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.
Conventional propellers need consideration
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 been 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.
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.
Blade choice offers optimal control
An FPP may be cast as a single piece or have detachable blades whereas a controllable pitch propeller 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 propeller 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.
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, Rolls-Royce 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 by 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 Rolls-Royce Marine Promas system 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 principal 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 principals but here the propeller boss has integrated vanes that break up the hub vortex generated behind the rotating propeller.
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 is planning to fit a larger version to a 60,000dwt bulk carrier. Test results from the tanker propeller apparently show significant improvements over the vessels 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 Bureau Veritas standards.
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.
“From day one, this project has been characterised by a good working atmosphere and team dynamics, so there were quite a few high-fives on board when we had successfully completed the tests!”