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TB 55-1900-232-10
5-5.2.2  Wire Rope. For wire rope in new or very good condition and used in conjunction with an automatic towing
machine, a minimum safety factor of 3 is appropriate for routine ocean tows in good weather See Table 6-4 To be on the
conservative side and allow for unforeseen occurrences, a value of 4 is a preferable factor of safety to use for routine
tows. Other conditions require higher factors as noted in the table. The calculated steady towline tension values are
multiplied by the safety factor to obtain the required minimum breaking strength of the wire rope hawser. With these
breaking strength values in hand, Appendix B may be used to evaluate the wire hawsers carried by candidate tow ships If
there is no good match, the assumed tow speed can be adjusted until a match between required hawser strength and
available tow ships is achieved For an individual towing ship, with a specific hawser, the problem may be reversed to find
the maximum allowable steady tension.
5-5.2.3  Synthetic Line. Subject to any other restrictions on their use, for synthetic lines in new condition and over 8
inches in circumference, a value of 12 is a reasonable safety factor when towing with synthetics is unavoidable. The
steady towline tension value calculated in Section 6-3 then is multiplied by a safety factor of 12 to obtain the required
minimum wet strength Appendix C provides data for use in evaluating one or more candidate synthetic lines.
NOTE
Appendix C provides the breaking strength values for synthetic line.
Manufacturer's tables usually quote values for dry nylon. Breaking
strength for wet nylon rope is about 15 percent less than for dry rope
and thus, the manufacturer's values generally must be decreased by
15 percent for towing or other "wet" uses. Wet strength reductions do
not apply to synthetics other than nylon.
When towing with an automatic rendering feature activated, the factor of safety for synthetics over
8 inches in
circumference may be reduced to 10.
Smaller lines, with a greater portion of their fibers exposed to abrasion and the effects of ultraviolet light and other
chemical attack, require higher factors of safety. Increase the factors listed in Table 5 4 by 2.
5-5.2.4  Fiber Rope Springs.  As a peak tension-reducing element, a spring or fiber-rope pendant or grommet
sometimes is inserted in the towline between the hawser and the wire rope pendant. Double-braided nylon line has been
the most widely-used material. A grommet is fabricated by splicing a line to form one continuous loop. The two sides of
the loop are pulled together around two thimbles, and seized with small stuff to form the grommet or strap shown in
Figure 5-14 The size of the line used to make the grommet must be such that the assembled grommet will have a total
safe working load that is equal to or greater than the main hawser's safe working load.
Although the line is doubled in the grommet, its strength is not the same as two lines of the same size or one line of the
same total cross-sectional area would have. There are losses in strength in the splices, and especially in the two eyes,
so that the assembled grommet is only 0.9 times as strong as a single line of the same cross-sectional area. For this
reason, the line used for the grommet must have a basic breaking strength equal to at least %5 of a single line spring in
order to have the same total strength when fabricated into a grommet. A factor of safety of 14 is required for the wet
breaking strength of a synthetic spring.
At present, there is no agreement on the method to calculate the proper length of a nylon towing spring. When used,
commercial operators generally use a spring of 200 to 400 feet in length. Furthermore, as knowledge about towing with
nylon has increased, requiring a larger size spring, the use of fiber rope springs has declined. As an example, with a 2-
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