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 condition): 6.

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 follows:

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 systems.

Application of the formula: determine the BS for a 2 1/2-inch nylon line in both pounds and kilograms:

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 kilograms

NOTE: The constant for the metric system is 172.8.

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 size.

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 shot .

WARNING

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 hitches.

Line Parts

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 discussion.