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 TB 55-1900-232-10
1,800 foot hawser will not reach that tension until separation is increased by almost 34 feet On the other hand, a 20-foot
"stretch" of the 1,800-foot hawser will increase its tension to only about 75,000 pounds. The longer hawser significantly
reduces the peak tensions caused by the same ship movements.
The classic catenary is limited in its ability to absorb tug and tow motions, even where there is a relatively modest
average hawser tension.
The quantitative data shown in Figure 5-13 are based on slowly-applied changes. The data are now somewhat suspect
because of newly-understood limitations on the effectiveness of the classic catenary in reducing dynamic loads. This is
because the hydro-dynamic resistance normal to the tow wire significantly impedes the rise and fall of the wire at typical
frequencies of dynamic seakeeping loads This causes the classic hawser catenary shape to flatten out somewhat.
Nonetheless, Figure 5-13 provides qualitative comparisons of different towline configurations acting under dynamic
loading.
The reader could prepare a similar analysis of the advantages of adding chain to the towline Using the methodology
shown in Table 6-2, prepare curves showing the effect of adding one or two shots of chain pendant to a given hawser
length. The calculation process is identical, except that the comparison will be between hawsers of the same length but
with different total length and unit weights, since the weight of the chain is distributed throughout the hawser length. The
analysis will demonstrate that adding only one shot of 2/4-inch chain pendant provides a considerably softer system that
develops lower peak tensions for the same change in the separation between tug and tow.
5-4.5.3 Use of Automatic Towing Machines. Use of the automatic payout and reclaim feature of the towing
machines installed in most towing ships is a very effective means of reducing peak towline tensions. Table 6-3 provides
the range of automatic settings available on various tow ships. Generally used when water depth precluded an adequate
catenary, the towing machine was often taken off "automatic" after sufficient towline catenary had been established in
deeper water. Now, however, with questions concerning real effectiveness of a wire catenary in reducing peak tensions,
it appears that the automatic feature is equally as important in deep water. The automatic mode should be used at all
times.
5-4.5.4 Use of Synthetic Towlines. The elasticity of synthetic hawsers, particularly nylon, will reduce the dynamic
loading of the towing system. Significant questions raised elsewhere in this manual dictate against use of nylon for
routine towing However, the use of polyester (Dacron) hawsers is under investigation, the results of which may justify the
use of synthetic hawsers in towing. Synthetic springs are discussed in Paragraph 5-5.2.4.
SECTION V
5-5 SAFETY FACTORS, SELECTION AND SIZING OF TOWLINE LINKAGE COMPONENTS
Towline linkage components are shown schematically in Figure 2-9. Of these components, the hawser, pendants,
bridles and connecting jewelry are the items to be sized based upon towing loads and factors of safety Connecting
components, or towing jewelry, must be given careful attention. Hawser size generally is fixed for a given tow ship. If a
specific size hawser is required, this, rather than the availability of towing ships, will determine tow ship selection.
5-5.1
paragraphs will assist in the application of Table 5-4.
5-5.2
HAWSER. The hawser is primary tensile-load carrying component of the towing system In the past, ocean tugs
have used
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