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Thread: Rope Physics Question

  1. #1

    Rope Physics Question

    Before I ask a question for some good insight, let me share some background.

    An adult newbie, 250-260 lbs, is joing our group on a route that has what has been described as either a 160 to 170 rappel. He has told me that he feels comfortable doing it as a double strand rap. He has had some experience doing several double strand raps before, but not as long.

    In my groups' arsenal, we have a 320 foot 9.5mm rope, and also a 160 foot 8mm and a 185 foot 8mm rope. The rest of the group will single strand down. Not having done the route, we are uncertain if the 320 by itself will be enough doubled over, or even if the other two single ropes connected will be enough.

    Here is the question. What would happen to the rope upon descent with the following conditions? The first rapper down is going to measure the distance for certain. What would happen if the length is found to be more accurately 170 feet, and the 320 foot rope doubled up is not long enough? Is the following unsafe in your opinion?

    We stone knot the 320 foot rope at the anchor, so that both strands are secure; but we have the two sides be uneven, so that at least one side reaches the bottom. The other side of course will be approx. 10-20 foot off the ground. What would happen if the beginner goes down the rope double stranded, both sides again stone knotted off, and then he hits the end of the one short side? He of course will then continue the remaining single strand. Is there any danger with that scenario? I imagine that there will be a jostle as his weight is now totally on the longer strand.

    And an additional question. If he were to need an emergency fireman's belay from the bottom, what happens to the rope when you would be pulling down only on one side, the other side being 20 feet off the deck? Both rope strands again are stone knot secured at the anchor. What would the rope do?

    We'll end up getting, borrowing two ropes that will be long enough, but I just wondered and curious about what would the rope physics be like on these two situations...?

    Thanks for your info.
    Only Dead Fish Go With The Flow

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  3. #2
    Take him to a bridge or other place and have him try a single strand rappel. Find a device that adds a lot of friction like the Totem. Rappelling doublestrand that isn't matched is asking for injury.

  4. #3
    So many things to discuss with this thread. The first thing any person who rappels needs to learn IMO, is to add and maintain the proper friction with the device they have. Period. Any noobie should learn to rappel bare handed with a competent top or bottom belayer to stop the person if his hands get hot. Have the person repeat this process over and over again until they can successfully rappel WITHOUT a glove, single strand. Once the person understands how to add friction at the top of the rappel and control their descent with the device they have, then they can move on to other steps like locking off and adding more friction on the fly. Seems like more training is necessary rather than more rope.

    My opinion is that the hardest place to control speed of descent is at the bottom. Typically, the top is where one would struggle because of too much friction. Odds are your friend will be tired when the double strand ends and then he will quickly crater when he hits the single strand. I would never recommend this approach. Ever. Have him buy a CRITR or some such device and teach him how to use it single strand. I am about 210 and with a full pack with heavy wet ropes (two of them yesterday) I was pushing at least 260 lbs. Yet, I single strand rapped with my new love, the CRITR, and glided down effortlessly because I know how to set friction for my weight and length of rappel. What you are proposing to me is a recipe for disaster.

    As for "emergency fireman's belay" I thought a belay was the standard in the business of canyoneering and not for emergencies. If no one is belaying, by the time you recognize an "emergency" - a fall - you will not have time to react by grabbing the rope and pulling it tight, particularly if he falls when the rope goes from single to double strand. Assuming you are belaying on a single strand with a stone knot, I would imagine that it wouldn't do much to the belay process or rope once the person is on the single strand. I have no idea what kind of havoc, if any, would be created with the free hanging rope if a belay was employed on one rope only.

    Again, teach your friend how to properly rappel. Seems to me that is what a good friend would do.
    Life is Good

  5. Likes pikan, Byron, Canyonbug, harness man, hank moon liked this post
  6. #4
    ^^^THIS^^^

    Tap'n on my smart phone.

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  8. #5
    100% agree with Scott. If it was me if the the 160 won't reach so you could not double with the 185, I would send single on 320 with a belay from the top on the 185. But honestly I would get some more training before someone goes splat. IMO when he transfers off the double to the single he is going to crater. I doubt your bottom belay could stop him.

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  10. #6
    Mind sharing what route your doing?

  11. #7
    the CRITR makes rapping single strand super easy

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  13. #8
    Well said, Scott!
    We created the CRITR specifically for big people/newbies on skinny ropes.
    Rappelling off a scissor lift at my shop, I have rappelled several times with myself (150 lbs) and 350 lbs. of weight attached (yes, that's 500 lbs!) with the CRITR on 8mm rope.
    With the correct friction setting, this is an easily controlled with one hand.
    ANY canyoneer who is "only comfortable rappelling double strand" should get a CRITR.
    Problem solved
    You can find CRITRs and more information at http://canyonwerks.com
    Best, Todd and Desi

  14. #9
    Quote Originally Posted by moab mark View Post
    I would send single on 320 with a belay from the top on the 185.
    Nice idea. This should do the trick. The guy has been practicing with an ATS descender. We'll keep working with him before we go.



    The CRITR looks like a Piranha with a little bigger arms.
    The canyon is Jolley Gulch in Zions. Anyone know the precise length of the first rap?
    Only Dead Fish Go With The Flow

  15. #10
    Quote Originally Posted by harness man View Post
    Well said, Scott!
    We created the CRITR specifically for big people/newbies on skinny ropes.
    Rappelling off a scissor lift at my shop, I have rappelled several times with myself (150 lbs) and 350 lbs. of weight attached (yes, that's 500 lbs!) with the CRITR on 8mm rope.
    With the correct friction setting, this is an easily controlled with one hand.
    ANY canyoneer who is "only comfortable rappelling double strand" should get a CRITR.
    Problem solved
    You can find CRITRs and more information at http://canyonwerks.com
    Best, Todd and Desi


    Descent control aside, there are other reasons to rap double strand, such as to reduce the probability of rope damage or failure. Big people on skinny ropes can be fine, if that person knows what he is doing. Big beginners on skinny single line should be avoided, regardless of descender used.

    A top belay is not necessarily a solution to the OP's problem unless carefully managed by people experienced in top belays. Things can go wrong other than outright belay failure: pendulums, ropes tangling, belay rope stuck in crack, hard "catch" due to excessive slack in static belay line, etc.

    Questions for OP:

    - How well do you know the big guy? Friend? Acquaintance? Friend of friend? Have you had a chance to evaluate his abilities/competence firsthand? Are you competent to do so?
    - Does the difficulty of the chosen canyon (Jolly Gulch) match the skill level of the least experienced person? If not, are the more experienced people prepared to effectively "guide" the less-experienced and/or deal with the consequences (e.g. injury, self-rescue, etc) if something goes wrong?
    - Could you have just as much fun doing another canyon that better matches the skill level of the least experienced person? Save Jolly for another outing? It will be there, waiting for you.

    Not asking for answers to these questions - just putting them out there for your consideration.

    Latest accident in Jolly Gulch, AFAIK:

    http://canyoncollective.com/threads/...y-gulch.19192/

    And, x4 to what Scott Card said.

    hank

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  17. #11
    Quote Originally Posted by taatmk View Post
    The canyon is Jolley Gulch in Zions. Anyone know the precise length of the first rap?
    Zion area canyons have recently been blasted by unusually high floods. I heard a credible report that the first rap in Birch has increased in length by about 30 feet due to water scouring.

    Jolly got hit pretty hard and published beta on rap lengths might need an update. Best practice is to be sure you can see the rope on the bottom before committing.

    Oh, and conditions report, please?

    hank

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  19. #12
    Just heard from Kuenn Drake, who did Jolly Gulch today.

    Bottom of 1st rap is full swimmer. Multiple swimmers after that. I wonder how much longer that first rap is (or will be, after drying)?

    hank

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  21. #13
    Quote Originally Posted by hank moon View Post
    Bottom of 1st rap is full swimmer. Multiple swimmers after that. I wonder how much longer that first rap is?

    hank
    Many thanks for the info....
    Only Dead Fish Go With The Flow

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