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 TB 55-1900-232-10
ship class also has a 5,000-foot, 1-inch auxiliary hawser.
f.
T-ATF 166 Class-This ship was conceived and specified to replace the Auxiliary Ocean Tugs (ATA)
and Fleet Tugs (ATF) for routine towing, and it also was intended to serve as a salvage tender,
hence, it has a large afterdeck, much as an offshore supply vessel. Although not normally carried,
various suites of special equipment can be installed on board the T-ATF for support of such services
as: air and mixed gas diving, beach- gear operations, off-ship firefighting and oil spill recovery
response This class is capable in rescue towing applications, but has limited salvage capabilities.
The class is not equipped with an automatic towing machine; rather, it is equipped with a single-
drum, diesel-driven winch-one which is relatively common to the offshore oil industry. This 7,200
horsepower tug class carries a 2,500-foot, 2/4-inch wire rope tow hawser At the time that the T-ATF
was being specified and designed, it was thought that nylon rope would be utilized for various towing
missions. To sup- port this, the T-ATF was also given a traction winch which can handle synthetic
hawsers up to 15 inches in circumference.
g. ARS 50 Class-These four ships are an upgrade of the ARS 38 Class. The automatic towing machine
on board the ARS 50 Class is a Series 322 double-drum machine built by Almon A. Johnson, Inc.
This double-drum automatic towing machine stores 2/4-inch diameter towing hawsers, 3,000 feet
long The ARS 50 Class ships also have Series 400 traction winch capability for handling synthetic
line up to 14 inches in circumference. The traction winch also is useful for mooring and ocean
engineering operations.
1-3.2
TOWLINE SYSTEM. After the tug, the next elements in the towing system are the hawser and an
attachment such as a post, bitt or hook. Through time, this combination has developed into a winch or towing
machine. The towing system also may include a chain pendant, wire-rope pendant and/or spring pendant as
part of the tension-member portion of the system. These elements are connected by shackles, links or other
connecting devices The towing system must not only serve as the tensile load-carrying link between tug and
tow, but also withstand dynamic peak loads, often called "shock" loads. Traditionally, there have been three
ways of coping with these peak loads in a towline:
a
Using a fully automatic towing machine
b. Using a deep catenary as a spring
c.
Inserting a spring or stretcher into the towline.
These items and techniques are discussed briefly in the following paragraphs.
1-3.2.1 Main Towing Hawser. The main towing hawser generally has been a wire rope, especially when
used with any of the peak load-reducing systems mentioned above. One can, however, utilize a fiber line
hawser of either natural or synthetic fiber. Such a hawser has enough elasticity of its own to act as a spring and
dampen the peak loads. In this connection, refer to Paragraph 2-6 2 and Appendix C for conditions allowing
use of synthetic fiber hawser.
The U S. Navy, along with all other ocean towing organizations, relied on past experience and empirical rules
of thumb for designing and utilizing tow- lines which depended upon the catenary or upon the automatic towing
machine for shock mitigation However, when attempts were made to use nylon line (pendants or complete
hawsers) as the peak load mitigation system, the Navy began to experience problems. It appeared that the
source of some of the problems was a lack of understanding that nylon has lower strength when wet than when
dry. Additionally, there has also been a realization that the Safe Working Load and Factors of Safety for the
use of nylon in the marine environment have not been adequately defined
1-5
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