Hi all, i was wondering if you could give me some advise on what knot is best to use on my cowstail to tie the krabs on with. I am currently using a double fishermans but i have seen a lot of people using a figure eight instead. Is there a reason for this i.e is one stronger/safer than the other? or is it just down to personal choice?

Cheers

The short answer is that it doesn't matter too much - either half a double fishermans (also called a Barrel knot) or a fig of 8 wil do the job.

However, there is an increasing trend among people (including me) to use the Barrel knot. It has a number of advantages, the most important of which to me is that in drop tests it absorbs shock slightly better than the figure of 8, reducing in the process the peak impact force passed to you and the anchor(s) in the event of a fall. It also holds the krabs in place and is (probably) a more compact knot using less rope and wearing less as a result.

Thanks for the reasurance, i think i will keep with the Barrel knot then. 

i use a fig 8 for various reasons including ease of swapping out the carabiners, or adding an extra crab with a shunt etc.

Barrel knot for me, for ease of swiftly cliping in and out of bolts since the krab is always held the right way round.

you say potato, I say patarto.
You say tomato, I say tomarto

etc

Ropes break at knots.

But actually ropes hardly ever break in normal caving use (excepting rub points and rock fall, of course).

Krabs do unclip, however, and certainly a few accidents have occurred where a caver wasn't attached to the rope when they thought they were. In some cases we can't ask the caver what went wrong, for obvious reasons...

I'd suggest that any arrangement which allows easy clipping, and keeps the krab in the alignment that the caver expects, is more important to safety than the precise type of knot used (within reason! e.g. barrel knot, overhand, fig. 8...engage brains, caving is dangerous etc etc etc). A krab that can twist in the cowstail loop, potentially allowing gate loading, or being opened against the rock in the event of a slip on a traverse line, seems to constitute a greater hazard than the strength lost through any particular knot choice.

Surprised that many cavers still seem to be unaware of the importance of clipping karabiners away from the rock when on traverses. Enough rant, anyway.

read http://british-caving.org.uk/rope/lanyard_tests_v6.pdf for the best set of data.  Some people also use cycle inner tubes to keep the knot in "place" on the crab.

See also http://www.cncc.org.uk/tg_warning.html for concerns over crab location with respect to anchor. 

Barrel knot for me 

However, it is important to regularly  untie the knot to check the condition of the carabiner. I untied a friends cowstails recently and found that the carabiner had oxidised sufficeintly to start to cause damage to the rope!

Excellent to see Bob giving the proper info! I'll not get into quibbles about falls on cowstails onto traverse lines etc - we've bored spectators to tears with this already.

I'll just add that Bump makes a good point - cavers are traditionally rubbish about checking and maintaining their kit. That's probably as big a risk as what knot you use in your cowstail, too!

seddon said:

Surprised that many cavers still seem to be unaware of the importance of clipping karabiners away from the rock when on traverses.

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and away from other bits of rope (e.g. when clipping into knots at traverse lines). I am convinced that human error is far more likely to cause accidents with modern kit, than any of the kit failing.

Bob Mehew said:

read http://british-caving.org.uk/rope/lanyard_tests_v6.pdf for the best set of data.

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To Bob Mehew and damian: an excellent report, thanks.

damian said:

I am convinced that human error is far more likely to cause accidents with modern kit, than any of the kit failing.

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I remain convinced also

For me, the report underscores the suitability of knotted cowstails.  Looseness of knots didn't play as significant a role in the strength tests as i had been led to believe, but I will continue to untie mine occasionally (to clean and inspect).  Replacing kit after a fall appears important as ever, as does not climbing above the anchor!

So some sound technical data to back up the accepted practices cool.

I saw someone doing a good job of instilling effective use of cowstails with some university novices last weekend. Don't know if it was a professional, or an excellent amateur training officer; but it did me heart good to see it!

Bump said:

However, it is important to regularly  untie the knot to check the condition of the carabiner. I untied a friends cowstails recently and found that the carabiner had oxidised sufficeintly to start to cause damage to the rope!

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That's not something I've come across, even though I use the same krabs for a fair while.
If you're worried about that, I suppose you could try wrapping the krab with insulation tape where it's going to be inside the knot.
Assuming the tape was nicely bedded on to the krab, and stayed in place, that should pretty much stop any hidden corrosion happening.

seddon said:

I saw someone doing a good job of instilling effective use of cowstails with some university novices last weekend. Don't know if it was a professional, or an excellent amateur training officer; but it did me heart good to see it!

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I think you'll find that was me  :-[

I too have had krabs corrode quite badly underneath a barrel knot. Krabs had probably been in regular use for about 3 years at the time, without inspection. I was replacing the rope because the sheath was getting worn. Last thing I expected was the corrosion of the Krab.
I guess its stays damp where the rope is kept tight against the krab by the barrel knot. You live (hopefully) and learn.
Inspect regularly now, and replace as necessary.

marysboy said:

Looseness of knots didn't play as significant a role in the strength tests as i had been led to believe, but I will continue to untie mine occasionally (to clean and inspect). Replacing kit after a fall appears important as ever, as does not climbing above the anchor!

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Ahem.  I can't show the graphs here (not yet sorted out a location to post them), but Dave Elliot did some work on dynamic strength of ropes back in the 80's including recording peak force.  These show that the peak force rises on subsequent drops until the rope breaks.  Hence if you have tighten your knots by falling on them or just using them on a trip, then relaxing them will reduce the peak force from a subsequent fall (which may save you breaking something in your body) and also allow you to properly clean the rope. 

I also consider that the breaking point is within the knot (see http://www.iop.org/EJ/abstract/-search=63377408.1/1367-2630/3/1/310 for the theory) as is backed up by some high speed camera work done by Jim Evans and me (see my avita).  Leaving the knot tightened means that this point will be subject to continuing forces even when your weight is not on it.  Is this good for the rope?  Also tightening up a loose knot absorbs a hell of a lot of energy from the drop which otherwise would go into deforming the rope.

potholer said:

If you're worried about that, I suppose you could try wrapping the krab with insulation tape where it's going to be inside the knot.
Assuming the tape was nicely bedded on to the krab, and stayed in place, that should pretty much stop any hidden corrosion happening.

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Not sure about using tape to protect rope from crab.  I recall PVC tape causing stress corrosion cracking in an albeit high temperature pipe which makes me want to avoid covered areas on safety critical components. 

Bob

Must now go and clean my cows tails and relax the knots  :-[

Taped areas round the loop of a fig-8 cowstail knot seem to be pretty clean when the tape is removed for rope replacement.

Possibly, in a barrel knot, the movement of the rope when tensioned, or when the knot is handled on a loaded krab has a tendency to roll round the krab, and rub on the surface layer of the metal in a way that wouldn't happen in the (taped immobile) loop of a fig-8 or overhand, where the movement might be limited to the tiny amount of stretch in the rope length at the very top of the loop?
Could repeated untying and tying barrel knots actually cause more rubbing on the metal surface?

marysboy said:

Looseness of knots didn't play as significant a role in the strength tests as i had been led to believe

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Apologies, I should have put 'stiffness'. :chair: Tightly tied knots act less dynamically (ie more stiff), raising the peak force.  It is this effect that was less than I had been lead to believe.

Bob Mehew said:

Ahem. I can't show the graphs here (not yet sorted out a location to post them), but Dave Elliot did some work on dynamic strength of ropes back in the 80's including recording peak force. These show that the peak force rises on subsequent drops until the rope breaks.

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As did the current document.

Hence if you have tighten your knots by falling on them or just using them on a trip, then relaxing them will reduce the peak force from a subsequent fall (which may save you breaking something in your body) and also allow you to properly clean the rope.

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Agreed.

I also consider that the breaking point is within the knot (see http://www.iop.org/EJ/abstract/-search=63377408.1/1367-2630/3/1/310 for the theory) as is backed up by some high speed camera work done by Jim Evans and me (see my avita). Leaving the knot tightened means that this point will be subject to continuing forces even when your weight is not on it. Is this good for the rope?

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Hmmm.  Hadn't considered the long-term effect of continued internal stresses.

Again, thanks for the info from those in the know  :bow:

Unlike with loop knots, where each side of the loop going over the krab is only taking half the load, with a barrel knot, the loaded rope is effectively going straight up through the centre, and then bends 180 degrees over the krab, and then wraps round itself on a fairly large radius.

Under load, does the loaded rope actually get gripped significantly by the rope wrapped around it, or does basically all the load make it up to the rope bend at the krab - is the rope bend at the krab the point where the rope breaks, or does it still happen nearer the entry to the knot, even with the skinniest likely krabs?

Interesting question, Potholer. I don't know the answer, but I suspect it breaks at the rope bend. Bob?