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Multi drop single rope

PostPosted: Dec 22, 2012 4:12 pm
by rebelfirefighter
This might come down to preference but I'm gonna ask any way. Doing a multi drop I've seen people anchor to the bolts with a figure 8. Wouldn't it be better to use an alpine butterfly. My reasoning is 1. the butterfly is a strong excellent mid line knot. and 2. The butterfly seems easier to untie once it has had weight put on it. It just seems to me more designed for the task. I know the figure 8 is an excellent knot too and has its place. I'm just curious what others think.

Re: Multi drop single rope

PostPosted: Dec 22, 2012 4:42 pm
by Bob Thrun
It does not really matter. Use whatever knot you like to tie.

BTW it is just Butterfly, not Alpine Butterfly, according to Wright and Magowan who invented the name in 1928.

Re: Multi drop single rope

PostPosted: Dec 22, 2012 6:37 pm
by trogman
I think the main reason is that the figure 8 is so easy to tie and most people are very familiar with it. At least that would be my guess. My favorite is the bunny ears with two bolts.

Trogman :helmet:

Re: Multi drop single rope

PostPosted: Dec 22, 2012 7:21 pm
by snoboy
At least one source* says that the alpine butterfly (my preferred name to differentiate it from a false one) is not really a great choice to be loaded as a loop knot. It shines when the main line of the rope is loaded, and there is a load added to the side also - not the case in a rebelay.

*_Life On a Line_

Re: Multi drop single rope

PostPosted: Jan 23, 2013 2:07 pm
by Mike Hopley
An alpine butterfly is not meant to be loaded in that direction; a figure eight is. See Al Warild's Vertical for details.

In the event that the rebelay fails (abnormal loading), the alpine butterfly is actually better -- but the primary concern should be how it's normally loaded.

Re: Multi drop single rope

PostPosted: Jan 23, 2013 4:22 pm
by Jeff Bartlett
rebelfirefighter wrote:This might come down to preference but I'm gonna ask any way. Doing a multi drop I've seen people anchor to the bolts with a figure 8. Wouldn't it be better to use an alpine butterfly.


Are you talking about rigging to a bolt pair with a double figure eight, or are you talking about watching people rig a separate figure eight to each bolt? IF you mean the latter, you are correct in that this is not the preferred way to rig a drop. But we need more info about the scenario you're describing.

Also, if you mean the latter, then it is correct a butterfly knot can be the best option for rigging a Y-hang with certain bolt configurations... but it's tricky. And if the bolts are close together, a double-figure-eight often works best.

Re: Multi drop single rope

PostPosted: Jan 23, 2013 4:23 pm
by NZcaver
Mike Hopley wrote:An alpine butterfly is not meant to be loaded in that direction; a figure eight is. See Al Warild's Vertical for details.

In the event that the rebelay fails (abnormal loading), the alpine butterfly is actually better -- but the primary concern should be how it's normally loaded.

Warild's book Vertical is a great reference (my first real caving book), but in my opinion the knot data section is misleading and some of the advice should be taken with a grain of salt. Even his later editions still quote rope testing data from 30 years ago, using ropes which lose more strength than modern equivalents when knotted. The knot chapter is probably best studied in conjunction with other rope/knot testing sources as previously discussed in this topic.

At the 2012 NCRC weeklong we conducted some informal, non-scientific destruction testing which included the Butterfly knot. With the Butterfly tied in one end of a short length of used 11mm PMI and a Figure 8 bight in the other end, the Butterfly did indeed break first when loaded in both configurations (into the bight, and across the knot). However the load cell figures showed that both failures were well within acceptable parameters for normal caving use:
Butterfly versus figure 8 bight - the butterfly broke at 3351 lbf (14.9 kN).
Butterfly versus figure 8 bight, both loaded across legs of knot - the butterfly broke at 3152 lbf (14.0 kN).

Video here:


Re: Multi drop single rope

PostPosted: Jan 24, 2013 11:54 am
by rebelfirefighter
Jeff Bartlett wrote:
rebelfirefighter wrote:This might come down to preference but I'm gonna ask any way. Doing a multi drop I've seen people anchor to the bolts with a figure 8. Wouldn't it be better to use an alpine butterfly.


Are you talking about rigging to a bolt pair with a double figure eight, or are you talking about watching people rig a separate figure eight to each bolt? IF you mean the latter, you are correct in that this is not the preferred way to rig a drop. But we need more info about the scenario you're describing.

Also, if you mean the latter, then it is correct a butterfly knot can be the best option for rigging a Y-hang with certain bolt configurations... but it's tricky. And if the bolts are close together, a double-figure-eight often works best.



In the scenario I'm talking about.... its one bolt, one knot

Re: Multi drop single rope

PostPosted: Jan 24, 2013 12:37 pm
by gindling
You're talking about multiple pitches that are close enough together that instead of using separate ropes only one rope is used right? So as you're going down the rope and you come to the next pitch a figure 8 was tied midline and used to connect to the single bolt?

Re: Multi drop single rope

PostPosted: Jan 24, 2013 2:12 pm
by Jeff Bartlett
rebelfirefighter wrote:In the scenario I'm talking about.... its one bolt, one knot


Got it. So: one bolt, one knot, I assume one pit. In that instance, a figure eight on a bight (F8B) is your best option, mostly because the knot is intended for this direction of pull and partly because of the slightly higher strength versus a butterfly. A properly-dressed F8B shouldn't be difficult to untie, so it's possible that the knots aren't being dressed correctly.

However, I might recommend against rappelling off a single bolt. Especially in TAG, many of them are very old, and ought not to be trusted singly. Even a new bolt ought not to be used on its own. Luckily, our caves tend to have many options for rigging backups to natural anchors, and it can almost always be avoided. I've only lived here since 2010, but I've caved pretty extensively during that time, and I've only found myself hanging on a single bolt once or twice. For very short pitches.

Re: Multi drop single rope

PostPosted: Jan 26, 2013 10:17 pm
by rebelfirefighter
we were backed up. Didnt have any trouble untying or other wise. . I was just curious if one was better than the other and trying to find my way through some stuff I had heard. Thanks

Re: Multi drop single rope

PostPosted: Jan 27, 2013 10:33 am
by Jeff Bartlett
rebelfirefighter wrote:We were backed up.

Then that's, potentially, different! Oftentimes, if your rope is tied off to a natural anchor and then a bolt at the lip, your best bet is to make sure the rope between the two anchors is basically taut when the rope is loaded, and tie off with a butterfly. This allows the backup to take some portion of the load -- but the real reason to do it this way is that if the bolt fails, you're not suddenly loading a figure eight in opposition. Depending on the relationship of the two anchors, putting a longish bight in the butterfly can help optimize the angles and minimize the distance the rope would drop if the bolt were to pop.

Here's an example of a common situation, though of course you can't actually see the rigging here (sorry). The natural backup at right is at the same level of the bolt (not visible, as my friend is rigging to it). You wouldn't rig an eight to this bolt, you'd pull the rope between the natural anchor and bolt tight, then rig a butterfly with as long a bight as you can put in it without making it difficult to get on or off of the rope. https://sphotos-a.xx.fbcdn.net/hphotos- ... 0413_n.jpg



rebelfirefighter wrote:Didnt have any trouble untying or other wise.

Ah. I was making a reference to your original post, where you said that the butterfly seems easier to untie when weighted.

Re: Multi drop single rope

PostPosted: Jan 28, 2013 5:48 am
by Mike Hopley
NZcaver wrote:Warild's book Vertical is a great reference (my first real caving book), but in my opinion the knot data section is misleading and some of the advice should be taken with a grain of salt.


Good point. Like any reference, it should be read critically.

I don't think the relative strength is especially relevant, however. Our ropes and knots are almost always strong enough -- even in the case of 8 mm rope. I would not worry about the breaking strength, so much as the behaviour of the knot under load. Knots can do strange, unexpected things when loaded in an unintended direction -- witness the recent French testing of the bowline-on-the-bight.

Would an Alpine butterfly collapse and slide, when heavily loaded in the "wrong" direction? I don't know. But it's a concern that doesn't apply to the figure-eight in this application (a single bolt rebelay). And we know a figure-eight works well here.

When using thin rope (less than 9 mm), a figure-eight may be difficult to untie. If this is a concern, a more suitable replacement might be the figure-nine.

Re: Multi drop single rope

PostPosted: Jan 28, 2013 7:18 am
by NZcaver
Mike Hopley wrote:I don't think the relative strength is especially relevant, however. Our ropes and knots are almost always strong enough...

Agreed.
I would not worry about the breaking strength, so much as the behaviour of the knot under load.

Agreed also.
Would an Alpine butterfly collapse and slide, when heavily loaded in the "wrong" direction?

Based on what I have seen during testing, no. A single bolt rebelay loaded 'normally' with the bight clipped in, no problem with either a Figure 8 bight or a Butterfly. Bolt fails, sudden abnormal loading across legs, I would still expect no great problem with either knot. This is one of the reasons I find the reference you quoted to be misleading. The difference comes down which is the slightly smaller knot, which is easier to tie/untie, which is more easily recognized as the regional norm when checked by other cavers in your party, etc. All ultimately more about ergonomics and personal preference.

Re: Multi drop single rope

PostPosted: Jan 28, 2013 10:36 am
by Mike Hopley
NZcaver wrote:The difference comes down which is the slightly smaller knot, which is easier to tie/untie, which is more easily recognized as the regional norm when checked by other cavers in your party, etc. All ultimately more about ergonomics and personal preference.


Indeed.

I've heard that Polish cavers often use a bowline-on-the-bight on single bolt rebelays, by the way. It's interesting to learn about other groups' norms and their reasons.


Based on what I have seen during testing, no. A single bolt rebelay loaded 'normally' with the bight clipped in, no problem with either a Figure 8 bight or a Butterfly. Bolt fails, sudden abnormal loading across legs, I would still expect no great problem with either knot.


That's encouraging. I'm not sure I'd want to be a pioneer, but if the knot will hold up well, I could imagine the butterfly being an improvement over the 8. Certainly I find it easier to undo.