Thruster Propulsion Systems
Typically these thrusters are rated at 2-3 MW/unit for continuous operation and built satisfy the requirements of various classification societies and authorities. Available bevel gear technology allows thrusters of about 6000 kW to be built.
To introduce thruster propulsion in a bulk/container vessel design is more of a conceptual novelty than a technical one.
In a Rotatable Thruster the following functions are generally included:
S A propeller system including a Controllable Pitch Propeller (Fixed Pitch optional)
S A power train including necessary bearings, shaft seals and shaft speed reduction gear(s)
S A step-less thrust magnitude / direction control through the CP-system
S A rudder /steering gear function through the azimuth control.
S A transverse thrust control by the option of selecting a transverse setting (90°) of the thrust direction.
S An optimized astern thrust by the option of turning the unit 180° (depending on the stern design)
S An integral remote control for required horizontal ship movements. Combined control of propeller pitch and engine RPM and / or constant RPM operation are available.
S Automatic engine load / overload control is an added standard feature whose function is based on true engine load sensing and pitch correction. Interface for an autopilot is easily arranged.
Hydrodynamic Performance
A new propulsion system should have approximately the same efficiency as the conventional one. In the case of thruster propulsion the position of the unit in the stern is normally further aft when compared with a conventional propeller. In general this allows for tie adoption of a relatively larger propeller diameter. Furthermore, the introduction of a High Skew propeller blade design allows for smaller tip clearance and larger propeller diameter. The above increase in diameter will, in combination with an optimized shaft speed, increase noise efficiency.
Two alternatives with regard to the installation concept are available:
Controllable Pitch (CP-type) thruster in combination with skeg
Traction thruster (CP - or Fixed Pitch - type)
Machinery Arrangements
The thruster propulsion offers a great flexibility regarding the propulsion machinery:
1) The propeller unit includes one built-in bevel gear. An upper bevel gear may be added. The gear ratio 'of these gears can be selected to give a total speed reduction from the engine to the propeller ranging from 3:1 to 18:1 (depending on the torque situation); because of this it is possible to select an optimum propeller shaft speed in combination with any practical r.p.m. of the power plant (diesel, electric motor etc.)
By choosing suitable length of the couplings / intermediate shafts between the engine-upper gearbox propeller unit, it is possible to locate the power plant at the most suitable position, both vertically and longitudinally.
The upper gear can be arranged for single, twin and triple input. When a CP-type thruster is installed, one can make full use of constant speed driven auxiliaries connected to the engine or gearbox power take-off shaft(s).
Diesel engines are natural for most ships. If, however, the vessel has a high demand on electric power capacity (dredgers, crane vessels, maintenance / service vessels, Hotels (floating hotels), etc., diesel-electric drives may be an attractive solution.
The simplest inexpensive thruster drive is arranged through simple non-synchronous a.c. motors with relatively high shaft speed in combination with a CP-type thruster. t* space allows, the motors can be direct-coupled to the thruster input shaft.
The thruster propulsion system is most suitable for remote control and remote monitoring (i.e., unmanned engine room operation) as the number of control areas are minimised when compared with the conventional ship's propulsion / manoeuvring system.
Thruster Machinery and Propeller Concepts
Basically two options for the machinery concept are available: diesel-mechanical or diesel-electric drive. Several factors and conditions influence the choice of a concept. Optimal overall operational economy is of course the design goal for any system. The diesel-mechanical machinery is a natural and cost efficient system for many applications.
However, depending mainly on the operation load profile and the demand for auxiliary electric power, the diesel-electric drive may also be an attractive solution. As discussed above, installation aspects and flexibility with regard to hull disposition may also be advantages of the electric machinery. The penalty of the diesel-electric system is the higher cost and higher transmission losses.
A multi-diesel generator-set power plant with its power management system provides for •primal operating conditions for the engines. This allows for operation with a higher efficiency close to the design conditions for diesels and generators. This in turn is advantageous with regard to wear and maintenance. One further advantage, of growing importance, is that the exhaust emissions can be minimized.
The rotatable thruster can be of the FPP type or the CPP type. Depending on the type of machinery, the main alternatives for the combination of machinery and propeller-type are as shown in Figure 17.19.
Diesel - Mechanical Drives
For a diesel-mechanical drive, CPP and FPP are possible. In the case of a fixed pitch slier, power control is accomplished by varying the engine shaft speed. Thrust reversal is rained by engine reversal or by turning the thruster 180°. If the thruster operates behind a skeg, turning is not possible and thus engine reversal is necessary. In the case of CPP, thruster power and thrust control is achieved by pitch control. This facilitates rapid and smooth thrust reversing without engine reversal or turning of the thruster. This means improved manoeuvrability and reduced stopping / acceleration time and distance.
A CPP also allows for constant shaft speed operation and the use of constant-speed-driven auxiliaries connected to the engine or gearbox power take-off. If reversal of the propeller rotation is avoided, installation of high skew blades is possible, which will provide for low-pressure pulses and hull vibrations.
Diesel-Electric Drives
The simplest and least expensive system is arranged through constant speed non-synchronous a.c. motors with a relatively high shaft speed in combination with CPP-type rosters. Depending on the operation load profile, two-speed motors could also be an interesting alternative. If operating for long periods at low power this may give the best economy.
For the FPP thruster, variable speed motors are required for power control. Different types of electric systems and concepts for shaft speed control are available. However, big interest is being shown for a system based on frequency conversion and a.c. motors. The main advantage of the variable speed system is improved fuel economy at fractional loads. However, the investment cost of the electric equipment is high compared with the extra cost of the CP propeller. The rather complex electronic equipment requiring expert knowledge and further training of personnel should also be considered.
Hydraulic motors are used for the steering of luxury cruisers. The diesel or turbine-electric drive system has an approximately 15 to 25,000 HP electrical motor built the actual propeller housing. The steering works in such a way that the entire propeller can be turned right round, i.e. through 360 degrees, driven by hydraulic motors. This means that the conventional rudder has been completely replaced.
To summarise, each specific ship requires an extensive evaluation of the above discussed and other conditions in order to arrive at the optimal machinery-propeller concept.
Reliability Service and Maintenance
As mentioned above, the rotatable thruster concept minimises the number of systems facilitates integration of functions. This makes the thruster propulsion machinery most suitable for remote control and remote monitoring (i.e., unmanned engine room operation).
It also seems logical to assume that the reduced number of components and separate systems will improve and augment system safety. If more than one thruster is installed, j system redundancy is obtained as independent operation of the thrusters is normally provided for. Integration of functions and a reduced number of systems will also simplify and reduce the amount and cost for service and maintenance work. The work to be carried out on board by the crew of the vessel is reduced which makes reduced manning possible. In case of complicated service procedures, specialists from suppliers or other land-based personnel could be utilised.
When including containerisation of the thrusters, and in the case of lightweight power units, the maintenance work can be further rationalised. In that case the lifting ashore of j engines and thruster units can be arranged in order to carry out major maintenance work. In I the case of multi-thruster/engine installation it may be economical to invest in spare units for quick exchange. Lifting can be facilitated by on board or land-based lifting gears.
If containerisation of the thruster is adapted, it is possible to inspect / maintain the outboard parts without dry-docking the ship. A need for dry-docking will only appear when the ship's bottom needs reconditioning. Since the need for dry-docking is considerably reduced, the vessel could safely operate for prolonged periods in areas with a minimum of yard / docking facilities.
Thrusters for Booster Propulsion of Existing Ships
Existing ships may for several reasons need additional propulsion power and / or improved manoeuvrability. In the first case exchange of the existing engine is one solution which, however, will often require a new gearbox (where applicable), shaft line with bearings and a propeller.
An alternative solution is the installation of one or two thrusters with propellers specially bid out for propulsion. As the original installation normally includes sufficient means for Bering in the free running condition, the additional thruster can be arranged for a fixed mounting, i.e. without steering gear.
Figure 17.20 shows the principal arrangement of additional thrusters for existing single and twin-screw vessels respectively. In both cases either a thruster mounted behind a skeg or thruster of the traction type with a guide-vane-shaped stay are solutions.
Figure 17.20 - Existing Single Screw with Additional Booster Thrusters
Note: there could also be two existing screws with one additional booster thruster.
Figure 17.21 shows the required propeller thrust versus ship speed and the delivered thrust versus ship speed of existing propeller equipment and additional booster thrusters. From the diagram the speed gain obtained from additional booster thrusters can be determined. Of course the speed increase greatly depends on the resistance curve of the ship in question. A typical 20% power increase could give a 12-14% thrust increase and a 5-10% increase of the ship’s speed. The choice of the prime mover for the booster thruster will depend on the overall situation of the main engine auxiliary power for the vessel in question. The most straightforward way of powering is the installation of a separate high-speed diesel. With suitable clutch arrangements the same diesel can be utilized for driving an alternator in case electric power is in demand.
Figure 17.21 - Speed Gain from Additional Thrusters
A diesel-electric drive by a simple a.c. motor is another possibility provided sufficient electric power is available. If there is a demand for improved manoeuvrability during docking / undocking the thruster should include a steering gear enabling the direction control of the thrusters in at least a transverse direction. Examples of installation of rotatable thruster are Danish single-screw ro-ro (roll-on-roll-off) ships in which two rotatable thrusters have been installed. The reason for this conversion was the demand for higher speed and improved manoeuvrability.