What is a Tug Boat
A tug (tugboat) is a boat or ship that manoeuvres vessels by pushing or towing them. Tugs move vessels that either should not move by themselves, such as ships in a crowded harbour or a narrow canal,or those that cannot 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.
Many tugboats have firefighting monitors, allowing them to assist in firefighting, especially in harbors
In literature, the effect of these ships on the whole maritime economy is often neglected but in fact their value is of paramount importance in manoeuvring during bad weather conditions in limited areas of a world port in particular.
When to use a Tug
Each port has its own particular features which determine the way of use of the tugs, such as
Tugs can be used to hold the ship against berth to reduce load on the moorings, during mooring that is docking the ship or when the weather condition require so.
Generally, the following conditions are crucial in choosing and using the tugs for towing or pushing: –
-volume of traffic in the port,
-types of ships serving the port,
-navigational obstacles in the port,
-conditions of environmental protection,
-local laws and habits,
-towing a vessel / ship
Types of Towing
1. Harbour Towing: This is towing, in sheltered waters, related to the manoeuvring of a ship to or from a berth.
2. Emergency Towing: Emergency towing means towing of a vessel without propulsion power at sea.
3. Escort Towing: Escort towing is a precautionary measure to use tug while navigating in restricted waters to protect the vessel and harbor or damage to other vessel, if Engine or steering failure occurs
4. Pull Back: This type of tug connection is done at stern to assist the ship having bow moored to prevent the ship from over running the single point mooring, e.g. SBM or other facility.
5. Canal Transit Towing: This is type of towing/connection is when vessel is transiting the canal.
Considering the above factors and types of towing, tug are decided to be used.
Principal Types of Tugs
1. Conventional tug
2. Azimuth stern drive tug
3. Tractor tug
1. Conventional Tug
One of the oldest types of tugs are the conventional ones. They are still present in almost all the world ports, although they are pushed away more and more by contemporary tugs with better manoeuvring capabilities.
Nowadays, they are equipped with diesel engines having one or two propellers respectively. With single propeller, depending on the type, the most common is the right-handed (although left- -handed are also quite often met), and are equipped with a classic rudder.
The power plant complex is placed at the stern with the towing hook in the middle of the tug. The main characteristic of these tugs is high reliability and efficiency of power expressed in the force of “bollard pull”.
Screw propeller — the “conventional” device used to develop thrust in water, usually driven by a diesel engine. Vessels are often described as single, twin or triple screw, depending on the number of propellers they have.
In this type of propulsion only the rudder and nozzle can move, the propeller is fixed.
– Simple construction and less maintenance than CPPs
– No support system needed
– Smaller propeller leading to increased open water
– Performance degrades rapidly at off-design conditions, where the vessel may operate majority of the time
– Not able to provide reverse thrust without stopping the engine, unless a reduction gear is installed
– Potential cavitation, strength & stability issues at highly loaded off-design conditions
Capabilities and Limitations
Conventional propeller/rudder configurations are more efficient for port-to-port towing.
Conventional tugs are used for push-pull assistance, in particular for towing on a line. Economic operations for using on small and medium sized vessels.
Nowadays, big vessels are not assisted by conventional tug boats, but by azimuthal stern drive(ASD) tugs.
• limited manoeuvring capabilities which is demonstrated in slow repositioning of tugs;
• amidships position of tow which results in the possibility of “girting“(capsizing) of the tug along the radius of tow and consequently could result in capsizing of the tug with fatal outcome for the crew. Tragic result of girting is the crew’s inability for prompt reaction due to high speed of the girting development. To avoid girting, the tugs can be fitted with a quick release hook
• The astern power of conventional tugs is generally low. When making fast near the bow of a vessel, interaction forces between the ship and the tug makes it very difficult for a conventional tug to connect.
Positioning of the towing hook
• Mainly due to the location of the towing point (generally 0.45 x LWL from the aft) the tugs have limitations regarding performance and safety.
• When using a gob-line, the towing point is moved further to the aft and reduces the manoeuvrability of the tug.
2. Azimuth stern drive (ASD) tug
These tugs are fitted with two thrusters at the aft of the ship. The thrusters can each independently rotate at an angle of 360°. So the thrusters can give thrust in every direction. Azimuth thrusters can be fitted with fixed pitch propellers or controllable pitch propellers(CPP). The advantage of having a CPP system is the possibility to have immediate reverse thrust and higher efficiency with different loads.
ASD tugs are fitted with a towing winch, which is located on the foredeck instead of the aft deck. An ASD tug is particularly towing over the bow. This means that as being a bow tug it is mostly sailing astern.
This type of propulsion system makes the tug highly manoeuvrable and very safe during making fast at the bow, because the thrust can be carried out to the side. The ASD tug can easily sail sideways but has to turn their rudder propellers in almost the opposite direction, to create sufficient power at the aft ship to drag the hull sideways through the water. This way of manoeuvring will reduce the towline force significantly.
Azimuth thrusters were invented in the late 50s. These thrusters are sometimes also called Z-drives. There are two different types of thrusters to choose from. There is the mechanical driven version and the alternative is the electrical driven version.
– Good maneuverability due to 360o turning propellers
– Low maintenance costs
– They do not require a lot of space in the engine room
– Excellent in combination with diesel generators, making a main engine redundant
– The blades are fixed
– Installing thrusters costs more than installing a conventional propeller
Capabilities and Limitations
ASD tugs have azimuthal thrusters fitted on the aft of the vessel, giving them excellent manoeuvring characteristics for towing over the bow.
One of the disadvantages are the fixed blades. However this is not necessary because the propeller is driven by an electric motor. Because of the latest technology in frequency controllers it is now possible to reduce an electro-motor its power delivery without losing efficiency
Azimuth thrusters are used mostly for ship docking and marine construction.
3. Tractor Tug (Voith-Schneider)
These types of tugs are fitted with two azimuthal thrusters or Voith Schneider thrusters at the bow (forward of the amidships). These tugs are fitted with a harbour towing winch which is located on the aft deck. The stern and bow is generally heavily protected by fenders so this makes it suitable for push and pull operations.
The large skeg is typical for tractor tugs and particular for Voith tractor tugs. It increases the course stability and brings the centre of the hydrodynamic pressure more to the aft. The last is a big advantage when towing on a line as an after tug, extreme high force can be originated.
The azimuth tractor tugs have almost the same characteristics as the Voith Schneider tractor tugs. Only sometimes the skeg of the azimuth tractors are smaller than the Voith tractor tugs.
Voith Schneider tractor tugs have great manoeuvring capabilities. They can turn on a place around their axis, deliver a lot of thrust in every direction and sail straight astern at high speeds.
The amount of thrust astern is nearly the same as the available thrust ahead. A lot of disadvantages of the conventional (particularly single screw) tugs, such as no or low side thrust, does not apply to Voith Schneider tugs. Because the availability of thrust sideways makes it much safer to make fast when a vessel is sailing at high speeds.
A tractor tug does not require a lot of power to tow sideways, because the propulsion point is close to the turning point.
The Voith-Schneider Propeller works as follows: it consists of a circular disk with movable and controllable blades. These blades are installed at a 90 degree angle on the disk that rotates at the vessel bottom. Below is shown a figure of the cycloidal drive.
The thrust force is determined by the speed of the rotation of the disk. The controllable angle of the blades determine the direction of the thrust. This means that there is no need for a rudder. It provides the vessel with very precise maneuvering, even in difficult conditions.
– Performs extremely well and fast at manoeuvring