Steam tug

A tug (tugboat) is a boat or ship that maneuvers vessels by pushing or towing them. Tugs move vessels that either should not move by themselves, such as ships in a crowded harbor or a narrow canal, or those that could not move by themselves, such as barges, disabled ships, log rafts, or oil platforms. Tugboats are powerful for their size and strongly built, and some are ocean-going. Some tugboats serve as icebreakers or salvage boats. Early tugboats had steam engines, but today most have diesel engines. Many tugboats have firefighting monitors, allowing them to assist in firefighting, especially in harbors.

Seagoing tugs (deep-sea tugs or ocean tugboats) fall into four basic categories:

Compared to seagoing tugboats, harbour tugboats are generally smaller and their width-to-length ratio is often higher, due to the need for a lower draught. In smaller harbours these are often also termed lunch bucket boats, because they are only manned when needed and only at a minimum (captain and deckhand), thus the crew will bring their own lunch with them. The number of tugboats in a harbour varies with the harbour infrastructure and the types of tugboats. Things to take into consideration includes ships with/without bow thrusters and forces like wind, current and waves and types of ship (e.g. in some countries there is a requirement for certain numbers and sizes of tugboats for port operations with gas tankers).

River tugs are also referred to as towboats or pushboats. Their hull designs would make open ocean operation 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.

Tugboat 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). 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. The engines are often the same as those used in railroad locomotives, but typically drive the propeller mechanically instead of converting the engine output to power electric motors, as is common for diesel-electric locomotives. For safety, tugboats' engines often feature two of each critical part for redundancy.

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