I'm not the one that designed or tested it, I just went looking for stronger stopper knots and found the information at the link provided. This link describes how he did the testing.Originally Posted by sargevining
http://www.bethandevans.com/load.htm
Thanks.
It looks like the fella did his homework and tested extensively before offering his opinion. Very believable that his configuration bumps the strength of the line.
Actually, it makes perfect sense. A soft shackle is a loop. When you load the loop, the two sides each carry half the load. So line tension in the loop is 1/2 the load. The loop is also effectively made from 2 loops of the base cord, except the the weak point - which is the constricting loop at the stopper knot. However loading on this loop isn't necessarily purely tension (which is what rope strength is rated in). So looking at the loading at the weak point, you can at most expect 1/2 the load on the total system (pure tension load at the weak point), which would make a breaking point of 200% of line strength feasible. Take into account that line strength varies significantly (see below), and 230% is completely possible in 1 sample.
1400lbs is minimum breaking strength. 1600lbs is average breaking strength. So you could assume 1 strand of 7/64 amsteel will at least hold 1400lbs, and possibly it could hold up to 1800lbs before breaking (or more, or less, it depends if its normally distributed or not, and what the range really is. I'm basically assuming 200lbs is 3 standard deviations).
So if they used the minimum breaking strength as the "line rating", 230% of that is 3220lbs for 7/64. Divide by 2, for the tension on one side of the loop, and you see that the line broke at 1610lb loading - right at the average breaking strength.
That's a quick and dirty analysis, and doesn't include a lot of other factor's, but it's enough for me to say, "ok, that's plausible". I wouldn't say ALL soft shackles will hold 230% (because this is just n=1 sample, and we already see large variation in line strength).
Last edited by Boston; 06-18-2015 at 12:37.
My understanding of the soft shackle is there are effectively 4 lengths of line sharing the load so it is possible to get over 2x the line strength. However, in the configuration described, the closed loop is still only a single line, so I agree, I don't see how it could be over 2x the line strength.
Not to be sensitive, but you're tone comes off as confrontational. I admittedly don't have any in the field experience, but I've been testing this in the backyard, so it's not completely theoretical. At this point, I've not added any knots, I have descender rings with the line wrapped around it as described by Grizz here and pictured here
Let's put aside if my suspension configuration is good or not, I haven't described it well enough. It really is fundamentally the same a traditional whoopie slings and a structural ridge line. I was really looking for help on a way to splice in a loop (to replace the descender rings) that would hold a hammock and not damage the main line. I think what I proposed wouldn't damage the line since it would pull the bury parallel to the main line, but you think it would significantly degrade the main line, more than the splice for the adjustable part of the whoopie?
I just don't understand the 2x or 4x line strength. A traditional soft shackle would only be 2 lengths of line sharing the load. I can't find anything (other than the link you provided) saying that a soft shackle actually improves line strength, rather than degrading line strength as splices do.
I suppose that Boston's interpretation of things is plausible - I just can't find anything that authoritatively says so.
I'm not discouraging you from experimenting - but an SLS is just about as experimental as one could go in the search for the perfect suspension.
"A foolish consistency is the hobgoblin of little minds." Ralph Waldo Emerson
A lot of the testing for soft shackles do show an increase in strength vs base line strength. The question is how much.
A lot of the testing I've seen reports the diamond knot failing before the shackle it self. If you look at this document, you see the knot portion remains intact, so that can account for the discrepancy. Basically, because of the splicing, the knot portion is extra reinforced, and this style of shackle fails at a different area.
the 4 lines is such: to make a shackle you effectively pass the amsteel through itself. That makes 2 lines. Then when you close it and load it, you have 4 lines (2 in each side of the loop).
Tension with-in the loop is only equal to the load one side of the loop is holding. This is the same effect as if you have a line and pulled on each end with 100lbs of force. The line tension would only be 100lbs, not 200lbs like you might think.
I can't see the document.
"A foolish consistency is the hobgoblin of little minds." Ralph Waldo Emerson
I love it when you talk engineering to me---
Its the one the OP posted when I asked him how it was tested:
http://www.bethandevans.com/load.htm
Actually I was referencing the original report: http://www.bethandevans.com/pdf/improvedsoftshackle.pdf Page 4 specifically, where picture of the failed shackle is shown.
Oh, daddy knows what you like....
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