UKC

As the title suggests, is this advisable?

Cheers

In reply to seanywillycocks:

If there's a significant difference in rope diameters controlling your descent may be a more pressing issue.

In reply to seanywillycocks:

overhands on similar diameter ropes with a tail is fine, however if there is more than a few milometers difference use a fisherman's knot (two back to back stopper knots).

This is because if you use an over hand to tie two different diameter ropes together, the knot will either roll/slip at a much lower loading weight.

In reply to seanywillycocks:

Double fisherman's if you're unsure

In reply to seanywillycocks:

If you are not pulling the ropes down from below then joining with a carabiner will work for any rope diameter.

In reply to seanywillycocks:



I'm not sure if it's advisable, but we did multiple abseils using this setup last year. We backed it up for the first person and removed the backup when it hadn't slipped for the second. We never saw any reason to doubt the choice of knot.

In reply to seanywillycocks:

this topic has been discussed previously on many occasions.

just search the forums for more info.

Davey

In reply to seanywillycocks:

Just be aware that the rope knot does not follow you down, keep an eye on the knot, as this happened to us on High Tor, and that was a 10.5 and a 9, lucky for my mate that I was watching from above and just put my foot on the rope before he got to far. this was with double Fisherman and stopper knots.

In reply to seanywillycocks:

i use Reef knot with double fishermans if rope are unequal thicknesses as its easier to undo after its been weighted.

In reply to seanywillycocks:
The major danger of using anything but a single overhand knot on multiple abs is the greatly increased risk of getting your rope jammed and being benighted/storm bound/having to prussick or solo back up to retrieve the rope

In reply to seanywillycocks:

Here's what I do. Since the failure mode of the overhand knot is rolling, tying it as illustrated with ropes of different diameters might well be more stable than with two ropes of the same size.

http://www.supertopo.com/photos/18/71/308671_1199_L.jpg
http://www.supertopo.com/photos/18/71/308672_1634_L.jpg
http://www.supertopo.com/photos/18/71/308673_8648_L.jpg

In reply to andrewmcleod:
Maybe I'm being dense, but I can't see the point in using a carabiner to join ropes. Surely whatever knot you use to tie the ropes to the 'biner could also be used to tie the ropes to each other!?

What am I missing?

In reply to Clinger:
Presumably you'd be using a knot with a bight. For example two figure of 8s with their bights together. This raises the possibility of the bights to rubbing and abrading the rope itself.

Either way, there are so many easy ways to join ropes I have no idea why you'd add the complication of a carabiner.

In reply to seanywillycocks:

Slight differences is perfectly safe and the overhand is surprisingly good for even very big differences.

Try the experiment - its quite interesting and I'm sure a lot of people don't bother and just repeat the same second hand stuff as fact. At the local wall I tried abseiling off a length of 6mm accessory cord tied to my 10.5mm wall rope using a single overhand knot. The knot did turn once and tightened and was steady after that. PS it was from the first bolt over a mat !

I'm not advising using this sort of different size rope in reality (you can't really control it in any case) but it does mean I'm more than happy to trust a single overhand to join a normal half rope to a single rope for instance. I am always careful to make sure the knot is properly snugged up and not the ropes aren't twisted but that only takes a second to do.

In reply to A Longleat Boulderer:



Carabiner is rather handy (perhaps even necessary) for abseiling off of a single rope, but you still want to retrieve it (with a lighter tag line).

In reply to SenzuBean:

Personally I'd not want to use a biner. I can see it getting stuck in every crack that comes close to it when you're fifteen pitches from the ground! But each to his own.

In reply to A Longleat Boulderer:


Agreed. It would be a terrible way to do it.


Exactly. The only thing I can think of would be for people who only know how to tie a figure-of-eight on the bight or something, in which case a better approach would be to learn a few simple knots.

In reply to Clinger:
Like I said, carabiner only a good idea if you aren't pulling the rope down from below (as pointed out above it may be useful for pull-cord abseil, although I don't think it is required). The only use I can think of would be some incredibly contrived scenario like joining an 11mm static onto 5.5mm dyneema for a long abseil where somehow those were the only spare ropes you had (possibly an emergency situation). Most bends would be inappropriate and joining bights requires you to worry about thin-cord-on-rope abrasion...

Nonetheless, it is guaranteed to work with any rope diameters (even 13mm statics, US style, and 2mm pull cord if you were crazy enough to try that) without worrying about the ropes rubbing (or indeed thin cord cutting through the thick one).

In reply to GrahamD:


If you had used the system I described with links to pictures above, it wouldn't have rolled even once. My guess is you had the roles of the big and small ropes reversed (as in the picture I labelled "incorrect") and it rolled into the more stable "correct" configuration. (My pictures are with 6 + 8.5, but just to see what happens I've tried 6 + 10.2 with no problems.)

In reply to rgold:

Interesting pictures, thanks. Of course its moot whether the knot rolling once is actually a failure ! I don't think it is. The tendancy to allow multiple rolls without tightening is a failure.

In reply to seanywillycocks:

If you're worried about the overhand knot (different diameters, wet/icy ropes, brand new "shiny" ropes, etc) then tie a second overhand in the tails, snug against the first knot. This will slightly increase the risk of jamming (but not as much as a reef+stoppers) but is much more reassuring.

In reply to GrahamD:


Stood on a tiny ledge in the middle of nowhere with your partner abseiling down to the next hanging stance way down below you... and watching the knot start to roll, might just change your mind on this, Graham...

In reply to A Longleat Boulderer:


I've used it once - and the carabiner came down a treat. The problem I did have was that my tag line was too stretchy (I used paracord) - and that made it hard to give a sufficiently hard yank on to the rope to force it through bushes.
If you are using the tag line method, I don't think you have a choice to avoid using the carabiner - as otherwise you'd need to have rope sliding over rope.

In reply to GrahamD:
Your test from the first bolt doesn't even begin to model what might happen during, say, a 60m rap---how do you know the knot would be "steady" in such a situation after the first roll?

Don't forget that people have died from a knot rolling. One case I know of is when a figure-eight rather than an overhand was used, and in that case it had been "obvious" to many climbers that the figure-8 was a "steadier" option than the overhand.

I'd say any rolling is a failure and a fairly terrifying one at that.

In reply to jon:

Thanks for your concern, Jon. I've done plenty of 60m abseils to hanging stances to have a fair idea of when I'm happy with a knot and when I'm not. Watching a knot start to roll sort of implies it carries on rolling which, of course, it doesn't

In reply to rgold:

I don't think saying just because one demonstrably unsafe knot rolls is evidence that all knots are unsafe if they roll. For an overhand, the possible roll is only part of the tightening up process that happens in all knots. In any case, I'm happy enough with my (and my most regular partners, of course) choice of joining ropes with a single overhand. Anyone is free to make the knot as complicated as they like, though.

In reply to GrahamD:

Of course it is up to you and your partners, although I wonder how many people would rap your lines if you told them that the knot would roll once and then tighten up just fine. I sure wouldn't; I want my knots to stay put when loaded.

As for complication, the shot I posted labeled "correct" is of a single overhand, but with the strands oriented so it doesn't roll, so no complication whatsoever there. For extra complication, I also posted a picture of the same knot with an additional overhand---but in only the small strand so very compact---that strangles off any possibility of rolling.

If I remember correctly, Moyers original tests suggested that rolling happened either sooner or more easily when the ropes were wet, so maybe you should test your setup that way too?

In reply to SenzuBean:

At least some of the cavers I know, when doing a 'pull-down/through' trip, tie a Fig 8/alpine butterfly in the middle of the rope (after passing through anchors) and thread the rope (from other side of anchor) through the knot. They then abseil down the tail; it is obviously critical to choose the right side of the rope! Cavers do this because their standard abseil devices are all single-rope.

In reply to andrewmcleod:


That's cool - didn't know they did that.

In reply to rgold: Nice thinking! A new one to me.

It just happens that I just abseiled off an ice pitch I soloed this morning using a single rope abseil with a pull cord (8.5mm rope and 5mm accessory cord). If in doubt you can always rig this way, although obviously with more chance of your blocking knot/krab getting stuck.

In reply to rgold:


The knot does stay put after you have done your trial loading and after you detach yourself from the anchor.

In reply to seanywillycocks:

http://user.xmission.com/~tmoyer/testing/EDK.html

In reply to Name Changed 34:

The flat double fishermans looks good, especially for different diameter ropes.

Anybody fancy doing some tests? DMM? Wild Country ? Lyon ?

In reply to seanywillycocks:

I've no problem doing this but always tie a double overhand knot with the two knots tight together. This does two things, stops any tendency for the knot to roll (more likely when it is wet or icy) and focusses your mind on what you are doing when you may be tired so that you tie the knot properly. The other important thing is to make sure that the rope that you will pull on is the narrower of the two which means that the fatter rope cannot get dragged downwards by the belay device as the knot will prevent movement in that direction.

In reply to Rick Graham:



You might like to read this: http://people.bath.ac.uk/dac33/high/16Knots2.htm#bigflatfish

In reply to David Coley:
Thanks for that.

I was thinking along the lines of using it to join ropes of very unequal diameters.

Either flat half fishermans , tie thick rope first, then triple? fishermans in thinner rope .

or overhand then flat fish combined ?



Might just try it on site between a 6 tonne dumper and 30 T excavator, should amuse the lads