Tugboats are an indispensable part of the global shipping industry, facilitating the safe passage of large ships through narrow channels, busy stretches, and shallow water, as well as performing several other vital duties, including firefighting and marine salvage.
You’ll find tugboats being used in all manner of industries worldwide, from the cruise industry to the oil industry. Quite simply, without tugboats, the entire maritime world would come to a grinding halt.
But what are tugboats? How do tugboats work? And what are the different tugboat types? You’ll find out the answers to these questions and much more when you read through everything we’ve got to say below.
What is a Tugboat?
The most basic question to answer on this page is what are tugboats? In simple terms, a tugboat is a vessel designed to manoeuvre other vessels by pulling or pushing them. This movement can be facilitated via direct contact between the tug and the other vessel or by using tow lines.
But why are these tugboats needed? They’re needed because larger ships can’t turn sharply at low speed, as insufficient water passes over the rudder to facilitate quick turning. Tugboats are therefore used to push and pull ships into position, such as into designated berths at ports.
However, not all tugboats are used in the way described above, as some tugboats are now used for roles not relating to the movement of vessels. For example, some tugs have been designed for search and rescue operations, while others are specially adapted to act as ice breakers.
Nowadays, most tugboats have two diesel engines. These are exceptionally powerful and can generate over 6,000 horsepower between them. However, electric tugs have recently started to be used, with the first being deployed in Istanbul in 2020. These are slightly less powerful than other tugs but are necessary for use in areas with environmental requirements.
But who works on the tugs? The average tugboat used to navigate ships in and out of port has several types of workers. The captain is in charge of the vessel and must have the necessary qualifications to haul specific tonnages. Mates work under the captain and are second in command, while engineers are required to ensure the tugboat functions correctly at all times. Some tugboats also have a crew of deckhands.
So, to recap, a tugboat is usually used for navigating larger ships by pushing or pulling them into position, although they can also have many other jobs. They’re mainly powered by diesel engines and are manned by several crewmen, all under the overall command of a tug captain.
Anyone looking to gain use of a tugboat has two options. They can either purchase one, which often means spending a large sum, or they can hire a tugboat. Ship chartering represents the most cost-effective option for most.
Bollard Pull: The Measurement of Pulling Power
Bollard pull is a measurement used to determine the pulling power of a vessel, so as you can imagine, it is exceptionally important for tugboats. A more technical definition of bollard pull would be the thrust developed by a vessel’s propulsion systems at zero ahead speed.
There are two types of bollard pull: static bollard pull and continuous bollard pull. The static bollard pull is measured shortly after the start of the test, when full power is possible thanks to the still water. The continuous bollard pull is then measured over 10 minutes.
The bollard pull of tugs and other vessels is calculated in kilonewtons (kN) or occasionally in different units of force like tonnes of force (tf) or short tons (stf). The average medium-sized tugboat has approximately 500-600 kN of bollard pull.
Every vessel that’s required to pull a load must have a bollard pull certificate from one of several classification societies, such as the Indian Register of Shipping (IRS) or the American Bureau of Shipping (ABS).
Importance of Bollard Pull
The bollard pull of a tugboat is exceptionally important, as without the figure, it’s impossible to work out whether a tugboat has sufficient power to navigate a specific ship safely. If the bollard pull is not known, tugboats could be deployed that don’t have the necessary power to bring a large ship to a halt or manoeuvre it quickly and accurately.
How to Calculate the Bollard Pull
To calculate the bollard pull of a vessel, the vessel is anchored to a pier using a bollard. The vessel then propels itself forward at maximum thrust, and the tension measured in the towline shows the total thrust achieved.
The bollard pull test can’t be carried out in all conditions. Instead, a few criteria must be present for the bollard pull test to be accurate. The most important are:
- Undisturbed water
- Enough clearance around the vessel
- Deep water
- An unhindered propellor wake and stream
It should be remembered that perfect conditions are rarely found. Therefore, the results of the test must have some margin of error.
It is also possible to calculate bollard pull using computational simulations. However, these are expensive, and few facilities can run the simulations.
How to Increase Bollard Pull
As you might imagine, the bollard pull of a tugboat is a significant selling factor. Therefore, manufacturers do all they can to design tugboats with the highest possible bollard pull. There are several ways the bollard pull can be increased, including:
- More powerful engines
- Increased number of engines
- Make the vessel design more streamlined to minimise resistance
It should also be mentioned that sellers of tugboats sometimes unscrupulously alter their vessel’s bollard pull score to make it more appealing to buyers. However, buying or renting from reputable companies all but eliminates the possibility of this.
Is Bollard Pull Always Accurate?
Bollard pull is never going to be completely accurate, and this is because conditions are never perfect. For example, the test might be carried out when the bollard and vessel are not perfectly horizontal with each other, or when the water isn’t perfectly calm.
Propellors also regularly yaw vessels to one side slightly, affecting the accuracy of the bollard pull calculation. Even the salinity of the water will influence the accuracy of the test.
To obtain the most accurate results possible, bollard pull tests should be carried out in deep water and away from the mouth of a river. They should also be carried out on a calm day, with minimal traffic in the area.
How Do Tugboats Work?
Tugboats generally work in one of three ways: direct towing, indirect towing, and pushing. Each is used in specific situations and has its unique advantages. Find out about them below.
This is the simplest type of towing. Quite simply, a towing line attached to a powerful motor is fastened to the vessel being towed. The tug then pulls the ship through the water. This is often used when the vessel is not able to propel itself. Generally, this type of towing happens at low speeds, typically 4-5 knots, and is governed by the bollard pull of the tug.
Indirect towing is another way of manoeuvring ships using a tow rope, but it differs considerably from direct towing. In this type of towing, the tow rope is attached to the vessel’s stern, which drags the tug. The tug uses thrust to keep a sheered position in relation to its heading, which allows the drag force to be higher than the bollard pull of the tug. Using this method provides more force when turning or decelerating. It generally happens at speeds of 6 knots or greater.
Most commonly used in North America, pushing is exactly what it sounds like. The tugboat pushes the vessel into position, using its impressive power to reposition ships many times larger than itself. Pushing tugs often have an attachment that fits into a specific receptacle of the vessel being pushed, plus tugs of this type are generally built with reinforced hulls.
Types of Tugboats
In general, there are two types of tugboats – escort tugboats and support tugboats. Below, you can find out a little about these two types of tugs.
Tugs of this type are the most recognisable, as they manoeuvre and escort vessels in dangerous and coastal waters. The escort tug is required to assist with the steering and stopping of large ships, plus they can also perform routine towing tasks in harbours.
An escort tugboat must have an optimum speed equal to or greater than the maximum speed of the vessel being moved, and it must also be able to carry out duties while moving at these top speeds.
Support tugboats, as their name suggests, provide support to towing operations. They are primarily used to assist with berthing and unberthing and don’t carry out the heavier work done by escort tugboats – although support tugs are capable of tethered towing of smaller vessels. They can also be used for untethered towing.
It is also possible to find offshore support tugs designed to support offshore operations, such as oil rigs, working in deeper water compared to regular support tugboats. As per regulations, all support tugboats can be used for firefighting purposes.
Tugboats generally use three designs: conventional, tractor, and Azimuthal Stern Drive. Each of these has its advantages and disadvantages, which we discuss below.
As the name suggests, tugs of this type are the way they’ve been for years, created using age-old principles. This makes them relatively easy to construct, plus they also require less maintenance than other tugboat designs. They also have a much lower operational cost, making them the economical choice. What’s more, they’re completely self-sufficient.
Tugs of this type can assist with pulling and pushing, but they are not suitable for on the line or indirect towing due to their lack of manoeuvrability – something that also limits their ability to reposition themselves. Conventional tugboats are less powerful than the other two types of tugboats mentioned on this page, and performance is further affected by the dated design principles used.
It should also be mentioned that conventional tugboats have more chance of capsizing, making them the most dangerous type of tugboat. A release hook is used to mitigate against this issue. Despite the many drawbacks to conventional tugboats, they are used in most ports worldwide.
Tractor tugs are the newer style of tugboats and are named after the traction their propulsion systems manage to achieve when pushing or pulling. This propulsion unit is multidirectional and gives tractor tugs tremendous manoeuvrability. The towing point is placed close to the stern, which allows the propulsion units to work optimally.
Tractor tugs can reposition themselves quickly, making them highly versatile, plus they have a high top speed. They’re the best choice in restricted areas and far less susceptible to capsizing, mainly thanks to their straightforward controls.
However, this design has a few downsides, such as the high purchase costs – which is why many instead look for tractor tug boats for hire – and the lower bollard pull compared to ASD tugs. Retraining of a tug master is also required to gain maximum use from a tractor tug.
Azimuthal Stern Drive
Azimuthal Stern Drive tugs – usually abbreviated to ASD – are a mid-point between conventional tugs and tractor tugs. They get their name from the two azimuth units which provide most of the tug’s propulsion. These are placed in the same way as a twin screw tug. ASD tugs can have two towing locations – one amidship and one forward.
In terms of efficiency, they are better than conventional tugs but not as efficient as tractor tugs. They are the most suitable tug type for working in open waters and seaways, thanks to their hull form, plus they can offer a large bollard pull. The average draft of these tugs is 3 metres, plus the maximum heel with side thrust is under 15 degrees – much lower than the 21 degrees of tractor tugs.
Even the most experienced tug masters will find ASD tugs to be challenging at first, mainly down to the complex control systems onboard. They can also be susceptible to capsizing or girting when using the aft towing position.
Uses and Applications of a Tug
While tugs are mainly used for the manoeuvring of ships, they also have various other uses, all invaluable to the maritime industry. We’ve listed some of the main tugboat types below, so you can see exactly how widespread their use is.
Ice-breaking tugs are used to clear a passage of ice, breaking up the ice by impacting it. Many ice-breaking tugs can also push and pull vessels. These tugs are built to be solid – they have unique hulls that can withstand heavy damage, plus the hulls are designed to resist metal embrittlement.
Tugs of this type usually use a twin-engine screw propeller with a Kort nozzle. This non-rotating nozzle can increase thrust by up to 90%, providing the extra power necessary to break through ice. Ice breakers also usually have lower freeboards thanks to the extra scantlings used.
Salvage/Search and Rescue
Marine salvage tugs are generally used to recover grounded ships or apply measures to stop them from sinking. They’re also regularly used for search operations when distress calls are received. Tugs of this type are usually tractor tugs and are manned by specialist crews.
Tugs used in this sphere of work are highly specialised, boasting many extra pieces of equipment. These can include specialised winches and high-powered pumps. They are also usually equipped for dredging operations, where they remove sediment and foreign objects from water channels, ensuring ships can pass through unhindered.
Towing and Workboats
The primary use of tugboats in ports, tugs of this type offer assistance in braking and steering to larger vessels navigating particularly challenging passages. Most tugs of this type have been adapted to offer both pushing and pulling capabilities.
It’s nearly always the case that tugs of this type use Azimuth Stern Drive propulsion systems – you can find out more about these later on this page. They are also often twin-engine powered to provide the extra power needed for operations.
Firefighting is something that every tugboat must be equipped to carry out, as per SOLAS regulations. This means that every tug can carry out basic firefighting duties. However, there are also specialised firefighting tugs operated by qualified firefighting crew, which are stationed strategically to ensure reports of fires can be responded to quickly.
As you might expect, specialist firefighting tugs are equipped with all the features necessary to fight A, B, and C-type fires effectively. These include high-powered pumps, as well as hose lines and specialist retardant materials. They often work with tractor tugs in situations where emergency towing is required.
These are exceptionally powerful tugs, often with three engines to increase bollard pull, which deploy, retrieve, and relocate anchors potentially weighing several tonnes. They work in deep water and many miles from any assistance, and as such are required to be self-reliant, able to deal with emergencies if they arise.
These tugs are mainly used to handle anchors for oil rigs, towing them to the necessary location and securing them. They also often undertake supply duties, provide towing help, and move dangerous objects from specific areas.
Ocean towage tugs are specialised vessels designed to handle long-distance towing operations, such as towing floating production units and drilling rigs. As you’d expect, they’re fitted with all pulling equipment necessary to perform optimally and are high-powered. Many tugs of this type are also equipped to deal with regular towing jobs.
Ocean towage jobs often take over 24 hours to complete, and the tugs head into deep, open water, meaning they must be specially modified for safety and effectiveness. One vital modification is a large fuel tank to make long journeys possible.
Yarding tugs are smaller than the average tugboat and only have moderate power. Because of this, they are only used for lighter towing activities, with the movement of dumb barges being a particularly common role for these tugboat types.
A single crew member can often operate tugs of this type. Because they are smaller and have less power, they have limited usage within larger ports, where giant vessels are regularly found. Larger tugs, which can also carry out yarding duties and other roles, are often preferred.
Line handling tugs are the least powerful and smallest of all tugboats and have very limited usage. Tugboats of this type are not strong enough to carry out pushing and pulling tasks and instead support larger tugboats, securing the hawser line of ships to facilitate the towing operation.
Tugs of this type can only be used in ports and sheltered waters. They’re often only powered by an outboard motor, although slightly more powerful line handling tugs can also be found. Like all other tugs, they have firefighting equipment and can also be used for oil spills and emergency response.
Survey and Research
Tugboats of this type are specially equipped to carry out many types of surveys and research, including, but not limited to: marine biology research, seismic activity research, hydrographic surveys, and oceanographic research. Tugs of this type must be able to spend extended periods away from shore, plus they need to be able to handle the conditions of many environments.
This type of boat is not technically a tugboat, but the construction utilises many similar principles. They are usually equipped with highly specialised instruments, some of which might be installed in a drop keel, plus they often boast highly advanced radar mapping capabilities.
Tugboats with the primary goal of patrolling an area of water are mainly used by the emergency services, namely the police and coast guard, as well as by disaster rescue operators. They’re also used by those with a need to patrol a harbour.
Tugboats for patrolling are built with speed in mind, and crafts of this type can reach speeds of 40 knots. Patrol boats operating between 20-35 knots often have a water-jet propulsion system fitted, although fitting one can cost large sums of money.
Tugboats of this type are specifically designed to transport personnel and cargo, usually to places such as oil platforms and drilling rigs. They range wildly in size – some are just 30-foot vessels that work in bays and inland waterways, while others are large 200-foot vessels capable of working hundreds of nautical miles from shore.
The capacity of crew boats differs from vessel to vessel, with the largest crew boats being capable of transporting up to 100 passengers, usually in airline-style seating. They typically have a higher GRT, plus they often have considerable space on deck. As they carry passengers, they’re generally designed to offer some level of comfort.
How much does a tugboat cost?
The price of purchasing a new tugboat can vary wildly – some can be bought for around $750,000, while at the other end of the scale, they can cost upwards of $10 million. When well-maintained, tugboats retain much of their value but purchasing second-hand is usually far more cost-effective. However, the cheapest way to gain use of a tugboat is to charter one.
What is it like to travel on a tugboat?
This depends on the type of tugboat being travelled on. Some tugboats are specially designed to carry passengers comfortably, while others are designed to be more practical and focus less on comfort.
Why is it called a tugboat?
As ships became larger and larger, there was the need to assist them when coming into port – the large ships simply didn’t have the manoeuvrability to do it on their own. This was solved by using small boats to provide something called a tug assist. The boats themselves soon became known as tugboats.
How powerful is a tugboat and how come it is able to tow large ships?
Tugboats vary in power, with the average tugboat having a bollard pull of 400-500 kN. This is a tremendous amount of power compared to their small size, and this is primarily caused by the mighty engines used. Also, several tugboats are sometimes used to move a large vessel.
As you can see, tugboats are a vital component of the maritime industry. Without them, large ships would find it impossible to navigate specific stretches of water, plus many essential roles, such as firefighting and marine salvage, would remain unfilled. You’ll spot tugboats in every port, and the fact that they’re ubiquitous only underlines further their importance.
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