How should I cut my rope?

[aricooperdavis]

TLDR: Visit how should I cut my rope to find a tool that evaluates different rope length options.

There's been loads of discussion on here about the best selection of rope lengths to have for your SRT adventures, including some great work analysing the frequency of different pitch lengths and so on. Whilst working on the what can I rig tool I realised that I was effectively developing an objective function that spits out a value for how "optimal" a certain selection of ropes are. With a bit of fiddling it's been fairly straightforward to turn this into a tool that allows you to plug your ropes in and get out a range of different options for cutting them.

Enter how should I cut my rope? This tool works out all the possible options for cutting your ropes and spits out the best of those options. It evaluates every rope combination in terms of the number of trips you can rig with it, and the efficiency of doing so (the proportion of rope that you're carrying for a given trip that you actually need, averaged across all possible trips).

This tool is driven by the same data that the what can I rig tool uses, which (at the time of posting) comprises 292 trips from Yorkshire, Derbyshire, and Scotland.

Here's a screenshot showing the results for some of the ropes I'd like to cut:

You'll note that it's not showing me results for many of the possible rope combinations. This is because if I displayed that many table rows on one web-page or data-points on one graph it would probably crash your web-browser. As such by default it only displays Pareto efficient combinations.

Other tweaks to improve performance include setting a resolution (5m by default) and having the option to disable the graph view.

Even so it can take a very long time to evaluate all the possible options, so it maybe best to start it running and go and make a cup of tea if you have a lot of ropes that you want to cut.

I hope this is somewhat useful, and if you'd like to contribute any more trip data please get in touch or do it directly on GitHub.

 

[Fulk]

Presumably this only shows what trips you can do without tying ropes together.

 

[andrewmcleod]

You could add a 'shrinkage' parameter...

 

[aricooperdavis]

Presumably this only shows what trips you can do without tying ropes together.

Indeed, and that's a whole can of worms I am happy to ignore. You can almost certainly rig using shorter ropes joined at intermediate bolts or even mid-pitch. However the data that drives this takes rigging guides and guide books literally (even including hand-lines, as it's too complex to determine whether a handline is "optional" or not.)

 

[aricooperdavis]

You could add a 'shrinkage' parameter...

Different ropes shrink by different amounts (particularly if you're cutting a rope you've already used for a while) so we would need a shrinkage parameter for each rope entered. Personally this seems slightly overcomplicated when you could just adjust for shrinkage before using the tool, or even pre-shrink your rope before cutting it (unless this is a bad idea for some reason?).

Edit: I'd accept a pull request though...

 

[Fjell]

The old recommendation from Elliot for the Dales was 2x10, 2x15, 2x20, 2x25, 2x30, 40, 50, 60. Which is what you get from 2x200m reels after shrinkage. Always seemed a good basis to me and I can’t remember the last time I had to actually pass a knot. I usually keep more than that but it just means I reduce the extra rope I need to carry, which is a minor luxury really. I have had to pretty much dump my entire rope stock a few times and this is what I start from.

 

[steviet_scg]

Looks interesting and I've enjoyed playing with the what can I rig tool!

Re. Passing knots - plenty of times you can (with planning, rather than finding yourself accidentally 10m off the deck) use the additional rope for the traverse so no knot pass is needed. Max length of rope between two anchors would be a useful variable to know 🤔

Whilst I like the efficiency metric my usual approach is to cut ropes to fit my most often visited trips. I'll cope with carrying more now and again to be bob on more often. Different sort of efficiency I guess.

For the guidance of the wise as per usual

 

[cap n chris]

Presumably this only shows what trips you can do without tying ropes together.

Hey kids, don't tie ropes together if you wish to avoid rescuing someone who is hung up. That's a proper palava. If you're brave, ignore me.

 

[aricooperdavis]

The old recommendation from Elliot for the Dales was 2x10, 2x15, 2x20, 2x25, 2x30, 40, 50, 60. Which is what you get from 2x200m reels after shrinkage.

The tool can't quite handle 2x 175m ropes at 5m resolution (p(175/5) is 14883, and 14883² is 221.5 million possible combinations for the tool to test), but 2x 180m ropes at 10m resolution gives:

Cut lengths	Possible trips (of 292)	Efficiency
[[90,50,40],[60,40,30,20,20,10]]	272	0.811
[[60,50,40,30],[90,30,20,20,10,10]]	270	0.827
[[60,50,40,30],[80,30,20,20,10,10,10]]	267	0.832
[[60,50,40,30],[70,30,20,20,20,10,10]]	265	0.833
[[60,50,40,30],[60,30,20,20,20,10,10,10]]	257	0.836
[[70,50,30,30],[40,40,20,20,20,10,10,10,10]]	255	0.839
[[60,40,30,30,20],[50,50,20,20,10,10,10,10]]	251	0.842

This would be Elliot's:

Cut lengths	Possible trips (of 292)	Efficiency
[10,10,15,15,20,20,25,25,30,30,40,50,60]	245	0.892

 

[Fulk]

Hey kids, don't tie ropes together if you wish to avoid rescuing someone who is hung up. That's a proper palava. If you're brave, ignore me.

Sorry, cap, I was under the impression that passing knots is an essential SRT skill . . . still, what the f*** do I know, given that I only started dabbling in SRT in 1972?

 

[Babyhagrid]

Hey kids, don't tie ropes together if you wish to avoid rescuing someone who is hung up. That's a proper palava. If you're brave, ignore me.

Knot passes are a taught part of learning SRT? And I've never found them to be that much of an issue. With a nice loop to clip into and some knowledge of down prussicing it's pretty easy.

 

[Fjell]

The tool can't quite handle 2x 175m ropes at 5m resolution (p(175/5) is 14883, and 14883² is 221.5 million possible combinations for the tool to test), but 2x 180m ropes at 10m resolution gives:

Cut lengths	Possible trips (of 292)	Efficiency
[[90,50,40],[60,40,30,20,20,10]]	272	0.811
[[60,50,40,30],[90,30,20,20,10,10]]	270	0.827
[[60,50,40,30],[80,30,20,20,10,10,10]]	267	0.832
[[60,50,40,30],[70,30,20,20,20,10,10]]	265	0.833
[[60,50,40,30],[60,30,20,20,20,10,10,10]]	257	0.836
[[70,50,30,30],[40,40,20,20,20,10,10,10,10]]	255	0.839
[[60,40,30,30,20],[50,50,20,20,10,10,10,10]]	251	0.842

This would be Elliot's:

Cut lengths	Possible trips (of 292)	Efficiency
[10,10,15,15,20,20,25,25,30,30,40,50,60]	245	0.892

Not sure how that works, but the only thing I can think offhand you can’t do without a knot is GG direct cos you need a 90m rope. Otherwise that rope set works for all caves (the ropes add up to about 50% longer than the longest cave, which is something like Meregill). Obviously you can join ropes at belays. I cunningly no longer have a 90m rope.

 

[cap n chris]

Sorry, cap, I was under the impression that passing knots is an essential SRT skill . . . still, what the f*** do I know, given that I only started dabbling in SRT in 1972?

They are. But it's amazing how many people can't do them without getting their knickers in a twist.

 

[Fulk]

Hmmmm . . . I wouldn't fancy GG direct on a 90-m rope! (And thanks for your reply, cap.)

 

[wellyjen]

The tool can't quite handle 2x 175m ropes at 5m resolution (p(175/5) is 14883, and 14883² is 221.5 million possible combinations for the tool to test), but 2x 180m ropes at 10m resolution gives:

Cut lengths	Possible trips (of 292)	Efficiency
[[90,50,40],[60,40,30,20,20,10]]	272	0.811
[[60,50,40,30],[90,30,20,20,10,10]]	270	0.827
[[60,50,40,30],[80,30,20,20,10,10,10]]	267	0.832
[[60,50,40,30],[70,30,20,20,20,10,10]]	265	0.833
[[60,50,40,30],[60,30,20,20,20,10,10,10]]	257	0.836
[[70,50,30,30],[40,40,20,20,20,10,10,10,10]]	255	0.839
[[60,40,30,30,20],[50,50,20,20,10,10,10,10]]	251	0.842

This would be Elliot's:

Cut lengths	Possible trips (of 292)	Efficiency
[10,10,15,15,20,20,25,25,30,30,40,50,60]	245	0.892

Impressive that Elliot got to such an optimal list just from his considerable experience. Storage space I have available and access to club rope means my personal store is pretty small these days and doesn't get used much, but what I have tends to the low end of that distribution. 10,10,15,20,35. Our club store and I reckon others around here have a 90m rope mainly for the pitch down to the Event Horizon in Titan. The local (and bigger ) version of GG.

 

[Fjell]

Hmmmm . . . I wouldn't fancy GG direct on a 90-m rope! (And thanks for your reply, cap.)

It’s on the current topo, even with P bolts. I don’t mean the 120m suicide drop under the spout inlet. Since I no longer have two 60m x 10mm I would use 60, 30+40 for Dihedral - if I could talk my wife into it after the last time, long ago, when she got flood pulsed when derigging.

 

[aricooperdavis]

They are. But it's amazing how many people can't do them without getting their knickers in a twist.

When I did risk assessments for taking students on SRT trips one of the highest consequence x likelihood risk factors was inexperienced cavers getting exhausted/hypothermic whilst tackling pitches. I absolutely wouldn't rig a knot pass on a trip where I wasn't pretty confident that it could be managed by everyone with ease.

 

[Fulk]

Sorry, Fjell, I didn't realize that you meant the Main Shaft route that involves traversing round the top; I thought that you meant going straight down or the Jib Tunnel route. (When I say straight down, this is something I did many years ago (before SRT in the UK got sophisticated), when I tied a rope round the big boulder near the edge, fed it down, and went down and back up without benefit of rebelays / deviations or any such techniques; I actually did it twice, the second time taking a girl friend . . . not that I'm recommending it, of course!)

 

[Steve Clark]

We had a go at doing this manually a couple of years ago when we got a new reel of 200m. Just flicking through topos etc. We ended up with something similar to the Elliot list for the Dales (from memory - 20,20,25,30,30,40,60 & two 90m's that we share with the climbers for sea cliffs).

Now we do quite a lot of trips as 2-3 people with camera gear too. Where our list really fell down was missing the 10s & 15s. We were often carrying an extra 10 or 15m on several ropes. The weight isn't much of a problem, but we were often spilling over into a 3rd or 4th bag which is much more awkward for a crawl to pitches.

You do naturally create some of these short lengths, particularly when you let a climber use a longer length on granite and you have to chop it up afterwards!

 

[mikem]

They are. But it's amazing how many people can't do them without getting their knickers in a twist.

Most routes you can just join the ropes at a bolt, so makes no difference to participants