A tug, or more commonly a tugboat, is a secondary boat which helps in the mooring or berthing operation of a ship by either towing or pushing a vessel towards the port.
A tug is a special class of boat without which mega-ships cannot get into a port. Along with the primary purpose of towing the vessel towards the harbour, tug boats can be engaged to provide essentials, such as water, air, etc., to the vessel.
Tug boat eases the manoeuvring operation of vessels by forcing or tugging them towards the port. Mega vessels can never be manoeuvred on their own. Also, with the increased boat size, they need tug boats to carry some of their domains and tow them through narrow water channels.
Tug boats have become essential for non-self-propelled barges, oil platforms, log rafts etc. These are small, rather powerful boats due to their strong structural engineering.
Their propulsion system is the main reason behind their enormous strength. Some secondary functions of a tug boat, along with easing mooring operation, are listed below:
- They can work as salvage boats and icebreakers.
- These can also have fire fighting accessories to provide fire fighting assistance in port or barges.
- These are the most critical marine element, as they act as saviours to the boat in hard times, such as in narrow canals and lousy weather, but on the contrary, the most neglected ones in the maritime industry.
The name Tug Boats gives a fair idea about the size and task of the vessel being discussed. These are relatively smaller but very powerful for their size.
These are primarily used to tug or pull vessels that cannot move by themselves, like disabled ships, oil platforms and barges or those that should not move like a big or loaded ship in a narrow canal or a crowded harbour.
In addition to these, tug boats are also used as ice breakers or salvage boats, and as they are built with fire fighting guns and monitors, they assist in the fire fighting duties, especially at harbours and, when required, even at sea.
With the developments in the shipping industry, the ships began to grow larger than they ever were in history. It is easy to manoeuvre them at sea. Still, it becomes exceedingly difficult in narrow sea strips and harbours because they can move forward and backwards with ease but usually have problems with sideways movement.
This was when the need for tug boats was felt, and thus these vessels were introduced to help the larger ships navigate the narrow waters. This came to be known as tug assist and hence the name of the boats.
Most of these boats can also venture out into the ocean, but some are not that strong, like the river tugs. The river tugs are towboats designed to help out in the rivers and canals. They have varied hull types that make it dangerous for these boats to venture into the open Ocean.
Renowned tug boat builders include Ranger Tugs, Florida Bay, American Tug, Nordic tug and Tugboat. They make ones with inboard, outboard, outboard-4S and other diesel and gas fuel systems propulsion systems.
Why are tug boats popular?
Tug boats are known for their moderate draft and broader beam, which make them suitable for an array of commercial and recreational boating activities in coastal, inshore and offshore locations. Tugboats can carry about 6 to 8 passengers or guests.
Applications Of A Tug
The usage and functions of tugs vary from port to port as different ports have different requirements and intakes. The common in all is pushing or towing mega boats or barges. Their usage depends on the following factors:
- Port traffic volume,
- Types of ships to be served by that tug,
- Navigational obstacles to be catered to,
- Conditions of environmental protection,
- Local laws and
- Domains to be carried by a tug
The tug boats were one of the first to have a steam propulsion engine, which the diesel engine has replaced today.
An average tug boat has 680-3400 horsepower engines (500-2500 kW), but boats which are larger and venture out into deep waters have engines with a power close to 27200 hp (20000 kW) and a power: tonnage ratio ranging between 2.20-4.50 for large tugs and 4.0-9.5 for harbour tugs.
These are incredibly high ratios, especially considering the ratio of the cargo ships or generalships that varies between 0.35-1.20. Such boats usually have engines quite similar to those used in railway locomotives. Still, with a notable difference: they mechanically drive the propeller instead of converting the output to an electric motor supply.
Since the manoeuvrability of a tug boat has been one of its assets, all the engine developments over the years have focused on improving it without compromising on the strength and power of the vessel. Thus the transition from the paddle wheels to the propellers.
The tug boats have been the topic of many literary works, especially children’s works like cartoons. They have inspired many to create characters that have long lived in people’s memory and attracted many to study them. The real lifeboats and crew are as resolute and lovable as their counterparts in fiction and cartoons.
Looking back, one can easily say that a tug boat is more than just a boat and an important jigsaw piece in the historical scheme of everything related to the sea.
Classification Of Tugs based on Purpose:
Based on purpose, they serve marine tugs can be of two types:
1. Escort Tugs: The tugs designed generally to escort and manoeuvre ferries and barges to their destination are known as escort tugs.
2. Support Tugs: These tugs provide support services for offshore and towing operations. These tugs play a significant role in berthing operations. Principal Types Of Tugs based on Design:
Mainly tugs used in the marine industry are of three types which are briefed as under:
- Conventional tug
- Tractor tug and
- Azimuth stern drive tug
1. Conventional Tug:
These are the type of tugs we have been using from the time of our forefathers. They are made on the oldest known principle of tug development. It is based on obsolete or old principles. The conventional tug has lesser manoeuvring capability but is still used in almost all ports worldwide.
But today, with technological advancements, they are fitted with diesel engines with either one propeller or a couple of propellers. Single propeller tugs are further classified into two classes: right-handed conventional tugs and left-handed conventional tugs. Right-handed conventional tugs are more common and have more chances to be used than left-handed tugs. These are highly reliable. Board pull force is used to express power efficiency. The essential components of conventional tugs are briefed as under:
- These have a classic rudder.
- A towing hook is provided in the centre of the tug. Along with other components of tug positioning, a hook is of enormous importance. The safety and performance of the tug boat mainly depend on the towing point location. When a gob line accompanies the towel hook, this point can be taken towards the aft; its distance is reduced from its original value of 0.45 times LWL to minimise tug manoeuvring.
- Its stern consists of the power plant complex.
Configuration of propeller used:
Screw propeller — In the “conventional” tugboat, the propelling device is generally diesel driven. Thrust is developed in water by the propeller, responsible for ships’ movement. These tugboats are designated based on several propellers in them:
- Single screw tug implies one conventional propeller tug,
- Twin-screw tug implies a two-propeller traditional tug and
- A triple screw tug implies three classic propeller tugs.
In conventional screw propeller tugs, a moveable nozzle and rudders are provided, but the whole propeller is immovable or fixed.
The main advantages or features of conventional tugboats are as follows:
- The construction of traditional tugs is quite simple and requires less maintenance, so the maintenance and construction costs of these tugs are meagre compared to the two types of tugs.
- These are self-sufficient and thus require no support system.
- Smaller is the propeller, larger be the open water.
- These tugs have the upper hand and maximum efficiency in towing for port-to-port.
- These assist with pushing and tugging operations while carrying towing on a line.
- The operation cost is relatively low and seems economical while working with moderate-sized vessels.
“Every coin has two faces”, so as the conventional boats. There may be certain disadvantages of conventional tugboats, and these are briefed as follows:
- The designing principle is ancient; thus, it degrades the performance standards.
- They are unidirectional and cannot work in the reverse direction until provided with a reduction gear to facilitate reverse flow.
- There are more chances of cavitation in these tugs.
- These tugs’ stability and strength are less than the other two.
- Due to obsolete design principles, their usage is limited to small and medium boats, so azimuthal stern drive and tractor tugs are employed to assist the mega ships.
- The repositioning of the tugs is relatively slow due to the limited manoeuvring capacity.
- There are more chances of capsizing or overturning the vessel, which results in fatal accidents. A release hook is required to overpower this issue of capsizing and halting the development of girting.
- Conventional tugs have low values of astern power
2. Tractor Tug:
The key to the tractor lies in using a 2-multidirectional propulsion unit, some of which are like large rotating outboard motors, with others consisting of rotating vertical blades. They enable the thrust units of the tug to be placed side by side, more or less under the bridge, facilitating spectacular manoeuvrability in the right hands.
The towing point can be placed much nearer the stern to get maximum output from the propulsion units, and therefore the thrust is always outside of the towing point, thus creating a positive turning moment. The rotating disc decides the magnitude of the force of thrust. The versatility of tractor tugs is further enhanced by working the tug’s towline directly from a winch drum with a remote control joystick control from the bridge. The tug master can thus alter the span of the towline at his will and with considerable ease. Their precision in manoeuvring makes them the most widely used tugs.
It is the most important type of tug, tractor tug has many positive points. Some of its advantages are briefed as below:
- These are the only tugs which provide full thrust over 360 degrees.
- They have rapid power-on response time and are well known for their outstanding manoeuvrability.
- In contrast to the conventional tug, tractor tugs are adaptable to repositioning swiftly on the pilot’s will or on demand of the towing job in which they are employed. So these are readily available tugs with high-efficiency values.
- Their straightforward control systems pose a very low risk of girthing or capsizing.
- These have an extraordinary performance with fast speed.
- They can effectively overcome the interaction forces with ships at close distances.
- They can work efficiently in sideways movement due to the close location of the propulsion unit to the turning point.
- They have improved operational capability in a restricted areas such as a lock flow channel.
- In these types of tugs, a rudder is not required is the use of a rudder can be eliminated.
- They have reliable and robust propulsion units.
The tractor tugs are the most effective and most potent sort of tugs. As everything has certain cons and pros, these have certain loopholes, mainly their complexity and cost. Other drawbacks of these tugs are as follows:
- These kinds of tugs have less bollard pull as compared to ASD tugs.
- They are considered to have very high capital investment costs, implying a lot of money is required to buy and maintain these tugs. In short, it confers that the initial and maintenance cost of tractor tugs is very high.
- The repair and maintenance of complex under-water units of these tugs are quite expensive
- Handling in an open seaway might be poor with the short distance between the pivot point and the thrust creating a short turning lever
- With some tugs, heeling angles with full side thrusts may be up to 21 degrees. Therefore, the risk of damage can exist when lying alongside a ship
- Sophisticate under-water units may be damaged on grounding
- A draft may be up to 5m which is large in comparison to conventional tugs
- The retraining of a traditional tug master is essential to maximise a tug’s potential fully
3. Azimuthal Stern Drive (ASD) Tugs :
These tugs are midway between conventional tugs and tractor tugs as these utilise some of the benefits of both traditional and tractor pulls. It can have two towing locations, one forward and one amidship, and the main propulsion is from two rotating azimuth units, which are placed like a traditional twin screw tug.
These are considered more efficient than conventional tugs but less efficient than tractors. The significant advantages of using ASD tugs are as follows:
- Better directional stability at speed
- They are more suitable hull form for open waters and working in a seaway
- They have improved bollard pull
- The azimuth units are easy to withdraw for maintenance and repair
- The maximum heel with side thrust is less than 15 degrees, which contrasts with tractor tugs having the value of twenty-one degrees.
- These tugs have a shallower average draft of 3m
These tugs are challenging to work with as compared to tractor tugs, and other drawbacks of working with these tugs are as follows:
- The side-stepping ability of ASD tugs is not so good.
- The squat at the stern and flooding of the aft deck has been acquainted with occurrence with certain types when backing with full power.
- They are still susceptible to girting or capsizing when using the aft towing position.
- These types of tugs are slightly more at risk from the interaction effects.
- These have slightly Complex control systems.
- These tugs are more susceptible to rubbish damage in propellers
- It is not uncommon for 99 % of all towing to be limited to a forward position
Other Miscellaneous Types Of Tugs
Along with these classifications mentioned above, some other miscellaneous types of tugs are briefed as below:
1. Tractor Tugs With Cycloidal Propellers
Before the Second World War, tugboats developed were of high power rating and large tonnage capacity, the propeller used were cycloidal to serve the manoeuvrability purpose; these were renowned as Cycloidal Tugboats. Voith Schneider propeller (VSP) is an advancement of a cycloidal drive. It is excellent at manoeuvring operations. So, it became a perfect choice for ferries and barges.
2. The Carousel Tug
The Carousel Tug was developed by the Dutch and emerged as an award-winning Maritime innovation in 2006. It consists of interlocking inner and outer rings in which the former are connected with a boat, whereas the latter with the tug’s body attached to towed ship through winches or hooks.
3. Reverse Tractor Tugs
The tractor tugs are termed reverse tractor tugs if they have Z-drive aft-mounted propulsion units. These reverse tractor tugs don’t have a skeg and work efficiently in escorting. There are fewer fatigue issues with these tugs than with tractor tugs due to the unavailability of flat plate skeg. Generally, ASD tugs serve as reverse tractor tugs.
A bow thruster and a nozzle with steering are fitted with a conventional Screw Tug to improve manoeuvrability. These modified tugs, along with additional devices like a thruster and steered nozzle, equipped with the latter devices are referred to as “Combi-Tugs”. Combi-Tugs are preferably the modification of twin-screw conventional tugs to have maximum efficiency.
It can have two towing locations, one forward and one amidship, and the main propulsion is from two rotating azimuth units, which are placed like a traditional twin screw tug. Azimuthing propulsion unit replaces conventional shafts and propellers, which allows 360-degree rotation about the vertical axis.
6. Giano Tug
The Giano Tug is highly stable and can serve the purpose of both support as well as escort tug. It is a high-tech tug which allows remote manoeuvring through VSAT or 4G connections. Its 360-degree rotation and excellent side-stepping speed made it to the top in the order of standards of tugs.
Hybrid technology tugs or tugs with LNG as a running fuel are categorised in the eco-tugs category. These tugs serve the same purpose of escorting and supporting as the conventional tugs do, but without harming and polluting the marine ecosystem.
8. Ice Tugs
These tugs escort ferries or barges and serve as ice breakers or salvage boats to make the ferries and barges speed up to their destination.
The Most Powerful Tug in the world
“The title of most powerful tugboat crowns Farstad Shipping’s Far Samson by the Guinness World Record, which achieved an astounding four hundred twenty-three tons bollard pull during testing”. Spanish shipyard launched a groundbreaking or innovative, powerful tug boat to supply it to the Luleå, Sweden Port.
But presently, Gondan Shipyard promises that they will build and deliver an ice-breaking towing tug which is assumed to bag the ticket for the most potent hybrid tug in the World.
Robert Allan’s design was the foundation for building this tug. To assist ships or vessels through tough regions, it employs a powerful hybrid power system, namely Wärtsilä’s HYTug power system. It combines “Lloyd’s Register class and a 3600-H design of RAL TundRA”.
All the super, as well as substructures of this giant, are very strong and efficient. The superb hull structure is provided, which beats even the class rules of Finnish-Swedish ice. The excellent bollard pull of this tug of about 100 tonnes emphasised the capabilities of the tug. It is operated in a diesel-manual mode to achieve maximum efficiency.
These can easily break a metre of ice with a comparatively high speed of about three knots. The thirty-six metres of span of the tug is equipped with a great propulsion system. Its propulsion system comprises batteries to store energy and run in a condition of certain emergencies, shaft motors, generators and two diesel key engines, etc.
The important features of this tugboat are as follows:
- It is flexible in operation and can switch swiftly.
- It emits less fuel under its hybrid conformations, and its maintenance is economical and easier.
- There are a variety of operation modes available in this tug due to its innovative and hybrid design.
- It can serve multi-purpose ice management, assisting ships and enormous barges during voyages, fire fighting, and towing in coastal regions and difficult paces.
Frequently Asked Questions
1. Why are tug boats so powerful?
They are powerful as they are used for towing purposes. Hence they have powerful engines. They have two towing locations and two rotating azimuth units for propulsion.
2. What are tugs used for?
A tug or tugboat is a secondary vessel used for mooring or berthing a big ship by either towing or pushing it towards the harbour. It is a special class of vessel without which cargo ships cannot dock in ports.
3. How fast can a tugboat go?
A tug measures over 80 cm and can easily carry 15-20 people. They can run at 15 knots when they are not fully loaded and are usually described in terms of horsepower.
4. Why do tug boats push and not pull?
The answer lies in physics. In terms of power and water resistance, pushing rather than pulling a barge or ship is much easier and less power-consuming.
5. Do cruise ships require tugboats?
If the winds are strong or there are strong currents that might sway the cruise, tugboats can be used to direct them into the harbour or marina. Also, some docks, such as Venice, insist on the use of tugs as the bow and stern thrusters of the ship could kick a lot of silt near the shore.
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Zahra is an alumna of Miranda House, University of Delhi. She is an avid writer, possessing immaculate research and editing skills. Author of several academic papers, she has also worked as a freelance writer, producing many technical, creative and marketing pieces. A true aesthete at heart, she loves books a little more than anything else.