1,000-foot and 1,800-foot hawser scopes. A 1,000-foot length of 2-inch wire, with initial tension of 50,000
pounds, will have tension increasing to the wire's safe working limit of 187,000 pounds (.65 x 288,000) when the
tug and tow are separated by an additional 9 feet, still a relatively low figure. Further stretch will permanently
damage the wire, and it will break when increased separation has reached a total of 15 feet beyond the
separation characterized by the 50,000-pound initial tension. The same wire, with initial tension of 100,000
pounds, can absorb increased tug-tow separation of only approximately 10 feet.
The figures for 1,800 feet of the same hawser at 50,000 pounds initial tension are somewhat more
advantageous. The system can absorb 19 feet of increased tug-tow separation before reaching its safe working
limit. Overall, the ability of wire hawsers to absorb changes in the distance between tug and tow is relatively
limited, compared to probable ship motions under strenuous sea conditions.
K-2.2 AUTOMATIC FEATURES ON TOWING MACHINES-GENERAL . The full-featured automatic towing
machine can be set to maintain hawser tension within a pre-selected range. It will pay out hawser if the hawser
tension exceeds the set point, and will recover hawser when tension falls below a second set point. Typically,
there are limitations on the total amount of hawser allowed to be paid out or retracted. Some towing machinery
will pay out, but not retract, automatically.
K-2.3 LIMITATIONS IN QUANTITATIVE UNDERSTANDING . The responsiveness of U.S. Navy automatic
towing machines to actual dynamic loading of their towing hawsers is not well-understood at present. The
automatic payout feature reduces the potential peak tension. However, quantitative data on towing machine
time constants and responsiveness need study. The landmark work leading to the data contained in Appendix N
is very recent. It significantly improves the understanding of the dynamic demands placed on the towing
Further work is needed in understanding the process with the automatic towing machine as part of the dynamic
situation. At-sea testing will undoubtedly be required as well, to validate the model predictions There is a
distinct possibility that this work will ultimately result in lowering traditional factors of safety, which are really
"factors of ignorance," while maintaining or improving efficiency and safety in towing operations.
K-3 TYPES OF TOWING MACHINES
This section provides an overview of the types of towing machines in use in the U S Navy, with additional
reference to other types of machines for technical interest.
K-3.1 CONVENTIONAL TOWING WINCHES AND MACHINES. These units, which store the unused wire
hawser on a horizontal drum, are the most prevalent.
K-3.1.1 Drum Arrangements. There may be either one or two main drums, one for each hawser if there is
more than one In the U.S. Navy, two-drum units locate the drums side-by-side. Commercial tugs often locate
the second drum forward of and above the first drum in a "waterfall" arrangement. The drums are always
powered, but capabilities of independent operation of the drums vary. They may be equipped with a level-wind
apparatus and may be strong enough to withstand the breaking tension of the wire applied directly onto the
drum. Some U.S. Navy units are equipped with one or two auxiliary drums to accommodate work on mooring
lines or long, target-towing hawsers.
K-3.1.2 Drum Securing Features. Towing hawser drums generally can be positively locked with a pawl or dog.
For control purposes, a brake system is also provided.
K-3.1.3 Drum Prime Movers. The more sophisticated units use DC electric motors to provide infinitely -
variable speed control. It is