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 TB 55-1900-232-10
to be issued so as to permit slow and orderly course changes. It is important not to subject the tow or towline to
excessive dynamic loading. Slow course and speed changes will prevent excessive strain. If an automatic towing
machine is installed, a low tension setting can be employed and the tow streamed as speed is increased. Once the
desired scope is achieved, the setting on the automatic towing machine may be increased to the desired value.
CAUTION
Small increments of rudder angle are recommended when changing course under tow. This will ensure that the
tug maintains control of the tow and prevents the tow from ranging up on the tug. Never permit the tow to pass
forward of the tug's beam, as the tug or tow hawser may be severely damaged.
3-5.1 TUG STEERING. The ability of the tug to maneuver itself under all conditions is essential.
NOTE
Maneuvering characteristics of the tug can be dramatically affected when towing another vessel.
The position of the tow point (that point at which towline tension is applied to the tug) and the tension on the towline
create a moment which opposes the rudder moment and hence restricts the turning motion of the tug. The tug's ability to
steer is increasingly hampered as the tow point is moved further aft. The affect is aggravated at low or zero speed. The
term "in irons" is used to describe the condition where the opposing moment of the towline is the same as or greater than
the turning moment created by rudder and other hydrodynamic forces. The tug is then rendered incapable of steering.
See Figure 3-8. Clearly, being in irons is undesirable, especially when maneuvering in confined waters or in a poor
orientation with respect to the sea. A tug also can be rendered in irons when she cannot make headway under her own
power because of the towline making contact with the bottom. In this case, the tug effectively is anchored by the stern.
Her tow is not anchored and the distance between the two vessels may rapidly close. To avoid being run down, the tug
should shorten the wire and regain headway at once.
Ideally, the position of the tow point should be located at the pivot point, to allow the tug maximum freedom of rotation in
steering. This is why the towing winch is mounted as far forward from the stern as practicable, and as close to the pivot
point as possible. Although it is doubtful that any towing winch is located at the pivot point itself, the pivot point without a
tow is usually located on the center line at about one-third of the tug's length from the bow. From a practical standpoint,
the towing point is designated as the towing winch or towing bitts, if installed. However, there are times when the towing
point is located farther aft-e-g., a Norman pin, hogging strap or stern roller. The operator should be aware of the possible
maneuvering restrictions imposed on the tug when towing in these configurations and should take the necessary
precautions to avoid being placed in irons.
3-5.2 KEEPING A TUG AND TOW IN STEP.
When a tug is at sea with a tow, the two vessels move distinctly and separately in surge, sway, heave, roll, pitch and yaw
in response to the surface waves. The degree and timing of motion which either vessel experiences with respect to the
surface waves depends on the individual vessel's characteristics. No two vessels will respond to the surface waves in
exactly the same pattern. In cases where the surface wave pattern is characterized by a single predominant wavelength,
it may be possible to minimize the difference in the timing of the tug and tow motions. This involves adjusting the towline
scope so as to place the tug and the tow on crests of the predominant surface waves at the same time. By placing the
vessels on the crest at the same moment, they will move in response to the waves in the same direction at
approximately the same time. Adjusting the timing of the vessel motions in this way will reduce the dynamic tension in
the
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