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TB 55190023210
relative wave directions, speeds and hawser surges. Figures L2 through L5 provide 100 standard curves
predicting extreme tensions based on average resistance The following section provides examples of use of
the tables and curves.
Use of the extreme tension charts and curves first requires a predicted or observed mean towing hawser
tension. Tb simplify the description of using the tables and figures, the following examples are based on the tow
of a 7500 LT DD 963 Class destroyer into seas generated by a 25knot wind Example 3, page G10, predicts
the hawser tension for several speeds, including the hydrodynamic resistance of 2,000 feet of 2'/4inch wire
towing hawser with one shot of 2 1/4 inch chain.
Note that the example shows fairly modest changes in hawser hydrodynamic resistance over the tension range
of interest. For the following examples, the effects of different scopes and different hawsers can be ignored
Alternatively, tow resistance alone can be considered and the wire resistance of the specific hawser in use
added back at the appropriate points. However, the method used here is simpler and will make the results
somewhat conservative.
Using the example from Appendix G, the maximum safe speed for each of the most common Navy Towing
ships will be determined.
The tables are developed for each towing ship class towing a DD 963 Class destroyer displacing 6,707 LT. The
assumed tow, being heavier at 7,500 LT, will experience somewhat less motion, the results will slightly
overstate extreme tensions, but are sufficient for estimating purposes.
L5.1 ATS 1 CLASS Appendix G predicts a maximum tow speed of 7.4 knots with a total hawser tension of
106,000 lbs.
Enter Table L8 at the 25knot/6knot section. Pick off the curve number for several hawser scopes, enter the
curves at 106,000 pounds to find several predicted extreme tensions as follows.
Wave
Extreme
Direction
Scope
Curve No
Tension
0
1,200
5
270,000 lbs.
0
1,500
4
234,000 lbs.
0
1,800
3
203,000 lbs.
0
2,100
29
177,000 lbs
60
1,200
33
235,000 lbs
60
1,500
31
198,000 lbs
60
1,800
30
217,000 lbs
60
2,100
29
177,000 lbs.
The ATS 1 Class towing hawser has a new breaking strength of 360,000 poundsUsing a 1.5 dynamic factor of
.
safety for evaluating extreme tensions, the allowable extreme tension is 240,000 pounds Note that the tension
with only a 0.1 percent probability of being exceeded in a 24hour period is less than the 240,000pound figure
for hawser scopes of at least 1,500 feet. Note also that predicted extreme tension is reduced in some cases
when the seas are met at 60 percent relative, rather than headon.
Not shown here is the extreme tension when towing downwind under these conditions.Assume tow speeds up
to about 9 knots with the same tension, because of the following wind and seas. Predicted extremal tension is
beyond the curve display limits of 400,000 pounds for all towline scopes listed.
The steady state, or mean, tensions must also be checked against the safety factors listed in Table 54. When
towing on automatic tension control, the minimum factor of safety is 3, and the allowable tension is.
360,000/3 = 120,000 lbs
This is greater than the mean tension of 106,000 pounds, so the tow described is within
L8


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