The ferry has two Azimuth Rim Driven Thruster (RDT) units developed and delivered by Brunvoll AS, one unit at each end. It is believed that these are the first ever main propulsion rim driven azimuth thrusters installed on a ship for commercial service.
The thrusters have variable speed by applying a true electric shaft where the thruster speed is controlled directly by the speed of the diesel generators. This is a drive solution developed and delivered by Inpower AS and is also a first of its kind.
Several development tasks
The concept of Rim Driven Thrusters has attracted a significant interest in the marine industry for a wide range of different applications. Brunvoll has previously delivered several pioneering rim driven tunnel thrusters.
The project for Fjord1 MRF has offered new opportunities for the further development of azimuth rim driven thrusters for main propulsion.
For Inpower AS the project with Fjord1 MRF is decisive for the validation and commercialization of the electric shaft system.
The project has been supported by Innovation Norway.
M/F «Eiksund» is an older ferry, 49 m long, and a capacity for 28 car units. Before the rebuild she had one main engine capable of driving two controllable pitch propellers, one at each end.
New double ended ferries are commonly equipped with azimuth thrusters, with either a mechanical shaft system or a diesel or gas electric drive system.
Compared to a direct driven propeller increased transmission power losses from the prime mover to the propellers are inherent in these designs. For a diesel or gas electric system there are power losses in the generator, frequency converter, electric cabling, thruster motor, and in the thruster unit.
However, azimuth thrusters are still preferred due to several other reasons like redundancy from two independent propulsion systems, improved manoeuvrability, improved arrangement solutions in terms of location of machinery spaces, flexibility in selection of engines, as well as flexibility to meet various operating conditions in terms of load profile on the engines.
The new Azimuth Rim Driven Thrusters and direct speed control with electric shafts offer additional attractive perspectives with regard to:
- System simplicity and associated reduction in investment and maintenance costs
- Minimal space requirements
- Improved propulsion efficiency
- Further improved manoeuvrability
- Reduced noise and vibration levels
Retrofitting a novel propulsion system
The main components of the retrofitted system are:
- Two resiliently mounted diesel generators. The generators are Permanent Magnet (PM) machines.
- Two azimuth Rim Driven Thrusters complete with steering machinery, ancillaries and control, alarm and monitoring systems. Two independent control stations were fitted on the bridge.
Testing and verification
Extensive testing has been carried out to validate the new propulsion system quantitatively and qualitatively.
- Reduced power requirements and fuel consumption for comparable conditions with similar load distributions between the aft and forward propeller, even when compared to a direct drive solution.
- Improved manoeuvring capabilities
- Excellent course keeping stability
- Excellent crash stop capability
- Extremely low noise and vibration levels from the propellers
- It has been demonstrated that the thrusters may be operated throughout the complete speed range from idling speed just by varying the speed of the connected diesel engine.
Environmental and Operational Gains
By replacing old and polluting diesel motors from its fleet, Fjord1 MRF actively contribute to reduced emissions.
In addition, for a ship speed in the same range as the maximum speed before rebuild, and at otherwise comparable operating conditions, the power requirements are in the same range or lower after rebuild. This implies that increased propeller efficiency has more than counterbalanced the increased transmission loss of a diesel electric system.
The technical director of Fjord1 MRF, Mr. Glenn Sandberg states, “The sea trials have confirmed that the new propulsion system meet our expectations. This includes reduced fuel consumption, improved manoeuvring capabilities and noise and vibration.”
The ferry will now go into commercial traffic. This will give valuable operating experience for a cutting edge project realised through cooperation between companies in a vivid Norwegian maritime cluster.