QC Lab: What is the strongest rappel knot?

I was climbing in Yosemite last summer and, while at a belay, was talking to a party from Bozeman, Montana. They noticed all the proto gear on my rack and deduced that I was from Black Diamond. My partner and I bailed (typical) and they were watching me like a hawk. I asked what they were looking at, and they said, "We want to see how the QA guy from BD raps." In particular, they were eyeing my rap knot.

There are advantages and disadvantages to each type of rap knot, but is there one that is superior in strength to the others? I put all three knots (with three different rope diameter scenarios) through a quick-and-dirty series of pull tests and have provided below some basic strength testing data based on my limited testing of the three most common rappel knots. (Note: only two data points per configuration.)
QC with KP rap knot chart

THE OBSERVATIONS

  •  The Double Fisherman's and Ring Bend had similar strength results
  •  The Euro Death Knot was the weakest: ~20-30% less than the Double Fisherman's and Ring Bend.
  •  The Euro Death Knot slipped a bit before failure at ~4000 lbf with the 10.2 and ~2000 lbf with the 8.1 in the system.


THE BOTTOM LINE

The reality is that all three of the methods for joining two ropes for rappelling that I tested were PLENTY strong for the forces seen during a typical rappel (i.e., bodyweight-plus, taking into account some shock loading when bouncing around, jiggering with tangled lines, not smooth rappelling technique, etc).

For what it's worth, when I started climbing I always used the Double Fisherman's, but now I've fully converted to the Euro Death Knot—it's fast to tie, plenty strong, less likely to get hung up when pulling and easy to untie. And finally, no matter what rap knot I tie, I always leave long tails (like at least 12 inches).

Climb safe,
Kolin Powick

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Kolin Powick (KP) is a Mechanical Engineer hailing from Calgary, Canada. He has nearly 20 years of experience in the engineering field and has been Black Diamond's Director of Global Quality since 2002. Kolin oversees the testing of all of Black Diamond's gear from the prototype phase through continual final production random sample testing. If you have a technical question for KP, please email him at askkp@bdel.com and he will TRY to respond.