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 TB 55-1900-232-10
are significant and are considered in Section 6-4. However, they do not impact on required towing power or selection of
the tug.
5-3.4 TUG SELECTION. The tug's available propulsion power determines the speed of the tow, and therefore, steady
forces on the towline.
The available pulling power of a tug is the difference between the total thrust from the engine-propeller combination at a
given speed and the thrust expended in propelling the tug alone at that speed. The available pull is maximum at zero tug
speed (the "Bollard Pull" condition) and zero at the tug's maximum free-running speed.
Each class of ship should have its own unique set of available tow tension curves that depend upon engine power setting,
ship speed, propeller RPM and propeller pitch for those ships with CPP. The problem is simplified for tow planning
because the maximum available tow speed is the figure of interest. Figure 6-1 provides the approximate pull available to
the tow hawser, versus speed of Navy tugs at each ship's maximum continuous power rating. These curves, when
compared to the several RT + Rwire values developed in Paragraphs 5-3.1 and 6-3.2, provide approximate maximum tow
speed for the assumed conditions. If the maximum speed available does not coincide with one of the assumed tow
speed conditions, additional tow resistance computations should be performed to achieve a balance between tension
required and tension available. A more direct method is to plot a curve of tow resistance (plus 10 percent for a wire
hawser) directly onto a copy of Figure 5-1 The tug and tow curves will intersect at the maximum speed attainable with
each tug for assumed tow conditions.
If the available tow speed exceeds the amount needed (usually for small or non-ship-type tows), the tow ship will require
less than maximum continuous engine power. In this situation, a less powerful tow ship can be considered. Conversely,
if available tow speed is less than required, a more powerful towing ship or multiple tugs must be selected. In the latter
case, there will be two or more towlines, so R ire, must be increased appropriately. Otherwise, the available towline
w
tension of the tugs is additive.
When the available tug is underpowered for the desired tow speed, the most important consideration is whether it has
sufficient power to keep the tow out of danger under the most strenuous wind and sea conditions that can be reasonably
expected For instance, it may be acceptable that a given tug is unable to make headway over the ground, while towing a
large ship in a sudden gale in the open sea However, the same tug may be considered inadequate for towing the same
ship under the same conditions near a lee shore. In the case of a planned tow of a large ship, adjustment to tow dates
and careful weather routing always are appropriate. For more strenuous cases, adjustment of the assignments and
schedules of other tow ships also may be required to provide the required towing capability.
For emergency or unplanned towing requirements, the tow will be initiated by the first available tow ship Procedures
outlined herein are useful in determining whether additional towing assets should be diverted to escort or take over the
tow.
SECTION IV
5-4 TOWLINE TENSIONS AND CATENARY
Paragraph 5-3 addressed the prediction of tow speed and tug selection based on tow resistance. This section discusses
the steady state and dynamic effects on the towing system, limitations imposed by the tow ship's hawser, selection of
towing rig components and the geometry of the towline catenary.
5-4
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