System Optimization: Tagline + Dynamic Double Rope Rappel, Pull Dynamic Line Down

So, I have been playing around with a system for double rope rappels:

Dynamic Single Rope 8.7mm joined with Pur Line 6mm using an EDK rappelling through a standard ATC with a 5mm accessory cord (I think polyester material?) for a third hand.

The issue I am having is 'slippage' of the entire Pur Line. It seems to me this slippage is movement through my third hand knot (I have tried an autoblock and a prussik, with the prussik knot working a little better). Yet, the slippage leads to movement of 20-50' of the Pur Line up and through the rings, leaving with a shorter rappel and more risk to rappelling off the end of one line. To clarify, the joined knot moves down the face, the Pur Line feeds through, and the dynamic line continues to stack more on the ground.

To visualize, the system starts like this (note: the rope in the pictures is a 9.4mm cragging rope, not my 8.7mm alpine):

And begins to creep or move in this direction (the picture shows some change, but for a full rappel, the Pur Line would continue to feed up and through the anchor master point for another 50’). 

The creep/slippage was even worse with my climbing partner’s hollow block.

I would strongly prefer to rappel both strands and pull the dynamic line so I have more lead line at the next station in case I need to climb up to free the ropes if they got stuck. If I went with a knot / carabiner block and pulled the tagline, I could be left with just tagline to free the stuck system. I do not think with this approach you could add a carabiner block to prevent movement.

With the below setup, I could either single line rappel the dynamic, or rappel both lines, and pull the tagline (But I am trying to avoid ending up with only the tagline).

With the following setup, I could single rappel the tagline and pull the dynamic line (which would satisfy what I want to achieve, but single rappelling the skinny line seems like introducing another variable requiring risk mitigation). If I rappelled with both ropes, I am in functionally the same situation as the first two pictures – just that my carabiner block will slowly creep down with the rest of the system. 

From some reading, joining and rapping both, while pulling the dynamic first, seems to be a semi-common, but 'off label', approach motivated by the reasoning above to keep as much dynamic line with you.

I have experimented with adding a second carabiner to the ATC as well, but that did not seem to help. I threaded the strands to individual barrels in my ATC, but I am wondering if I thread both through the same barrel if that would help with the different feeding rates causing the slippage.as possible for self rescue. 

So what could be done to resist movement of the entire system? Different third hand cordage? Different rappel device setup? 

Or am I just trying to have my cake and eat it too? In other words, do I need to make a compromise in my goals or move to a larger second rope?  

Stacked rappel? Second person’s device keeps the system from creeping while the first raps. Then the first person down fixes the ropes to the next anchor which prevents rope creep?

Julian Davis wrote:

Stacked rappel? Second person’s device keeps the system from creeping while the first raps. Then the first person down fixes the ropes to the next anchor which prevents rope creep?

Great idea. I personally wouldn't trust the tag line from slipping without a mule+overhand to tie it off. At that point, might as well do a BHK to isolate the strands.

Fixing the rope at the bottom is good idea, but it has some issues:

• If doing a double strand rappel, the tension in the tag line might provide a firefighter's belay and prevent movement.
• If doing a single strand rappel, the person up top can't personally verify the tag line is fixed. Yeah, they can test it and trust their partner fixed the rope appropriately, but we do partner checks for high consequence situations for a reason. Even with static tag lines, there might be enough stretch in the system to prevent proper testing of the system before committing. 
• The lower station may not be suitable for an upward pull. Not a huge deal (the lower person can just fix the rope to their belay loop, providing a counter-balanced rappel), but not ideal.

I’ve done this with an Alpine ATC and two biners on the Purline to add friction. I got basically no creep with this setup. 

I agree stacked rappel is the best option. I’ve been the top person when I saw this start happening and quickly threw a prussik on the downcreeping strand and clipped it to the anchor. Then yelled for my partner to fix the strand at the anchor below when they got there and that worked fine.

If you’re not concerned about pulling the dynamic line first then you can obviously biner block the other side and/or rap on one strand, but I understand your thought process regarding possible stuck rope. 

Andy Kirkpatrick covers this situation in his book Down. The simplest “solution” is to accept the creep. Just make sure the purline either has a giant knot/carabiner in the end or the end is clipped to you. That way, it’s fine if the purline ends short. You just keep rappelling as the rope creeps back the other direction. Also, I would recommend using the Gibbs/Half Gibbs if you’re joining purline to another rope.

Julian Davis wrote:

Stacked rappel? Second person’s device keeps the system from creeping while the first raps. Then the first person down fixes the ropes to the next anchor which prevents rope creep?

Yes, this was one thought my friend had. I will have to try. 

Adam had some things to consider about communication + unintentionally creating a tensioned fireman's belay for fixing above/below which had made me a bit wary about approaching this 

Zach K wrote:

I’ve done this with an Alpine ATC and two biners on the Purline to add friction. I got basically no creep with this setup.

Zach, to confirm, your set up is Alpine ATC, two biners, and combining a Purline + Dynamic lead line.

Are you using a BD Alpine ATC or what brand? I was thinking about buying one of those but was wondering if this was a gear or a skill issue. 

Are you using a third hand knot? If so, what material and thickness are using for cord and what knot are you tying? I was seeing some clips of guides using some skinny (~4mm) cord of techy materials. I am not sure if that is needed.

This would be about the system I would want to use. I am fine carrying a specialty rap device (and having one for a partner) if it makes rappelling pretty close to 'standard'   

Leif Mahoney wrote:

Andy Kirkpatrick covers this situation in his book Down. The simplest “solution” is to accept the creep. Just make sure the purline either has a giant knot/carabiner in the end or the end is clipped to you. That way, it’s fine if the purline ends short. You just keep rappelling as the rope creeps back the other direction. Also, I would recommend using the Gibbs/Half Gibbs if you’re joining purline to another rope.

Leif - yeah, the 'accept' method is what I did in the moment. But the ropes moving while being waited is obviously not my preference as that accelerates wear.

The Gibbs / Half Gibbs is interesting. I see an article here discussing the details: https://www.andy-kirkpatrick.com/blog/view/rappel-knots-1

To note, when I was joining the lines, I had a full arms-length of tail an EDK and a second overhand (EDK) just after my first. I either read this somewhere else or saw it in Petzl's manual (below). And I agree, that is a good reminder to back-up the single overhand due to a rolling knot (and like the Petzl diagram shows to avoid the F8)

Thanks for the input, everyone! I am reading each comment closely.

Nate Grygo wrote: 

Zach, to confirm, your set up is Alpine ATC, two biners, and combining a Purline + Dynamic lead line.

Are you using a BD Alpine ATC or what brand? I was thinking about buying one of those but was wondering if this was a gear or a skill issue. 

Are you using a third hand knot? If so, what material and thickness are using for cord and what knot are you tying? I was seeing some clips of guides using some skinny (~4mm) cord of techy materials. I am not sure if that is needed.

This would be about the system I would want to use. I am fine carrying a specialty rap device (and having one for a partner) if it makes rappelling pretty close to 'standard'  

Yes, I'm using a BD Alpine ATC. I have two biners on the Purline, and only one on the lead line. The goal is to add extra friction to just the Purline side to prevent the system from creeping.

For a third hand I use plain old 5mm nylon since that's what I already own. With an extra wrap or two it grabs well enough for me to go hands free.

Zach K wrote:

Yes, I'm using a BD Alpine ATC. I have two biners on the Purline, and only one on the lead line. The goal is to add extra friction to just the Purline side to prevent the system from creeping.

For a third hand I use plain old 5mm nylon since that's what I already own. With an extra wrap or two it grabs well enough for me to go hands free.

Cool stuff. Thank you. I mocked up a few versions at home based on "two biners on the Purline, and only one on the lead line".

There are a few ways I could see approaching this: 

Scenario A

3 carabiners (to prevent the lead line rubbing/catching): 

Scenario B 

2 carabiners (with the Purline contacting 2, the picture below has the lead line connected to just the back one. This could be flipped, not sure if there would be a difference. My first reaction is that the lead line might get 'pinched' between the two carabiners and that would create more friction where we are trying to avoid it. But, it would be best to test that theory.):

Scenario C

2 carabiners (1 Small, 1 Regular sized). With a smaller carabiner, it sat best with the small carabiner in front and with the lead line clipped to the regular sized carabiner. This might help with avoiding pinching?

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This may be something that I need to go out and test to see if there is any setup that works best. Of the scenarios above, do you have any thoughts on which setup is most similar to what you used? Or if not what would be the best way to rig (while keeping things simple to reduce opportunity for error)?

If you just use half ropes, you have no problems to resolve. And you gain some advantages. 

Eric Craig wrote:

If you just use half ropes, you have no problems to resolve. And you gain some advantages. 

Yes, definitely a well-proven system but it does have its own pro's + con's

Potentially another scenario to add friction solely to the tagline, extend the rappel and a second device with one or two carabiners. 

Pro's for this would be the initial setup of the rappel system would be the standard process (no potential misclipping) and then you could add the system above. 

I am going to find some time to test these options and share thoughts but would be open to any feedback before. 

I typically don't like solving problems by buying gear, but the Edelrid Rap Line  solves your problem completely, albeit with more weight than the Pur Line. Since you can lead on it, it doesn't really matter which rope is through the rings.

Similarly, a super skinny half rope might be worth its added weight (36 vs 20 g\m for the PUR) as a tag line to save the faffing\risk involved with what you want to do.

Adam Fleming wrote:

I typically don't like solving problems by buying gear, but the Edelrid Rap Line  solves your problem completely, albeit with more weight than the Pur Line. Since you can lead on it, it doesn't really matter which rope is through the rings.

I suppose I'd rather lead on a single strand of Edelride Rap Line than Pur Line but isn't the idea that you're suppose to fold it in half and climb use it like a twin? 

Andrew Giniat wrote:

I suppose I'd rather lead on a single strand of Edelride Rap Line than Pur Line but isn't the idea that you're suppose to fold it in half and climb use it like a twin? 

Yes. 1/2 the distance is better than none in my opinion. I guess the point is moot when the rope gets stuck at the anchor, but the sentiment still stands. 

Yes, interesting Edelrid has a tagline with a backup reserve. It seems like a glaring shortsight in a system to end up in a position with just a bit of a tagline and a stuck rope. Yet, this is how the most commonly recommended setup is? Maybe for someone buying a new tagline this is something to keep in mind? 

And not to be a gram weenie but with 36 vs 20 g/m that ends up being nearly double the weight (and I'd assume) double the size. Which could require me to take a pack bigger than my small ~12L pack that sits tight and small on my back. Additionally, with something like a static tagline, you have the option of tagging/hauling with it as well. So, if you are approaching with that in mind, it could be a good multi-use item, which is always helpful for bigger days. 

Nate Grygo wrote:

I would strongly prefer to rappel both strands and pull the dynamic line so I have more lead line at the next station in case I need to climb up to free the ropes if they got stuck. If I went with a knot / carabiner block and pulled the tagline, I could be left with just tagline to free the stuck system. I do not think with this approach you could add a carabiner block to prevent movement.

...

Or am I just trying to have my cake and eat it too? In other words, do I need to make a compromise in my goals or move to a larger second rope?  

Nate, it might be the case that for your specific goals, the Pur line is not the best option.  It's my understanding that the Pur line was specifically designed to be ultra slippery, so that it's less likely to get caught on the pull.  When you combine the small diameter and super slippery exterior, you have a recipe for rope creep when trying to use it the way you're indicating, which is probably why Petzl doesn't recommend that.

There are probably other tag lines out there that would be better suited to this specific application, perhaps the Rad line or other options that are more textured and/or larger diameter.  Perhaps someone else would be able to make a more specific recommendation for your needs, most of my experience is with small and slippery tag lines that are best used with a blocking system.

(I was reminded of this thread last week, when my partner and I did ~20 rappels with a Pur line and a lead line, and I was thinking about how ridiculously slippery that thing is, and how I would be pretty spooked to try to use it as an actual part of the rappel system; we blocked all of our rappels, which was fairly conducive in the clean/slabby terrain we were in).

Colin Haley made a video about this topic the other day. The tl;dr of it is that he does a biner block and pulls the Pur line.

A lot of comments in here about the slipperiness of the PUR line in specific, and I think that’s a valid consideration. RAD line is a good but expensive alternative for something with a grabbier sheath, and there are plenty of other options out there as well. That said, this phenomenon is still something you’re going to have to mitigate for. Slippage may be less if you’re using something with a rougher sheath, but it’s just physics that the skinny cord is going to try to move thru the device faster than the lead line. 

In Colin’s video he is showing the situation where the knot would be sucked into masterpoint, not creep away (single rope threaded vs tagline threaded, respectively).  I dont see the need for the blocking carabiner there unless rings are unusually large.  I do agree that adding carabiner block when tagline is threaded is a good idea (every other rappel), but is totally unnecessary and adds snag risk (i know he stated it doesn’t increase snags but my brain struggles to get on board with that) when single rope is threaded thru standard hardware and you’re equipped to do a double strand rappel.

You will always have access to the end of the tagline when threading it and pulling from previous rappel.  You can tie a stopper or a bight knot and tie it to yourself and then u can deal with the creep in the Kirkpatrick manner described above (let it happen) and deal with it if needed once you are at the stopper knot in skinny rope).

As referenced above, unfortunately knot migration with a Pur Line joined with a dynamic line led to a fatal accident. There is some relevant discussion here:

https://www.mountainproject.com/forum/topic/203665581/rappel-accident-involving-knot-migration

A possible safegaurd discussed in the tread is the "J Loop" technique.