seas to angles of 40 degrees relative The 60-degree results can be used for seas from 40 to 70 degrees
relative. Results are not provided for predominantly beam seas between 70 and 110 degrees relative, because
the tow's rolling and sheering present much larger difficulties than towline dynamics for these angles The 120-
degree results can be used for quartering seas between 110 and 140 degrees relative, respectively, whereas
the 180-degree results are appropriate for following seas between 140 and 220 degrees relative.
L-4.2.6 Wind Strength and Wave Height. If the seas are not fully-developed, the relationship between wind
speed and wave characteristics will be different from that used in the computer program. Factors for which seas
would not be fully-developed include small fetch or changes in wind speed or direction, since it takes many
hours for the sea state to reach equilibrium with the wind.
When seas are not fully-developed, data for wind speeds corresponding to actual wave heights should be used,
as listed in Paragraph O-3 3.2, instead of data for existing wind speeds. For example, a sudden 45-knot wind
can develop waves estimated at 9 feet. Enter the tables at the 25-knot wind speed, which assume H 1/3 at 9 1
L-4.3 RESPONSE TO WORSENING SEA CONDITIONS. When encountering rising seas, the towing ship
Commanding Officer has several options.
L-4.3.1 Reduce Speed. This probably is the single most effective action, because of the multiple effects in
extreme tension' reduction of towline stiffness, with consequent dynamic component reduction; and reduction of
the base to which the dynamic components are added.
L-4.3.2 Increase Towline Scope. While not as effective as a reduction in tow speed, increasing towline scope
usually is the first action taken, assuming that water depth and towline total length permit. This reduces the
stiffness of the system, and therefore the dynamic component of the extreme tension.
L-4.3.3 Change Course. Examination of the tables and curves reveals many examples of changing course to
encounter the waves on a different relative heading. Sometimes head seas are better; and at other times, seas
from 60 degrees relative are better For instance, examine the ARS 50/DD 963 tow, 30 knot head wind, 1,500-
foot hawser scope at 3 and 6 knots tow speed, and 9 knots with 1,200-ft. Scope. In general, stern seas at a
given average tension are worse, but in this case, the tug can reduce RPM and might still achieve headway
over the ground with acceptable extremal tensions. Specific examples will have to be worked out carefully.
L-4.3.4 A Sheering Tow. A tow sheering badly to one side or the other will raise the average tension. This is
due to significantly increased hydrodynamic resistance of the towline and (possibly) increased resistance of the
tow because of a relatively long-term yaw angle from the course of the tug. Such sheering movements
generally occur at a low frequency, so that they in themselves do not generate dynamic effects. The tug,
typically with a constant-torque engine setting, will simply slow down to compensate for the increased average
resistance. Sheering is not allowed for in the extreme tension model, which assumes that the tow yaws about
the track of the tug With a sheering tow, the tow ship should observe the average tension over a minimum of
30 seconds, when the tow is at its extreme deviation from the tug's track. This figure should be used to enter
the curves to determine the extreme tension. If a badly-sheering tow is also rolling heavily, and has a high bow,
increasing the dynamic factor of safety to 2 0 is appropriate.
L-5 EXTREME TENSION EXAMPLES
Tables L-1 through L-15 identify the appropriate extreme tension vs average tension for various tug/tow
combinations, wind forces,