|River Rat Tug Boat|
A tugboat (tug) is a boat used to maneuver either by towing or pushing other vessels (see shipping) in harbors, open sea, or through rivers and/or canals. Tugboats are also used to maneuver disabled ships, barges, or oil platforms
Tugboats pack a strong punch when considering their size. The earlier tugboats had steam engines (see steamboat), but today they run on diesel engines. Their engines produce typically from 500 to 2,500 kW (cca 680 to 3,400 hp), but larger tugboats which are used in deep waters can have power ratings as much as 20,000 kW (cca 27,200 hp), and usually have an extreme power/tonnage ratio. Typical cargo and passenger ships have a P/T ratio of 0.35-1.20, whereas large tugs have typically ratio of 2.20-4.50 and small harbour-tugs have the ratio of 4.0-9.5. Often times the engines used in tugboats are the same as the engines used in locomotives , but usually drive the propeller mechanically, unlike the locomotives where the engine output gets converted to power electric motors. Due to safety concerns, tugboats’ engines often have two of each critical parts for redundancy.
The power of a tugboat is determined by the engine’s horsepower and its overall Bollard pull.
Tugboats are highly maneuverable, with numerous propulsion systems which help increase the maneuverability alongside increasing safety. The very first tugs were fitted with paddle wheels. They were soon replaced with propeller-driven tugs. In order to increase thrust per kW/hp, Kort nozzles have been added. Soon after that nozzle-rudders followed, which omitted the need for a conventional rudder. Prior to World War 2 the cycloidal propeller was developed, but was only used occasionally in tugboats because of its maneuverability. After World War 2, it was linked to safety due to the invention of the Voith Water Tractor, which was a tugboat configuration that could not be pulled over by its tow. The Z-drive (azimuth thruster) was developed during 1950s. Although it was mostly reffered to as the Schottel system, there are many brands that exist: Schottel, Z-Peller, Duckpeller, Ulstein, Wärtsilä, Thrustmaster, etc. The propulsion systems that are installed on tugboats help with tasks such as marine construction and ship docking. The conventional propeller/rudder configurations are used more in port-to-port towing.
The Kort nozzle is designed to be a sturdy cylindrical structure around the propeller, with minimum clearance between the propeller blades and the inner wall of the Kort nozzle. This way the thrust/power ratio is enhanced, because the water approaches the propeller in a linear way, and exits the nozzle the same way. It was named after its inventor, but many other brands also exist.
Inventions in Netherlands have lead to creating a new type of a tugboat. The carousel-tug is designed in a way where the flexibility and effectiveness of the tugboat’s maneuvers is not determined by the propulsion system, but rather by a steel construction on the deck, which consists of two steel rings. One of the rings is fixed to the ship, and the other one rotates freely and caries a hook or a winch. Because of this the ship can maneuver freely and independently of the towed ship. Since the towing point rotates freely towards the point that is nearest to the towed ship, the tugboat is almost incapable of capsizing. There is only one prototype that has been built as of right now, but the new types of tugboats are expected to sail out in spring of 2007.