In reply to kritter:
> having a prusik on the rope coming up through the quickdraws from belayer to anchor.
> > would the prusik round both strands not behave the same way?
I was talking about being lowered off a route here not abbing. In the lowering off case, it wouldn't make any sense to put a prusik around both strands of rope as they're moving in opposite directions.
In the abbing case, if your top anchor fails and you have removed all the other gear as you go down (like you said you were doing) you will just fall - the rope isn't attached to anything anymore so the prusik won't help. The rope would just pull through the gear that it's still clipped to lower down.
The one scenario I can see where it would help is if you also had the rope anchored at the bottom of the pitch (sorry if I've missed you saying this somewhere), then in which case it would be equivalent to taking a leader fall, but with the link from the rope to your harness being the prusik. This is the same scenario as would happen if the top anchor fails in a lowering off scenario with a prusik around the 'up' rope that I was talking about before.
As to whether a prusik linking rope and harness can cope with a leader fall, probably yes. Most 5mm cord breaks at about about 5kN I think, you have two strands in the prusik so get 10kN, but then the double fishermans knot reduces the strength, I can't remember by how much exactly but lets go for a conservative 50%. I don't know how much the wrapping around the rope in the actual prusik knot reduces the cord strength but I doubt it's as much as 50% and as it's in series with the double fishermans it's a weakest link scenario - only the biggest reduction matters. So we have 10kN x 50% = 5kN which is a breaking strength larger than the force generated at the harness in most leader falls. A lot of ropes will only give an ~8kN force in the UIAA FF 1.7 test, so if it's not a high FF fall you'd probably be fine. Using a 6mm prusik cord would obviously increase the max force it could take.
Post edited at 02:23