They move up and down the rivers, through the Gulf and Atlantic Intercoastal Waterways and across oceans. Tug boats tow loads astern on mile long cables or push them ahead, locked together like a ship. A tow two or three barges wide and 1,000 feet long is a familiar sight on the Mississippi. A tug may even move a single barge by its side, like a mother with a baby on her hip.
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"Towing astern" means the tug is in front of the barge or string of barges, called a tow, and pulls them after itself. The tow astern presents special manoeuvring problems, since the two cannot stop quickly. This means that, should the tug stop, the tow might run over the tug. The tug towing astern carries two white mast lights in a vertical line as well as the red and green navigational lights on its port and starboard sides. It will also have a yellow towing light above a white stern light. A tow more than 200 meters long is marked with an unlighted diamond shape on a staff at all times. The tow is illuminated at night by a spotlight from the tug. A tow less than 200 meters long must have side lights -- red to port and green to starboard -- and a white stern light.
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Towing by Pushing Ahead
Towing by "pushing ahead" means the tow is secured to the front of the towing vessel and is pushed through the water. The tug pushing ahead will have two yellow towing lights arranged vertically above a white stern light. If the tug and barges are not locked together, each barge has its own red and green navigation lights and a flashing "tow" light at the bow of the lead barge. If the tow is more than 200 meters long, a diamond will be displayed on the lead barge.
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When towing alongside, also known as towing "on the hip," the barges are fastened to the side of the tug. Towing on the hip is not used for large tows if only a single tug is available for power, since the tug is unable to apply power to both sides of the tow evenly. The barge or barges will be lit with their own navigation lights and the tug will carry two white mast lights in a vertical line. The tug will also display two yellow towing lights vertically, above a single white stern light.
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A composite tow, called an integrated tug-barge or ITB, is a special kind of tug that locks into special ship-shaped barge, to form a single ship-sized vessel. Most often seen along the U.S. East and Gulf coasts, the composite unit offers shippers an alternative less costly than a large ship. Should you encounter a composite unit, remember that it is lighted as a single vessel, under Rules 23 and 24 of the navigation rules.