Saturday, April17, 2021
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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

The goal is to replace the two bowlines on my 7/16 jib sheets with something that will not hang up on the rigging during a tack. I put eye splices in the ends of the jib sheets and am planning on using soft shackles made from 3/16 Amsteel. 3/16 Amsteel is rated at 5400 pounds. Each side of a shackle carries half the load on the ends of the shackle and in this case each side is made of two lines. The strength of a soft shackle is then 4 times line strength degraded by the real world losses due to bends and the diamond knot. These tests show that the soft shackle is at least as strong as the line it is made from or 5400 pounds in the case of 3/16 Amsteel. The 7/16 XLS is rated at 5800 pounds but a bowline degrades 40% to 3500 pounds. link. It would thus seem that my change will be stronger as well as hang up less. Just to be sure, I did some testing to see how strong these soft shackles were and to make sure they didn't open up under load.


I made two soft shackles out of 1/8 inch Amsteel. I made one of the Kohlhoff style and one of the Colloigomarine variety. The one to the left is the Colloigomarine version.

>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

The test setup had the two soft shackles on each end of a section of Amsteel that had eye splices on the end. The idea was that if the Amsteel broke in the center of the test section, then the soft shackles were at least as strong as the line they are made of. As you can see from the picture on the left, that was the case. This is not surprising as without any degrading for knots and such, the soft shackle should be 4x as strong as the line it is made of. Of course, as I said above, the knot degrades the strength my more than 50%. Another point of interest is that neither splice broke. It is commonly stated that splices cut 10% of the line strength or that a spliced line is 90% of the strength of the line. My testing and that of others I have talked to show that the splice is stronger than the line so that the point of failure is not the splice but the line. I see now that the manufactures are starting to say 90-100% for a splice. Of course, you cannot get the spliced line to be stronger than the line but I would assert that the splice does not weaken the line assuming the taper is gradual.

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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