Author Topic: Grant Prattley testing of rope joining knots (for abseiling)  (Read 3500 times)

agent_smith

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Grant Prattley testing of rope joining knots (for abseiling)
« on: August 22, 2018, 04:04:23 PM »
KNOT TEST REPORT: (Summarised from a Blog Grant's website)

Tester details: Grant Prattley
Title of report: Which bend for joining ropes
Link: https://canyonsandcrags.com/pdf/which-bend.pdf
Company: Over the edge rescue
Application context: Recreational climbing/abseiling/caving/canyoning
Publication date: August 2015 (reported June 2016)
Testing lab type: 'Pseudo lab' (not a certified lab and not accredited by a third party agency)

This is another test report which bases all conclusions purely on the default mindset of the MBS yield of the joining knot (ie pull-it-till-it-breaks mindset). This pull-to-failure mindset permeates virtually the entire climbing/rope rescue sub-culture and there appears to be no other way of conceptualizing knot security and/or reliability. The default viewpoint is; "If it has the highest MBS yield, it is by definition 'superior'.

Some criticisms of this report (list is not exhaustive):

1. There are no photos showing precisely how the knots were tied and in what rotation state (ie orientation of the knot to trap and crush the tails). Only given computer drawn images.
2. He begins by mentioning that the expected load on a joining knot within a retrievable abseil system is approximately 100kg. However, in his conclusion, he does not revisit that concept. No further emphasis is given on how much load the joining knot is expected to withstand - ie, given that a load of one (1) person is nominal, and the joining knot is typically located on one side of a retrievable abseil system - the knot will only be subjected to 50% of the overall load. It is left to the reader to conceptualize this loading profile (ie assumed that readers understand this concept). A diagram showing the position of the joining knot within the retrievable system would be helpful to aid in understanding. So his conclusion somewhat contradicts the opening remarks about how much load the end-to-end joining knot is expected to withstand.
3. Grant Prattley appears to be completely unaware of the effects of rotation. His testing regime should have used #1410 Offset overhand bend as a control in its mid-rotation state. He should have tested this knot in 2 different rotation states and compared his results to the control. He did not do this.
4. He does not appear to understand the concept of 'offset' - and its definition - since he refers to the name 'flat' to describe the structure.
5. He missed an opportunity to investigate properties other than the default mindset of MBS yield point. For example, he could have investigated jamming threshold - ie the load threshold at which jamming is triggered. He could have measured the instability threshold - ie, the load threshold at which instability is triggered. However, due to the all pervasive mindset of 'pull-it-till-it-breaks', other ways to conceptualize knot performance are ignored.
6. Grant Prattley did not investigate the effect of different rope diameters and the position of each rope segment relative to the other. For example, larger diameter 'above' smaller diameter versus larger diameter 'below' smaller diameter. It is presumed that all tests used 9.0mm diameter rope joined to 9.0mm diameter rope. This is a missed opportunity because it is likely that climbers will join different rope diameters.
7. His sample size of three (3) is better than just one (1) but, some may criticize that his sample set should have been larger to gain more confidence in his results. As he is not a fully certified test lab operating in accordance with third party rules of its accreditation, he is free to use whatever test sample size he thinks appropriate.

DerekSmith

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Re: Grant Prattley testing of rope joining knots (for abseiling)
« Reply #1 on: August 22, 2018, 08:22:49 PM »
The T Fishermans, three stacked Strangle knots.

Even under modest loading the Strangle has the habit and reputation of bedding down to a solid nub.  How on earth did he remove them?

And this knot made it into his book?  What was he advising - use it at your ropes peril I hope.

Dan_Lehman

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Re: Grant Prattley testing of rope joining knots (for abseiling)
« Reply #2 on: August 28, 2018, 10:24:46 PM »
The T Fishermans, three stacked Strangle knots.
WRONG, i.e., if one were to give original-namer
the rights to say ... :: Josh GUDELIUS so-named
a stacking of single fisherman's knots
--cf. www.gudelius.de/spst.htm
(where there is now a different name, I think prompted
by my comments (or others'?) to him).
(Edelrid did the testing, and there might be an English
version somewhere around --quick search by me, now.)

Quote
Even under modest loading the Strangle has the habit
and reputation of bedding down to a solid nub.
How on earth did he remove them?
He was working with Real Stuff(tm) and not reputations/myths!
The guard knot is loaded w/pulling apart strands running
into it --its own SPart and that of the middle knot,
which in turn presses against the guard's nub qua
stopper knot which also doesn't so much tighten
the strangle form.  Note that with the single OH
("fisherman's") version Josh/Edelrid got an interesting
capsizing !

(And note that my choice of "guard" --the first-loaded
blue knot set before the grapevine bend-- is just a matter
of perspective : one might see the latter as the base
and having a stopper finish!  (In both cases, there is no
loading as defines the grapevine --it's either offset
or qua stopper !  (Which by some strict nomenclature
would require us to use different names!) )


--dl*
====