The propulsion system is an integral part of ships with shafted propellers being the most popular configuration, as it is effective over a broad range of applications, despite all the hindrances mentioned in my previous post. Nonetheless there are many alternatives introduced over the years, attempting to improve the efficiency and limit the negative aspects.
In the mid thirties Kort nozzle, or ducted propeller, was presented which was quickly adopted by tug boats as it is highly efficient at lower speeds, provides an increment to the bollard pull of around 30%, adds to the operational stability of the vessel and simultaneously lowers the vulnerability to debris. Further more it is possible to eliminate the rudder, when the Kort nozzle has the ability to rotate and thrust in different directions. On the down side, all aforementioned benefits are diminished when the speed goes over 10knots, where it is also very susceptible to cavitation.
Over the last decade advances in technology of capacitors and their applications have developed a variation of the ducted propeller, often referred to as rim thrusters, although main players in the field, like Rolls Royce and Voith, have their own patented designations, namely Permanently Magnet thrusters and Voith Inline Propulsor respectively. The concept has a nozzle equipped with motor windings comprising the stator, while the rim holds permanent magnets inducing the rotation. The propeller inside the rim is creating the thrust. The system is using electric power to activate the windings and is controlled by frequency converters, matching input to the required output.
It is novice approach on a proven system, with many advantages.
- As mentioned in the previous paragraph, it is controlled by frequency converters, therefore power produced equals requirements, effectively regulating energy consumption;
- As it is powered from electricity, it is open to alternative energy storage methods, i.e. it has the potential to be extremely environmentally friendly;
- Compared to the shafted propeller configuration, it takes very little machinery space as the whole azimuth unit is outside of the hull;
- It is directing the thrust 360°, therefore rudder and respective installation is also excluded, adding more space to the payload;
- The thrust produced at 0° and 90° is the same, improving manoeuvrability and facilitating dynamic positioning for the ship;
- Gear losses are eliminated, while the thrust overall is improved, with reported 7% added efficiency compared to a more conventional system. Surely this value is given from the manufacturers, but there are projects also validating this proposition. For example “Gunnerus”, an ocean research vessel of the Norwegian University NTNU that changed from a shafted propeller to a permanent magnet from Rolls Royce states that 1knot as added across the whole range of the power generation;
- One significant headache of the shafted propeller is the vibration transmitted to the ship’s structure. By excluding the shaft, theoretically the source is excluded, yet the phenomenon is comprised by multiple parameters and it is not completely absent. Experience has proven that it is reduced;
- Moreover structural borne noise should also be minimised, as vibration levels drop, yet on “Gunnerus” similar noise readings are reported;
- As both the nozzle and rim are watertight, repairing them doesn’t require dry-docking;
Indeed there are numerous advantages that support further adoption of the system, yet it is not free of challenges that have to be taken into account.
- Smaller units, up to 0.5MW, are widely available and been tested for some time. Larger ones have been introduced more recently in various application, especially from Rolls Royce and Kongsberg, the latter only as a tunnel thruster;
- Smaller units need no cooling system, as the main source of temperature are the motor windings that are cooled from the sea, which is projected as extra machinery space. Larger units though require cooling arrangements;
- Normal industrial motors operate commonly at 50Hz, yet larger rim thrusters units operate between 100-300Hz, necessitating custom feed;
- As mentioned in the introductory part, Kort nozzle has been most advantageous below 10knots and it is not clear whether the same applies for this configuration. In the example of “Gunnerus”, service speed is at 9.4knots with maximum at 12knots, implying that we are looking at the same range of speeds;
- Very much like pods the whole unit protrudes below the hull, introducing a drag that is responsible for cyclic loading on the structure. So there is concern whether rim thrusters will also be susceptible to units lost during a voyage, as do pod installations. Off course forget a single thruster installation, under these conditions;
- Last but definitely not least, there is also a financial aspect to consider, as the owner would be looking into double the investment for them. This has to be justified in sufficient return at an acceptable timeframe;
I personally like the magnetic applications, believe in their potential and would love to see further expansion of this system, or modifications of other system with similar principles. The more we try out, more concepts will be build and develop a reliable mean to have our ships propelled in an efficient and more environmentally friendly way.