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Avoiding an Accident with a Tug and Towed Vessel

Boating on waters where there is significant commercial traffic requires skill and knowledge. Barge traffic and large commercial freighters are rarely seen on any inland waters save for the Great Lakes. If you are primarily an inland waters boater and you travel to areas where commercial traffic is common, it would behoove you to brush up on the common vessels you might encounter. This can be accomplished by reviewing the Coast Guard Navigational Rules book.

For example, you might think that these slow moving vessels would present an opportunity for your fast moving vessel to travel between the two vessels rather than turning and going behind the second vessel.

tug with tow

This is exactly the scenario that occurred years ago in Puget Sound. An express cruiser attempted to pass between the two vessels at night. It was torn apart and several people on the express cruiser died. The captain of the express cruiser failed to recognize that this was a tug towing a barge. The partially submerged tow line was not readily visible and the metal tow line tore the express cruiser apart. Let's more closely examine the photo above to see what the captain of the express cruiser might have looked for.

This photo of the second vessel clearly shows the steel towing bridle attached to the bow of the barge. Because of the mass of the chain, it has "catenary" or drooping of the chain making it less visible.

Towed Vessel with attached tow chain

This should tip you off that the vessel you are observing is being towed. The captain of the express cruiser did not see the towing bridle because he was traveling at night when there was limited visibility.

The tug also should be examined to see if it has an active tow. This photo clearly shows the tow line attached to the tug.

Tug with attached tow line

The tow line would be difficult to see at night although even during the day, it is not that obvious unless you know what you are looking for. Another indicator is that this vessel has the typical appearance of a tug. What else could you look for to suggest that this tug might be towing another vessel?

Tug with towing lights on

This photo shows two clear indications that this is a tug with a tow that exceeds 200 meters from the stern of the tug to the stern of the towed vessel. It has three vertically oriented white lights, visible at night, and a vertically oriented black diamond ( Navigation rule 24).

Note that according to Navigation Rule 24 this tug is also required to have a stern light visible for 135 degrees (67.5 degrees port and starboard of dead astern) and a yellow towing light also visible for 135 degrees mounted vertically above the stern light. These lights are not visible from this photo because it was taken directly abeam of the tug.

Tug

The Coast Guard Navigation Rule 24 summarizes the requirements for a tug towing a vessel that has a tow length exceeding 200 meters from the stern of the tug to the stern of the towed vessel. Diagram from Coast Guard Navigation Rules.

Towing Diagram

Finally, this discussion would not be complete without further mention of "catenary". "Catena" is latin for "chain". A chain, when suspended between two points, has a typical droop. This droop, in fact, can be described by mathematical formulas. The first mathematical description of catenary was by Joachim Jungius in 1669. The amount of catenary, or droop is directly related to the mass of the chain. This is why a nylon towing bridle has little catenary, while a chain catenary has considerable catenary to the point where most of the chain between the towing and towed vessels is not visible. This is the reason the express cruiser did not see the chain between the two vessels. The catenary in a towing chain is relatively small when the tow begins since dynamic tension is increased. Once the tug and the tow reach a steady speed the dynamic ( or acceleration forces ) are decreased and only the steady tension state remains. The catenary then takes its normal shape. The catenary will only change when subjected to new dynamic forces such as changing sea conditions, acceleration or deceleration of the tug. For further information about towing see the Navy Towing Manual.