tug

tug, a specially designed vessel which tows other vessels, or dumb barges, though it pushes them, too. In the West the earliest known ones were paddle-wheel boats, and among the first to use steam propulsion were two Royal Navy tugs, the Comet and Monkey. These were purchased in 1822 and were employed to tow ships of the line out of harbour when the wind was unfavourable. The port or harbour tug aids large vessels to enter and leave their berths; the larger ocean-going one is mainly concerned with salvage work, though nowadays seagoing tugs also fight oil pollution at sea, and can be used for fire-fighting as nearly all are suitably equipped for this. There are also the tugs which service the offshore oil and gas industry. One kind positions and anchors offshore drilling rigs, another supplies them. Often these roles are combined into one powerful anchor-handling supply vessel. This can be a ship of about 2,300 tonnes and about 74 metres (240 ft) in length which is capable of carrying on its long afterdeck over 1,000 tonnes of cargo. There are also various forms of pusher tugs—known in the USA as towboats—designed for inland waterways, and the ‘notch tug’. This pushes a specially designed barge which has a notch in its stern into which the tug's bows are secured.

The traditional tug had the bridge right forward and a pronounced overhang on its counter. Most of the length of the vessel was taken up by the after working deck which was clear of anything that might obstruct the bows of the towed vessel, and allowed the tug crew to handle the tow ropes safely. Today a tug's profile has altered (see Fig. 1), its manoeuvrability and power more so. Different kinds of propulsion systems have largely replaced the traditional single or twin screws fitted to rigid propeller shafts at the stern, while the early diesel engines have been replaced by ones that are often turbocharged and have six to eight cylinders. These engines have considerably increased a tug's bollard pull—the power, expressed in tonnes, produced by a tug when pulling against a fixed object like a bollard. They also power the vessel's propulsion system, whether it be azimuthing—a propulsion unit with a conventional propeller which can be rotated about its vertical axis to achieve the maximum thrust—the Voith Schneider Cycloidal propeller system (see Fig. 2), or the tubular-shaped Kort nozzle which can improve a tug's bollard pull by up to 30–40%. There are two forms of Kort nozzle within which the propellers rotate: the fixed nozzle is part of the hull structure and has the ship's rudder immediately aft of it; the steerable one has a rudder, or rudders, incorporated into it which fits around the tug's screw(s).

With the development of these propulsion units have come different types of tug. The combi-tug has a conventional single propeller on a fixed shaft, but also has a small retractable azimuthing unit beneath its bows; the azimuthing stern-drive tug (ASD) is similar to a conventional twin-screw tug except that the propellers and fixed shafts have been replaced by azimuthing propulsion units (see Fig. 3); and the tractor tug, known as a Z-drive tractor tug. This has either two Voith Schneider Cycloidal propeller systems, or azimuthing propulsion units, that are fitted, not at the stern, but about a third of its length from the bow. This allows it to push as efficiently as it pulls which makes it ideal for ship-handling. The tractor tug also has a large skeg fitted beneath its stern for directional stability and those with the Schneider Cycloidal system have a plate to protect the propellers. The plate also acts as a nozzle effect to the flow of water which improves the tug's performance.

Bibliography

Gaston, M. , Tugs Today (1996).