Pods and other alternative propulsion systems
A podded system comprises a pod located outside of the hull inside of which is housed either an electric motor or a system of gears driven from an electric motor or drive shaft inside the vessel. The pod can have propellers at either end or at one end only.
This type of propulsor is relatively new and comes in a variety of sizes and types from a number of manufacturers. The first was installed in 1990 and was a small 1MW unit fitted to a buoy service vessel. The larger versions up to 25MW used to power some cruise ships and specialist merchant vessels are often referred to generically as Azipods but in reality this is a brand name belonging to ABB. Early podded systems suffered teething troubles and some had reliability issues but these generally appear to have been resolved by makers still active in the market.
Podded propulsors can be fixed or azimuthing and if the latter they will combine the propulsion and steering for the ship within a single unit. An azimuthing type will be able to rotate fully or partially allowing the direction of thrust to change and thus removing the need for a rudder to steer the vessel.
Rolls-Royce has developed a smaller Azipull podded system which, as the name suggests, is fitted with a pulling rather than pushing propeller. This system is designed for use on offshore vessels and ferries.
Podded propulsion systems have been installed on ice-classed cargo vessels and tankers employing the double acting principle whereby a ship navigates stern first in ice and conventionally in open seas. This concept hints at the early genesis of podded systems as propulsion systems for icebreakers.
ABB has regularly improved and expanded its range. The Azipod D range launched in early 2015 is gearless and available in two versions with power outputs of between 1.6MW and 7MW. The D range built upon experience with the earlier range of compact Azipod C and drew also on the Azipod X range. It is targeted at segments such as offshore drilling, construction and support vessels and ferries.
At SMM in 2016 ABB launched a new model: the linear-flow Azipod XL which is said to improve efficiency by around 10% over previous models. This is achieved by introducing a nozzle with stator plates that direct the water flow from the propeller to reduce the turbulence and energy loss and to give optimum thrust. That model is available for power output of up to 17.5MW.
Two years later at SMM in 2018 a new podded system was introduced but this time by Rolls-Royce. The ELegance podded system was designed to meet market demand for smaller, more compact units. There are two versions – one with an open propeller, the other ducted – and both make use of permanent magnet technology. The pods feature a ‘twin tail’ concept to improve efficiency while significantly reducing cavitation-induced noise and vibration. A new integrated hull fitting interface also allows a compact head-box to be used, which minimises drag and further improves hull efficiency. This also allows the height and tilt of the pod to be adjusted, allowing operators to select the optimum propeller size for the vessel.
The 1A Super and Polar Code 6 ice class pods have also been designed with installation and maintenance simplicity in mind. For example, should the pods need to be replaced they can be lifted and mounted in water. Maintenance is optimised by way of fewer moving parts within the below-the-waterline housing and the only components that require dry-docking service are the pod’s bearings and seals. These however have a life expectancy of ten years or more before they need replacing.
Now more than 60 years since first developed by Schottel, thrusters are well established and in terms of ship numbers second only to conventional propellers as the choice of final propulsion. The biggest users have been offshore ships but the depression in the offshore sector has meant that sales have decreased which may slow development.
Thrusters have a propeller that can be of the fixed or controllable type mounted inside a nozzle and share many similarities with podded systems in the method of drive and operation. Market leaders in the thruster sector include Schottel, Berg (now part of Caterpillar), Brunvoll, Rolls-Royce and Wärtsilä but there are many more manufacturers.
As with pods, thrusters can be fixed or azimuthing but they could be considered even more flexible as they can also be retractable and able to be withdrawn inside the hull when not required. Thrusters may also be located in tunnels across the ship when they are used for manoeuvring.
Today, most commercial vessels aside from the smallest are fitted with tunnel thrusters (sometimes referred to as bow thrusters) allowing them to manoeuvre easier in port without needing to employ tugs. As well as this use, tunnel thrusters are also essential for ships with dynamic positioning systems as they allow the vessel to counteract thwartship current and wind effects. Brunvoll has developed a Combi Thruster that acts as a normal thruster when extended but when retracted locks into place inside a tunnel so can also be used as a conventional tunnel thruster.
Moving the motor to the edge
A recent development in thruster technology is the advent of rim drive. Rim-driven thrusters (RDTs) have been understood and prototypes tested for several years. The design features permanent magnet motors in a rim driving a propeller in the centre.
Permanent magnet (PM) azimuth thrusters consist of three main assemblies – the PM motor/propeller/nozzle underwater unit, the hull mounting system, which includes the azimuth bearing and duplicate frequency controlled electric steering gear, and the inboard power unit that feeds electric power to the thruster.
The permanent magnet motor consists of two main parts – a stator that carries a number of electrical coil windings, and a rotor fitted with a number of very strong permanent magnets. A rotating magnetic field is created by the stator which interacts with the fields of the permanent magnets on the rotor, generating force to drag the rotor around, providing the mechanical power.
RDTs have several advantages over conventional designs, including a power increase of around 25% for the same size propeller, reduced noise and vibration and easier maintenance. Another benefit is the freeing of space directly above the unit normally used for the motors. Some makers have chosen to do away with a central hub but Rolls-Royce has preferred to continue with this feature.
This is a comparatively new form of propulsion for merchant vessels and tends to be found only on fast ferries. It is also applied to fast naval vessels and private yachts of varying sizes. In essence the workings of water jets are revealed by their alternative name of pump jet. A simple water jet consists of a pump that can be either shaft or motor driven, an intake, a nozzle and a steering mechanism. The types fitted to vessels also include a reversing bucket for running astern.
The intake is usually under the hull and will be fitted with a guard to prevent foreign bodies being sucked into the pump which may be of the axial or centrifugal type with the latter usually being chosen for highspeed craft. The water pressure inside the inlet is increased by the pump and forced backwards through a nozzle. The nozzle provides the steering for the vessel and its effectiveness can be enhanced by small rudder-like plates attached to it that are moveable and help direct flow.
Rapid changed from forward to reverse are possible with the reversing bucket. This is a device that is bowl shaped and which can pivot into the flow of the jet from above. When disengaged, the thrust from the jet is directed astern driving the vessel forward. When engaged it is pivoted into the flow and directs the force back towards the vessel and therefore reverses its course. The actual position of the bucket is controllable and when partially engaged it will add a braking effect to the forward motion of the vessel. Another advantage is that when travelling astern by using the reversing bucket, steering is not inverted as it would be in propeller-powered ships.
The market for water jets is not large but it does support a number of players including Rolls-Royce, Wärtsilä, Hamilton (founded by the man who made the concept viable) and Doen.
The Voith Schneider propeller (VSP) is a unique system dating back to the early days of motor ships but still favoured for certain craft where high manoeuvrability is required. Tugs are particularly suited to this form of propulsion.
A VSP consists of a driven circular plate underneath the vessel with movable and controllable blades installed at a 90° angle to it. The angle of the blades will determine the direction of thrust and the speed at which the circular plate is rotating will determine the magnitude of thrust. Since the blades are located under the vessel they could be exposed to risk of damage from grounding. For this reason a guard plate is fitted beneath them and has the added advantage of increasing the thrust obtainable from the VSP.
Because of their ability to give infinite variations of thrust and direction, VSPs are also considered as being suited to vessels where DP is a desired feature. They have not however achieved the same degree of popularity in the offshore sector where azimuthing thrusters are the preferred choice.