Updated 22 Oct 2019
The final component of the drive system is the propulsor, of which there are many types. After paddle wheels became obsolete due to their inefficiency, propellers or screws became the dominant means of propulsion. The term ‘screw’ harks back to the very first versions which, unlike the bladed type of propeller, were in the form of Archimedean screws.
Bladed propellers, whether conventional or as part of a podded propulsor or thrusters, are the most common means of propulsion but there are some more exotic variations such as waterjets, Voith Schneider propellers and rim-drive thrusters.
With propellers being seen as one way to achieve the additional efficiency demanded by regulations such as EEDI, far more thought is now given by naval architects to matching the propeller more exactly with the hull form, the chosen power train and the intended operating profile of the vessel.
Even if shipyards would prefer to continue the traditional practice of fitting almost any standard propeller to their newbuildings, the buyer is entitled to a ship that does meet the minimum EEDI requirements so more care needs to be taken. On newbuildings, an appropriate propeller choice can allow for a de-rated engine to be fitted without a detrimental effect on the ship’s speed in comparison to existing ship types.
The effectiveness of a propeller depends upon several factors, but diameter is one of the most important. Theoretically, a single bladed propeller would be the most efficient but the vibration that would result makes this an impractical option. However, there is little agreement as to how many blades are the best options with the result that very similar ships may be fitted with quite different propellers based on personal choice and complex mathematical calculations.
Even for existing ships, an exercise in checking the performance of the fitted propeller may reveal that a change to a different form could considerably improve efficiency. In many ships, the improvement could easily reach double figures with 17% having been achieved in one instance and 20% or more on the cards in a small number of cases.
There is debate over whether two or more propellers make for more efficient operation than the usual single propeller but any savings in efficiency need to be weighed against the additional cost of a more complex hull design and multiple drive trains. The choice can be made easier now that computational fluid dynamics (CFD) has become part of mainstream ship design and experience with it is increasing.
There is however little chance that many owners of large vessels would be persuaded as to the merits of multiple propellers, although some have and twin-skeg vessels are not especially rare. This is sometimes done so as to retain overall propeller blade area but with a lower draught that would preclude the use of a larger diameter single propeller. This allows vessels to enter ports where water depth might otherwise have been a limiting operational factor.
Propellers have come in an incredible variety of forms since their first use but development is still continuing and efficiency improvements continue to be made. They can either be fixed pitch propellers (FPPs) or controllable pitch propellers (CPPs) and although the latter are more capital intensive, they do provide many benefits for ships that spend more time manoeuvring in port than in open waters.
An FPP may be cast as a single piece or have detachable blades whereas a CPP will always have detachable blades because they are adjustable to give the variation in pitch which their name suggests. In a CPP, the blades are separate components that can be adjusted to present different surfaces to the flow giving fine tuning over speed and engine loadings. A CPP has the added advantage that, in the event of blade damage, a replacement can often be fitted underwater eliminating the need for expensive drydocking and time lost in removing and repairing the damaged propeller. The behaviour of a CPP can be duplicated to a large extent on an FPP if a controllable slipping clutch is incorporated into the drive train.
Regardless of propeller type, one of the most important considerations during design is to limit cavitation. This occurs when vapour bubbles are generated as water boils in response to a drastic drop in the water pressure. The bubbles instantly implode and a giga-pascal-order collapse in pressure occurs. This causes erosion on the propeller surfaces and ship vibrations. Preventing cavitation and cavitation-induced erosion is a top priority to maintain ship safety and propulsion capability. The ability to reduce cavitation has been much enhanced by the use of CFD design tools.
There are several independent makers specialising in propellers, among them MMG, Zollern and Piening from Germany, BT Marine Propellers and Stone Marine from the UK, and Van Voorden in the Netherlands. In addition there are several Asian manufacturers often connected with shipyards in Japan, South Korea and China.
The relationship between engines and propellers is a very good reason why some of the main engine manufacturers have their own propeller-making divisions. Among those that do are Wärtsilä, Kongsberg (having acquired the former Rolls-Royce Marine in early 2019) and MAN Energy Solutions, which increased its Alpha range with its takeover of Kappel propeller in 2013. Compared to conventional designs, the Kappel propeller blade designs offer fuel savings of up to 6% due to the reduction in tip vortices formed due to the difference in pressure between the pressure and suction side of the propeller.
Some propellers are designed for use in conjunction with specific rudder types for improved efficiency. Possibly the two most well-known are the Kongsberg Promas system (also developed by the former Rolls-Royce Marine) and Wärtsilä’s Energopac but other companies working alone or in tandem with others have similar systems on the market, one example of this co-operation is that between German propeller maker MMG and Dutch rudder maker Van der Velden. Typical efficiency savings of between 2% and 9% are claimed with the wide range due to different vessel types.
As well as looking at the propeller itself in terms of size, blade shape, number of blades and the speed at which the propeller turns, some research has gone in other directions and resulted in more complex propeller configurations such as contra-rotating propellers and attachments such as boss cap fins.
A contra-rotating propeller has two powered shafts with one inside the other and rotating in opposite directions. The outer shaft carries the main propeller and a secondary propeller is mounted on the inner shaft behind. The principle behind the contra-rotating propeller is that the rotational energy losses behind the first propeller are recovered by aft propeller.
A propeller with a boss cap fin works on similar principles but here the propeller boss has integrated vanes that break up the hub vortex generated behind the rotating propeller.
Propeller material choices
Most propellers are made from bronze alloyed with other metals, notably aluminium, manganese and nickel. The use of such non-corrosive metals is essential for a piece of equipment that will spend most of its life submerged or semi-submerged in seawater. The exact composition will depend upon the desired weight and size of the propeller aimed at ensuring best performance.
At various times, trials of other materials have been made and, as with many areas of ship construction, composites, particularly carbon fibre reinforced plastic (CFRP). Most vessels fitted with composite propellers have been small ships and craft but, following a retrofit project of a 2.12m CFRP propeller on a small tanker in 2014, Japanese maker Nakashima has added CFRP propellers to its product range. Test results from the tanker propeller apparently show significant improvements over the vessel’s performance compared with its original nickel aluminium bronze propeller.
In September 2017, additive manufacturing – better known as 3D printing – was used to produce a prototype propeller. The 1,350mm diameter propeller – named WAAMpeller – is the result of a cooperative consortium of companies that includes Damen Shipyards Group, RAMLAB, Promarin, Autodesk and Bureau Veritas.
The propeller was produced from a nickel aluminium bronze alloy with the Wire Arc Additive Manufacturing (WAAM) method using a Valk welding system and Autodesk software. The triple-blade structure uses a Promarin design that is used on Damen’s Stan Tug 1606. With production complete, the WAAMpeller was CNC milled at Autodesk’s Advanced Manufacturing Facility in Birmingham, UK.
This prototype 3D printed propeller represents a steep learning curve of the understanding of material properties. Because 3D printed materials are built up layer by layer, they display different physical properties in different directions – a characteristic known as anisotropy. Steel or cast materials, on the other hand, are isotropic – they have the same properties in all directions. Because of this critical difference, one of the first steps was to carry out extensive testing of the material properties of the printed material to ensure compliance to BVstandards.
In November 2017, the propeller was fitted to a Damen tug and subjected to testing. The testing programme included bollard pull and crash stop testing in addition to speed trials. Apparently the WAAMpeller displayed the same behaviour as a conventional cast propeller in all the tests.
Another project using similar technology is being conducted by the French Naval Group where blades have been produced using WAAM. The blades were produced with the aim of reducing the environmental impact of ships, as part of the Horizon 2020 (H2020) project, RAMSSES (Realization and Demonstration of Advanced Material Solutions for Sustainable and Efficient Ships).
Podded and 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 each 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 this is in fact 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 Marine 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 and is now being produced and developed by Kongsberg, along with the other propellers, thrusters and waterjets previously manufactured by Rolls-Royce Marine.
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. It 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.
In 2019, ABB extended its range by introducing a new midsize range of Azipod propulsors covering the 7.5-14.5MW power range. The range is aimed at the ferry sector but will also be applicable for larger offshore construction vessels, midsize cruise ships and shuttle tankers.
At the core of the new mid-power range are ABB’s latest fourth generation permanent magnet motors which increase power and efficiency. The mid-power range propulsion units have been designed for low onboard height, which allows the placing of the Azipod system under the car deck of ro-pax vessels At SMM in 2018, a new podded system was introduced by Rolls-Royce, one of its final pre-Kongsberg innovations. 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.
Podded propulsor systems have been installed on numerous vessel types but have their largest volume sales in the cruise, ferry and offshore sectors. As far as commercial vessels go, tankers and LNG carriers feature, especially ships with high ice class. In 2019 ABB received its first order from the bulk carrier sector when Oldendorff opted to install twin 1.9MW systems on a pair of 21,500dwt bulkers.
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, Kongsberg 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 can 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 although they are also fitted at the stern of many ships) 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.
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 a 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 Kongsberg Maritime 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, 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 changes 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 this is 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 Kongsberg, 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 horizontal plate underneath the vessel with movable and controllable vertical blades. The angle of the blades varies as the plate rotates to 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.