Winch rope breaking strength - Knots vs Splice
Factor 55
On the whiteboard today are Samson 3/8 Dyneema test specimens from our last destructive test session. By now you know that we recommend splicing a broken synthetic winch rope whenever possible. Although tying a knot is quick and easy, it can reduce the overall rope strength by over 50%. We decided to validate the reduction of SK75 rope strength due to knotting.
We tested the two most common knots that we have witnessed being used on the trail, the Bowline, and the Overhand Loop knot. Unfortunately we could not get the Bowline knot to stop slipping during the test, so we were not able to obtain any test data on the Bowline knot. The chart below, from Marlow Ropes, shows that the Bowline knot can reduce rope strength by 65%.
Strength test chart
The whiteboard shows a comparison of the common overhand loop knot, to the proper long bury (22") spliced rope sample. The proper long bury spliced sample broke at 20,120 pounds and the overhand loop knot sample broke at a mere 6,410 pounds. The overhand loop knot reduced the rope breaking strength by 68%. As the knot tightens, the tremendous bending and compressive forces on the rope fibers cause this early failure. Do yourself a favor and learn how to properly splice your synthetic rope, and realize the full strength of your winch rope.
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