Login with Facebook
 ADVANCED
Multiple knot effect on rope strength.
View Latest Posts in This Forum or All Forums
Page 1 of 2.  1  2   Next>   Last>>
Follow replies to this topic? Notify me at the top of web site.
1

Email me.
 
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!
DVan
Joined Feb 27, 2014
20 points
Feb 27, 2014
Top of Rainbow Buttress
You will be fine. Andrew Yasso
From Las Vegas, NV
Joined Sep 9, 2012
188 points
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.
mark felber
From Wheat Ridge, CO
Joined Jul 30, 2005
65 points
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. Ben Huber
Joined Dec 19, 2013
7 points
Feb 27, 2014
on top of the RNWF June 2012
you could tow an aircraft carrier with a 11 mil static, I wouldn't worry about people hand over handing down it. Keenan Waeschle
From Bozeman, MT
Joined Feb 8, 2010
217 points
Feb 28, 2014
prindle
a knot only weakens the rope at the actual knot, not the overall strength of the cord everywhere else. Seth Kane
From Bozeman, Montana
Joined Sep 22, 2013
102 points
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? JeffL
From Salt Lake City
Joined Jun 14, 2012
19 points
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.
Mark E Dixon
From Sprezzatura, Someday
Joined Nov 29, 2007
200 points
Feb 28, 2014
Youre better off using a thinner rope doubled round the tree, forget the knots and learn to classic abseil (rappel). Jim Titt
From Germany
Joined Nov 10, 2009
195 points
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.
John Husky
Joined May 10, 2011
3 points
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: traditionalmountaineering.org/...

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.
wivanoff
Joined Mar 3, 2012
121 points
Feb 28, 2014
Rrrrr
to knots or knot to be, no problem.

maybe dix will be at the base with a little feel-good kicker
Buff Johnson
Joined Dec 19, 2005
1,499 points
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.
Mark E Dixon
From Sprezzatura, Someday
Joined Nov 29, 2007
200 points
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!
Mark E Dixon
From Sprezzatura, Someday
Joined Nov 29, 2007
200 points
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. Dylan B.
Joined Mar 31, 2006
487 points
Feb 28, 2014
If its that low angle you body rap? It would be more secure than scrambling and retrievable. Dave006
Joined Feb 16, 2013
0 points
Feb 28, 2014
BD Fuel
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]
rocknice2
From Montreal, Quebec
Joined Nov 27, 2006
3,008 points
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.
Mark E Dixon
From Sprezzatura, Someday
Joined Nov 29, 2007
200 points
Feb 28, 2014
BD Fuel
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.
rocknice2
From Montreal, Quebec
Joined Nov 27, 2006
3,008 points
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.
Mark E Dixon
From Sprezzatura, Someday
Joined Nov 29, 2007
200 points
Feb 28, 2014
me
have fun tying a bunch of knots along an 11mm static rope. Can you say BAD HAND

:)
5.samadhi
Joined Jul 20, 2013
27 points
Feb 28, 2014
BD Fuel
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.
rocknice2
From Montreal, Quebec
Joined Nov 27, 2006
3,008 points
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.
Mark E Dixon
From Sprezzatura, Someday
Joined Nov 29, 2007
200 points
Feb 28, 2014
BD Fuel
Reread the bold part as many times as necessarily. rocknice2
From Montreal, Quebec
Joined Nov 27, 2006
3,008 points
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.
Robert Cort
Joined Oct 12, 2009
804 points
Feb 28, 2014
Rrrrr
you didn't account for the hog-rider effect Buff Johnson
Joined Dec 19, 2005
1,499 points


Follow replies to this topic? Notify me at the top of web site.
1

Email me.
Page 1 of 2.  1  2   Next>   Last>>
Beyond the Guidebook:
The Definitive Climbing Resource
Inspiration & Motivation
to Fuel Your Run
Next Generation Mountain
Bike Trail Maps
Backcountry, Sidecountry
& Secret Stashes
Better Data. Better Tools.
Better Hikes!