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 TB 55-1900-232-10
K-2 TOW HAWSER SURGE LOADING AND AUTOMATIC TENSIONING
This section provides a historical perspective and brief discussion of surge loading of towing hawsers and the
advantages of automatic-tensioning towing machinery.
K-2.1 SURGE LOADING. Most seamen are unaware of the surge motions of ships at sea because there is no
reference from which to measure the motion. However, connecting two ships by a tow hawser does provide a
reference, and towing people long have been aware of the relative motion of tug and tow in ocean wave
systems For small tugs and/or tows, the hawser itself exerts considerable restraint to the motion of the ships
from their completely independent states. If the tug and tow are both relatively large, it is apparent that a
strenuous sea state can easily impart sufficient relative motion to cause hawser failure.
K-2.1.2 Early Automatic Towing Machinery .
Experienced tug seamen have known the dangers of load surges for at least a century. Steam power led to
larger, more powerful tugs and to wire hawsers as manila hawsers grew to unmanageable sizes. As steam deck
machinery was developed, it was naturally applied to a winch for the towing hawser. The throttle to the winch
steam engine could be cracked open by trial and error to provide an automatic feature to the winch. When the
steam pressure behind the pistons was overcome by the tension of the hawser, the winch would pay out;
likewise, slack would be taken in automatically when the load was eased. Simple controls were added to
quantify the set point and to limit the total amount of wire paid out, or taken in, without human intervention
Through this arrangement, large potential surges in hawser loading were significantly reduced with the
"automatic" steam towing winch This improved safety and wire wear, and permitted use of more power than
would be available otherwise.
K-2.1.3 Electric Towing Machinery. Dieselization of large tugs, commencing in the 1930s, was a major
advancement for propulsion power and endurance. But it was a setback for automatic towing machinery
because the steam-powered winch no longer was an option Electric-driven automatic towing machinery was
developed, but it tended to be relatively expensive and complex. While the U.S. Navy was a leader in the use
of automatic towing machinery, commencing early in World War II, much of the rest of the world returned to
non-automatic towing machinery for its non-steam-powered tugs, and much of it remains there today!
The arguments against automatic towing machinery are many. To list a few:
a. "My seaman's eye is more accurate and reliable than any automatic winch."
b. "Automatic towing machines are too heavy and too expensive."
c. "Automatic towing machines are unreliable, difficult to understand and Impossible to maintain."
d. "Automatic towing machines are noisy, making it difficult for the crew to sleep at night."
Each reader will be able to add other often-heard complaints. Nevertheless, the arguments for automatic towing
machines are even more compelling, as described in the following paragraph, which quantify the magnitude of
surge tensions.
K-2.1.4 Wire Surge Example. Section 6-4 provides data on catenaries of wire towing hawsers. As the strain
increases, the catenary becomes flatter, with less spring available. In fact, if it were not for stretch of the wire
itself, it can be shown that a 1,000-foot, 2-inch FC hawser, with a steady tension of 50,000 pounds, would break
if the tug and tow were separated by only an additional 2 feet! Fortunately, the hawser has considerable
elasticity Figure 5-3 compares tug-tow separation to hawser tension for
K-3
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