Remember that the strength of a line will decrease
with age, use, and exposure to excessive heat, boiling
water, or sharp bends. Especially with used line, these
and other factors affecting strength should be given
careful consideration and proper adjustment made in
the breaking strength and safe working load capacity
of the line. Manufacturers of line provide tables to
show the breaking strength and safe working load
capacity of line. You will find such tables useful in
your work; however, you must remember that the
values given in manufacturers tables apply to NEW
LINE used under favorable conditions. For that
reason, you must PROGRESSIVELY REDUCE the
values given in the manufacturers tables as the line
ages or deteriorates with use.
The safety factor of a line is the ratio between the
breaking strength and the safe working load. Usually
a safety factor of 4 is acceptable, but this is not always
the case. In other words, the safety factor will vary,
depending on such things as the condition of the line
and circumstances under which it is to be used. While
the safety factor should never be less than 3, it often
must be well above 4 (possibly as high as 8 or 10). For
best, average, or unfavorable conditions, the safety
factors indicated below are usually suitable.
Best condition (new line): 4.
Average condition (line used, but in good
Unfavorable condition (frequently used line, such
as running rigging): 8.
Breaking Strength of Nylon Line
The breaking strength of nylon line is almost three
times that of manila line of the same size. The rule of
thumb for the breaking strength of nylon line is as
BS = C squared x 2,400
NOTE: The symbols in this rule are the same as
those for fiber line in both the English and metric
Application of the formula: determine the BS
for a 2 1/2-inch nylon line in both pounds and
Solution: BS = 2.5 x 2.5 x 2,400 = 15,000 pounds
or BS = 6.35 cm x 6.35 cm x 172.8 = 6,967
NOTE: The constant for the metric system is
Nylon line can withstand repeated stretching to
this point with no serious effects. When nylon line is
underload, it thins out. Under normal safe working
loads, nylon line will stretch about one third of its
length. When free of tension, it returns to its normal
When nylon line is stretched more than 40 percent,
it is likely to part.
The stretch is immediately
recovered with a snapback that sounds like a pistol
The snapback of a nylon line can be as
deadly as a bullet. This feature is also true for
other types of lines, but overconfidence in
the strength of nylon may lead one to
underestimate its backlash; therefore, ensure
that no one stands in the direct line of pull when
a heavy strain is applied to a line.
The critical point of loading is 40-percent
extension of length; for example, a 10-foot length of
nylon line would stretch to 14 feet when underload.
Should the stretch exceed 40 percent, the line will be
in danger of parting.
Nylon line will hold a load even though a
considerable number of strands are abraded.
Ordinarily, when abrasion is localized the line maybe
made satisfactory for reuse by cutting away the chafed
section and splicing the ends.
KNOTS, BENDS, AND HITCHES
The term knot is usually applied to any tie or
fastening formed with a cord, rope, or line. In a
general sense, it includes the words bends and
A BEND is used to fasten two lines together or to
fasten a line to a ring or loop. A HITCH is used to
fasten a line around a timber or spar, so it will hold
temporarily but can be readily untied. Many ties,
which are strictly bends, have come to be known as
knots; hence, we will refer to them as knots in this