Types of Tug Boats
There are two groups of tugboats, either Inland or Oceangoing.
1. Inland Tugboats
a) Harbor Tugs: Harbor tugs are the most typical of the tugboats that people recognize. They are used worldwide to move ships in and out of berth and to move industrial barges around waterfront business complexes. Their job has remained the same but their design and engineering has changed much over the the decades. Harbor tugs have evolved from paddle wheelers, to the conventional tug known by all, and now to the Ship Docking Moduals and tractor tugs in the modern industry.
b) River Tugs: River tugs are also referred to as towboats or pushboats. They are designed as large squared off vessels with flat bows for connecting with the rectangular stern of the barges. They are large and powerful, most commonly seen on the big rivers of the world. They are capable of pushing huge fleets of barges that are lashed together into "tows". Some tows can be up to 1000 feet long and 200 feet wide. Smaller push boats are often seen handling only a few barges on inland waters. Despite their size, they are designed to push their tow rather than tow from the stern.
2. Oceangoing Tugboats
a)The Conventional Tug: The conventional tug is the standard seagoing tugboat with a model bow that tows its payload on a hawser. A hawser is the nautical term for a long steel cable or large synthetic fiber rope. It operates independently and is used to tow various loads, e.g., cargo barges, ships, oil rigs, and etc. This is the most versatile method of towing since the conventional tugboat is able to move its load three ways: Pushing from behind, secured to the side of the towed vessel, or by towing astern, all achieved by the use of various lines and cables in various configurations. They are importantly recognized as the design of choice for salvage and assistance of wrecked ships and in the rescue and safe return of disabled ships from the high seas.
b) Notch Tug: The notch tug is a conventional tug which is assigned to tow and push a specific barge, usually built to the shape and specifications of that tugboat. A notch tug has a large towing winch on its stern but it gets its name from the deep notch built into the stern of the barge. This notch is built in the exact shape of the tug's forward hull and can be quite deep, up to 90 feet, sometimes more. The tugboat fits snugly into the notch of the barge and with the use of various lines can be secured firmly enough to push the barge at much higher speeds than it would if it were towing. The towing hawser remains rigged during pushing. In the event that the seas get too rough to push safely, the tug merely releases any securing lines and backs out of the notch while extending its towing hawser. Once in calmer waters, the tug can maneuver back into the notch and resume pushing.
c) Articulated Tug and Barge: The articulated tug and barge, or ATB, is a specially designed vessel, comprised of a tugboat and a barge which are coupled using specially designed machinery. The tug is connected to the barge inside a notch, similar to the notch boat, using a system of heavy pins, clamps, and/or side pads. ATB's remain coupled all the time; the tug pushes its barge in all but the roughest seas. The advantages of this system are speed, safety, and cost efficiency. As a unit, the ATB can push much faster than a tug can tow from astern and the use of a coupling system eliminates many of the hazards associated with towing winches and cables. The unit is considered by authorities to be coupled in a "semi-rigid" manner and, thus, regulated by laws governing tugs and barges, rather than ships. This makes the ATB a less expensive vessel to operate. To be considered articulated, the two vessels may roll simultaneously but must pitch independently. There are three popular systems to achieve this, each having a method to lock the tug onto the barge and secure it's side to side movement, while allowing the tug to pitch freely.
d) Integrated Tug and Barge: The integrated tug and barge, or ITB, is a rigidly connected tug and barge. This means that it fits so tight into the stern of its barge that it will roll and pitch in the same manner with the barge. The systems used to couple the two vessels are varied, but they are similar in that the connection point is virtually seamless and for all practical purpose, they appear to be a ship. These units stay coupled under any sea conditions and the tugs usually have poor designs for sea keeping and navigation without their barges attached. Vessels in this category cannot pitch independently from the barge and so are legally considered to be ships rather than tugboats and barges. As a result of this classification, they are regulated by authorities as ships.