Tugboats usually have an extreme power: tonnage-ratio normal cargo and passenger ships have a P:T-ratio (in kW: GRT) of 0.35 to 1.20, whereas large tugs typically are 2.20 to 4.50 and small harbour-tugs 4.0 to 9.5. Tugboat diesel engines typically produce 500 to 2,500 kW ( ~ 680 to 3,400 hp), but larger boats (used in deep waters) can have power ratings up to 20,000 kW (~ 27,200 hp). Steam tugs were put to use in every harbour of the world towing and ship berthing.
Compound steam engines and scotch boilers provided 300 Indicated Horse Power. In the 1870s schooner hulls were converted to screw tugs. Paddle tugs proliferated thereafter and were a common sight for a century. She had a steam engine and paddle wheels and was used on rivers in Scotland. The first tugboat, Charlotte Dundas, was built by William Symington in 1801. Modern ship assist tugs are "tractor tugs" that employ azimuthing stern drives (ASD), propellers that can rotate 360 degrees without a rudder, or cycloidal drives (as described below).Ī tugboat on the Arakawa River in Tokyo, Japan These tugs may also be used for ship assist as needed. 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. The port generally mandates a minimum horsepower or bollard pull, determined by the size of the escorted vessel. Also, in many ports, tankers are required to have tug escorts when transiting in harbors to render assistance in the event of mechanical failure. In some ports there is a requirement for certain numbers and sizes of tugboats for port operations with gas tankers. Svitzer Tyr, a Danish tugboat, built in China in 2011, pictured in 2018 in Ystad harbourĬompared 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. The typical American ATB displays navigational lights of a towing vessel pushing ahead, as described in the 1972 ColRegs. ATBs are generally staffed as a large tugboat, with between seven and nine crew members. ATBs generally utilize Intercon and Bludworth connecting systems. The tug slips into a notch in the stern and is attached by a hinged connection, becoming an articulated vehicle. "Articulated tug and barge" (ATB) units also utilize mechanical means to connect to their barges.These vessels must show navigation lights compliant with those required of ships rather than those required of tugboats and vessels undertow. Vessels in this category are legally considered to be ships rather than tugboats and barges must be staffed accordingly. These units stay combined under virtually any sea conditions and the tugs usually have poor sea-keeping designs for navigation without their barges attached. The "integral unit", or "integrated tug and barge" (ITB), comprises specially designed vessels that lock together in such a rigid and strong method as to be certified as such by authorities (classification societies) such as the American Bureau of Shipping, Lloyd's Register of Shipping, Indian Register of Shipping, Det Norske Veritas or several others.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. Model bow tugs employing this method of pushing nearly always have a towing winch that can be used if sea conditions render pushing inadvisable. 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. This configuration is generally used in inland waters where sea and swell are minimal because of the danger of parting the push wires. 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.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 standard seagoing tug with model bow that tows almost exclusively by way of a wire cable.Seagoing tugs (deep-sea tugs or ocean tugboats) fall into four basic categories:
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