Soft Shackles
Soft Shackle Testing
I did some testing of soft shackles. I made a scale models using 1/8 Amsteel of both the Colloigomarine and Kohlhoff styles. My goal was to see if they were stronger than the raw Amsteel and the bowlines that they would replace. I made a section of Amsteel with two eye splices that had a center section without anything inside it. This would give me a calibrated known strength point. I used sections of 3/8 double braid with bowlines to tie very thing together simulating the actual application. The double braid scaled up to 9/16 which is more than the 7/16 line I would be using.
Soft Shackles on Jib Sheets
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Testing

>Note: Other tests I and others have done show that the weak point is the knot and that the knot degrades the line to 30 to 45% of line strength. As there is a 4x multiplier, that means the soft shackle is 120 to 180% of the strength of the line it is made of. My testing shows that a soft shackle is only slightly stronger than the line it is made of. We tested 3 samples at the test facility at New England Rope. They failed at 1.04, 1.09, and 1.10 times line strength. The 180% number quoted on other sites is optimistic.
NOTE: This 180% number came from non marine sites testing for tow ropes, hammocks, winch lines, etc. I have not found a marine site where I could get testing data and I have asked.
Soft Shackle test

Video of test
To have a section of known strength, I put a section of Amsteel with eye splices on each end but a couple inches in the center of straight 1/8 Amsteel. This video shows the fourth test I did. The first three broke the test setup rather than any of the Amsteel. For this test, I used a Barient 22 winch with 1/2 inch double braid that led to a 3:1 purchase block. This pulled on the Amsteel. The 4 things under test, the soft shackles, and the two types of eye splices did not break. The reference section of Amsteel between the two eye splices broke, just as I would have predicted. The first test broke the bowline knots I used to tie things together using a car as the force. The second eliminated the bowlines and the car but I could not get enough force with the winch to break anything. The third added a 3:1 purchase using 3/8 double braid, which broke. This one uses 1/2 inch double braid in the blocks that make up the 3:1 purchase that is in front of the winch. The Amsteel is rated at 2500 pounds but I would estimate I put more like 4000 pounds on it.
The first test attempt (not successful)
This is my first attempt at testing soft shackles. I tied them to one of my cars with some double braid and to the other with some more double braid. It took a fair force with the car to break... The double braid.
There has been some more scientific testing done as well on one of the forums. I have it here. It rates the small Amsteel at 175% of line but it uses 8 strand and not 12 strand. Still more than line strength. However, my own testing does not support that conclusion. I have shown that a soft shackle is not as strong as 128% of line strength. Here is a page with that test.
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