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TB 55190023210
limits, the actual control being the towing capacity of the ATS 1 at the speed of 7 4 knots.
L5.2 ARS 50 CLASS. This class uses the same tables as the ATS 1 Class. Therefore, the curve numbers will
be the same. But the ARS 50 can tow the 7500 LT DDG 963 Class destroyer at only 7.2 knots with a mean
hawser tension of 98,000 pounds. Therefore, extremal tensions will be less than for the ATS 1 at 7.4 knots. For
example, with a wire scope of 1,800 feet, the extremal tension prediction is 186,000 pounds. The ARS 50 has a
stronger hawser than the ATS 1, and the example needs to be carried no further to check factors of safety. The
tow will be limited by the maximum towing speed attainable by the ARS 50.
L5.3 TATF 166 CLASS. The Appendix G example predicts that the TATF can conduct the tow at 7 75 knots
with a hawser tension of 113,000 pounds. However, the TATF towing winch has no automatic tension feature.
The minimum factor of safety, from Table 54, therefore is 4, and the allowable mean tension is
360,000/4 = 90,000 lbs
This will limit the towing speed to about 6 9 knots.
The extreme tension must also be checked with a mean tension of 90,000 pounds using Table O3.
Wave
Wire
Extreme
Direction
Scope
Curve No
Tension
0
1,200
4
195,000 lbs
0
1,500
3
170,000 lbs
0
1,800
30
162,000 lbs
0
2,100
29
148,000 lbs.
Applying the dynamic factor of safety of 1.5, the maximum allowable predicted extremal tension is 240,000
pounds. Therefore, predicted extreme tensions from ship motion theory is satisfactory for this tow at 6 9 knots.
The 90,000pound steadystate tension condition limits this tow.
L5.4 ATF 76 CLASS. New hawser strength for this class is 288,000 pounds. When towing with the automatic
tension feature engaged, the allowable mean tension is:
288,000/3 = 96,000 lbs
The maximum allowable tension predicted from ship motion theory is:
288,000/1.5 = 192,000 lbs.
The Appendix G example predicts the ATF 76 can tow the 7,500 LT DDG 963 destroyer at 5.7 knots into 25
knot wind and seas, with a steadystate hawser tension of 68,000 pounds. This is less than the allowable mean
tension, so the extremal tension predictions should be examined. The tables do not include ATF 76 Class tows
Use the ARS 38 tables, since that is the ship closest in size, and remember that the extreme tension
predictions may be slightly understated. Enter Table L13 in the 25knot wind/6knot speed block to develop the
following data for an average tension of 68,000 pounds
Wave
Wire
Extreme
Curve No
Tension
Direction
Scope
0
1,000
4
149,000 lbs.
0
1,200
3
128,000 lbs.
0
1,500
30
134,000 lbs
0
1,800
29
121,000 lbs
All of these predictions are less than the 192,000 pounds allowed, and the tow can be performed with
acceptable risk at 5.7 knots at 68,000 pounds mean tension
Note that the tables and curves predict an increase in extreme tension when increasing hawser scope from
1,200 to 1,500 feet. This does
L9


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