Page 36 - Randy Penn Everything Knots Book

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ALL ABOUT ROPES AND TWINE

Rope is best used at or below its “working load” level. Using
a rope near its breaking strength will cause what is called “fatigue,”
and the rope will have lower strength as a result. The working load
rating is many times less than its breaking strength and is often
labeled on rope packaged for sale. This is the amount of load that
can be used on the rope repeatedly without causing fatigue.
As important as strength is to your choice of rope, it may
actually be the last criteria that you use. That is because most
ropes come in many sizes. You may choose a rope based on
other factors such as whether or not it floats, its durability, ten-
dency toward stretch, and so on. After narrowing your choice
down to a single type or rope, just choose the size that corre-
sponds to the strength needed.

QUESTION?

How do I determine the strength of a rope?
The rope’s strength is determined by the area or cross section
of the rope. An otherwise identical rope with twice the
diameter will have four times the strength, and vice versa.
Moreover, the bigger rope will have less stretch when put
under the same amount of strain as the smaller one.

Stretch under Strain
Ropes vary in how much they stretch under strain. Some appli-
cations require very little stretch, while some benefit from it. Nylon
is an example of a cordage material that stretches, and polyester
is a common material used in low-stretch applications. A low-
stretch rope can help you avoid shock loading in an object by
firmly holding it in place.
Some applications can only use ropes of low stretch. If the
rope holding up a sail on a sailboat were able to stretch when the
wind picked up, it would allow the sail to move and thus change
shape. A hammock should be made of, or slung from, low-stretch

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