A tug or more commonly a tugboat is a secondary boat which helps in 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 in the purpose of providing 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 by their own. Also with the increased size of the boat, they need tug boats to carry some of their domains and tow them through narrow water channels.
Also tug boats become essential elements for non-self-propelled barges, oil platforms, log rafts etc. These are small rather powerful boats due to strong structural engineering behind them. Their propulsion system is the main reason behind their enormous strength. Some secondary functions of 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 so as to provide fire fighting assistance in port or barges.
- These are the most important marine element, as they act as saviour to the boat in hard times such as in narrow canals and bad weather, but on contrary, the most neglected one 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 but 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 is 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 thus the name of the boats.
Most of these boats can also venture out in the ocean but some of them are not that strong like the river tugs. The river tugs are towboats designed to help out in the rivers and canals. They have a hull design that makes it quite dangerous for these boats to venture into open Ocean.
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,
- 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 in today’s time has been replaced by the diesel engine.
An average tug boat has an engine of 680-3400 hp (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 extremely high ratio especially considering the ratio of the cargo ships or generalships that varies between 0.35-1.20. Such boats usually have engines which are quite similar to the ones used in railway locomotives but with a notable difference that they mechanically drive the propeller instead of converting the output to 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 the aspect of improving on 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 provided inspirations to many to create characters that have long lived in the memory of the people and attracted many to study them. The real lifeboats and their crew are as resolute and lovable as their counterparts in fiction and the 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 things about everything related to the sea.
Classification Of Tugs On The Basis Of Purpose:
On the basis of purpose, they serve marine tugs can be of two types:
1. Escort Tugs: The tugs which are designed generally to escort and manoeuvre ferries and barges to their destination are known as escort tugs.
2. Support Tugs: These are the tugs which provide support services at offshore along with towing operations. These tugs play a significant role in berthing operations. Principal Types Of Tugs On The Basis Of 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 which we are using from the time of our forefathers that are the tugs made on the oldest known principle of tug development in its early stages. It is based on obsolete or old principles conventional tug has lesser manoeuvring capability, but still in use in almost all ports of the world.
But today with advancement in technology they are fitted with diesel engine with either one propeller or a couple of propellers. Single propeller tugs are further classified into two classes namely: right-handed conventional tug and left-handed conventional tug. The right-handed conventional tugs are more common and more chances to be used than to the left-handed conventional tugs. These are highly reliable. Board pull force is used to express power efficiency. The important components of conventional tugs are briefed as under:
- These have a classic rudder.
- In the centre of the tug, there is provided with a towing hook. Along with other components of tug positioning of a hook is of huge importance. The safety and performance of the tug boat mainly depend on the towing point location. When towel hook is accompanied by gob line this point can be taken towards the aft that is its distance is reduced from its original value of 0.45 times LWL from the aft so as to reduce 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 which responsible for the movement of ships. These tugboats are designated on the basis of a number of propellers in them as:
- Single screw tug implies one propeller conventional tug,
- Twin-screw tug implies two-propeller conventional tug and
- Triple screw tug implies three propeller conventional tug.
In conventional screw propeller tugs, 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 conventional tugs is quite simple which requires less maintenance, so the maintenance and construction cost if these tugs are very low as compared two other two types of tugs.
- These are self-sufficient thus requires no support system.
- Smaller is the propeller larger be the open water.
- These tugs have an upper hand and maximum efficiency in towing carried for port-to-port.
- These provide complete assistance with pushing and tugging operation, while carrying towing on a line.
- The operation cost is quite 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 very old thus it degrades the performance standards.
- They are unidirectional and cannot work in reverse direction until provided with a reduction gear to facilitate reverse flow.
- There are more chances of cavitation in these tugs.
- The stability and strength of these tugs is less as compared to the other two.
- Their usage is limited to small and medium boats due to obsolete design principles so azimuthal stern drive tugs and tractor tugs are employed to assist the mega boats.
- The repositioning of the tugs is quite slow due to restricted manoeuvring capacity.
- There are more chances of capsizing or overturning of the vessel which results in fatal accidents. To overpower this issue of capsizing and halting the development of girting, a release hook is required.
- Conventional tugs have low values of astern power
2. Tractor Tug:
The key to the true tractor lies in the use of 2-multidirectional propulsion unit, of which some are rather like large rotating outboard motors with other 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, thereby facilitating spectacular manoeuvrability in the right hands.
The towing point can be placed much nearer the stern so as to get maximum output from the propulsion units, and therefore the thrust is always outside of the towing point, thus creating a good 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 being the most important type of tugs, 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 will of the pilot or on demand of the towing job in which they are employed. So these are readily available tugs with high efficiency values.
- Their control systems are very simple and they pose very low risk of girthing or capsizing.
- These have an extraordinary performance with fast speed.
- They can effectively overcome the interaction forces when interacting with ships at some 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 area such as a lock flow channel.
- In these types of tugs, a rudder is not required that is use of rudder can be eliminated.
- They have reliable and robust propulsion units.
The tractor tugs are the most effective and most powerful sort of tugs. As everything has certain cons along with pros these too have certain loopholes mainly their complexity and cost and other drawbacks of these tugs are as follows:
- These kind of tugs have less bollard pull as compared to ASD tugs.
- They are considered to have very high capital investment costs it implies a lot of money is required to buy these tugs and maintain these tugs. In short, it confers that 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
- Heeling angle with full side thrusts may be up to 21 degrees with some tugs. Therefore, risk of damage can exist when lying alongside a ship
- Sophisticate under-water units may be damaged on grounding
- Draft may be up to 5m which is large in comparison to conventional tugs
- The retraining of conventional tug master is essential in order to fully maximize a tug’s potential
3. Azimuthal Stern Drive (ASD):
These tugs are midway between conventional tugs and tractor tugs as these utilizes some of the benefits of both conventional and tractor tugs. It can have two towing locations, one forward and one amidships and main propulsion is from two rotating azimuth units which are placed rather like a traditional twin screw tug.
These are considered more efficient than conventional tugs but less efficient than tractor tugs. Major 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 degree which is in contrast with tractor tugs having the value of twenty-one degrees.
- These tugs have shallower average draft of 3m
These tugs are difficult to work with as compared to tractor tugs and other drawbacks 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 to occurrence with certain types, when backing with full power
- They are still susceptible to girting or capsizing when using aft towing position.
- These types of tugs are slightly more at risk from the effects of interaction
- 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 forward position
Other Miscellaneous Tugs:
Along with these above-mentioned classifications, some other miscellaneous types of tugs are briefed as below:
1. Tractor Tugs With Cycloidal Propellers:
Before 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 operation. 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 interlocking inner and outer rings in which former are connected with boat whereas latter with tug’s body connected to towed ship through winches or hooks. The only difficulty in working with these tugs is in capsizing otherwise these are a superior choice for towing and support.
3. Reverse Tractor Tugs:
The tractor tugs are termed as reverse tractor tugs if these 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 as compared to tractor tugs due to unavailability of flat plate skeg. Generally, ASD tugs serve as reverse tractor tugs.
A bow thruster and a nozzle with steering is fitted with conventional Screw Tug to get improved manoeuvrability, this sort of modified tugs along with additional devices like a thruster and steered nozzle, as fitted with the latter devices these tugs 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 amidships and main propulsion is from two rotating azimuth units which are placed rather 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 categorized in an 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:
The tug boats escort ferries or barges and serve as ice breakers or salvage boats to make the ferries and barges to speed up to their destination, are termed as ice tugs.
The Most Powerful Tug in the world:
“Farstad Shipping’s Far Samson is crowned by the title of most powerful tugboat 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 a most powerful hybrid tug in the World.
Robert Allan design was the foundation to build this tug. To assist ship or vessel through tough regions, it employs a powerful hybrid power system namely Wärtsilä’s HYTug power system. It is assembled to a combination of “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 by diesel-manual mode to achieve maximum efficiency. These can easily break a metre of ice with comparatively high speed of about three knots. The thirty-six metres of the 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 emergency; 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 by virtue of its hybrid conformations and its maintenance is quite economical and easier.
- There are variety of operation modes available in this tug due to innovative and hybrid design.
- It can serve multi-purpose of management of ice, assisting ships and enormous barges during voyages, fire fighting, towing in coastal regions and in difficult paces.
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