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 TB 55-1900-232-10
a chain pendant, Table 6-4 would require a shackle factor of safety of 4. Appendix D and Tables D-4 and D-5 show that
the minimum Grade B shackle size is 3-inch, with a 3 -inch pin. The 1 -inch available plate can be used with a larger
hole, taking care to maintain the minimum (L) required by the load and plate thickness
The below-deck structure must be checked or altered to transmit towing stresses to the ship's structurals. Simply welding
the padeye to the deck plating is not enough.
5-6.2
EMERGENCY INSTALLATION OF ATTACHMENT POINTS.
Suggested attachment points of sufficient
strength to tow in an emergency include: using the ship's anchor chain, using installed bitts or padeyes, wrapping a
foundation structure such as gun mount or winch with chain, or welding a padeye to the deck. The preferred methods are
to use the ship's anchor chain or installed padeyes. The other methods are to be used in emergency situations and may
be necessary due to damage to the tow, or other unusual operational constraints.
If there is no designated towing padeye, the ship' anchor chain stopper padeyes are possibilities. See Paragraph 2-5.6
s
for a description of emergency towing from the tow's anchor chain.
Mooring bitts are a possible choice for securing a tow hawser. U.S. Navy bitts are designed to withstand the breaking
strength of double-braid nylon rope for which they are designed, with a factor of safety of 3 on yield strength. Thus, the
safe working loads for U S. Navy bitts are as follows.
Bitt Size
SWL
4 inches
23,000 pounds
8 inches
60,000 pounds
10 inches
100,000 pounds
12 inches
164,000 pounds
14 inches
265,000 pounds
18 inches
375,000 pounds
The allowable load can be applied to either barrel (not both), in any direction.
The strength criterion for bitts m commercial ships is similar, except that older ships and Navy support craft often have
been designed for manila mooring lines. Consider this when employing bitts for towing of commercial or older Navy
ships. In all cases, the strength must be discounted in the presence of obvious corrosion or poor maintenance
Attaching a chain directly to the typical-sized bitts found aboard ships is feasible, but removing slack is difficult. Such a
connection is susceptible to shock loading from sudden rendering and has a higher possibility of failure.
An improved connection can be made using wire the same size as the towing pendant. Furthermore, it is easier to
minimize slack when rigging with wire and backing up to other bitts is quite feasible. See Figure 5-17.
In Figure 5-17, note that the chain provides the towing chafing protection at the deck edge, but wire is used to make the
final connection. The reason for using backup bitts is to share the load. To accomplish this, the loaded part should make
only one turn around one barrel of the first bitts. That first turn will absorb 50 to 75 percent of the total load on the wire,
depending on the coefficient of friction, and pass along 25 to 50 percent to the backup bitts. The wire can be secured to
the second bitts in the normal fashion. Backing up to a third set of bitts is not worthwhile
If two turns had been taken around the first set of bitts, only about 6 to 12 percent of the total load would be passed on to
the second bitts, and the whole reason for backing up, i e., load sharing, would have been voided.
The same principles are applicable to fiber rope load sharing.
5-33
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