Alpine Butterfly

[Cave_Troll]

1) rope strength:
new rope has a strength of about 30kN
Tie in a knot (60%) - 18kN
10:1 safety margin is 1.8kN = 180Kg
ie in an emergency , rescue, hauling or mid rope rescue situation, with two slightly large cavers, you are eroding into your safety margin and hope that you don't shock load the rope.

2) cavers butterflies.
I've just tied one on a bit of rope on my desk. Set it and pulled it tight all three ways.
I then undid it by repeatidly flexing the knot , pushing the rope in, from either side. This is not a good idea and is the reason you always tie a stopper knot on a bowline (unless its in the middle of the rope)
Be aware of this knots limitations. Any cavers i teach are taught to avoid it like the plague.

 

[Dep]

As far as I can see the 'false' or 'cavers' butterfly is horribly like a sheepshank in the way that it uses two half-hitches to lock onto the loop under load.
These are two independant hitches, lose one and you have a simple slipknot. Sometimes this can be a handy feature of this knot - sometimes its downfall.

The AlpineBK has these two locking half hitches not just crossed but more specificially interlocked - each prevents the other from pulling out.

I appreciate the other is quicker and fine for tying on a bag or something non-critical, but for rigging I would use the ABK every time.

 

[Dep]

GeoffR wrote: Alpine Caving Techniques (dont mean to advertise)

Actually do you have the ISBN no?
This is a nice book, I haven't had much chance to look at it but what I did see was good - I want a copy.

 

[Geoff R]

ISBN = no need, ask any caving shop.

3-908495-10-5, Georges Marbach and Bernard Tourte, first ENGLISH edition, 2002.

 

[MSD]

1) rope strength:
new rope has a strength of about 30kN
Tie in a knot (60%) - 18kN
10:1 safety margin is 1.8kN = 180Kg
ie in an emergency , rescue, hauling or mid rope rescue situation, with two slightly large cavers, you are eroding into your safety margin and hope that you don't shock load the rope.

A safety factor of 10:1 for the rope itself is pretty irrelevant. The reaon being, that most abseiling and prussiking devices will fail at much lower loads. A chain is only as strong as its weakest link.

A new caclulation:

strength (new) of rope ~20kN (I'm assuming 9mm rope here)
strength (worn) of rope ~10kN
tie a knot ~6kN

6kN is about the same order of magnitude as the maximum force you can apply with jammer. You can't actually apply much more force than this, because either the sheath breaks (at which point the jammer slides down the rope) or the entire rope severs at that point. So the links of the chain are in approximate balance with one another.

Actually, the maximum force you can apply does not vary much with rope diameter. If you look at Petzl's data, an ascension will slip at 5.5kN on 9mm rope and 6kN on 10.5mm rope (for static loading). For an 80kg FF1, maximum force is 5.1kN and 5.4kN respectively. Sheath breakage occurs for all four cases. 8mm rope gets distinctly dodgier, because it severs with the 80kg FF1. It seems that 8mm rope is on the edge of the design envelope for the jammer.

Safety margin with a static rescue load of 180kg in this situation is about three. That's not hugely generous, but if you want more, you are going to have to change all the links in the chain. That's why I cave mostly on 9mm rope, but regard 8mm rope as a bit on the edge (although I have occasionally used it).

I take the points raised about the false butterfly by both Cave_Troll and others. I think I'll do a few experiments in the garden. It's never too late to change your opinion about somethng!

Mark