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 TB 55-1900-232-10
fiber is considerably less than that of natural fibers, more tucks are needed to produce a safe splice.
The traditional standard end fitting for manila was a tear-drop wire rope thimble. However, with the advent of high-
strength synthetics, the eye of the line could stretch sufficiently to allow the thimble to capsize out of the eye. In addition,
the higher strength of the synthetic line caused thimbles to crush and fail To resolve these problems, a variety of solid
thimbles have been developed. These thimbles have become the standard end fittings used on synthetic line. See
Figure 2-42.
SECTION VI
5-6 ATTACHMENT POINTS
Attachment points on tugs and tows transmit the towing load from the tug to the towline and to the tow The tug
attachment point will not be considered here since the vessel already will have been designed for towing The attachment
point on the tow, how- ever, may not have been designed and fitted for towing. If a tow attachment point exists, it must
be checked for adequacy; if it does not exist, it must be designed, fabricated and installed.
Paragraph 2-5 5 of this manual discusses the various types of attachment points on tows and describes the types of
loading to which the various types of attach- ments may be subjected. Every possible effort should be made to ensure
that the tow's attachment point is subjected to only one type of load at a known point, and in a known direction Horizontal
and vertical padeyes should be subjected to a force only perpendicular to the axis of the pin. See Figures 2-21, 2-22 and
2-23 for examples of towing padeyes.
The safety factors to which the attachment points should be designed and built should be in accordance with the General
Specifications for Ships of the U.S. Navy (Ref. 16). The criterion generally applied is that the padeye, bitt, cleat, chock,
etc., will withstand the breaking strength of the line for which it is sized, while not exceeding 35 percent of the yield
strength of the structure
WARNING
If time and the situation permit, a detailed analysis of the padeye and connection shall
be made in order to avoid unexpected failure of either.
5-6.1
suitable connection point exists To use Figure 5-16, enter the chart at the estimated force (F) that the padeye is expected
to see. The force m the case of a tow attachment point will be the approximate towline tension as determined m Section
6-3 Taking an estimated towline steady state force, move across to the desired plate thickness (t). Reading vertically
from the intersection of lines (F) and (t), the minimum hole diameter (d) is read off the top of the chart The minimum
distance to the edge (L) is found by reading up from the intersection of (F) and (t) to the intersection of the diagonal line at
the upper left side of the chart. Read L at the point where this line is intersected. Read padeye length (1) by entering the
chart with force (F) across to the fillet thickness (T), then up to determine (1).
For example, assuming an estimated towline tension of 80,000 pounds, and the plate available for the padeye is 1
inches thick, the hole/pin size must be at least 2 inches in diameter with at least 4 inches of plate forward of the hole.
The padeye length of over 7 inches will permit welding to the deck with -inch fillet welds.
The example is satisfactory for an 80,000-pound steady state tension where the tow will be performed using an automatic
towing machine The factor of safety requires a 3 x 80,000 = 240,000 pound proof-load shackle, or a 2 -inch Grade B
shackle The pin for this shackle is 2 inches and will just fit the hole in the padeye. If the tow were to be performed
without an automatic towing machine, but with
5-31
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