The Azipod turning shaft is located in the place of the old rudder vertical shaft. This allows full azimuthing angles and enough clearance to the baseline as well as sideways. The Azipod attachment to the ship hull is carefully designed according to the model scale tests and full scale experience. The synchronous electric propulsion motor design values, power and torque curves were kept identical to the sister vessels although savings in propulsion efficiency were expected.
3.2.1 Propellers
The fixed-pitch propeller diameter is 5,2 meters, as on the sister vessels. These propellers are the most powerful pulling propellers ever built. New operation modes created new challenges for the design. The task was successfully performed by making detailed hydrodynamic and FEM-calculations and by model scale tests.
The hydrodynamic optimization procedure resulted in inward rotating propellers. The Azipod was slightly inclined (six degrees) downwards in order to give a good inflow for the propeller. The results based on model scale tests estimated a several percentage improvement in propulsion efficiency.
3.2.2 Steel structure
The structural dimensioning was based on two basic load conditions: max. continuous loading in normal service and extreme loading (abnormal operation, e.g. if the control of the Azipod fails). The key design point was to adjust the dynamic behavior. The excitation forces and moments were known from earlier projects. Excitation level is low due to the good wake field. The steel structure of the Azipod and ship hull was dimensioned so that the resonances were avoided in critical areas and in full powers. A special emphasis was placed on the Azipod attachment to the hull.
3.2.3 Steering system
The steering of the Azipod units is done by an electro-hydraulic steering system. A total of two to four hydraulic motors give the pod sufficient turning speed and redundancy. Steering logic had to be rethought e.g. the stability of the Azipod units, steering angles versus ship speed, power limitations and crash stop characteristics. Ship behaviour calculations during extreme maneuvering were done. The black-out situation was analyzed: Azipod system behaviour, dimensioning of the emergency network and starting sequence of the steering motors.
Also the redundancy was studied in order to prevent any failure in one pod to stop the other. The Azipod units are mechanically, electrically and hydraulically independent.
3.2.4 Layout Modifications Onboard the Elation
Changes in the layout were kept to a minimum, except for the Azipod itself. The propeller motor room, no longer needed, actually offers new design possibilities because it makes available an additional 1200 m2 of space. This space is as wide as the ship (32 m), 20 m in length, and two decks high, the height of the propeller motors. In the case of the Elation, the room was excellent for additional waste handling equipment:
An incinerator and a gray water treatment plant were installed in the propeller motor rooms of the original design and the old shaft tunnels were converted into fresh water tanks. Changes in the lay-out of the machinery spaces were minimal compared to the sister vessels. This was an advantage to the shipyard and the owner.
3.3 Verification of design data in full scale
Propulsion efficiency
The following results were recorded during the sea trial in December 1997 in the Gulf of Finland. The increase in propulsion efficiency compared with the existing Fantasy class ships was 8%. The hull lines are the same on the Elation as on the previous ships in the Fantasy-series. The only changes were the local modification around the Azipod units and the closing of the stern holes for the thrusters.
Later the owner has found out that the real fuel savings on a rote that has been operated early with a sister ship are over 10% on week's cruise.
3.3.1 Maneuverability
The maneuverability of a ship is best demonstrated by its full speed turning circle. The diameter of the turning circle of the Elation was about 30% smaller compared with the previous Fantasy-class vessels. The ability to turn the ship fast gives the master a better margin for maneuvering in tight situations and increases the safety of the ship.
The other important feature in a ship is its crash stop performance. The test was performed by reversing the propellers. An additional safety feature of a ship with Azipod units during a crash stop, is that the ship can be steered towards the desired stopping point.
3.3.2 Passenger comfort - vibrations
Reduction in noise and vibrations were also observed during the sea trials. This is mainly due to the very good wake field of the pulling propeller and resulting reduction in pressure pulses from the propeller to the hull. The passenger will observe the biggest difference in the confined area operations and during harbor maneuvers. The absence of the stern thrusters and the rudders makes a big difference in passenger comfort.
4. THE CONTRA ROTATING PROPELLER (CRP) CONCEPT
The Contra Rotating Propeller (CRP) concept has been widely studied and used in a variety of vessels. The idea of applying the CRP concept and an electric podded drive or azimuthing thruster in place of a rudder is not new although it has never gained popularity. The development of the electric podded propulsion systems and experience of pods at higher vessel speeds give new possibilities in utilizing the concept in a wide range of vessel applications.
The main benefits of the concept are:
・ Fuel economy
・ Propulsion redundancy
・ Maximum propulsion power capacity of a conventional single shaft hull form
・ Manoeuvrability
・ General arrangement of the machinery areas
・ First cost
・ Life cycle cost
The popularity and acceptance of electric propulsion systems for ships has been accelerated by the introduction and success of the podded propulsion concept. First installations of the electric podded propulsion systems took place in the early 1990's. In ten years the operating experience has proven the concept to be ideal for a variety of vessels.
