The idea of the Smooth Operator can be found here. This pic is modfied from the BluGnome website:
(NOTE THIS PICTURE DEPICTS THE TIME AFTER THE LAPAR HAS REMOVED THE SAFETY CARABINERS!!)
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]
Addendum: Pulling a smooth operator out of a stone hitch under body weight The LAPAR disconnects the safety
carabiners on the smooth operator, then descends carefully, so as not
to pull the device out during that last rappel! Before the LAPAR
removes the biners, the partners at the bottom try to make sure the
amsteel cord will be free from interference with the rap line.But
surely you have wondered--what would it take to pull it out as you were
rapping?-- say by tangling the Amsteel with your body.
First, recognize that Amsteel is incredibly slick; the surface
coefficient of friction is like that of teflon. It takes very tight
turns to bring in the substatial bending friction, and make this stuff hold knots.
In the video shown below, I suspend my body weight (149 lbs with
clothes
and harness) from a stone knot in yellow/red polyester "8mm" rope
(Maxim kermantle cord, ~8.2 mm, quite slick). The brown rope is a very
static 9mm technora/polyester cord, and has a loop to anchor the stone knot
in the yellow cord, plus has a nylon prusik to catch me when the stone
knot collapses. This is one of 3 tests.
First I try to make a 2 turn footwrap with amsteel, and push against it
to simulate tangling; as I weight the amsteel, it just pulls off my
foot. Then I directly pull on the bottom loop of the load cell -- and
the knot fails at 54 lbs force. In the other 2 tests, it failed at 48
and 80 lbs, the higher number when I had bounced in my harness to cinch
the knot more tightly.
In other words, the 2500 breaking strength of Amsteel is massive
overkill for this purpose. The slickness likely helps keep a slight
tangle from holding to your body, but will likely not help if you got a
massive tangle caught in a crack before the rap. For the latter, I've
had to pull with full body weight to free the tangles, which could be deadly here.
