DARGAUD Guillaume (Dargaud@Corcaigh.GI.Alaska.Edu) wrote: > In article <19950411172407.A.Strang@sp0078.kub.nl>, A.Strang@kub.nl > (STRANG A.) wrote: > > I have a 60 meter 9.8 mm rope and I plan to climb some long walls next > > summer. For rappelling I want to buy a second rope of 60 meters. Someone > > told me that some people use a thin rope of about 7 mm together with a > > rope of, I think a rope of, normal size (= 11 mm). > Yeah, you can do that, just make sure that the knot is on the right side of > the rappel anchor. i.e. the rope that goes through the carabiner/sling/ring > is the 9.8mm one. Otherwise, as you go down the thin rope will slide a > little and you may end up with a couple feet missing.
There's more to it than that. In general you always want to pull your lead rope, which means the knot is on the other side than described above, but you have to deal with the slippage. Also, the knot can become stuck or "pop over" if rigged as described above.
Pulling the lead rope is easier since the larger diameter is easier to grasp, it saves wear & tear on your good rope, and if something gets snarled you have a length of rope in your hand instead of a bunch of 7mm static line!
So now you have slippage. The 7mm rope doesn't present as much friction to the rappel device as the 11mm, causing the ropes to slide past each other. This is exacerbated if you have a old fuzzy 11mm and a nice new 7mm.
Slippage is *bad* if you're rapping from slings; if the rope cuts through the slings you're history. Also, as mentioned above, you may end up with several feet less 7mm than 11mm at the bottom of the rappel. Slippage of only 5 feet means you may still be 10 feet from the next anchor when you have to stop! Tying the ends together will help wandering minds not rap off the end of the rope. It won't get you that extra 10 feet, but it will give you time to figure out how.
Some rap devices are better than others in preventing slip. Do some experiments in a safe situation to get familiar with how yours performs with ropes of very different diameter. An 8mm static line may be a good compromise. Be extra careful in tying them together; make sure to tighten the knot firmly.
Lord Slime