of 4. The result is an SWL of 2,250 pounds. A safety factor is always used because the breaking strength of line be comes reduced after use and exposure to weather conditions. In addition, a safety factor is required because of shock loading, knots, sharp bends, and other stresses that the line may encounter during its use.
If tables are not available, the SWL may be closely approximated by a rule of thumb. The rule of thumb for the SWL, in tons, for fiber line is equal to the square of the line diameter in inches (SWL = D
2 ). The SWL, in tons, of a 1/2-inch-diameter fiber line would be 1/2 inch squared or 1/4 ton. The rule of thumb allows a safety factor of approximately 4.
Synthetic-fiber lines, such as nylon and polyester, have rapidly gamed wide use by the Navy. They are lighter in weight, more flexible, less bulky, and easier to handle and stow than manila lines. Also, they are highly resistant to mildew, rot, and fungus. Synthetic lines are stronger than natural-fiber line; for example, nylon is about three times stronger than manila. When nylon line is wet or frozen, the loss of strength is relatively small. Nylon line will hold a load, even though several strands may be frayed. Ordinarily, the line can be made reusable by cutting away the chafed or frayed section and splicing the good line together.
Table 1-1. - Properties of Manila and Sisal Line
|Nominal Diameter (inches)||Circumference (inches) Lb per ft||No. 1 Manila - Breaking Strength (pounds)||No. 1 Manila - Safe Load (pounds) F.S. = 4||Sisal - Breaking Strength (pounds)||Sisal - Safe Load (pounds) F.S. = 4|
|1 1/8||3 1/2||.360||12,000||3,000||9,600||2,400|
|1 1/4||3 3/4||.418||13,500||3,380||10,800||2,700|
|1 1/2||4 1/2||.600||18,500||4,620||14,800||3,700|
|1 3/4||5 1/2||.895||26,500||6,625||21,200||5,300|
|2 1/2||7 1/2||1.35||46,500||11,620||37,200||9,300|