Manufacturers are integrating permanent magnet motors in thrusters for electric-powered and hybrid propulsion tugs
Thruster manufacturers have developed Z-drive and L-drive units for propulsion systems in tugs, enabling hybrid systems, all-electric power and new fuels including LNG. Azimuth thrusters in both drive configurations are available in power ranges from 50 kW to 3.5 MW for all types of tugs with bollard pulls up to 100 tonnes.
Veth Propulsion manager of projects and product manager for thrusters Bastiaan van Zuijlekom says selecting the correct propulsion system is essential for tug design and construction. “Design and engineering need to start with the thrusters,” he tells Tug Technology & Business. “Then you can maximise the space for other applications.”
Azimuth thrusters provide the tug’s manoeuvrability and bollard pull power, with thrusters able to complete 360° turns in just eight seconds enabling tight turning circles.
“Thrusters are the problem solvers in a lot of applications and vessel types. They can save weight and take away a lot of equipment for propulsion,” he says.
Designers and shipyards should collaborate with propulsion manufacturers from the earliest design stage on new tug projects. “There are a lot of benefits and synergies from co-operating at an early stage,” says Mr van Zuijlekom.
“We will be their knowledge partner in choosing the most suitable propulsion solution. It should be a triangle between the shipyard, designers and thruster manufacturer to develop the best solution,” he says.
The latest development in tug propulsion is including an electric motor inside azimuth thrusters. “The electric motor is inside the steering gear box within the rudderpropeller and included in the assembly, so there is no need for the shipyard to insert it separately,” says Mr van Zuijlekom.
“The motor is not below the waterline or in front of the propeller, it is integrated into our L-drive thruster for applications where space is tight below the deck.”
The integrated unit will make the installation work for the shipyard much easier as there are no long shaft, intermediate bearings and bulkhead seals required.
“We will make vessels sustainable, safe and efficient. With L-drives, there is no upper gearbox as in Z-drives,” says Mr van Zuijlekom.
Most thrusters installed on tugs are for diesel-electric systems with main engines turning shaft lines to drive motors, but this could change in the future. “Demand for L-drives will pick up with electrification,” he says.
Permanent magnet motors
Steerprop has developed ducted propulsion for tugs and permanent magnet (PM) motors for electric propulsion. Its development of Steerprop W was driven by operator requirements to enhance performance capabilities and cost efficiency in azimuth propulsion, says Steerprop director for sales and project management Juho Rekola. Steerprop W-series is “mechanically robust and has high hydrodynamic efficiency due to the shape of the frame” he says.
“When high bollard pull is needed, nozzles with outstanding hydrodynamics can be added,” he explains.
Some of the key design principles for the Steerprop W-series were to produce azimuth propulsors with higher durability, more flexibility and simplified construction. “These are available with an integrated PM motor and in shallow draught versions, together with a dual-input hybrid solution,” says Mr Rekola.
Steerprop has integrated PM motors within its azimuth Steerprop LM propulsors to deliver better efficiency compared to traditional induction motors. “By integrating the PM motor to the propulsion unit, we get more room in the vessel for other components and can avoid upper gear excitations in the hull,” says Mr Rekola. “We are working in close co-operation with our partners to make Steerprop LM units possible for larger units,” he adds. In its development, the upper gear was replaced with an electric motor integrated straight to propulsor’s vertical shaft.
“For maximum reliability and service access we wanted to place the electric motor inside the vessel hull, instead of outside the hull below the waterline,” says Mr Rekola.
To achieve this, a vertical electric motor with PM rotor was selected. “The PM motor is lighter compared to induction or an externally excited motor,” he explains. “Its simple rotor structure has reduced weight and inertia, making the motor construction simple and more robust.”
Its air or water cooling units are easy to connect to a tug’s water-cooling circulation. “Most importantly, the PM motor has high efficiency through the whole speed area, especially at the important lower powers and speed areas,” Mr Rekola explains.
Kongsberg Maritime has collaborated in ground-breaking projects to develop mechanical, hybrid and pure-electrical power transmissions, with applications from engines to thrusters.
“All these solutions are designed to increase operational flexibility and safety – and reduce emissions – through better control of thrust and more efficient use of generated power,” says Kongsberg Maritime general manager for propulsion sales Jussi Kuusisto.
“Thruster technology has been a pivotal component in the drive to cut fuel costs and lower emissions while improving manoeuvrability.”
Kongsberg Maritime’s US range of azimuth thrusters offers 18 different propeller diameters. It has responded to the growing popularity of hybrid technology by developing two alternative solutions (dual and twin inputs) to combine electrical power and direct-diesel drive to thrusters.
With twin inputs, the electric motor is fitted to the aft end of the thruster, whereas with dual inputs the electric drive operates in parallel to the input connected to the main engine.
“This kind of hybrid solution is not only applicable to newbuilds; the hardware can be also retrofitted at low cost, if a hybrid-ready thruster is specified at the construction stage,” says Mr Kuusisto.
A fully electrical version has also been developed, where a vertical PM electric motor replaces the top gear of the Z-drive unit to form an L-drive system (PM L-drive).
“These systems permit more sophisticated integration of energy storage, distribution and management systems, allowing tugs to combine high- or medium-speed liquid and gas fuel engines with battery usage to reduce fuel consumption and CO2/NOx emissions,” says Mr Kuusisto.
“The flexibility of these solutions allows more efficient operation at partial or low loads – a frequent mode of operation for tugs – and thus helps to reduce emissions in ports.”
There are numerous recent examples of such units in use. The first fully-electric Damen RSD 2513 ship-handling tug comes equipped with two US255 azimuth thrusters with the PM L-drive solution. A multi purpose, hybrid-propulsion tractor tug recently delivered to Baydelta Maritime in San Francisco, US, is fitted with the twin input version.
Electric tractor tugs
Voith has incorporated a PM motor into its propeller drive for electric-powered tractor tugs. This electric version of the Voith Schneider Propeller (eVSP) combines the technology of a conventional VSP with the Voith inline thruster.
“With the eVSP, we are making an important contribution to the electrification of the driveline in marine applications and thus to even more resource-saving shipping,” says Voith vice president of research and development for marine applications Dirk Juergens.
Its new eVSP uses a PM synchronous motor as its main drive, reducing the required oil volume and complexity of the propeller. The complete omission of gears reduces noise to a minimum, lowers maintenance and frees up critical space on a vessel.
It enables stepless operation and virtually loss-free conversion of the electrical drive power with dynamic response characteristics. An eVSP would enable a vessel to be driven and controlled with one unit in wave heights of up to 4.5 m.
Schottel has developed Z-drives for diesel-electric and hybrid propulsion. Its latest development, Sydrive M, enables two thrusters to be drive by one engine in a hybrid system. “The other engine can then either be idling or used to drive a fire-fighting (FiFi) pump,” says Schottel vice president of sales Roland Schwandt.
“This results in reduced engine running hours and less fuel consumption and, consequently, lowers emissions,” he says. Sydrive M takes up considerably less space than a hybrid system with an e-motor.
“Tugs have quite diverse operating profiles and rarely require full power for propulsion,” says Mr Schwandt. “More operators rely on integrated slipping clutches to control the propeller at low input speeds or even enable them to drive the propeller and FiFi pump at the same time.
“This integrated technology, in particular, serves to meet the requirement of a compact system on a vessel where machinery space is limited.”
Schottel has also achieved high efficiency with a high torque gear (HTG) version to maximise bollard pull at minimum power. “Besides manoeuvrability, bollard pull is one of the most important criteria for tugs,” says Mr Schwandt. “The HTG is able to transmit up to 15% more torque without enlarging the diameter of the gear.”
A further improvement of propulsion efficiency has been achieved by the high-efficiency nozzle SDV45 with the patented Schottel ProAnode system. “This has higher nozzle efficiency and enhances protection of the anodes against external impacts,” says Mr Schwandt. “By moving the position of the anodes from the outside surface into the cross-section of the nozzle tail.”
Integrating thrusters with engines in complete packages
Thrusters should be considered as part of wider propulsion packages, consisting of main engines or generator sets running on LNG or diesel, or electric motors for electric propulsion.
Wärtsilä director for propellers and gearboxes Elias Boletis says the company supplies energy storage batteries and energy management systems for hybrid or electric propulsion and provides dual-fuel engines and LNG fuel systems for tug applications.
“By matching all components in the systems to the operational requirements of the customer, these hybrid tugs are capable of operating more energy efficiently, with lower emissions, less maintenance and higher reliability,” says Mr Boletis.
Selecting the propulsion system depends on the performance requirements for the tug, its operational efficiency and manoeuvring capabilities. “The noise and manoeuvring impact the ability of the vessel to operate under certain conditions, in the vicinity or within ports,” explains Mr Boletis.
Wärtsilä’s hybrid solutions combine mechanical and electrical propulsion, according to the tug’s operational modes and support tools. It provides the propulsion interfaces and controls.
“Beyond our main portfolio products we also provide digital tools, which improve the performance of tug operations and machinery availability during its lifetime,” says Mr Boletis.
Caterpillar also supplies integrated engine and thruster systems. This can be combined with hybrid solutions, with power take in and take off devices, or with Caterpillar’s advanced variable drive.
In May 2020, Caterpillar agreed to sell its thruster business to investment group Gula Skrinet, led by former Berg Propulsion chief operating officer Stefan Sedersten.
This deal includes manufacturing operations in Sweden and Singapore and offices in Shanghai, China, and Dubai, United Arab Emirates.
It brings back the Berg Propulsion brand for mechanically and electrically driven propulsion systems. It will continue to support sales of MaK and Cat engines while developing its own products and services.
Berg Propulsion manufactures marine azimuth thrusters in L-drive and Z-drive configurations and marine transverse thrusters with rugged gears, bearings and customised blades.
Thrusters developed for shallow draught tugs
Another trend in tug propulsion is developing thrusters for shallow-draught vessels. According to environmental reports, global warming is leading to lower seasonal precipitation and water levels in channels and harbours. Designers, shipyards and owners need to create pusher tugs with shallower draughts to access these waterways year-round, says Veth Propulsion manager of projects and product manager for thrusters Bastiaan van Zuijlekom.
“Now is the time to act,” he says. “We should be prepared for less rain and limited ship operations with shallow draught solutions,” he says. Shallower waters can affect propeller efficiency and raises the risk of damage from debris and silt. “Propulsion will need to be protected from this,” says Mr van Zuijlekom.
Veth designed its propulsion to be enclosed within the vessel’s hull instead of free hanging below the hull. “Our integrated propulsion is inside the aft making it possible to sail with 1.4-m diameter nozzled propellers in 1.45-m water depth,” says Mr van Zuijlekom. “Being inside, people could think it would lose efficiency, but we found we can install larger nozzles and gain efficiency. We are ready for the future of inland shipping and pusher tugs with this shallow draught solution.”
Veth is supplying propulsion to Concordia Damen as it is building pusher tugs for inland waterways.
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