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Multiple knot effect on rope strength.
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By DVan
Feb 27, 2014

Hi All,

Im a hiker, not a climber. Now and again I have to scramble down steep slopes in our local woods, and am considering using a rope to take some pressure off my knees. I often see ropes--each with several overhand knots for hand grips--tied around trees. These tend to be useful for descending (not rapelling, but walking or scrambling). Id like to use a static rope (11mm) in a similar but retrievable configuration for easy descents. My question is: If I have a, say, 35 foot 4000 lbs strength rope with overhand or stopper knots spaced every 16 inches or so as hand holds, how will the knots affect rope strength? I know there are stats for strength reduction of a single knot on a rope, but will the rope strength half every time another knot is added?!

Thanks in advance!


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By Andrew Yasso
Feb 27, 2014
Top of Rainbow Buttress

You will be fine.


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By mark felber
From Wheat Ridge, CO
Feb 27, 2014

Putting one knot in a rope creates one weak point. Adding more knots doesn't make that weak point, it just gives you another weak point, of roughly the same strength (or weakness) as the original knot. So no, the rope won't get weaker as you add knots.

Wouldn't a pair of trekking poles take a fair bit of pressure off your knees with a lot less work? That's what I do, and so do an awful lot of people.


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By Ben Huber
Feb 27, 2014

As a further reassurance, if each time the rope was knotted, you lost 50% of the strength below it, then over the 35 ft. you would have ~23 knots if placed every 16 inches, which would mean that the strength of the rope would be 5K lbs * .5 ^23 =~.0006 lbs. Since that makes no sense, I think you're ok. I'm confident that the physics behind this are quite a bit more complicated, but in which ways, I have no idea.


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By Keenan Waeschle
From Bozeman, MT
Feb 27, 2014
on top of the RNWF <br />June 2012

you could tow an aircraft carrier with a 11 mil static, I wouldn't worry about people hand over handing down it.


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By Seth Kane
From Bozeman, Montana
Feb 28, 2014
prindle

a knot only weakens the rope at the actual knot, not the overall strength of the cord everywhere else.


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By JeffL
From Salt Lake City
Feb 28, 2014

It might be tough to retrieve a rope that has overhands in it. How are you planning on setting this up without fixing the rope to a tree?


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By Mark E Dixon
From Sprezzatura, Someday
Feb 28, 2014
At the BRC

JeffL wrote:
It might be tough to retrieve a rope that has overhands in it. How are you planning on setting this up without fixing the rope to a tree?


You could hang it like a rap line, tie knots in one side, but not the other, tie the two ends together, scoot down, then pull down the knotted side after getting to the bottom and untying the joining knot. You'd have to hang onto both sides of the rope as you went down of course.


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By Jim Titt
From Germany
Feb 28, 2014

Youre better off using a thinner rope doubled round the tree, forget the knots and learn to classic abseil (rappel).


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By John Husky
Feb 28, 2014

Mark E Dixon wrote:
You could hang it like a rap line, tie knots in one side, but not the other, tie the two ends together, scoot down, then pull down the knotted side after getting to the bottom and untying the joining knot. You'd have to hang onto both sides of the rope as you went down of course.




WHAT?! Jesus Christ man. Think about this one for a second please. You will end up in a pile at the bottom with rope burns and compound fractures.

OP: tie one end to a tree if you must. Tie knots as you wish. The rope is strong enough. Don't try to rappel without proper instruction.


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By wivanoff
Feb 28, 2014
High Exposure

I see Jim beat me to this.

Low angle terrain where you want to scramble down? I would absolutely use a dulfersitz and have used it many times: www.traditionalmountaineering.org/FAQ_Dulfersitz.htm

On steep terrain this will hurt/rope burn. But this is perfect for low angle. Practice in a safe place before you use it in anger.


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By Buff Johnson
Feb 28, 2014
smiley face

to knots or knot to be, no problem.

maybe dix will be at the base with a little feel-good kicker


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By Mark E Dixon
From Sprezzatura, Someday
Feb 28, 2014
At the BRC

John Husky wrote:
WHAT?! Jesus Christ man. Think about this one for a second please. You will end up in a pile at the bottom with rope burns and compound fractures. OP: tie one end to a tree if you must. Tie knots as you wish. The rope is strong enough. Don't try to rappel without proper instruction.


I'm picturing a slope that's not steep enough to rappel.
He's a "hiker not a climber."
There are plenty of spots a short knotted rope is fine, but if he's venturing into true climbing terrain hoping to get away with just a knotted rope, he might want to reconsider.
Pretty much his call.


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By Mark E Dixon
From Sprezzatura, Someday
Feb 28, 2014
At the BRC

Buff Johnson wrote:
maybe dix will be at the base with a little feel-good kicker


I can give him my cast after it gets cut off next week!


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By Dylan B.
Feb 28, 2014
Orgasm Direct, Devil's Lake, 5.11a  c. 2008

You'll never break a good-condition, 11mm static rope with just body weight, no matter how many knots you tie into it. Never.


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By Dave006
Feb 28, 2014

If its that low angle you body rap? It would be more secure than scrambling and retrievable.


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By rocknice2
From Montreal, Quebec
Feb 28, 2014
BD ice tool fusion2

Mark E Dixon wrote:
You could hang it like a rap line, tie knots in one side, but not the other, tie the two ends together, scoot down, then pull down the knotted side after getting to the bottom and untying the joining knot. You'd have to hang onto both sides of the rope as you went down of course.

^^^ Definitely don't do this!! ^^^

If you only knot one side but require to hold both sides to descend then there is no point to the knots. Might even be worse. Either knot both sides, which will get stuck easier or no knots at all

Your best off to use zero knots and do a Dulfersitz [body rappel]


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By Mark E Dixon
From Sprezzatura, Someday
Feb 28, 2014
At the BRC

I don't think you guys do enough sketchy dirt/tree scrambling!


Furthermore, 11 mil static is total overkill. A thick poly ski rope is easier to hold onto and probably wouldn't even need to be knotted.


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By rocknice2
From Montreal, Quebec
Feb 28, 2014
BD ice tool fusion2

Mark E Dixon wrote:
I don't think you guys do enough sketchy dirt/tree scrambling! Furthermore, 11 mil static is total overkill. A thick poly ski rope is easier to hold onto and probably wouldn't even need to be knotted.

I don't think you realize why your first post was just plain wrong.

You are correct that 11mm static is overkill.


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By Mark E Dixon
From Sprezzatura, Someday
Feb 28, 2014
At the BRC

rocknice2 wrote:
I don't think you realize why your first post was just plain wrong.


That's always possible!

You would certainly not want to hold onto just one side of the rope- if you did, it would be an exciting ride. What else am I missing?

I guess he could knot both sides, but would still have to hold onto both strands.

Do you have a better suggestion for using and retrieving a knotted handline?

Suggesting he rappel is evading the question.


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By 5.samadhi
Feb 28, 2014
me

have fun tying a bunch of knots along an 11mm static rope. Can you say BAD HAND

:)


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By rocknice2
From Montreal, Quebec
Feb 28, 2014
BD ice tool fusion2

Mark E Dixon wrote:
That's always possible! You would certainly not want to hold onto just one side of the rope- if you did, it would be an exciting ride. What else am I missing? I guess he could knot both sides, but would still have to hold onto both strands. Do you have a better suggestion for using and retrieving a knotted handline? Suggesting he rappel is evading the question.

What you're missing is the fact that you need to hold both ends of the rope, so tying knots into just one end is pointless if not more dangerous. Depending on how steep the descent is a simple hand over hand could very well be enough. Next would be a body rap then if very steep a full on harness and rappel device.


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By Mark E Dixon
From Sprezzatura, Someday
Feb 28, 2014
At the BRC

rocknice2 wrote:
What you're missing is the fact that you need to hold both ends of the rope, so tying knots into just one end is pointless if not more dangerous. Depending on how steep the descent is a simple hand over hand could very well be enough.


I don't think I'm missing that you need to hold both ends, I believe I mentioned that every time.

However, whether you find it easier to hand over hand with no knots or with knots on one side seems like a matter of taste to me. DVan should let us know how it goes.


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By rocknice2
From Montreal, Quebec
Feb 28, 2014
BD ice tool fusion2

Reread the bold part as many times as necessarily.


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By Robert Cort
Feb 28, 2014

OP: First off, you're fine.

If you need the technical reason: A good rule of thumb is that knots reduce the strength of rope by 30%. Some knots more than others, but don't worry about that, if Technical Rescuers can use the 30% rule, so can you.

Added knots will not will not reduce the strength any further. Each knot reduces the strength the same. More knots adds more weak(er) points, but the overall reduction is still just 30%.

Now let's do the math: 11.1 mil static rope is good for 6,000 lbf. (Usually more, for example sterling 11.1mm superstatic has a minimum breaking strength of 6850lbf).

Subtract 30% for the knots = 4000lbf remaining.

You weigh, let's assume 200lbf. If you are hanging in free space, you have a factor of safety = 20:1.

Now, since you're just using it to downclimb, let's say a 45% slope, then your force on the rope is only your weight times cosine of 45% or 141 lbf (assuming 200 lb). So actual factor of safety = 28.

You're fine.


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By Buff Johnson
Feb 28, 2014
smiley face

you didn't account for the hog-rider effect


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