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 TB 55-1900-232-10
SECTION VII
5-7 FUSE OR WEAK LINK IDENTIFICATION
Many trade-off decisions must be made in the final selection of the towline system. We know that the peak dynamic
loads will be significantly greater than the steady-state, or average, load. The preferred method for mitigation of dynamic
towline loads is the use of an automatic towing machine. The next-best method is the use of a long wire-rope hawser with
a chain pendant. The elasticity of nylon provides an excellent means for damping out the dynamic effects on towline
tension. However, concern over the current state of knowledge regarding synthetic fiber lines, particularly nylon, has led
the Navy to discourage the use of an all-synthetic towline system. In addition, the required safety factors require an
unreason- ably large synthetic spring See Paragraph 5-5 2.4.
5-7.1
CRITERION FOR FUSE OR WEAK LINK IDENTIFICATION. When a weak link is used as a component m the
towing system, its primary purpose is to ensure a known location and mode of towing system failure in the event of an
overload. It is unfortunate that the term "weak link" has been used to describe what is really analogous to a safety valve
or a circuit breaker Sometimes the term "fuse pendant" is used In all cases, the most probable point of failure must be
identified, whether or not it is designated or intended as a weak link.
CAUTION
Good judgment is required in the use of a weak link. When operational requirements
dictate maximum strength, such as a tow along a lee shore, an intentional weak link may
jeopardize the mission.
A weak link is the component in a towing rig that is most likely to fail at a predicted load and/or at a known point Whether
or not it was designed into the tow system, it always exists It is necessary to know where the weakest part of the system
is, and that information be provided to the officer responsible for the tow Some designs intentionally designate the
location of the weak link in the towing rig so that a more critical portion of the tow system will be protected from the
overload A weak link usually is designed to protect the tow hawser. If a towing system overload occurs, the failure will not
damage the tow hawser, which then can be reconnected. A weak link should not fail under the anticipated tensions of a
planned tow. The weak link's primary characteristic is its predictability. Tow preparing activities should specify the
weakest link in the rig for which they are responsible
When a weak link is intentionally used in the tow design, the towing pendant is the usual selection, being sized to have a
10 to 15 percent lower breaking strength than the main tow hawser. Selection of these figures considers the
hydrodynamic resistance of the towline itself, which places a higher tension at the tug end of the hawser Such a pendant
shall never be placed in a position subject to chafing or other unusual service.
SECTION VIII
5-8 TUG CAPABILITY. Much too often tug assignments have been based almost completely on availability. A tug must
have many special attributes other than availability It must be properly manned by competent personnel and have:
a.
Adequate power for the tow at hand
b.
Proper towing gear for the towline linkage system from the tow's hard point to the tug's hard point
c.
Sufficient endurance to complete the tow
The following paragraphs summarize some of the information on power and towing equipment
5-8.1
TUG POWER. The principal measure of a tug's power is its ability to deliver tow- line pull Generally, a tug's
power plant and propeller will have been designed to deliver
5-35
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