Relaxing Cowstail Knots

[Bob Mehew]

interesting stuff but could you make it clear that at this stage this is unofficial advice from Bob Mehew and not approved advice from the BCA. Obviously that might change in the future once it has gone past the E&T Committee.

The simple answer is no I will not.  I say no because making any such statement of "unofficial advice" will in my opinion immediately create a link that you wish to deny.  I write on this forum in my personable capacity.  If I were to formally make a statement as Rope Test Officer, then it will be flagged as Bob Mehew, BCA Rope Test Officer not just Bob Mehew.  I accept that any such formal statement would be likely to follow closely what I say personally but there could be differences.  If not just because debate on a forum like this can and does change my mind.

Apologies to other readers for getting political and going off topic.

 

[Bob Mehew]

I do not recollect seeing any accident involving a rope breaking under an accidental load in the past number of decades of UK caving history (excepting the polyprop rope at GG Main shaft).  I also only recollect one incident involving an anchor failing leading to a severe injury (3 crushed vertebra).  I have not scanned USA accidents but don't recollect seeing any such reports in Nylon Highway.  There was one report from France of a rope breaking in unusual circumstances but I never was able to get any detail.  I have had one report of a relatively unused rope breaking in Mulu after some months in cave but no other details.  (I would be grateful if any one knows anything else to inform me.) 

I have tested the short side of a few cows tails (the BCA Rope Test Rig is not long enough to cope with the long side of the cows tails).  Please forgive me for not recalling the specific results nor looking them out as I have tested well over 500 samples.  But the head line result from all this work is that I have yet to have a sample which breaks on the first drop.  But as Glenn emphasises this advice is also about improving your chances of surviving the fall with less or no injury.  I will also add that all bar around 10 samples broke in the knot.  Hence the emphasis on the knot.

We have done some work on barrel knots which makes me hopeful but not yet the specific test suggested by MikeM.

 

[Fulk]

have not scanned USA accidents but don't recollect seeing any such reports in Nylon Highway.

I came across a report in American Caving Accidents many years ago, when a 'kosher' nylon rope broke as someone was abbing on it . . . fortunately he did not fall very far and got away with relatively light injuries.

It turns out that the guy was a geologist, and always carried a little phial of hydrochloric acid with him, to test for carbonate rock by seeing if it bubbled (with evolution of CO2) when acid was poured (or more likely dripped) on it; unfortunately he got some on his rope! So it doesn't really count as a rope-breaking accident through fair wear and tear.

 

[Bob Mehew]

It was conjectured that the Mulu rope broke because of calcite deposits within the rope given the location / high temperatures etc. whcih created a sort of sand paper effect within the rope.  Regrettably no sample was brought back for analysis.  But for any one reading this who has access to a scanning electron microscope, we could do a scan of the interior of a used rope to see if grit does penetrate.  We would just need to sort out how to cut the rope without cross contamination.  :-\

 

[Mike Hopley]

Regularly untying cowstail knots is paranoid. It's also impractical, at least if you're doing SRT where the short cowstail gets repeatedly tightened by body weight. I need a spanner or skewer to undo my cowstail knots, and it takes me ages (say, 30 -- 60 minutes).

The FFS testing used knots that were pre-tightened to 3 kN. Despite this, all the cowstails produced acceptable impact forces when subjected to a factor 2 fall -- except for some manufactured cowstails, such as the Spelegyca.

Looser knots reduce the impact force even further. But if you are that worried about impact force, you would be better off using a more effective solution, such as a Via Ferrata energy absorber.

You can in fact create caving-style cowstails that incorporate an energy absorber. Simply replace the middle knot with a friction plate (like this one). This will not be as effective as proper Via Ferrata kit, but it will be much, much more effective than loosening your barrel knots. I have used this setup (for a different reason), and it works fine.

As for cleaning your cowstails: it's not difficult to get most of the grit out from the knot crevices; just soak it in water. And because my cowstail knots are really tight, less grit can get inside the knot. If your cowstails are looser, more grit can find its way inside.

Cowstails should be replaced regularly anyway -- say, once every two years at least.

Anecdotally, I have seen one example where novices were given cowstails that were lethally loose, and could easily have untied themselves when loaded. Was this due to relaxing the knots? I don't know. But tight cowstails are secure cowstails!

 

[Speleokitty]

Regularly untying cowstail knots is paranoid. It's also impractical, at least if you're doing SRT where the short cowstail gets repeatedly tightened by body weight. I need a spanner or skewer to undo my cowstail knots, and it takes me ages (say, 30 -- 60 minutes).

I was my cowstails with my ropes in the washing machine when they are dirty (which is quite often since I mostly cave in Derbyshire) and the cowstail knots either fall out or are very loose when they come out of the machine.  No need to pry the knots apart with spanners or skewers which can't do the rope much good.

 

[Mike Hopley]

I was my cowstails with my ropes in the washing machine when they are dirty (which is quite often since I mostly cave in Derbyshire) and the cowstail knots either fall out or are very loose when they come out of the machine.

I've also done that. My cowstail knots remain tight after coming out the washing machine. 

 

[Speleokitty]

I've also done that. My cowstail knots remain tight after coming out the washing machine. 

Thats interesting. My central overhand knot always loosens and the barrel knots fall apart. I wonder if it is rope dependant.

I use 11mm which I think was Edelrid but it was just whatever 11mm dynamic I could buy by the metre.

 

[Mike Hopley]

Thats interesting. My central overhand knot always loosens and the barrel knots fall apart. I wonder if it is rope dependant.

I use 11mm which I think was Edelrid but it was just whatever 11mm dynamic I could buy by the metre.

Ah -- mine is 9 mm, which binds much tighter than 11 mm. I would imagine untying 11 mm cowstail knots is much less annoying.