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 = D²). 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 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 Circum- Lb per ft No. 1 Manila Sisal diameter ference (inches) (inches) Breaking Safe Breaking Safe strength load strength load (pounds) (pounds) (pounds) (pounds) F.S. = 4 F.S.  =  4 1/4 3/4 .020 600 150 480 120 3/8 1 1/8 .040 1,350 325 1,080 260 1/2 1 1/2 .075 2,650 660 2,120 520 5/8 2 .133 4,400 1,100 3,520 880 3/4 2 1/4 .167 5,400 1,350 4,320 1,080 7/8 2 3/4 .186 7,700 1,920 6,160 1,540 1 3 .270 9,000 2,250 7,200 1,800 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 6 1.08 31,000 7,750 24,800 6,200 2 1/2 7 1/2 1.35 46,500 11,620 37,200 9,300 3 9 2.42 64,000 16,000 51,200 12,800 1-3