A single bolt rebelay - what knot do you use?

Me and a mate are having a massive argument about this.

I've used both fig 8/9 and an alpine butterfly to do this. I much prefer using an alpine butterfly in this situation as it uses less rope, is easier to adjust and isn't a pig to untie after 3 or more people have loaded it. Also, since only one of the standing ends is being loaded, there's no danger of it capsizing (I know that alpine butterflys can reverse if both the rope going in and out of the know is loaded.)

So what do people think?

Vance said:

Me and a mate are having a massive argument about this.

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Cavers? Arguing? Seems unlikely.

Vance said:

Also, since only one of the standing ends is being loaded, there's no danger of it capsizing (I know that alpine butterflys can reverse if both the rope going in and out of the know is loaded.)

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Is this something that you've seen? My alpine butterflys keep their shape fine when both ropes coming out of the knot are loaded.

Would use a fig-8 or fig-9 in this situation.

Alpine butterfly's used as end loops are less strong than other end loop knots e.g. fig 8 or 9. In some venerable journals their use as such is frowned upon.

Have a look at the Life on a line thread, download the sample chapter (or buy the whole eBook) and read the advice there. I won't try to interpret it as I've only half read it myself so far.

Vance, which knot(s) did your argumentative mate suggest and why didn't he like your choice?

My mate is dead keen on figure 8s and as I've said, I've used these before and know that they're perfectly adequate for the job. Except when you get 20 odd people going up and down them and need to cut them out.

His objection to using Alpine Butterflies is that they're designed for the load to pull sideways from the knot, not vertically downwards. I don't see this as an issue, as long as both the incoming and outgoing ropes aren't loaded opposite to one another, the knot will rotate a little and wont capsize.

Knot strength isn't really that much of an issue as the forces required to make the difference in strength between a fig 8 and alpine butterfly relevent would pulp the unfortunate caver, rip out anchor points, split descenders or cause toothed ascenders to shred the rope

To reply to the life on a line suggestion, yes they say that fig 8s are ideal, but they also ban using bowline on the bights. This shows that it's useful, but is for rescue rigging, not sport rigging.

Please be nice to me, I can see some people objecting strongly to my comments 

Vance said:

My mate is dead keen on figure 8s and as I've said, I've used these before and know that they're perfectly adequate for the job. Except when you get 20 odd people going up and down them and need to cut them out.

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I have been known to put a bowline on the bight at single-bolt rebelays if there is going to be a lot of traffic. Having two loops both going through the maillon reduces wear at the maillon and it is also quite easy to untie afterwards.

Personally, I normally use figures of eight, although I have used both figures of nine and alpine butterflies before.

I don't see much difference really.

I think your arguement for using an alpine butterfly (that it's easier to undo) is perfectly valid. I often use it myself for the same reason. Where does it say that butterfly knots are designed to be loaded sideways; I can't see that it should make any difference.

Just mulling something over. Not done it (because I've never used an Alpine on a single bolt belay nor have I caved with anyone who does), but if you were to clip your cowstail into the rope loop during the changeover it'd be interesting to see what happened to the knot when weighted (and there are many good reasons to clip the rope loop as opposed to the Crab/maillon/bolt).

Furthermore just this weekend I had to help someone out of a sticky spot. With a handy bit of rope and my hard earned training, I got them out. The rope loop I clipped into was a fig 8 which momentarily took both our combined weights. Interesting to see how the Alpine would look in such a situation.

I might try this at work tomorrow!

I have to admit when I first learnt the Alpine all those years ago I thought it was the dog's nuts. Nowadays I'm not so sure. I have some views that are contradictory to what is seen as Gospel. But at least you know I have a prejudice!!   

Vance said:

I don't see this as an issue, as long as both the incoming and outgoing ropes aren't loaded opposite to one another, the knot will rotate a little and wont capsize.

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Now I'm really intrigued cos you've said it again! The Alpine Butterfly is symmetrical and I really can't see how loading both sides of the knot will cause it to distort. Are you saying that having two cavers hanging on a traverse line either side of a bolt where the rope is attached with an alpine butterfly is going to cause the knot to distort? Have I got the wrong end of the stick entirely?

AFAIK the false butterfly isn't symmetrical. Maybe there is some confusion between the two knots?

Francis

The Life on a Line sample chapter I referenced above says the opposite - that an Alpine Butterfly should be loaded with both ends being tensioned in opposite directions. If the two ends are pulled in the same direction (at an acute angle to each other) it collapses. So says Life on a Line - in a few minutes testing, I haven't managed to capsize it myself, but then I'm not really loading it fully.

Read the chapter (page 11) and see if that's how it read to you!

Chris

Vance said:

A single bolt rebelay - what knot do you use?

Me and a mate are having a massive argument about this.

I've used both fig 8/9 and an alpine butterfly to do this. I much prefer using an alpine butterfly in this situation as it uses less rope, is easier to adjust and isn't a pig to untie after 3 or more people have loaded it. Also, since only one of the standing ends is being loaded, there's no danger of it capsizing (I know that alpine butterflys can reverse if both the rope going in and out of the know is loaded.)

So what do people think?

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I'm with your mate. I'd use a fig8
ABK is for when the loop is at right angles to teh main rope and load.

Fig8s are not that hard to untie with practice, it's all about twisting the  knot at the right angle until it opens up enough to work out the loop.


But to be honest it isn't that critical, F8, ABK BOTB even an Overhand loop knot would do the job although not recomended... The key point is the knot should be tied as intended, carefully dressed and secured with a safety knot if appropriate.

JB said:

Vance said:

I don't see this as an issue, as long as both the incoming and outgoing ropes aren't loaded opposite to one another, the knot will rotate a little and wont capsize.

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Now I'm really intrigued cos you've said it again! The Alpine Butterfly is symmetrical and I really can't see how loading both sides of the knot will cause it to distort. Are you saying that having two cavers hanging on a traverse line either side of a bolt where the rope is attached with an alpine butterfly is going to cause the knot to distort? Have I got the wrong end of the stick entirely?

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I'm basically going off what life on a line is saying. If you were to load (and I mean a dynamic load, not just your weight. I hope) both ends of an alpine butterfly so that they're both pulled directly downwards (or near enough) there's a good chance the knot will turn inside out. I've never seen it actually happen as I've always avoided using the knot in that way. I did see it happen to a figure 8 once that was used incorrectly.

Maybe I should rephrase my question - is the ABK unsafe for use at a rebelay and why?

Intregued (or something similar-can't spell!) wots considered wrong with a BLOTB for re belays? It was the standard knot for attaching ring hangers mid rope & I still use it - mind you I am an old git!

Vance said:

...
Maybe I should rephrase my question - is the ABK unsafe for use at a rebelay and why?

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Not so much safe as just sub-optimal.

The knot is not being loaded in the way it is deisgned to work.
Load it sideways (ie two ropes pulled apart) and it is at its strongest - from any other direction it isn't!

Simple as that.

The Fig8 is exactly the same case but with the good/bad loading directions reversed.

For confused SRT Novices (like me) its worth people agreeing on a default solid answer that wont fail even if your not clever and abusing the knot (like novices do), rather then trying to work out the answer from some of the technical answers given.

To me if someone just wanted a simple answer I would say Figure 8 for re-belays I would use the ABK for traverse lines, Y hang (figure 8 on upper P bolt then at the right height ABK at lower like in ledge pitch of Hunters). ABK's for clipping in etc.

I'm thinking until I'm a rope expert I cant go wrong with that idea.

** Hang on If I use a ABK as the lower connection of a Y hang and the top one fails, what happens to the ABK? It would be loaded like a re-belay wont it??????

The reason we've gone against an ABK in a rebelay is purely one of tensile strength - a F8 or F9 is stronger, so why add trouble when you don't need to...

The capsizing issue happens with an ABK where the standing parts (the two long ropes) are pulled together in the same direction (take a rope, tie an ABK in the centre, hang it from a hook, pull down both ends at the same time...). As long as there's more than about 60 degrees between the two, the knot doesn't capsize. If it's been loaded beforehand, it tends to stay in shape until a dynamic insult hits it. If you only load ONE side of the ABK (as you will be doing in a rebelay) then it doesn't capsize, it just deforms a little bit and that's perfectly safe, with no loss in strength. A capsized ABK on the other hand is substantially weaker (it can get as bad as 40%) but of course it won't physcially release anything, so on a rebelay used for personal SRT the ABK "safe" in that sense. Just a little silly when there's a stronger knot that's just as easy to tie.

The argument for rebelays in LOAL is based on rescue rigging (twin lines, heavy things) but in personal SRT you have to consider the potential top-bolt failure event (caver above the rebelay.. bolt fails above the caver = fall factor of 2.0). In most situations it'll be straight out fatal (especially on ascenders) but if you plan on surviving at all then having the strongest knot at a rebelay is arguably a good idea!

As to an ABK being easier to untie - it depends on the rope. F8s are always nasty on thin, soft rope and the F9 always releases better simply because it's long enough to be broken by bending. The ABK in stiff rope is trivial to release but in a thin soft rope it can be just as horrible as a F8.

LifeOnALine said:

The argument for rebelays in LOAL is based on rescue rigging (twin lines, heavy things) but in personal SRT you have to consider the potential top-bolt failure event (caver above the rebelay.. bolt fails above the caver = fall factor of 2.0). In most situations it'll be straight out fatal (especially on ascenders) but if you plan on surviving at all then having the strongest knot at a rebelay is arguably a good idea!

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Out of interest, are the comparisons of knot strengths quoted for various knots as a result of a shock loading or as a result of a static pull until faiure occurs?

If you were to suffer a FF2 due to the failure of the anchor above with a rebelay below you, I would say the comparison of varying knots and their strengths would be highly academic - especialy as you will be attached to the rope (as you say) using jammers when prusiking or by a descender when abseiling which will either fail or cut the rope long before the rope fails at the knot (that is if you don't just slide down the rope!) - not to mention the resulting internal injury.

Don't forget - rigging for rescue and rigging for "normal" caving are two different things.