggest the best place to find out about industry best practice would be from the IRATA International Code of Practice, freely available from the IRATA website, www.irata.org.

Mark


Offline Jon

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Re: Which knots for Y hangs?
« Reply #54 on: February 16, 2019, 05:21:26 pm »


I wouldn't consider the single strand as being unsafe to clip into with a cow's tail. We are all happy enough to be attached to a single rope with a toothed ascender that will probably break the rope sheath at +/- 450kg. This could easily be achieved with a 100kg caver falling just 45cm regardless of the Fall Factor.


I would argue any traverse line approach you have to put an ascender on is awkward. Ideally, traverse line should be fairly tight, which precludes using your descender to approach the pitch head and makes ascending tricky. If the pitch head is that much lower than the traverse bolts, then possibly a short rebelay from the traverse bolt/bolts would be easier anyway, but it is often much easier if the traverse line goes straight to the pitch head.

Leaving the traverse line slack enough so that you can reach means you don't have a tight traverse line and you use extra rope anyway.

The first drop going from the right-hand wall at the top of Aldo's in the Berger is where I have regularly used an ascender just to give me that extra 25cm or so to reach the traverse line on the left-hand wall. I totally agree this is not ideal rigging and can be quite awkward but the traverse is higher than the pitch head so there isn't much choice. We just need to be sure we are being as safe as is reasonably practicable.

I would argue that a tight traverse line is unsafe due to the significant Vector forces which could be applied to the traverse line and its anchors. If a tight traverse line gave a 160 degree angle at the point of suspension then the resulting load at each anchor point would be +/- 288kg with a 100kg mass. If your cow's tail reduced the potential falling forces on you to, e.g. 4.5kN then the traverse lines and their anchor points would be subjected to a force which could be alarmingly close to the breaking strength of a typical 9mm rope with Fig. 8 termination knots!

The forces applied to traverse lines and their anchors rigged tighter than this will increase exponentially. A 100kg mass hanging (not falling) on a traverse line with an angle of, e.g. 179 degrees could result in anchor point loads of +/- 5,747kg!!! This figure would be halved with a 178 degree angle but still way in excess of the rope's breaking strength.

In an industrial environment, and professional caving would come into that category, traverse lines should never allow angles greater than 120 degrees. 10% sag between anchors would usually achieve the 120 degrees. Safety nets should always be rigged with 10% sag for this reason. 

Depends on their experience really. I can imagine various situations where they wouldn't be interested in rigging, either due to training time constraints or what they want out of the trip.

I would argue that time constraints are irrelevant in a professional SRT instructional caving situation and whether 'clients' are interested in rigging or not is no excuse for disregarding best practice. I'm not the one who would be stood in front of the judge trying to explain why the 'client' died, I would more likely be the expert witness for the prosecution. 

Mark

The rest of your post is really useful and thought provoking but I don't understand how you've leapt from an instructor teaching a client SRT but not rigging and then taking then caving to the client dying because they haven't been taught rigging. It's perfectly possible to teach SRT but not how to rig a cave and then take that person caving safely.

Offline Jon

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Re: Which knots for Y hangs?
« Reply #55 on: February 16, 2019, 05:23:48 pm »
Anyway, we're a bit off topic. Seems like I need to persuade someone to drop test the fusion knot and the triple bowline in the configurations mentioned above.

Thanks for all the help.

Offline mikem

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Re: Which knots for Y hangs?
« Reply #56 on: February 16, 2019, 06:21:55 pm »
Also, what happens if you load the fusion with no bolts attached (i.e. that bolt fails)?

Online andrewmc

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Re: Which knots for Y hangs?
« Reply #57 on: February 16, 2019, 07:54:10 pm »
I wouldn't consider the single strand as being unsafe to clip into with a cow's tail.

Because if the other bolt (the one with the double loop) fails there is every chance that your nice big cowstail carabiner will slide over the central knot and other stuff and send you down the pitch.

Quote
We are all happy enough to be attached to a single rope with a toothed ascender that will probably break the rope sheath at +/- 450kg. This could easily be achieved with a 100kg caver falling just 45cm regardless of the Fall Factor.

Have you got a reference for that? (The physicist in me says you mean 450kg-force, but that doesn't really matter).

And happy is a relative term - unless I've got two bolts with minimal extension on failure or a bucket load of rope above me, I am not happy about being on ascenders...

Online andrewmc

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Re: Which knots for Y hangs?
« Reply #58 on: February 16, 2019, 08:11:34 pm »
I would argue that a tight traverse line is unsafe due to the significant Vector forces which could be applied to the traverse line and its anchors. If a tight traverse line gave a 160 degree angle at the point of suspension then the resulting load at each anchor point would be +/- 288kg with a 100kg mass. If your cow's tail reduced the potential falling forces on you to, e.g. 4.5kN then the traverse lines and their anchor points would be subjected to a force which could be alarmingly close to the breaking strength of a typical 9mm rope with Fig. 8 termination knots!

A tight traverse line doesn't mean it has to be a large angle, and in any event large angles are actually really quite hard to do anyway since you might start at 120 degrees for a (small e.g. 1 foot spaced bolts) but once you've actually loaded it it's nearly always less than 90 degrees anyway.

Has anyone ever damaged a rope from overloaded at a tied Y-hang style anchor system with no hauling or mechanical advantage? I think even for a Tyrolean from one wall to another, just tying into anchors on opposite sides of the wall will always end up with so much stretch and slack from the knots that you never end up breaking the rope. I could be wrong on that though.

Having a taut (maybe a better word than 'tight' traverse line reduces the size of a fall from a slip as there is less slack and the attachment point is higher.

The traverse line also only needs a very small amount of slack in it to not take most of the force as well.

Offline Chocolate fireguard

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Re: Which knots for Y hangs?
« Reply #59 on: February 16, 2019, 08:22:36 pm »

I would argue that a tight traverse line is unsafe due to the significant Vector forces which could be applied to the traverse line and its anchors. If a tight traverse line gave a 160 degree angle at the point of suspension then the resulting load at each anchor point would be +/- 288kg with a 100kg mass. If your cow's tail reduced the potential falling forces on you to, e.g. 4.5kN then the traverse lines and their anchor points would be subjected to a force which could be alarmingly close to the breaking strength of a typical 9mm rope with Fig. 8 termination knots!

The forces applied to traverse lines and their anchors rigged tighter than this will increase exponentially. A 100kg mass hanging (not falling) on a traverse line with an angle of, e.g. 179 degrees could result in anchor point loads of +/- 5,747kg!!! This figure would be halved with a 178 degree angle but still way in excess of the rope's breaking strength.


The numbers are correct but misleading in a caving situation.
Caving ropes stretch. A typical figure for 9mm rope is 3.9% for a 150kg load (about 1500N tension in the rope).
Someone with a firm footing and muscles like Garth might get to a third of that if they really tried to rig a tight line (I believe 300N is taken as a good personal contribution on a haul line, but I am willing to be corrected).
So they get perhaps 1.3% stretch. And that's a really tight line!

Someone coming along and putting a little weight on the centre of the line is going to increase the tension considerably - as Mark has pointed out with an included angle of 179 degrees the tension in the rope is over 50 times the weight placed at the middle.
So on the face of it a 20N weight (sack with a few metres of rope in it?) will increase the rope tension by 1kN.
Just as, on the face of it, a heavy person would snap the rope.

The reason these things don't happen is because the rope stretches and the included angle decreases, so allowing the weight to be supported by a much lower rope tension.
In the tight line above an increase in rope tension of 1kN to 1500N would increase the stretch by about 2.5% so the load point at the centre would fall by 22% of the span and the included angle would fall to 155 degrees. At which point the tension in the rope is just over twice the weight being supported.

But coming up with accurate figures for exactly what the tension is in a loaded traverse line is very complicated, and I don't know how it can be done. I have put in quite a few hours on the problem.


Online andrewmc

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Re: Which knots for Y hangs?
« Reply #60 on: February 16, 2019, 08:37:45 pm »
Having recently seen an example of the supposedly professional caving 'best practice' documents,

I'm guessing that unfortunately these documents are not currently in the public domain for our shared critique :p

Offline Hammy

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Re: Which knots for Y hangs?
« Reply #61 on: February 16, 2019, 09:39:35 pm »
Having recently seen an example of the supposedly professional caving 'best practice' documents,

I'm guessing that unfortunately these documents are not currently in the public domain for our shared critique :p

As far as I know the only professional caving ropework best practice document is this one, available in the public domain.

http://british-caving.org.uk/wiki3/lib/exe/fetch.php?media=training:ropework_jan19.pdf

Is this the one you are referring to Mark?


Offline Mike Hopley

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Re: Which knots for Y hangs?
« Reply #62 on: February 17, 2019, 06:30:40 pm »
I wouldn't consider the single strand as being unsafe to clip into with a cow's tail.

I'm puzzled by this. Are you aware that this connection depends entirely on a single anchor, as explained by andrewmc?

Quote from: andrewmc
Because if the other bolt (the one with the double loop) fails there is every chance that your nice big cowstail carabiner will slide over the central knot and other stuff and send you down the pitch.

Offline Mike Hopley

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Re: Which knots for Y hangs?
« Reply #63 on: February 17, 2019, 08:26:33 pm »
Seems like I need to persuade someone to drop test the fusion knot and the triple bowline in the configurations mentioned above.

To create a Y-hang with an integrated clip-in loop, I like the triple bowline best. :thumbsup: Finish it with a Yosemite tie-off, so that the clip-in loop loads correctly.

I don't know whether this knot is subject to the same issue as the bowline on the bight, where clipping just one of the Y loops could lead to the caver travelling rapidly back down the pitch. But since you're providing a convenient central clip-in loop, it seems unlikely that anyone would do that.

This knot is a little more fiddly to tie and adjust than more conventional options, but a lot less fiddly than the BFK (which also uses at least a mile of rope). ;)

(I suppose in principle you could make a "triple fusion" instead of a triple bowline, but now I'm really veering into uncharted waters...)