Knots for "Smooth Operator" Dyneema Pull Lines; 1048 lbs for Barrel Knot, 1648 lbs for Girth-Hitched Multiple Lines

The idea of the Smooth Operator can be found here. This pic is modfied from the BluGnome website:


These devices are used in canyoneering, either for "ghosting" or just to minimize the frictional wear between rock and rope. Amsteel cords  (8-strand hollow braid made from Dyneema) have been used as the pull line attached to a drilled eye in one end of the lexan "Smooth Operator." The upper part of the rope is looped around a tree, a sling in a hanger, or some other good anchor. After all have rappelled down, the Amsteel cord is pulled, the Smooth Operator is pulled out of the knot, and the stone knot collapses, allowing the rope to be pulled down (without pulling the entire length up and over the rock). The thickness of the lexan on the end of the eye is 0.375", about the same as many carabiners.

Usually the Amsteel cord is 1/8" (~3.2 mm). This Amsteel cord is very strong -- about 2500 lb bs (defined as the strength of cord between two bury-spliced eyes). Knots greatly weaken Amsteel; but the force needed to pull a smooth operator is not very great -- if more than body weight is needed, something is very wrong. But Amsteel is also very slippery, and there are concerns that some knots will simply come undone.

A friend recently bought 300' of Amsteel as a pull cord, and planned to tie it to the end of eye of the Smooth Operator with a barrel knot at the lexan eye. I had two thoughts:

1) How strong would the barrel knot be?

2) do you really need to buy a continous 300' piece? For example, could you get three 100' pieces, put small eye splices on each end of each piece, then girth-hitch them through the eyes as needed?

Below are the two specimens for the break tests, which were pulled between  my jeep and a load cell (linescale 3) attached to a solid anchor at the end of my driveway. The breaking strength of the barrel knot (1048 lbs, 42%) is more than enough for the use. The breaking strength of the girth-hithed sample (1648 lbs) is also more than enough, and the girth hitches are small and unlikely to catch on the way down. [click photos for larger images]