Know the Ropes

ArticleJune 20, 2011

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. 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 Sterling Static Rope 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%.