Podded Propulsor - an Overview - Essay Example

?Podded Propellers 508581) Introduction Marine Propulsion systems have been making giant strides in new designs that are capable of working at optimum efficiency at the same time generating good speeds. Since there are a number of products like heavy cargo and crude oil that can only be transported via sea, the need for efficient sea transport shall always be in demand. Source: King Julia, Journal on Technology, Viewed on 10th March 2011 Conventional Propeller and Steering Arrangement The early propulsion systems followed the propeller shaft with a conventional propeller having a series of blades fixed onto its hub. The work done by the diesel engine caused the main engine shaft to rotate. This propeller shaft which was geared to the main engine shaft subsequently rotated causing the propeller blades to swirl in a circular fashion. The momentum created by this swirling of water towards the rear end of the ship generated a forward thrust helping the ship to forge ahead. This water thrust at the rear end of the ship was made to flow across a rudder plate kept a certain angle; the angle fixed from atop the bridge by the ships navigator. This thrust achieved was at a particular angle thus helping the ship move ahead in the required direction. (Mc George H.D, 1995) Most of the marine ships use diesel engines with low power to weight ratio. Hence the power generation is intermittent; the flywheel attached to the propeller shaft ensuring uniform rpm. Another area of great concern for conventional propellers is the challenge to overcome cavitation. The presence of cavitation limits the ship speed to 35 knots. Increasing the speed of the ship creates low pressure areas around the propeller leading to the creation of small bubbles around the propeller. The bubbles on collapsing around the propeller cause a type of pitting corrosion on the propeller surface.( Ying Xiong, 2008) Apart from this, the defects on the propeller surface caused the ship in experiencing minor vibrations and noise. The efficiency of the propeller also reduces. Podded Propellers Podded Propellers are hubs with slightly larger diameters located at the rear end of a ship. This design incorporates both the manovereability and the propelling functions. The propulsion is achieved by the fixed pitch propeller that is rotated by an electric AC synchronous motor fitted inside the pod. The rotation of these blades arranged in pusher or tandem configuration generates the required momentum. The steering on the other hand is achieved by rotating the pod by means of slewing gears that are attached to the hull. Rotation of the pod deflects the flow of water to a new direction thus achieving change in direction. (Beijerinck H.C.W & Terwisga T.J.C, 2006) Since the pod can be rotated about its mounted axis, it achieves thrust capabilities in any direction. This therefore facilitates the forward motion, reverse motion and the rotation of the vessel about a small turning radius with the same efficiency. A propulsion pod consists of Fix pitch propeller, Electric AC synchronous motor, Exciter, Propeller shaft, Propeller shaft thrust and support bearings, propeller brake, Propeller shaft seals, Bilge pumps and the control equipment on the inside. The electric motor that is used in pod is stimulated by a frequency converter which is capable of developing full torque in either direction across the entire speed range. (Azipod Propulsion, 2002) Source: ABB, Viewed on 10th March 2011 This facilitates in fixing the speed of the propeller for different liquid hydrodynamics and therefore achieving optimum propelling efficiency. The outside part consists of the Steering system, lubricating oil equipment and the ventilation unit. The first series of propulsion pods were equipped on M/V ‘Seili by ABB in 1990. (Azipod Propulsion, 2002) Advantages Podded Propellers offer the following advantages over the conventional Propellers. 1. The degree of automation achieved is higher and chances of mechanical upsets are reduced due the system employing electrical energy. 2. The exhaust gas emissions when compared to conventional systems are lower. 3. There is lot of flexibility in this type of arrangement since the podded propeller is capable of turning around 360 degrees. Thus a certain degree of torque can be attained in any direction at any particular point of time. (King Julia & Ritchey Ian, 2011) 4. Due to the non requirement of reduction gears a lot of weight and wear and tear is reduced. 5. This system offers increased hydrodynamic efficiency which directly affects the quantity of fuel consumed. A saving is thus made on fuel consumption. Source: ABB, Viewed on 10th March 2011 6. This system offers reduced noise and vibrations thus proving a boon to sea life and the environment in general. 7. Compared to conventional propellers ships these have exceptional manoeuvring capabilities. Ships run by conventional propellers tend to be berthed or docked onto the shore using tugs that direct it into the harbour by using ropes tied onto either side. This is because conventional ships have limited navigability in crowded areas offering low turning radius. Thus podded ships or tugs have superior berthing qualities.( King Julia & Ritchey Ian, 2011) 8. There is a lot of space saved in the general arrangement of the podded propulsion system when compared to its counterpart. ((Raynor S.J, 1998) 9. Construction of the podded propeller in the shipyard is considered easier compared to the conventional propeller. 10. Crash distance is the distance the ships travels after impact with the entire momentum of the ships propellers in the opposite direction to minimise the damage. Ships employing conventional propellers have the major disadvantage that the crash distance is more compared to podded propellers. (Focus on Propulsion Pods, 2004) 11. Cruise ships that employ propulsion pods are more comfortable for the passenger to travel. Disadvantages The Propulsion pods have a few limitations which makes reluctant ship owners adopt this technology. 1. The initial cost of setting up this propulsion system is quite high. 2. The electric power that needs to be generated to run this system and the power losses that are subsequently incurred is high. 3. The power achieved per screw is around 30MW which is lower than that achieved from a conventional propulsion screw which is around 70MW. (Focus on Propulsion Pods, 2004) 4. Fitting this system on a two stroke diesel powered engine is not ideal since the propulsion efficiency obtained is very low. 5. The thrust and support bearings are areas that have a tendency to fail over a period of time. 6. The other areas include the shaft seal. Failure of this seal would cause sea water to seep into the system and cause great damage.( King Julia & Ritchey Ian, 2011) 7. Lubricating oil has a tendency to get contaminated by the oily waste and sea water. 8. Most of the propulsion pods use roller bearings which are highly susceptible to vibrations and inefficient lubrication. 9. Since these pods are located in areas that are inaccessible by human personnel any damage to the electrical circuitry within the pod when the ship is at sea renders the propulsion system incapable of repair. 10. Any repair that would be needed can only be achieved during dry dock. Most of time the repair of the propulsion pod would require the removal of propeller shaft from the pod. Since this a highly sensitive system, the work needs to be performed by highly skilled engineers who would be brought in from the technical team of the parent company. (Focus on Propulsion Pods, 2004) Innovation in Podded Propeller Design- SCHOTTEL SIEMENS SSP New Innovations are currently being undertaken in increasing the efficiency of the Pods. The Scottel Siemens have come up with the following advanced features (Focus on Propulsion Pods, 2004) 1. The use of a permanent magnet has done away with the use of exciter required for generating a magnetic field. This has helped in saving space and increase body shape. 2. Air cooling of the permanent magnet rotors has been replaced with fresh water cooling. 3. The rotor is attached to the propeller shaft by two lamellar couplings. This prevents any kind of axial stress on the bearings from the proper shaft since there is sufficient room for expansion of the rotor. 4. Bearing failures in previous design has led to the provision of two support bearings and two thrust bearings. 5. Fins have been placed between the two propellers which absorb the swirls from the first propeller and prevent tangential velocities affecting the performance of the second propeller. 6. The tandem propeller arrangement is an arrangement of two propellers in line on the same shaft with same speed and rotational direction. Scottel Siemens makes use of this technology which imparts increased efficiency to the ship. Conclusion The efficiency of the propulsion pods when compared to the conventional propulsion systems is a matter of debate. However there is no doubting the fact the navigation capability of the ships with podded propellers is quite high. The propeller pod variants that are presently available in the market are 1. ABB AZIPOD 2. ROLLS ROYCE – ALSTOM MERMAID 3. SCHOTTEL- SIEMENS SSP 4. STN – WARTSILA DOLPHIN Research is being carried out into evolving more efficient pods that could have a controllable pitch along with increasing the robustness and reliability of the entire propulsion system. Reference Lists 1. Azipod Propulsion, 2002, Azimuthing Electric Propulsion Drive, ABB, Available at:http://www.abb.com/marine [Accessed 10th March 2011] 2. Beijerinck H.C.W & Terwisga T.J.C, 2006, Model-scale Podded Propellers for Maritime Research, Maritime Research Institute Netherlands 3. Focus on Propulsion Pods, 2004, Studio di Ingegneria Navale e Meccanica, Available at:http://www.sinm.it [Accessed 10th March 2011] 4. King Julia & Ritchey Ian, 2011, Marine Propulsion-The transport technology for the 21st Century, Journal on Technology 5. Mc George H.D, 1995, Marine Auxilliary Machinery, Butterworth Heinemann 6. Raynor S.J, 1998, The Benefits of Podded Propulsion in the Off shore Market, Thrusters and Drive Systems. 7. Ying Xiong, 2008, Prediction of Podded Propeller Cavitation using an unsteady surface panel Method, Science Direct, Journal of Hydrodynamics Read More