TOWING SYSTEM DESIGN
This chapter addresses technical aspects of towing, including resistance of the towed vessel, effects of statics and
dynamics of the tow on the tow hawser and selection of components to be used in the tow connection. Data presented
will be useful to both the crew of the towing ship, in sizing the towing components and fittings, and to the Commanding
Officer in better understanding options he has available to minimize risk during strenuous tows and heavy weather. The
shore-based planner will find useful data for predicting operational aspects of alternative towing ships, where there is a
choice, for a given tow. This applies whether the operation is pre-planned or conducted under emergency conditions.
Finally, the chapter discusses navigational lights and flooding alarms.
5-2 TECHNICAL DATA
When planning a tow and designing the tow system components, important considerations are:
a. Expected or required towing speed.
b. Selection of the towing ship.
c. Towline tension as determined by the total resistance of the tow and respective seakeeping motions of the tug
d. Towing hawser system specifications (eg., type, diameter, expected maximum tension and scope) and
e. Maximum practical towline length, as determined by navigational and hydrographic restrictions on towline
f. Tug's endurance required to complete the mission.
g. Unique characteristics of the anticipated tow.
These factors are interdependent. For example, in theory the desired towing speed would largely determine the required
tow hawser size. But in practice, there is little choice of tow hawser for a given tug class. Hawser choice, therefore, is
governed by the ships available for the towing assignment. For larger tows, those fully utilizing the propulsion power of
the tug, the tug determines the potential speed of the tow. If this is insufficient, alternative tug arrangements will be
required. Given the tug and the resulting speed of the tow, the tow hawser size can be checked and an initial towing rig
design can be accomplished.
There are three steps in the design of a towing system.
a. Select the desired towing speed. Then calculate the towline tension for several speeds above and below
recommended (or desired) speed and at different wind/sea combinations. This will assist in determination of
towing speed. Section 6-3 and Appendix G provide the methods for predicting the resistance of the tow.
b. Review and evaluate available tug/hawser combinations by comparing them to requirements established by the
towing resistance and desired towing speed. Account for effects of weather, type of towline, dynamic load
mitigation, etc. Safety factors used in the calculations must be appropriate for materials and equipment
involved, anticipated weather and other conditions of the particular towing mission.
If calculated requirements for
Recheck the refined calculations against the towing ship's capabilities.
power or towline.