Monday, January25, 2021
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Soft Shackle Strength Testing


I know from pervious testing that a soft shackle is stronger than the line it is made of. I know this because I took two soft shackles and a piece of the same line with two eye splices in it. Between the eye splices was a section of single thickness line. When pulled to breaking, the line broke in the middle. Neither soft shackle showed any sign of weakness. The question remained, how much stronger? I decided to do a test to see if it was stronger than the next increment that I could test.

The Test

It is possible to do testing on fancy machines, and I have some results coming soon, but if what you are interested in is if one thing is stronger than another, why not just test one against the other and see which breaks? If you test using the machine, you get a strength number of what you are testing but unless you also test a sample of line at the same time, you don't really know how the test sample compares to the line it is made of. It is that philosophy that led to this test. I know that a soft shackle breaks at the diamond knot. If everything in a soft shackle was perfect, it would be four times as strong as the line it is made of. The only thing that could possibly break at the lower numbers we are talking about is the diamond knot. What I am reporting on here is a test of the diamond knot pulled on much as it would be in a soft shackle with a small adjustable eye splice. For this test, I tested a diamond knot of 1/8 Amsteel against a straight section of 1/8 Amsteel. I really wanted to test against 7/64 Amsteel and will do that next.

The Numbers

This test will show if the diamond knot with two strands is stronger than the line it is made of. That would mean that the soft shackle would be twice as strong as the line it is made of.

The Confession

My intention was to use 7/64 as the test link and I had some dark gray 7/64 Amsteel so I made up my test link out of dark gray Amsteel. Unfortunately, I apparently also had some dark gray 1/8 inch Amsteel and made the test link out of that so instead of testing for strength greater than 1.28x, I tested for strength greater than 2x. I am out of line to use for testing so more results will have to wait for more line.

Here is the test material. The line under test is the gray diamond knot stopper loop.
The test setup has the diamond stopper loop secured to some high load Amsteel that ties off to a cleat on the test fixture. The test fixture is a 4x4x8 feet that has a cleat on one end and a Barient 22 on the other. For safety sake I clamp a board between the line under test and the winch.
The other end of the test section is secured to a 1/2 inch double braid that goes to the winch. The blue line in this picture is a safety line that prevents the bowline from breaking the safety board (again).
The business end of the test fixture is the Barient 22 winch. I must say that breaking this line took a considerable force on the winch handle.
The test broke the diamond knot.

The Result

The diamond knot broke so the soft shackle is not twice the strength of the line it is made of.


A soft shackle with a diamond knot is greater than the strength of the line it is made of and less than 2 times the strength of the line it is made of. These numbers are not that close together. I need to test with a difference reference line.

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