Ascending Double Strand Rappel

[Turtle]

In a separate thread (http://www.canyoneering.net/forums/showthread.php?t=473) discussing the Great Falls of the Fox incident at the San Gabriel Rendezvous



Today would not have had the potential for harm if our 1/9 could have safely ascended the rope just ONE foot.
I'm wondering how this would have been done, given that the rappel was rigged as a double stranded rappel. Specifically, I've heard that a Prusik knot doesn't grab a double strand very well. On the other hand, I've heard that Prusiks work best with "thin grabbing thick", and two is certainly thicker than one. Moreoever, I've tried (in my living room anyway) attaching a Prusik to a double strand (6 mm on 10 mm), and it seemed to grip just fine -- though I wasn't inclined to commit my generous bodyweight to my stairway railing anchor.

As for mechanical ascenders, I can certainly see how many of these would prove challenging when confronted with a double strand not fixed at the anchor.

So, my general question is:

1) How does one ascend a rappel line that is rigged as a double strand and not fixed at the anchor?

And more specifically:

2) Can Prusik slings be used succesfully?

3) Are there any rigging tricks that would allow the use of a mechanical ascender?

Finally, is this all just one more argument in favor of rigging a single strand rappel from a contingency anchor? Presumably by the time the last person descends on a double strand, much uncertainty will have been removed by those who have rappelled before and the likelihood of ascending will be greatly diminished.

[mtngoat59102]

Finally, is this all just one more argument in favor of rigging a single strand rappel from a contingency anchor? Presumably by the time the last person descends on a double strand, much uncertainty will have been removed by those who have rappelled before and the likelihood of ascending will be greatly diminished.

Well, if you have the option and you know you are ascending all the way to the top you can tie a blocking knot or a variety of options and pull your rope through the anchor until it blocks itself and then ascend the single cord. If you are uncertain you will be able to cleanly pull your 'block' up to the anchor don't use this method unless it's your last option. Another legitimate option is to anchor the off side someplace on the ground. This will make a fixed single side and still let the party on the ground control rope pull at the anchor. Does this answer your questions?

[rcwild]

I had a conversation with someone yesterday who said he could recognize ACA students because they tend to carry a Petzl Shunt. A Shunt will work on two strands of rope -- as long as they are identical diameters.

Prussiks (and klemheists, bachmans, etc.) will work on two strands of rope.

If you only have mechanical ascenders designed for one strand, and your rope is rigged toss and go -- rig one of your ascenders on each strand and connect them to your harness. Set them up so you can move them both up at the same time. Take a loop of rope below you and wrap it under your foot. This is your improvised foot loop. Stand up, move your ascenders up togehter, then come to rest on them. Adjust your foot loop so you can stand up higher. Repeat.

Or ...

If the people at the top know you are stuck and you only have mechanical ascenders designed for one strand, they can fix the strands. Quickest way to do this is with a Shunt (if ropes are identical diameter), but it can also be done with a prussik on each rope with each prussik secured to the anchor.

This scenario is one of the reasons why it is good to use a contingency anchor when beginners are involved. (A) They are more likely to become stuck on the rope, and (B) they are less likely to know how to become unstuck.

[Paul Nelson]

Another legitimate option is to anchor the off side someplace on the ground. This will make a fixed single side and still let the party on the ground control rope pull at the anchor. Does this answer your questions?

Be very carefull if you rig toss-n-go and then later tie-off one side of the rope at the bottom because the anchor at the top is now a 'pulley'. This results in force being multiplied on the top anchor. If the anchor is marginal, now you have a new, and perhaps a worse, problem! :eek:

As Rich keeps saying, if you have uncertainy, use a block. That will prevent the anchor from becoming a pully.

[mtngoat59102]

Be very carefull if you rig toss-n-go and then later tie-off one side of the rope at the bottom because the anchor at the top is now a 'pulley'. This results in force being multiplied on the top anchor. If the anchor is marginal, now you have a new, and perhaps a worse, problem!

Good point. To my understanding though the force multipliers here are not off the chart like in an 'American triangle' etc. Once you start doing the mental exercises of what if this and what if that I hope it becomes blindingly obvious why the 2 seconds required to put a block in one side of your rope is important. It can save you all kinds of headaches later.

However, as long as we are what if'ing things to death. If the rope pull is questionable the last person down should remove the block to minimize the risk of it snagging and sticking your rope. What else did we miss? What if, there is a meteor shower and you're rappelling into a hydraulic full of crocodiles and your rope is 2 feet short? :D

[rcwild]

Neil provided two options. Both assume you are already on the bottom. To clarify ...

(1) Tie figure eight knot in one end of the rope. Clip the loop to the other strand of rope with a carabiner. Pull on the strand without the biner to pull the knot and biner to the top, thus creating a block. This technique does not create the 2:1 pulley problem Paul described.

(2) Tie one end of the rope to an anchor at the bottom and ascend the other strand. This works, but creates a 2:1 mechanical disadvantage. If the ascender puts a 200 pound load on the side he/she is ascending, the anchor at the bottom must also hold 200 pounds to offset the load. The pulley (rap ring) at the top must hold 400 pounds. Be sure you have a solid anchor.

[mtngoat59102]

I had a conversation with someone yesterday who said he could recognize ACA students because they tend to carry a Petzl Shunt.

Guilty. After taking my first ACA class I went right out and bought a shunt. After more mileage I now only carry it while I'm doing canyons with beginners. If you are working around an anchor, moving people on & off rap and belaying from the anchor it's really slick to have around. It works for ascending but isn't very high on my favorite list due to it's rope limitations. You can do a lot more with other items that weigh a lot less.

If I was guiding, I would always carry one. My 2 cents. YMMV

[dccampen]

To my understanding though the force multipliers here are not off the chart like in an 'American triangle' etc.
I don't know what an "American triangle" is or what you consider to be "off the chart" but fixing one end of the rope as you suggested does double the force on the anchor; I consider this to be significant. This is even more significant considering that someone is going to be ascending the rope; likely jerkily and while using a steel cable like rope such as Canyon Pro.

[mtngoat59102]

I don't know what an "American triangle" is or what you consider to be "off the chart" but fixing one end of the rope as you suggested does double the force on the anchor; I consider this to be significant.

There are many anchors that will serve a working load of 400-500 lbs very easily. Understanding that you are doubling your weight is an important thing to consider.

When building anchors the angle and direction that the load is equalized between the attachment points can rapidly multiply load forces. I won't be good at describing this here in this post. (Anything above 120 degrees starts what is nick named the 'American Triangle') Does anyone have a good link to an internet site that describes this? This does not relate to ascending as in the above posts but more to anchor construction. This is an important concept to understand if you do not.

There is a brief description of this in 'Freedom of the Hills' in the Climbing Fundamentals section. It is in the part where they talk about equalizing your anchors.

[rcwild]

Run a piece of webbing through two bolt hangers. Grab one strand and pull it down to form a "V". Leave the other strand tensioned laterally between the two hangers. American Death Triangle. A serious load multiplier. Downward force also creates inward force on the hangers. In addition, there is a 2:1 mechanical disadvantage working on each hanger because they are functioning as pulleys.

Always better to grab both strands of webbing and pull them down to form a single "V". Angle of the "V" affects force on hangers. If angle is zero degrees, each hanger will hold 50% of the load (in theory). If angle is 30 degrees, each hanger will hold 53% of load. At 60 degrees, each hanger holds 58%. At 90 degrees, each hanger holds 70%. At 120 degrees, each hanger holds 100% of the load. Climbing books suggest keeping angle below 90 degrees. Even better to keep it below 60 degrees.

[dccampen]

At 60 degrees, each hanger holds 58%. At 90 degrees, each hanger holds 70%. At 120 degrees, each hanger holds 100% of the load. Climbing books suggest keeping angle below 90 degrees. Even better to keep it below 60 degrees.
Yes, this is basic stuff that I know quite well. I had just never heard the term "American Death Triangle".

[dccampen]

There is a brief description of this in 'Freedom of the Hills' in the Climbing Fundamentals section. It is in the part where they talk about equalizing your anchors.
I am not familiar with this book. The books that I study are: "On Rope", "On Call", "Alpine Caving Technique" and "High Angle Rescue Technique". None of them use the term "American death Triangle".

[skianddive]

Here is a depiction of the infamous American Death Triangle:

[catware11]

I am not familiar with this book.OMG!!! That book is the bible!

[rcwild]

"Freedom of the Hills" might be the Bible of mountaineering, but "On Rope" is the Bible of caving. Until someone finishes the Bible of canyoneering, I believe "On Rope" is just as relevant, maybe more relevant to canyoneering than "Freedom of the Hills".

[catware11]

Ahhh, forgot that there are just as many (if not more) moles as eagles in this club. Having spent most of my outdoor life on the sides of mountain slopes I'd forgotten about the subterranean dwellers here! :D

I thought that french canyoneering book in Monterrey that Koen passed around looked really good. I wish they'd hurry up on a translation!

[Paul Nelson]

Ahhh, forgot that there are just as many (if not more) moles as eagles in this club. Having spent most of my outdoor life on the sides of mountain slopes I'd forgotten about the subterranean dwellers here! :D

I thought that french canyoneering book in Monterrey that Koen passed around looked really good. I wish they'd hurry up on a translation!

Umm, I prefer the term Troglodyte rather than mole! :D

To me, canyons are similar to caves but with sunlight. However, most of time we have to go back up that rope!

I plan on doing a night-time canyon trip.

Paul