2-6.2 SYNTHETIC LINE. When synthetic fiber line was developed for commercial applications, it began to replace
manila rope for towing springs and hawsers on small tugs. Synthetics also gained acceptance as open-ocean towing
hawsers, often at the expense of wire rope. However, knowledge about the strength, service life, degradation and
elasticity of synthetic line now is such that nylon is limited in its use as the main towing hawser. Synthetic line currently is
approved for use for open-ocean towing in tow-and-be-towed and emergency towing operations, with craft of less than
600 tons displacement, or other unique/special tows as approved by the originator on a case basis. NAVSEA is
continuing to investigate the use of improved and composite designs of synthetic hawsers that may lend themselves to
towing applications. See Appendix C for more data on synthetic ropes and their use.
2-6.3 CHAIN. Even though chain may not be viewed as a stretchable component in a towline, it does add "stretchability"
to the towline system as a result of its weight, which provides increased catenary in the towline. Chain is discussed in
Appendix D of this manual
2-6.4 FACTORS FOR CONSIDERATION. The preceding paragraphs have concentrated on the characteristics of
strength and elasticity of wire, rope and chain. Reference has been made to strength-to-weight ratios, abrasion
resistance and ease of handling. The following is a list of factors that influence selection of the flexible components of a
a. Strength (static loads, dynamic loads, fatigue)
b. Elasticity (includes stretch vs. load over a full range of loads and over the lifetime of material, as well as set or
c. Predictability (strength and compliance factors)
d. External abrasion resistance
e Internal abrasion resistance (related to fatigue life)
f. Weight and specific gravity
g. Survivability against environment (effects of corrosion, ultraviolet light, acids, temperature, moisture, etc )
h Ease of handling (surface character: slippery, sticky, etc.)
i. Stowability (volume shrinkage upon drying, flexibility, etc)
j. Adaptability to fittings and terminations
k Compatibility of fittings and terminations.
In various towing applications, one or more of these factors may have a predominant influence on the choice of material.
Chain, for example, often is selected as a chafing pendant or bridle because of its abrasion resistance and survivability.
As a lead chain, its specific gravity and flexibility in bending are dominant factors since they provide elasticity through
catenary action rather than through material stretching. Likewise, nylon may be suitable for use as a spring, but would
not be selected as a chafing pendant. Wire rope is favored for use as a tow hawser on ocean tugs because of its
strength and reasonably high abrasion resistance, but its flexibility, stowability and ease of handling also are important
reasons for its use
2-6.5 END FITTINGS, CONNECTING LINKS AND ADAPTERS.
Shackles and other fittings frequently come with cotter pins which should be replaced with locking bolts with
two jam nuts.
a. Towing adapters. A variety of devices and assemblies for connecting the various towing system components are in
use today. They include shackles, plate shackles and detachable links. They all are included in