The resolution of the towline design problem is now seen to require not only improved knowledge of the
characteristics of nylon and other synthetic fibers, but also better basic engineering tools and a better
understanding of the towline system itself as well as its response to the marine environment
Towards this end, the U.S. Navy has instituted the Synthetic Fiber Rope Safety Program. The final out- come
of this effort should provide sound decisions on the use of synthetic lines in towline systems (e.g., wet versus
dry strengths, nylon versus polyester or other synthetics, all-synthetic towlines, and Safe Working Loads and
Factors of Safety for synthetics). In fact, a major value of the research effort will be the provision of a stronger
engineering base for the design of any towline system.
In the meantime, the U.S. Navy does not approve the routine use of synthetic towing hawsers for open- ocean
towing See Paragraph 2-6.2 and the introductory paragraphs of Appendix C. Refer to the current edition of the
U.S. Navy Salvage Manual for guide- lines on use of synthetic lines in salvage.
1-3.2.2 Bitts, Padeyes and Hooks. Initially, the connection of the tow hawser to the tug was simply to belay
the natural-fiber manila line to bitts at the stern of the tug Later, wire rope began to be used for hawsers and
metal came to be a more prominent part of both the ship's structure and its fittings. Then padeyes were
installed, to which eyes in the tug end of the hawser could be secured The padeyes eventually evolved into
towing hooks over which thimbled eyes of both fiber and wire rope could be hooked. When wire rope came into
use, the hooks began to be equipped with heavy springs to help replace some of the natural elasticity that was
lost in the change from fiber line to wire rope.
1-3.2.3 Winches and Towing Machines. Although wire rope was somewhat easier to handle than was wet
manila line of equal strength, it still could not be faked out on the deck as a manila hawser could be when
hauled in Powered winches were a natural evolution, providing the in-haul and storage features for wire rope
hawsers and for the attachment point formerly provided by the bitts and hook. The winch soon became the
standard equipment for the hard- point, m-haul, and stowage functions for most ocean tugs Winches have
since developed along two paths. On one path, a control system was added which would pay out the towline
when the tensile load on it exceeded a set value; then, as the tensile load decreased, the control would haul in
the hawser. These actions tend to smooth out the peak loads and maintain a more nearly constant tensile load
on the towline system.
The other path of winch evolution was the development of a multi-sheave traction winch for use with very long
synthetic fiber hawsers. Rather than stowing the hawser on a drum which serves as both hard-point and
stowage, the traction winch, with assistance, feeds the hawser into a stowage bin. Some traction winches are
now equipped with automatic controls which pay out hawser to relieve high tensile loads However, the control
generally does not provide for automatic reclaim on the traction winches. Traction winches for wire hawsers are
also found on some larger commercial ships. Appendix L contains more detailed information on towing
machines, towing winches and traction winches.
1-3.2.4 Catenary. When the change was made from manila to wire rope for towing hawsers, the catenary
became the primary means of relieving the peak dynamic tension loads. The weight of a wire-rope towing
hawser, either alone or in combination with a short segment of chain cable at the tow end of the towline, causes
a catenary in the towline between the tug and the tow. Variations in the towline tensile load tend to be
smoothed out in the catenary. Temporary decrease of the distance between tug and tow, or a decrease in
tension, is absorbed by a deepening catenary depth. An increase in the separation between tug and tow
causes the catenary to decrease, and the hawser tension to increase. Thus, the wire catenary tends to act as a
spring, softening the tug-tow interaction.