Are sewn slings stronger than knotted ones?
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Point is, a knotted sling might appear strong because the webbing is doubled through the knot, but in actuality it is weak because the bends in the knot are stress-concentration points. Typically, a knot such as the grapevine or water knot will reduce sling strength by 20 to 30 percent, although reliable data is lacking. Meanwhile, a sewn sling is actually stronger at the sewn point because the stitching effectively weaves the overlapped ends together, reinforcing them.
Regardless, neither type sling is apt to break under normal circumstances. Recently, however, an 8mm dyneema sewn sling did break under body weight. While the jury is still out about what caused the failure, the sling was girth hitched through another sling, and was statically loaded. The combination of the super-thin webbing, the girth-hitch, which might be the weakest of all knots as its tight bends cause the webbing to cut into itself, and the static load apparently exceeded the poor runner's tensile strength. Lesson: Avoid the girth hitch by linking slings with a carabiner.