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TB 55-1900-232-10
fluence upon mean tension. To account for the less important influences of speed, different curves are
specified for different speeds.
L-3.3.5 Yawing and Sheering. In towing nomenclature, sheering is the movement of the tow off to the side of
the towing track This motion can be steady, with the tow staying to one side. Alternatively, it can be unsteady,
with the tow moving from one side to the other, taking several minutes to go through a complete cycle.
Occasionally, sheering is called "yawing." This term should not be confused with "yawing" in seakeeping
nomenclature, which is a wave-driven variation in heading that occurs at the frequency of the waves. The
effects of yawing are included in the computations, but effects of sheering are not.
L-3.4 DISPLAY OF THE DATA . The computations use fully-developed wave characteristics associated with
wind speeds of 15, 20, 25 and 30 knots. For every combination of speed, wave characteristic and angle, ship
combination and tow speed, the computation procedure assumes a tow speed of 3, 6 or 9 knots. Mean towline
tensions were assumed to be the following:
a. 10,000 pounds
b. 20,000 pounds
c. 40,000 pounds
d. 80,000 pounds
e. 120,000 pounds
A very large amount of data is developed and is displayed in Tables L-1 through L-15. To simplify the
presentation, from 100 different curves a "standard" curve is selected (Figures L-2 through L-5) that provides
the best fit to the results of comparing extreme tension to average tension. When there is not an obvious
choice, the program selects the curve that best minimizes the error at higher tension ranges-
i.e., at about 70 percent of the strength of the wire used by the tug. Precise accuracy at higher tension levels is
not needed, since there is too much risk associated with towing at these tensions. Errors at lower levels are not
a problem, since the wire is not highly-loaded.
For each tug/tow combination, the data are presented in 12 tables-one for each of three tow speeds at each of
four different wind speeds (therefore, wave characteristics). Within each table, data are listed for five towline
scopes against each of four wave-encounter angles. The individual data point identifies the standard curve that
best describes extreme tension vs. average (computed or observed) towline tension.
As an example, for a T-ATF using 1,800 feet of hawser, towing a ship comparable to a 40,000-ton LHA at 3
knots, into head seas generated by 30-knot winds, the table identifies curve number 6. If the predicted average
towline tension is 52,000 pounds, curve 6 predicts that there is only a 0.1% probability that a tension of 140,000
pounds will be exceeded in a day of towing. Assume that the tow now sheers out to one side, remaining off the
tug's quarter for a significant period. The tug's towing winch tension-meter now reads 70,000 pounds. Curve 6
predicts the extreme tension of 195,000 pounds still well below the allowable tension for the T-ATF's towing
hawser.
L-4 USE OF EXTREME TENSION DATA
This section presents the extreme tension curves and tables for various tug/tow combinations and provides
guidelines on their use.
L-4.1 ALLOWABLE EXTREME TENSION . The tables and curves presented are based on computations that
modeled the actual wire towing hawsers associated with the tugs involved See Table B-1. Computations
assume that the final connection to the tow uses one shot of 2 1/4-inch chain, 20 feet of which is on the deck of
the tow
L-5


 


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