Strength Testing a New Prusik Knot

A few weeks back we received a note through Facebook from James S. asking us to take a look at a new way of tying a purcell prusik that involved a simple overhand sandwiched in the middle of a double fisherman knot. This technique is intended to serve as a method of creating two separate loops with a fixed size. James advised this purcell knot was easy to visually verify but wanted to know how the extra overhand in the middle affects strength. 

For the test, we tied eight loops of 6mm nylon cord; four with the sandwiched knot and four without. We then put them through our pull test machine to see how they compared. 

How to tie the purcell prusik with keeper knot: 

Test_Purcell_prusik_step1-2.jpg

 

Test_Purcell_prusik_step3-4.jpg

 

Test_Purcell_prusik_step5-6.jpg

 

   Results: 

6mm Purcell without keeper knot (in kN)

6mm Purcell with keeper knot (in kN)

14.84

10.29

14.56

9.43

13.55

9.91

14.95

9.90

Average: 14.475

Average: 9.88

 

The result was slightly unexpected but not surprising. The prusik with the keeper knot had a reduction in average tensile strength of almost 32%. We initially did not expect to see such a dramatic loss in the loop strength and as such, we can see no distinct advantage in the use of the sandwiched overhand.

This is a perfect example of a seemingly good idea being properly tested before being put into use in the real world.  While it did not work out in this case it doesn’t mean we should stop exploring new ideas and techniques.

Thanks to James S. for reaching out to us to test his knot. Contact us if you have unique testing requests. 

 


Sterling Rope