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Does a knot in the kite line really half its strength?

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mogthedog
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Does a knot in the kite line really half its strength?

Postby mogthedog » Wed Nov 30, 2011 8:49 pm

I got a knot in one of my back lines and have been told by a dull chum that a knot in the line will half the strength of the line.

Is this true or is my chum being dull??

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Re: Does a knot in the kite line really half its strength?

Postby davesails7 » Wed Nov 30, 2011 9:04 pm

Worse, an overhand knot reduces the strength of a line by 60%!

Do a search for topics on how to get the knot out and get it out before kiting again. Kiting more on it will just make it tighter.

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Re: Does a knot in the kite line really half its strength?

Postby BWD » Wed Nov 30, 2011 9:10 pm

to say the least it's true.
for spectra lines it can even be worse.
so get the knot out - pick it with fingers, teeth, squeeze with pliers, poke thumbtacks under the loops of the knot and work them back and forth (favorite method), use a fid or whatever it takes.
A friend claims tapping the knots with a ball-peen hammer works well, no kidding....
Good luck.

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Re: Does a knot in the kite line really half its strength?

Postby Séb » Wed Nov 30, 2011 9:10 pm

He is right and even if it's on your backline where there is much less pressure you should try to remove this knot asap.

''A study on rope break strength vs. knot break strength done by the Cordage Institute in Texas showed that a knot in a line reduces its strength up to 50%''.

''In general, knots reduce a line's breaking strength by about 50% (Jacobson 1999)''

http://www.geospectra.net/kite/knots/knots.htm
http://boatsafe.com/marlinespike/breaking.htm

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Re: Does a knot in the kite line really half its strength?

Postby RickI » Wed Nov 30, 2011 9:19 pm

Yes they do. Get it out without causing untoward damage or replace the line. Sudden line failures can cause a good deal of trouble. Q line is a nice option for replacement. You can cut it to length, tie larks heads with no need for sewing or sleeving. You do need to pre-stretch it however.

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Re: Does a knot in the kite line really half its strength?

Postby edt » Wed Nov 30, 2011 9:28 pm

dull chum is right, my favorite paper is the one done by black diamond & Tom Moyer, Tusting and Harmston here

http://user.xmission.com/~tmoyer/testin ... h_Cord.pdf

A figure 8 knot (very strong knot) is 90% on nylon but this excellent figure 8 is only 50% breaking strength on dyneema or spectra and on an overhand knot like you get in your kite lines, it's even less, probably below 40% !

Purewind taught me the trick of sticking the knot in your mouth and chew until the knot releases, it does work.

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Re: Does a knot in the kite line really half its strength?

Postby acctx » Wed Nov 30, 2011 9:37 pm

mogthedog wrote:I got a knot in one of my back lines and have been told by a dull chum that a knot in the line will half the strength of the line.

Is this true or is my chum being dull??
Tap on the knot with a hammer very gently on a clean smooth surface. This will ensure that you dont break any of the individual strands (vs chewing, pliers etc).

Keep rotating the knot, it should loosen up enough for you to undo it in under 10 taps or so.

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Re: Does a knot in the kite line really half its strength?

Postby TheJoe » Wed Nov 30, 2011 10:33 pm

The breaking strength is reduced by a few factors. The type of rope, the diameter of the rope, type of knot, and the angle of pull on a knot. A larks head and the figure 8's we connect to the kite with reduce the strength of our lines. I am on the rescue crew at my plant so we have to study all this.

Would be interesting if some one would actually test the different lines we use with the various knots to find the breaking points. Might be a good study for some of the college students :wink: .

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Re: Does a knot in the kite line really half its strength?

Postby eree » Wed Nov 30, 2011 10:36 pm

well i'm not convinced it is true.

if you beleive this 50% strength reduction crap is true, so you must avoid the connecting the line with the help of larkhead knots.
why? because the stress on the fibers is still the same (riders weight) and the ange of the bent line in the knot is pretty much the same.

so, i suspect the producer of the kite lines just trying to sell more lines. it is important to know methodology of the tests and who performs the tests.
did anyone perform the stress tests on larkhead knot connections?

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Re: Does a knot in the kite line really half its strength?

Postby OzBungy » Wed Nov 30, 2011 11:03 pm

Yes it is true. It is not a belief. It is a fact. You can test it.

There are certified standards for testing the breaking strength of lines under steady load and shock load. Google it.

The skydiving industry is full of knowledge of the behaviour of lines under stress, friction and knotting.

There is a well documented accident of a paraglider test pilot who died after his knotted lines broke.

A lark's head is not a knot in a line. It's a loop and it's sheathed. In a loop the tension clamps down on the line it's around. The force is mostly in a straight line. In a knot the tension is clamping on itself and the force is around a point load with a sharp reversal in direction.

If you've got a knot in your back line why not remove it? It's easy to do. There's no such thing as a knot that cannot be undone.
Last edited by OzBungy on Wed Nov 30, 2011 11:16 pm, edited 1 time in total.


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