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 TB 55-1900-232-10
J-1.1.3 Freeboard/Depth. High freeboard forward improves seaworthiness. Freeboard aft is a compromise between
the desire to provide a work space that is safe and dry, versus one that is located conveniently close to the work site,
which often is near the waterline.
J-1 1.4 Beam. Beam improves stability, provides the internal volume for storage and other functions, and promotes
efficient work spaces. Too much beam, however, handicaps free-running speed and increases fuel consumption.
Salvage tug beams are 45 feet or more, with the largest over 50 feet.
J-1.1.5 Crew. The crew of a salvage tug is significantly larger than crews of other tugs. Commercial salvage tug crews
will vary from 15 to 25. Less tangible is the experience level of the crew. The best salvage and towing people often
gravitate toward the salvage tugs, man-for-man, the experience level often is superior in these ships.
J-1.1.6 Towing Equipment . Towing equipment, as described in Appendix N, is the "main battery" of a salvage tug.
Some salvage tugs have automatic tow- mg machines Most commercial salvage tugs have an automatic rendering
winch, at a minimum This is an important clue to the capabilities of salvage tugs. Appendix N has a more complete
discussion of tow- mg machines and winches. Section 5.5 discusses strength of towing hawsers and related equipment.
J-1.1.7 Power. Power obviously is an attribute for tugs because it provides for prompt transit to the location of the
casualty, assists in refloating the stranded ship, and facilitates towing the casualty. The citation of horsepower rating for
tugs varies, and this must be understood to make a valid comparison between tugs. This subject is addressed in Section
J-1.2.
J-1.1.8 Bollard and Towline Pull . Bollard and towline pull are measures of maximum pull while dead in the water and
available pull when the tow is under- way. These attributes are also discussed in Section M-1 2 since they are closely
related to power.
J-1.2 POWER, BOLLARD PULL AND TOWLINE PULL . Towing is a very competitive endeavor, with business often
sold on the basis of tug power. Custom, with regional differences, results in different methods of reporting a tug's
attributes, as follows.
J-1.2.1 Power. The power of a tug can be quoted in shaft horsepower, horsepower, indicated horsepower or kilowatts,
as follows:
Shaft Horsepower (SHP) is the power delivered to the propeller. Generally, only Navy tugs utilize SHP to describe their
power; however, this is the truest measure of power delivered by the tug.
Horsepower (HP) generally is the Brake Horsepower (BHP) of its propulsion engines-i e., the power delivered at the
engine' shaft This description ignores the reduction gear and propeller shaft losses, which may be considerable. Most
s
American owners and the offshore oil industry, worldwide, use the HP description for tugs. Some foreign shipowners use
kilowatts as units of power. Kilowatts (1 KW = 1.341 HP) may be assumed to be measured at the engine.
Indicated Horsepower (IHP) derives from the days of reciprocating steam engines and ignores heat, friction, valve and
engine-driven auxiliary losses within the engine Furthermore, some owners may add the generator engines and thruster
power to the total, so IHP may exceed HP by a factor of 1.2 to 1.6 or more. Of course, an owner reporting in HP also
may add generators and other power sources/users to his total Most European and Asian salvage operators report tug
power in IHP and no one wants to be the first to report otherwise, for fear of losing a competitive advantage. It has been
noted, however, that more owners are now reporting in kilowatts to avoid the confusion of HP vs. IHP.
J-2
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