TUGBOATS AND PROPULSION SYSTEMS
Tugboats are often used in pushing and towing operations to manoeuvre the other marine vehicles or facilitate their manoeuvring, as well as to attract large floating structures such as non-motorized naval vessels (barges etc.) and oil platforms. Tugboats can perform operations like search-salvage, firefighting (Fi-Fi), supporting (assisting), transport, escorting and manoeuvring when demanded another marine vehicle in tugboat’s service areas. In addition, there are special tugboats, which can be used for icebreaking operations. Therefore, a tugboat can be designed for performing one or more functions.
Tugboats are original designed maritime vehicles for towing, pushing, manoeuvring (positioning) with their capabilities, power and equipment. The main engine powers of tugboats are quite higher according to their length. It can be understood by the total horsepower and bollard pull capacities.
The design and dimension of tugs; several main engines or engine powers; rudder, propeller and propulsion systems; deck equipment’s (winches, capstan, windlass, towing hook etc.) arrangement; fender types and fender arrangements affect the efficiency of the tugboats bollard pull and manoeuvre capabilities according to their operations and service areas. The type of tug, propulsion system and bollard pull is the most important properties of a tugboat.
Tugboats have one or more strong winches for towing operations and a high manoeuvre capability design with a durable structure, which have less centre of gravity to avoid capsizing. Also, have a profile to provide superstructures from damages during pushing operations. The hydraulic pin, which arranged aft deck, absorbs the sudden forces in towing rope and prevent rupturing of rope. The different types of tugboats propulsion system or different configurations of tugboats propulsion systems can provide higher power with fewer energy consumptions.
Tug Boat Types
Tugboats are generally classified according to their work duties and the characteristics of the waters in which they operate. Mainly we can categorize them into 3 main types as seagoing, river and harbour tugboats.
The seagoing tugboats are designed for seagoing operations with their higher lengths, powerful main engines, higher bollard pull capacities and sufficient design for open ocean conditions like excellent seakeeping ability and enormous fuel capacity. The seagoing tugboats also should have a suitable design for long unassisted open ocean routes. Thanks to that, the properties of seagoing tugboats have the capability for performing operations like towing, pushing, search-salvage, backup, anchor handling, supply transports, and attract barges on open ocean waters. In addition, some seagoing tugboats can perform special operations like Fi-Fi, oil spill response and ice-breaking when equipped with required equipment’s. An example of seagoing tugboat is given in Figure 1.
The river tugboats are designed for river operations like towing or pushing (Towboat-Push boat) of barges with their medium lengths, high bollard pulls and deformed stern form which suitable for pushing barges. An example of river tugboat is given in Figure 2.
The harbour tugboats are designed for harbour operations with their smaller lengths, powerful main engines, higher bollard pull capacities, higher manoeuvre capability, different hull forms and different propulsion systems. The harbour operations can mainly write as ships berthing and departure operations in harbours, ship-escorting operations inside harbour borders or inner seas, firefighting operations, emergency response operations. An example of habour tugboat is given in Figure 3.
In addition, the ice-class tugboats can perform ice-breaking operations too. However, ice-class tugboats have to meet the extra requirements like hull form requirements, balancing and extra power for ice breaking.
Harbour tugboats can be categorized according to the configuration or type of propulsion system. There are mainly three types: conventional, azimuth stern drive (ASD) and tractor tugboats.
Conventional tugboats the oldest types of tugboats. They are still serving all the world ports successfully. The story of these tugs started with steam-powered tugboats and with the development of initial combustion engines, steam propulsion abandoned the diesel engines having one or more propellers. The most common type of conventional tugboats is equipped with a diesel main engine, a single propeller and a classic rudder. The key characteristics of the conventional tugboats are the high reliability and efficiency of power expressed in the force of “bollard pull”. A conventional tugboat series profile view is given in Figure 4.
Azimuth stern drive (ASD) tugboats
Azimuthing Stern Drive (ASD) tugboats can be defined shortly as a tugboat with steerable propulsion units located in aft of the vessel. Generally, these ASD tugboats have a couple of propulsion units placed in the turning nozzles. In addition, these propeller units are known as Z-Drive units due to the shape of the nozzles. Sanmar Boğaçay ASD tugboat series profile view is given in Figure 5.
The fundamental difference between the conventional and ASD tugboats is the manoeuvring system. The conventional tugboats manoeuvre with the help of a classical rudder, but the ASD tugboats have remarkable improved manoeuvring capabilities with azimuth propulsion that rotates their propulsion unit (Nozzle and Propeller) at any horizontal angle to manoeuvre.
In addition, there are relatively new versions of ASD tugboats which have arranged one or more Z-Drive unit in amidships or fore of the tugboat. This arrangement provides more manoeuvring capability during towing operations and maximum efficiency in harbour ship handling.
Nowadays, they are equipped with different types of propulsions like diesel engines, LNG engines and hybrid propulsion configurations.
While increased bollard pull capacity per KW of propulsion system power, high manoeuvrability, and simple M&R procedures of the nozzles are the advantages; difficulties on side movement, sensitiveness to interaction with a ship, complex z-drive unit components, and the possibility of rope entangling and damaging the propellers in stern side towing operations are disadvantages of the ASD tugboats.
Tractor tugboats are equipped with the drive units (Voith Schneider Propeller or ASD) located in the bottom of the vessel, approximately one-third of the length from the bow. Generally, one or two multidirectional propulsion systems combination is preferred for tugboats, that works in the cycle of 360 degrees on the principle of two vertically built propulsion mechanism with rotating blades.
Voith Schneider Propeller (VSP) are propeller units that combine propulsion and steering in a single unit and provide the propulsion of the ship by rotating a circular disc placed on the bottom of the hull. There are several movable and controllable blades on this circular disk. While the angles of the blades determine the direction of the thrust, the rotation speed of the disc determines the size of the thrust. It is one of the leading propeller systems that provide the highest manoeuvrability in use today.
This type of propulsion systems provides excellent manoeuvring characteristics, maximum performance in ship handling and escorting operations of tractor tugboats. With approximately one-third of the length from the bow location of this system on the tugboat, the positive turning moment is always present. Because the propulsion system is always in front of the tow. Therefore, the direct control of tow from Tugboat Bridge has a great advantage. When VSPs are operated at a low speed, their service life increases and M&R costs decrease.
The full power at turning 360 degrees, excellent manoeuvring capabilities, instant application of full power, better response with interaction with the hull of the ship, and reliability of propulsion are advantages while the sensitiveness to damages of underwater propulsion mechanism, stability issues, high capital cost, reduced bollard pull capacity per KW of propulsion system power, bad steering at open seas, and higher M&R costs are disadvantages of the tractor tugboats.