Hi, I have had several discussions about this to myself. Below is the result of one such discussion (me, myself and I). Perhaps the oracles on this forum who I am not worthy of belaying can comment on my reasoning:
Firstly: Everything in trad climbing is about compromises. One of the key reasons for climbing using the half rope technique is to help minimise rope drag. But this comes at the cost (read compromise) of carrying more weight (2 half ropes weighs quite a bit more than one single rope – but not twice as much). There are other reasons too such as a bit more insurance to cope with sharp edges / rope severing incidents (think rock falls or ice climbing with sharp axes and crampons) as well as a potentially lower impact force (think less solid protection).
On trad pitches with significant amounts of zigg-zagging strictly alternating the clipping of half ropes to pro will probably result in greater rope drag and eliminate much of the advantage of using double rope techniques (i.e. you could have just taken a single rope). On traverses and long pitches etc. I therefore find that I inevitably need to clip the same rope two or three times consecutively to minimise rope drag and allow longer pitches to be climbed.
In my layman’s mind the difference in the UIAA standard (see below) between half ropes and single ropes to me translates into a situation were a half rope cannot take the same level of severe fall as a single rope before breaking (however I cannot quantify it in mathematical terms). Hence to my mind you need to clip more conservatively (i.e. frequently) for a half ropes compared to a single rope (at least during the first part of the pitch where a fall combined with gear pulling can more easily generate high fall factors). My interpretation of the standard is also that you absolutely do need two ropes to stop significant falls to offer a similar amount of protection compared to a single rope (if one half rope was sufficient to stop significant falls similar to what a single rope would there would simply be no need for the UIAA to have a separate double rope standard).
Therefore in double rope technique when consecutively clipping only one of the ropes without clipping the other there are a few consequences I try to keep in mind when weighing up the various compromises: 1) I am increasing the distance I fall before the other rope comes into play in stopping the fall (remember half ropes stretch quite a lot, a further fall means a greater chance to hit something on the way down). 2) I am also increasing the potential fall factor on the other rope in case things go really pear shaped (e.g. if your gear pulls on the rope you have been clipping last). 3) When I am clipping in a fashion where one of the roper is ‘simply dragged along’ I need to realise that I am stretching the limits.
My understanding of the UIAA standard for single and half ropes are summarised below but in short the standards are similar but a ‘lesser standard’ holds for half ropes (there are more to it than this of course):
- Single rope: 80kg load dropped on a rope with a +- 1.8 fall factor must generate a maximum of 12kn impact force plus the rope must have limited stretch and withstand X number of such falls before breaking
- Half rope: 55kg load dropped on a (single) rope with a +- 1.8 fall factor must generate a maximum of 8kn impact force plus the rope must have limited stretch and withstand X number of such falls before breaking
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