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Karl Lew

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The Girth Hitch (spectra/spectra)

Chris Harmston of Black Diamond was kind enough to reproduce his sling girth hitch test on three runner samples that I mailed to him. In his original post, Chris reported that girth hitches reduced sling strength significantly and that the combination fails at the knot. I was curious to see if the knot could be reinforced by tying it in a slightly different way. My hypothesis was that adding an extra loop might strengthen the knot.

I sent Chris three samples of brand new (urk) Blue Water 12" runners purchased from REI:

  • Sample #1 was a single sewn loop rated at 27kN. The stitches failed at 27.5kN. This test provided a baseline for comparison.
  • Sample #2B was girth hitched to sample #2A to repeat Chris' original test. Runner 2B failed at 17.8kN (66% rated capacity) and was the runner with the girth hitch fold in it. Runner 2A did not fail, but looked somewhat worse for the wear with little bits of runner 2B melted on to it.
  • Sample #3A was girth hitched to a doubled loop in sample #3B. The double loop increases the size of the overall knot, which hypothetically lessens the strain on any given strand. Sample #3A failed at the knot at 18.6 kN (69% rated capacity). Sample #3B did not fail--it was the runner with the double loop.

Chris reports: "Fairly strong. Significantly weaker than rating. Very difficult to generate 4000lbf (17.8kN) in the field, but possible". For comparison, harnesses are typically rated to fail at 15kN. People die at 12kN, which would be about 18kN at the anchor.

One outcome of this test that surprised me was that failure happened on the runner with the girth hitch (2B & 3A). I had expected that the unmodified runner (2A & 3B) would fail because of the sharp angle produced. I am now curious to see how this might affect nylon/spectra runner girth hitches. In particular, I'd like to see what happens when the double loop is placed on the nylon runner. Earlier posts on rec.climbing indicated that the nylon runner was sliced by the spectra runner. My hypothesis for this future test is that the nylon will be strengthened by the double loop.

Many thanks to Chris Harmston and the staff at Black Diamond who've been really great in their support for my curiosity.

Lastly, on a humorous note, I report the following conversation with my son:
SON (looking at broken slings): What's that?
DAD: Oh, I asked a nice man to break these for me.
SON: Why?

continued...

DISCLAIMER: This test is NOT statistically valid as it consists of single samples. The results presented here are a product of personal correspondence only and are NOT an official statement of Black Diamond Ltd.