UKH

I recently took a trip to the high tatras (highly recommended) & found myself in an interesting dilemma about best practice .. 

On the first day we completed a route and ab’d off. I threw the ropes clear & began to descend. At some point a loose rock fell and through bad luck landed/bounced off one of the ropes (mammut twilight 7.5mm dry). When I got down to it the rope was essentially severed through the core, and really only holding together on 1/4 of the sheath. We got down safely & decided to get rid of the rope. A friend leaving the mountains took it down and left us a chunky single he had been using (something like 10.5mm-11mm).

i lead the following routes on the single but it came the question about the best practice for ab’ing with a thick single and a skinny half. 

Just used a normal overhand knot with long tail for joining the ropes at the stations.

however I wasn’t convinced how a Prussic round both ropes would bite. I considered using separate Prussics on each rope but didn’t.  In the end I ab’d using a reverso with the thin rope Prussic’d only as the thick rope ran so poorly through the reverso. It worked well, the only issue being that it tended to slightly run the knot at the top away from the anchor ring (but only a few meters).

Does anyone have advice/best practice for this situation? & I mean constructive advice, naturally I know that the ‘safest’ option would be to not climb/ab, or drop down to a store & replace the damaged rope.

A Prussic will grab just about anything you put it around. Just feed the ropes through the plate deliberately so they run at the same speed, and if you don't think you can do that reliably make sure any differential movement of the rope through the belay plate can't harm the belay, by for example sawing a sling as differential tension is relieved by the ropes slipping.

Whenever I've done it I've joined them with double fishermans knots (as opposed to the usual overhand) then otherwise treated them perfectly normally. Never been much of a believer in worrying about best practice, good enough is good enough.

jk

If you use a bolted abseil anchor it makes sense to thread the thick rope through the maillon rapide and put the knot on the side of the thinner rope. This way the knot will be pulled against the anchor whenever the reverso/ATC/whatever grabs only the thick rope. 

Otherwise I agree, best not overengineer things...

CB

Just use the thinner rope as a tag rope; and ab, with your Prusik, on the single.

Prussic is an acid, do not use this with nylon ropes.

As JK says, good practice is good enough. Best practice will leave you chasing the pot of gold at the end of the rainbow.

I've never tag line abbed, but I always wonder about it: don't you need some sort of bulkier arrangement than an EDK at the anchor to ensure it doesn't pull through? And doesn't this significantly increase the risk of rope snags when pulling?

Just add a screwgate clove hitched to the tag line side and clipped into abseil rope side. With smart rigging it shouldn't snag much worse than a normal EDK. As you know snagging abs are just like the Spanish Inquisition, it always happen when you least expect it!

Make sure you tie the bottom ends together, too, in case the thinner one runs through faster than the thick one.

Good practice anyway, regardless of rope diameters.

Thanks all,

yeah I should have said that the anchor was bolted ring, so the slight run through of the rope wasn't too concerning, also had the bottom of each rope knotted, therefore in a crazy case where i ran to the end of one I could thereafter equalise it I figured.

again perfect advice on knotting so that it catches on the anchor ring, it was a three ab descent so had this set up for two, but didn't bother retying the knot for the second stance.  That would have been a good option also.

I have also never tag line ab'd, I will look into this

There is a short video describing a good technique for doing this where the rope is threaded through metal work here:

youtube.com/watch?v=tjhnQ_fFwHY&

I certainly wouldn't choose to use it on featured slabby ground as it would catch quite easily, however I used it quite regularly during a 3 week trip to Arapiles and didn't have any problems with the ropes getting caught when I did that, but the abseils there are all either very steep or down pretty straightforward slabby ground.

I find the biggest problem with abbing off two very different diameters - especially with an ATC or similar is that the thinner rope never gets sufficiently pinched enough between the HMS and the ATC - due to the thicker rope being pinched at a greater diameter. . - This means the thinner rope will just run through if it isn't held with a prusik - Either a figure of 8 device - or tag line abbing resolves this.

Not sure about 'best' practice but you did pretty much what I'd have done (with the exception of 7.5mm ropes - I'm too scared to contemplate a fall on those !).

The mantra "smoothly does it" comes to mind.

Yeah I agree, my partner had a fig 8 with him & had far more smooth descent