For a while Bob Mehew has been advocating that we should all relax our cowstail knots between trips. I have been meaning to discuss this with him for a while, because I have concerns about it. I apologise to Bob for posting this here rather than talking to him first, but I think it is probably worth a "public" debate.
Imagine this: I've just arrived at the first pitch underground having been a good little caver and relaxed my barrel knots after my previous trip. I clip my cowstails to the traverse line and soon find the footholds disappearing, so I gently lower myself onto my short cowstail. Unfortunately the barrel knot was not properly tightened - because I'd loosened it as per instructions - so it came undone and I am now falling onto my long cowstail (assuming it is also connected), or to the base of the pitch if the long cowstail was not connected.
I think I understand Bob's rationale ... a tight knot absorbs less shock, so the rope (or more likely the caver!) will be more likely to break following a fall. However, in my view, the advice fails to take account of the fact that barrel knots in particular need to be body-weight tight against the krab to stay done up. On any normal trip cowstails probably only have about 1Kn of force on them anyway (i.e. body weight), and I can't imagine that this is going to have much of an impact on the peak shock loading in a fall. Of course, there is no benefit in having loosened them between trips from the point at which you have hung on your cowstail on the next trip (which is often pretty much the 2nd anchor of an SRT trip).
Any thoughts?
Imagine this: I've just arrived at the first pitch underground having been a good little caver and relaxed my barrel knots after my previous trip. I clip my cowstails to the traverse line and soon find the footholds disappearing, so I gently lower myself onto my short cowstail. Unfortunately the barrel knot was not properly tightened - because I'd loosened it as per instructions - so it came undone and I am now falling onto my long cowstail (assuming it is also connected), or to the base of the pitch if the long cowstail was not connected.
I think I understand Bob's rationale ... a tight knot absorbs less shock, so the rope (or more likely the caver!) will be more likely to break following a fall. However, in my view, the advice fails to take account of the fact that barrel knots in particular need to be body-weight tight against the krab to stay done up. On any normal trip cowstails probably only have about 1Kn of force on them anyway (i.e. body weight), and I can't imagine that this is going to have much of an impact on the peak shock loading in a fall. Of course, there is no benefit in having loosened them between trips from the point at which you have hung on your cowstail on the next trip (which is often pretty much the 2nd anchor of an SRT trip).
Any thoughts?