Until high-quality synthetic fibers like polypropylene, nylon and
polyester were developed in the 1950s and 60s, most ropes used
for outdoor activities like boating and climbing were made from
natural fibers. As you can see from the information below, synthetics
have revolutionized safety and rescue rope application and techniques.
Information in this article was provided by Sam Morton, Rescue/Safety
Manager for Sterling Rope Company.
Rope Construction
The rope construction used for most modern rescue and climbing
ropes is referred to as “kernmantle.” The braided
sheath (mantle) surrounds and protects the twisted parallel
core (kern) fibers.
Ropes for different applications have their own unique design
for maximum performance. Matching design with construction becomes
a balancing act that leads to many considerations: how much it
can stretch, its ability to absorb impact, strength, handling
qualities and durability.
Important characteristics for
ropes
used in many boating applications are: ability to float, visibility and strength. Water has a specific
gravity (SG) of 1.0, so anything with an SG less than that floats
in it and those with a higher SG sink in it. Polypropylene and
its derivatives have a specific gravity less than 1.0, making
them ideal for throw ropes. Our floating rescue ropes are all
brightly colored, in yellow, red or a combination of the two
colors.
Ultra High Molecular Weight Polyethylene (UHMWP) fiber
has an extraordinarily high tensile strength and relatively low
stretch. Dyneema® and Spectra® are trade names for
this fiber. Pound for pound, it’s stronger than steel.
It’s used in our high-strength rescue ropes, increasing
the strength of similar diameter standard polypropylene ropes
over 2.5 times. It also has a SG less than 1.0, so it floats.
You’ll notice that this fiber is only used as the core
(kern) of our ropes. The main reason for this is that UHMWP
is very slippery and won’t hold a knot. Polypropylene,
which does hold a good knot, is used for the sheath (mantle)
of these high-strength ropes.
Not all rescue ropes need to float.
Our
½” Sterling
Static Rope, which is also used in the
NRS
Z-Drag Kit, is made
of 100% polyester fiber. Important characteristics of polyester
for this application are that it is hydrophobic (fibers don’t
absorb water, which can weaken a rope) and it has very low
stretch. The low stretch factor makes this rope very efficient
in a Z-drag application. The definition of “static rope” is
a rope with a maximum elongation of 6% at 10% of its minimum
breaking strength.
NFPA Certified
A term you see on some of our ropes is “NFPA Certified.” The
National Fire Protection Association (NFPA) is a non-profit
organization that sets standards for much of the equipment
used by fire fighting and rescue agencies. They don’t
do the actual testing; that is done by third party organizations
such as Underwriters Laboratories (UL). Many rescue agencies
require the use of NFPA Certified rope and hardware in their
work.
Care and Storage of Rope
Rope used during boating gets wet, of course, and gets dirty.
After a trip, rinse your ropes in clean water and allow to
dry before putting them away. Store your ropes in a cool,
dry place away from chemicals and direct sunlight.
Regularly
inspect your ropes. Do this visually and by sliding the rope
through your hands. If the rope is excessively abraded
or you have core coming through the sheath, it is time to retire
the rope.
Knots and Strength Loss
The fibers in ropes, in the kern and in the mantle, are oriented
to line up with the length of the rope, for maximum strength.
The measure of this strength is commonly referred to as “tensile
strength.” They have low flexural strength, meaning
they are not strong along their horizontal axis, which is
why ropes lose significant amounts of strength when tied
in knots. This loss of strength occurs when a rope is bent,
as in a knot or going through a carabiner or pulley. Four
inches is the magic number for maintaining full strength
in a rope. Any bend tighter than four inches reduces the
rope’s strength. Common knots used in rescue situations
can reduce a rope’s strength by 20-40%.
