High Strength Soft Shackle
Recient work by Brion Toss, Evans Starzinger, and myself has led to the development of a high strength soft shackle that Evens has tested to 230% of line strength. The secret to this added strength in primarily an increase in the strength of the knot, the weak point in conventional soft shackles. I should point out that Evans testing shows conventional soft shackles, with diamond knots, test at 170% of line strength, considerable above the "higher than line strength" number I have been using. While these two statements are consistant, the more percise number is considerably higher and higher than the testing I had done at NE Rope.
Back to the added strength. Once the knot is no longer the weak point, the eye becomes the limiting factor. The strength of the eye is related to the bend radius and these new high strength soft shackles have larger bend radius as a byproduct of the improvements in the knot as the improvement is gotten by burying the tail inside the section of knot that the eye goes around, effectively making double braid out of the line. There are several versions, one of which I present here. This one is based on a button knot developed by Brion Toss. I have simplified the process for tying it and have documented that on a web page: How to tie a Button Knot
How to MakeThese instructions will be for the simplest form of soft shackle, one without buried sections along the body. use one of the soft shackle calculators to get an idea of how much line to use. Double the line over and pass one line through the other just far enough down to form an eye. The the button knot using the two ends and dress to position the knot for the target length shackle. See the button knot instructions. Tighten the knot by hand and then lock it. I lock mine by placing a strong flat washer in a vice and passing the shackle through it. Then I place a awl in the eye and position a board so that I get maximum leverage pulling the knot as tight as I can. You can also close up the shackle and pull on two ropes, one anchored, and the other on a winch. Then bury each tail as close as possible to the knot extending down the body at least 10 diameters (16 for the tested strength). In other words, if you are using 3/16 line, bury at least 1 7/8 inch of line with 3 inches being the as tested length. Realize that the line you are putting the burry in will shorten so you need to exit your splice down about 25% further down the line than the length of the bury. Before you bury the tails, tighten the button knot as much as you can. Pull with a winch if you can. Cut the buried part at a slight angle and milk it back to that it disappears into the body. After you do the bury, lock stitch the ends so that as the knot tightens more, it will do so evenly.
The Strength TheoryA soft shackle shares the load over four strands of the line. Theoritically, it would be 400% of the strength of the line it is made from. But the knot is generally the weak point. A diamond knot strength is somewhere between 25% and 40% the strength of the line it is made from. That means that a soft shackle made with a diamond knot will be between 100% a nd 160% as strong (actually a little higher by measurements) as the line it is made of. You get from 25% to 100% because there are two strands in the diamond knot each carying 1/4 of the load of the shackle. The other limit is the eye. It is bent around a the body of the shackle and the turning radius is about the diameter of the line it is made of. This 1x bend radius makes the eye about 50% as strong as the line it is made of which would limit the soft shackle strength to 200% of the line, 50% of the 400% theoritical maximum (the eye only has half the load and an eye is two strands).
What about the short taper? You always hear to have a long taper to get maximum strength in a splice. The buried sections are very short and the taper is also short. But now we have 2 buried sections carrying half the load of the shackle so each has only 1/4 the load. Each of these should be twice line strength if they had a full bury and long taper. That means that we are starting with a strength that would give 800% line strength so the loss of a short and abrupt taper is insignificant given that the eye will break long before the body of the line. The loss from an abrupt taper brings the strength to 70% of line so that would be 280%, above what we need.
What about the short bury? The bury length is not that important as important as it is on a splice because we are only asking for a 35% increase in strength, not 100% as in a splice. Calculations say that the bury should be more than 6 diameters (6 is not enough) with 10 being enough. More is better and 16 is certainly enough.
Final thoughts on strengthThis soft shackle is 35% stronger than the normal variety by tests. It has a very nice looking button knot. But personally if I wanted a stronger soft shackle, I would use the next size line up. But there are cases where this would not be the case. If I were mountain climbing or if I needed to pass a very strong soft shackle through a confined space, like an anchor chain. This is another trick in your bag of tricks but in my opinion, not one that replaces the easy to use better soft shackle.
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