I pulled a few loop shackles in an MTS machine to get their failure load. I tested one specimen made from 3mm Lash-It and it failed at about 2,200 lbf. The remaining samples were made from 7/64" Amsteel blue. Failure loads for three samples with diamond knots were consistently around 2,800 lbf. All samples failed at the knot and the spliced loop remained intact. The samples were intentionally made such that they had different sized loops, i.e., the legs of the shackles were made different lengths. The very tight loop showed substantially more damage than the other two, but the knot remained the weak point.
I also tested the "cinch wind" method suggested by lonetracker. While I personally find it less convenient than the spliced loop, the shackle also held to about 2,800 lbf before failure at the knot.
The Ashley stopper knot and the overhand knot failed at between 600 and 700 lbf. The tail was pulled through the knot and the shackles were perfectly intact with no damage after the test, but no longer had the stopper knot in them.
- Use a diamond knot for loop shackles made from Amsteel.
- It is not critical to have the legs of the shackle perfectly balanced.
- The loop shackles have a failure load around 175% of the line's rating (190% for the 3mm Lash-It).
- The Ashley stopper knot and the overhand knot are not suitable for this application.