Soft Shackle Testing Part 3
Overview
Brion Toss became interested in my unique soft shackle designs and offered to have them tested at New England Rope. He had NE Rope send me some of their Endura-12 in both 3/16 and 5/16 sizes. I made up three each of my improved soft shackles and double soft shackles in the 3/16 size and one each in the 5/16 size. I also made up one more double soft shackles which I will explain below. While I had hoped to have all of these tested, we tested only two each in the 3/16 size and one each in the larger size. I assume Brion kept one of each size as samples of the design.
The improved soft shackle testing is straight forward, pull until they break and record the strength. The double soft shackles are a bit more complicated because they are composed of two sections, the double shackle and the diamond stopper knot. I knew from my own testing that the shackle itself was stronger than a diamond stopper knot if the stopper knot was made from the same size line. I also knew that using the next size larger line for the stopper knot was a bit of a mismatch and probably not the desired solution unless it gave a significant increase in strength. But if I tested with the same size line, I would just be breaking the stopper knots. Because we also know that the soft shackles break at the knot, the testing of the single soft shackles will tell us all we need to know about the diamond knot. Thus the double soft shackles were tested with larger stopper knots so that we would be testing the shackle and not the knot. We would then have all the information we needed to determine the strength of various combinations.
Material Tested
Test Results
| # | Shackle | Line size | Breaking force | Rated strength | strength % of line strength | % of theoretical |
| 1 | Single | 3 / 16 | 7,082 | 6,100 | 116% | 29% |
| 2 | Single | 3 / 16 | 6,288 | 6,100 | 103% | 26% |
| 3 | Single | 5 / 16 | 15,995 | 14,500 | 110% | 28% |
| 4 | Double | 3 / 16 | 4,457 | 6,100 | 73% | 37% |
| 5 | Double | 3 / 16 | 4,029 | 6,100 | 66% | 33% |
| 6 | Double | 5 / 16 | 12,027 | 14,500 | 83% | 41% |
Conclusions on Single Soft Shackle
The most striking thing about the test is how much of the soft shackle strength is lost in the diamond knot. Almost 3/4 of the strength is lost. That makes them stronger than the line they are made of, a fact uncovered in testing part 1, but not significantly stronger. I found this result surprising. I want to compared a 1/8 inch diamond knot with a 7/64 inch line. That would determine if the soft shackle is 28% stronger than the line it is made from. I have found that the soft shackle is not twice the strength of the line it is made of in test 2 but I really need to make more tests.
Conclusion on Double Soft Shackles
The double soft shackles did a bit better being at the lowest 33% of theoretical vs 26% for the straight soft shackle. This confirms that the knots in the double soft shackle are more efficient than a diamond knot. But the double soft shackle puts all its force on the diamond stopper knot rather than half the force that you get with a normal soft shackle because the force is split between the two sides of the shackle. The first conclusion then is that the base of the double soft shackle is stronger than a diamond knot so if the same size line is used for both the body and the stopper knot, the stopper knot will fail first and the overall strength will be about half the strength of the line it is made of.
If the stopper knot is made from the next larger line size, the body will fail first and the double soft shackle will be about 2/3 of the strength of the line it is made of. (65% is just under twice the 33% number which is the worst of the three tests. I use twice because there are two strands).
Is it worth using a larger stopper knot? Comparing the two strengths, the double soft shackle goes from 50% to 65% an increase of 30% in strength by going to a stronger stopper loop. That is equivalent to going about 1/2 a line size up. In summary, starting with the base of using the same size line for both the base and the stopper loop, you can get half way to the strength of the next size line up by changing just the stopper loop to the next size up. Depending on the application, that may or may not be desirable.
Summary*
| Shackle Type | Approximate Breaking Strength | 5/32 Amsteel (approx) | 3/16 Amsteel (approx) | 1/4 Amsteel (approx) |
| Soft Shackle | Line strength | 4,000 pounds | 5,400 pounds | 8,600 pounds |
| Double Soft Shackle | Half line strength | 2,000 pounds | 2,700 pounds | 4,300 pounds |
| Double Soft Shackle w/ larger stopper loop | 65% of line strength | 2,600 pounds | 3,500 pounds | 6,200 pounds |
| Stopper Knot | Half line strength | 2,000 | 2,700 | 4,300 |
* These numbers are based on limited number of samples. See CYA notice below. Use appropriate safety factors.
Statement of CYA
These are just my opinion. Your safety is your responsibility, not mine. You should do your own testing and use safety factors that are consistent with the application. If you are using these for life lines, use 11:1. If you are using them for a boom vang on a light boat, perhaps something close to 2:1. 5:1 is a typical safety number but even here, the choice of your safety factor is your responsibility, not mine. I am providing this information for your amusement, don't bet your life on it.A Note of Caution
There is someone on eBay selling soft shackles with high strength ratings. He is basing his numbers on 70% of line strength from theoretical (Theoretical being strength of 400% of line strength due to the four strands sharing the load). These tests show 26% of line strength. He may be rounding down from the 70% number and I don't know what line he is using, but it is safe to say his shackles are not close to the strength he is advertising, probably not half as strong.
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