Tugboat. A tugboat or tug is a marine vessel that manoeuvres other vessels by pushing or pulling them, with direct contact or a tow line. These boats typically tug ships in circumstances where they cannot or should not move under their own power, such as in crowded harbors or narrow canals, or cannot move at all, such as barges, disabled ships, log rafts, or oil platforms. Some are ocean-going, and some are icebreakers or salvage tugs. Early models were powered by steam engines, which were later superseded by diesel engines. Many have deluge gun water jets, which help in firefighting, especially in harbours. Fleet tug USS Tawasa which towed a nuclear depth charge as it was detonated in Operation Wigwam in 1955 Tug and barge connected Seagoing tugs fall into four basic categories: The standard seagoing tug with model bow that tows almost exclusively by way of a wire cable. In some rare cases, such as some USN fleet tugs, a synthetic rope hawser may be used for the tow in the belief that the line can be pulled aboard a disabled ship by the crew owing to its lightness compared to wire cable. The notch tug can be secured by way of cables, or more commonly in recent times, synthetic lines that run from the stern of the tug to the stern of the barge. This configuration is generally used in inland waters where sea and swell are minimal because of the danger of parting the push wires. Often, this configuration is employed even without a notch on the barge, but in those cases it is preferable to have push knees on the tug to stabilize its position. Model bow tugs employing this method of pushing nearly always have a towing winch that can be used if sea conditions render pushing inadvisable. With this configuration, the barge being pushed might approach the size of a small ship, with the interaction of the water flow allowing a higher speed with a minimal increase in power required or fuel consumption. The integral unit, or integrated tug and barge, comprises specially designed vessels that lock together in such a rigid and strong method as to be certified as such by authorities such as the American Bureau of Shipping, Lloyd's Register of Shipping, Indian Register of Shipping, Det Norske Veritas or several others. These units stay combined under virtually any sea conditions and the tugs usually have poor sea-keeping designs for navigation without their barges attached. Vessels in this category are legally considered to be ships rather than tugboats and barges must be staffed accordingly. These vessels must show navigation lights compliant with those required of ships rather than those required of tugboats and vessels undertow. Articulated tug and barge units also utilize mechanical means to connect to their barges. The tug slips into a notch in the stern and is attached by a hinged connection, becoming an articulated vehicle. ATBs generally utilize Intercon and Bludworth connecting systems. ATBs are generally staffed as a large tugboat, with between seven and nine crew members. The typical American ATB displays navigational lights of a towing vessel pushing ahead, as described in the 1972 ColRegs. Svitzer Tyr, a Danish tugboat, built in China in 2011, pictured in 2018 in Ystad harbour Compared with seagoing tugboats, harbour tugboats that are employed exclusively as ship assist vessels are generally smaller and their width-to-length ratio is often higher, due to the need for the tugs' wheelhouse to avoid contact with the hull of a ship, which may have a pronounced rake at the bow and stern. In some ports there is a requirement for certain numbers and sizes of tugboats for port operations with gas tankers. Also, in many ports, tankers are required to have tug escorts when transiting in harbors to render assistance in the event of mechanical failure. The port generally mandates a minimum horsepower or bollard pull, determined by the size of the escorted vessel. Most ports will have a number of tugs that are used for other purposes than ship assist, such as dredging operations, bunkering ships, transferring liquid products between berths, and cargo ops. These tugs may also be used for ship assist as needed. Modern ship assist tugs are tractor tugs that employ azimuthing stern drives, propellers that can rotate 360 degrees without a rudder, or cycloidal drives. River tug pushing a log raft in British Columbia near Vancouver River tugs are also referred to as towboats or pushboats. Their hull designs would make open ocean operations dangerous. River tugs usually do not have any significant hawser or winch. Their hulls feature a flat front or bow to line up with the rectangular stern of the barge, often with large pushing knees. A tugboat on the Arakawa River in Tokyo, Japan The first tugboat, Charlotte Dundas, was built by William Symington in 1801.