How do tugboats work?
Updated: Oct 26, 2021
Tugs are special boats that assist other vessels into and out of port. The primary purpose of these boats is to help move larger ships by towing, pushing, and guiding. Many even have fire suppression and other systems to help the larger ships. But how is it that these seemingly tiny tugs move boats a thousand times their size, and why can’t those boats move themselves? Let’s take a look at what makes tugs special, and why they’re an integral part of shipping.
Image Credit: MarineLog
Why are tugs needed?
Tugs fill a few different roles:
Acting as salvage boats
Acting as ice breakers
Tugs carry firefighting equipment to provide assistance in case of a fire
Moving barges, rigs, and other floating equipment that does not propel itself
Helping a boat through narrow areas, in inclement weather, or docking and mooring
The last role is usually the largest, but why do large ships need help getting into and out of port? It’s easy to maneuver a large ship at sea, they can keep up speed and have plenty of room to adjust course. The larger ships grew, the more they could carry long distances, but the less agile they became at low speeds and in tight spaces. Moving sideways is particularly difficult for vessels that can be hundreds of meters long. Often, when huge vessels like container ships and LNG ships get close to port, they are escorted by a tugboat, ensuring they take the safest route. Once in port, it’s again up to the tugs to take them on the last leg of their journey.
How do they move large ships?
Depending on the size of the ship and the tugs, once in port, one or more tugs can tow via a tow line or push the ship into dock. At low speeds, large vessels don’t have enough water passing over the rudder to turn quickly, so this assistance is needed for the final mooring of the ship, especially in crowded ports. The tow line can be used for direct or indirect towing. With direct towing, the tug is pulling the vessel straight ahead, likely due to the ship not having the ability to propel itself. With indirect towing, the tug can act as a rudder and pull the ship to one side, helping rotate the ship as it moves. Of course, tugs can also cruise right up to the side of the ship and push. Large ships usually have designated areas where tugs can push safely.
Image Credit: Workboat
What are the types of tugboats?
Tugboat designs have changed over the years, the oldest being the conventional tug. These are in use all over the world and consist of a diesel engine and have one or multiple propellers. They use a screw propeller design with nozzles and rudders that move to help them maneuver. The benefits to a conventional tug are that they are simple, low maintenance, and have a proven design. Downsides are they do not have the performance of new tugs, some only go in one direction, they are more prone to capsizing, and can’t work with the largest ships. They are very reliable and still in use today.
The tractor tug is next up, it uses a “2-multidirectional propulsion unit, of which some are rather like large rotating outboard motors with other consisting of rotating vertical blades,” according to Marine Insight. These are more versatile and agile compared to conventional designs. They can provide 360 degree thrust, have low risk of capsizing, and can work sideways. Downsides are less pull compared to newer ASD tugs, high maintenance cost, and complexity.
Azimuthal Stern Drive (ASD) use some of the benefits of both the conventional and tractor tugs. They have two towing locations and use two rotating azimuth units for propulsion. Benefits of these tugs are lots of bollard pull, shallow draft, and better stability at speed. Downsides are they are a little more complex to control and limited in some maneuvers. You can see a tugboat with azimuth thrusters in the photo below.
Image Credit: Reddit
Most tugboats are still powered by diesel engines. Many newer and larger tugs have two diesel engines, some with a combined total of over 6,000 horsepower! There are also hybrid tugs that combine diesel and electric power, and LNG-powered tugs.
Are there electric tugs?
Yes! In September 2020, the first fully electric tugboat went to work in Istanbul. ZEETUG, which stands for Zero Emission Electric Tug boat, built the Gisas Power, owned by GISAS Shipbuilding. It operates in the Port of Tuzla Aydınlı Bay, where environmental and navigational needs drove the demand for the electric tug. Plugboats has specs on the Gisas Power, “The Gisas Power has an overall length of 18.7 metres • 61 feet, a beam of 6.7m • 22′ and 32 tonnes of bollard pull (BP), which is how the power of tugboats is rated. Different tugboats are designed for different jobs, but the ‘average’ tug has a BP rating of about 40-45 tonnes and the strongest are rated at 60-65T BP.” ZEETUG’s electrical propulsion system can be built to specifications from 5T BP all the way up to 75T BP. The ZEETUG-30 is the first all electric tugboat - it gets power from two 1,450kW battery packs that are located separately, forward and aft, and air-cooled. More electric tugs and even hydrogen tugs are on the way.
Image Credit: PlugBoats
What about Autonomous tugs?
There are autonomous tugs too! In March 2020, Wartsila and PSA completed initial sea trials with an autonomous tugboat for the IntelliTug project. The PSA Polaris was retrofitted for autonomous navigation and tested to ensure it could avoid virtual and real-life obstacles. They are continuing R&D on the new tug throughout 2020.
Image Credit: Splash247
As you can see, tugboats are an important part of getting large ships into and out of port, through dangerous situations, moving all types of non-powered or disabled vessels, firefighting, and even search and rescue. Check out the video below for a day in the life of a tug operator in Dublin Port.
Video credit: Dublin Port Company