Slackline tangling

Question

This question is about random formation of knots in a systematically tied rope. I will give some background, hope that does not make it off-topic.

In climbing (and sailing) one has to store very long ropes (over $25m$) and have them quickly available for use. There are several methods for tying these ropes into bundles. In lack of a better way of explaining it the method I use is described in this video (important part starts at 1:25, watch till 2:30).

It has always worked fine for me and I know that a lot of climbers use it and do not have any problems. Now recently I picked up another hobby: slacklining. The slackline I currently have is about $100m$ long and I use the same method for tying it up. But when I try to untie it, I always find it entangled with knots having formed... and they are sometimes highly non-trivial.

The main differences between a slackline and a climbing rope is that the former is a flatband and the latter is a round rope and that they are made of a different webbing (climbing rope's surfaces are usually a bit rougher).

Why do these properties make a difference? Is there possibly a way to adapt the method of tying it up?

Related: Why do earphone wires always get tangled up in pocket?

Answer 1

I love this question! I had never thought about it before, but you made me...

I suspect that the difference is in the flexibility. If you have a "flat" line (which is how you describe your slackline), then it will bend rather easily about the "thin" direction. By contrast, when a rope is braided so it is circular, it has a certain innate stiffness. This can be further exacerbated by changing the friction properties of the braid: the rougher the braid, the more it becomes a stiff jacket for the rope. If the braid is very smooth, it will slide easily and accommodate bending.

Once we decide that there is little energy penalty for bending, you make available to your rope many more twisted configurations; and one of those will pull tight into a knot...