Oops
Hello all,
My apologies for neglecting to report on the break tests. No excuse; I've had the results, but just hadn't gotten around to organizing them. So, a brief, most belated summary:
First, rate of pull. In previous tests, at New England Ropes and elsewhere, the hydraulic cylinder was set to move at about 12"/minute, while the cylinder at TreeStuff, where the current tests were done, moved more than twice as fast, at 27.5"/minute. At the slower speed, the knot has a chance to "set" before a serious load comes on. At the higher speed, we wanted to see if the set of the knot and the rate of pull had an effect on the strength of the shackles; in the real world, people might not take the time to set shackles they build, and the load will tend to come on faster than in the lab. Sure enough, the average break strengths were lower than had been reported in previous tests. The "standard" Lanyard Knot configuration averaged 106% of rope strength, instead of at least 120%, for instance.
Interestingly, the results for the button-knot-and-bury shackle were not significantly different than the slow-rate tests; I am guessing that this is because all four parts of the button version draw up under load, while only two parts of the lanyard version do.
We can't do much about bringing the load on gradually, but it seems pretty clear that setting a new shackle is an important step, at least for the lanyard version.
Next, effects of diameter. In all of our tests so far, the bigger the rope, the lower the relative break load. For instance, with 5/32" rope it is not uncommon to get breaks of 280% with the button version, but by the time you get to 11mm results are down to about 160%. The only explanation I have is that it is much harder to produce a fair, snug knot in larger materials than in smaller. This scaling factor seems to explain the range in break strengths we seen from other people's tests. For instance, the estimable Evans Starzinger was getting, as I recall, about a 150% average for the lanyard version, while New England was showing more like 120%. With larger rope than Evans could break on his machine.
If further tests confirm this scaling effect, then we will need to scale shackle strength to rope size, instead of using a blanket average number for all sizes. Further, the scaling effect argues for the use of the inherently stronger button version, because with it we can use smaller rope, with a higher efficiency, for the same load. This effect also seems to undo, to some extent, the advice to use the lanyard knot version, but with larger rope, if greater strength is required, because greater rope size does not seem to result in correspondingly greater strength.
I am trying to avoid confirmation bias here (the button version is my baby), but must also note that smaller rope simplyfits into places that larger rope can't, which matters in things like connecting rope to chain for rodes. It is also easier to get an acceptable radius for the bearing point of smaller rope. So even though the button version is more difficult to make, it seems to be worth the effort for many applications.
Next, operator skill. For a given configuration, how well the knots were formed made a huge difference. One tyer, skilled at making the lanyard version, and with a 135% break average to prove it, got a less-than 100% average for his attempt at the button version, instead of the over 200% results that skilled tyers of that knot obtained. His buttons looked malformed to my eye, but not everyone knows how to judge that kind of thing. Therefore I urge people to get destruction tests done. Or, if they are buying soft shackles, ask to see test results from the vendor.
Finally, the +300% Grail. In no previous tests have we gotten over 300% efficiency for any shackle, but a Mr. Gardner came up with the idea of a"bifocal" version, in which two pieces of rope are used, instead of one. Sadly, his version averaged only 99%, but the concept was compelling, so I played around with it. One variation averaged 328%(!). I have some further samples ready to test, and will try not to delay in getting them out. Or in getting back to you re results.
If you submitted samples for the tests, please be in touch for detailed numbers on your work.
Again, I regret taking so long in getting these results out.
Fair leads,
Brion Toss
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