Strength Loss in Luggage Tag

[thinwater]

My GoogleFu is failing me.



IF you luggage tag a halyard, for example, to a shackle, or to another rope eye of the same size, how much strength is lost? Assume for the purposes of the question that the radius is the same as the rope (if it is smaller, I know the math for that) D/d = 1.


Since each leg is carrying only 50% of the load, the loss around the ring should be zero. The loss where the eye wraps around the two legs of the rope should be zero since D/d = 2. The loss at the bend caused by the loop around the legs should also be zero because there are two legs.


So the loss should be very small. But I cannot find data and want to be certain.


This question comes up frequently with JSD discussion, but I could only find people's guesses.



If no one can find data I'll have to splice up a few dog bones and break them.

[Macblaze]

Not sure what you are looking for exactly but did you try googling girth hitch?

Theres some info here": https://www.edelrid.de/en/knowledge-...s-by-knots.php although its more about the material than the knot

Originally Posted by thinwater

My GoogleFu is failing me.



IF you luggage tag a halyard, for example, to a shackle, or to another rope eye of the same size, how much strength is lost? Assume for the purposes of the question that the radius is the same as the rope (if it is smaller, I know the math for that) D/d = 1.


Since each leg is carrying only 50% of the load, the loss around the ring should be zero. The loss where the eye wraps around the two legs of the rope should be zero since D/d = 2. The loss at the bend caused by the loop around the legs should also be zero because there are two legs.


So the loss should be very small. But I cannot find data and want to be certain.


This question comes up frequently with JSD discussion, but I could only find people's guesses.



If no one can find data I'll have to splice up a few dog bones and break them.

[GordMay]

Is this what you mean?
The Girth Hitch weakens a sling, particularly if two straps are directly knotted to each other. This loss of strength is worse, e.g., close to 50%, when tied in the form used in the picture, where one strap passes straight through, and the other forms a collar around itself. To join two slings when strength is a concern, use a carabiner – not a Girth Hitch.

[thinwater]

Originally Posted by GordMay

Is this what you mean?
The Girth Hitch weakens a sling, particularly if two straps are directly knotted to each other. This loss of strength is worse, e.g., close to 50%, when tied in the form used in the picture, where one strap passes straight through, and the other forms a collar around itself. To join two slings when strength is a concern, use a carabiner – not a Girth Hitch.

Right. Girth hitch. The redductions in a climbing setting is well know. I knew that.



But Dyneema climbing webbing is a special case for a number of reasons...
* Girth hitching a carabiner changes the leverage, prying them open. A special case.
* The D/d ratio is different with another sling vs. a carabiner.


Webbing experiences almost no loss in strength around a carabiner because the bend radius is so great relative to the thickness of the webbing. A sling sewn from 18mm webbing (2800#) will hold will hold 4900# in sewn sling, or 88% efficient. If you girth hitch it it holds 60% of that, or about 2900 #. If instead sew it as a rabbit runner (eye in the end) it tests ~ 2500# because of losses in the sewwing. If you girth hitch it, and loss 50% of 4900 pounds in the eye only, it is still 2500 pounds, because the eye (sling portion) was stronger than the main body of the rabbit runner in the first place.



If you do this same test with rope, without a thimble and a D/d ratio of one, the rope can break either at the splice or the carabiner. Rope loses 50% in a tight bend. Webbing does not. That is a big part of why climbers use webbing instead of rope for runners; it is not weakened as much by an edge.



A girth hitch in a loop and a girth hitch in an eye are NOT the same thing.



It is common practice to girth hitch JDS drogues and lifelines, and I can't find a single piece of test data regarding rope spliced as an eye. All of it is climbing webbing (40-55% reduction) or lifting slings (choker, 12-20% reduction). If the reduction is 50%, we might as well just tie a knot.


Gordo, show us your powers! Girth hitch strength, spliced eye. The stuff I found suggests 15-20% loss, not 50%.


---


I found this. Not directly applicable, but it implicates compression of the second rope. A different problem.https://www.samsonrope.com/docs/defa...rsn=f0eceef6_2

[thinwater]

Found it!


Samson rope manual, page 33.https://www.samsonrope.com/docs/defa...rsn=f0eceef6_2


The loss is only 15%, not much different than an average splice. As I explained, webbing and rope are different, and slings vs. eyes are different.

[GordMay]

Perhaps, answers might be found in this meta-study, which includes the Girth Hitch:
“A Review of Knot Strength Testing” ~ by Thomas Evans
➥ http://www.paci.com.au/downloads_pub...Evans_2016.pdf


I didn't see anything about only 15% loss, in the Sampson link.

[thinwater]

I posted the wrong link.

http://www.elishawebb.com/Samson%20R...sersManual.pdf


On page 33:


COW-HITCH CONNECTION: Ropes can be attached with a cow-hitch connection. This allows ropes to be disconnected without having to re-splice. However, this is a less efficient method and results in strength loss of approximately 15% for ropes of similar size.




Everything in the Evans study was about slings, which are very different from rope eyes.

[Seaworthy Lass]

Originally Posted by GordMay

Perhaps, answers might be found in this meta-study, which includes the Girth Hitch:
“A Review of Knot Strength Testing” ~ by Thomas Evans
➥ http://www.paci.com.au/downloads_pub...Evans_2016.pdf


I didn't see anything about only 15% loss, in the Sampson link.

Hi Gord
The correct Samson link is:

http://www.elishawebb.com/Samson%20R...sersManual.pdf

It is on page 33 of this.

Edited to add: Thinwater beat me to it .