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TB 55-1900-232-10
The relationship depends upon the metallurgical properties of the material
Safe Working Load (SWL) frequently is used for rigging components and systems including such material. The
concept of SWL is similar to the use of a "safety factor," and is appropriate where the load is fairly well-known
and dynamic loads are limited. The typical use of SWL is for lifting purposes The safety factor inherent m SWL
for Navy safety shackles, compared to proof load, is 2 for Grade A and 2 5 for Grade B shackles. This is
insufficient for use in towing systems where the dynamic loads are more difficult to predict than for simple
rigging purposes However, applying the safety factors from Table 6-4 in addition to SWL is overly conservative
and will result in unacceptably large components. Therefore, in designing towing systems for strenuous
conditions, the safety factors listed m Table 6-4 for shackles should be applied to proof loads listed m Table D-
As an example of the above, consider a predicted steady state tow resistance of 80,000 pounds. This is
appropriate for a 2-inch fiber core towing hawser under automatic towing machine control Table 6-4 requires a
safety factor of 3 for shackles. If this factor is applied to SWL, 4-inch Grade B safety shackles, weighing 310
pounds, would be required m the rig' Applying the required factor of safety to proof load requires more
reasonable 22-mnch Grade B shackles
Plate shackles frequently are used in salvage and towing operations because they are simple, efficient and
easily fabricated from commonly available materials Plate shackles are efficient because many connections of
chain to wire and chain to chain would require two safety shackles, back-to-back, whereas one plate shackle
will accomplish the task. The cheeks of towing plate shackles are fabricated from "medium" (ABS Grade A or
ASTM A-36) steel, the most readily available classification, and the pins are fabricated from 150,000 psi
minimum yield strength bar stock, also readily available. Appendix I includes drawings of plate shackles for use
in towing. Certain salvage ships can be outfitted for heavy-lifting operations. In this case, stronger plate
shackles than shown in Appendix I, may be required. Check the specific rigging plans for the specified shackles
for heavy lifting


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