Analysis Of Using Podded Propulsors

Over the past few decades the concept of podded propulsion in merchant ships has gained significant acceptance in the merchant marine industry. Mostly due to the fact that podded propulsion systems offer several advantages in contrast to the conventional, shaft drive arrangement systems. The benefits we will refer to are mostly maneuverability, lower noise generation and more available space for cargo in merchant vessels. However before discussing the advantages and disadvantages of a podded propulsion system we must first explain what is meant by “podded propulsion” by briefly analyzing its mechanical components and how they interact with the vessels water resistance and thrust efficiency. Nonetheless in order to fully realize the benefits provided by a podded propulsion system, we also need to briefly examine several aspects of conventional propulsion systems such as conventional single and double screw propulsion systems, systems using fixed pitch propellers, contra-rotating propellers, overlapping propellers and controllable pitch propellers which are most commonly used today in the propulsion of merchant and navy vessels.

Podded Propulsion Systems

A podded propulsion system refers to a gondola shaped pod located at the stern of the vessel mounted on a leg which is capable of rotating in a 360 degrees angle. Inside the pod there is an electrical motor capable of producing thrust by using a fixed-pitch propeller. The entire system is azimuth since there is no need for a rudder or any other steering arrangements (Appendix 1,2). This type of propulsion system was created in the late 1980s by the Kvaerner Masa-Yards in cooperation with the ABB Stromberg Drives and the Finnish Maritime Administration. The first podded propulsion system was mounted onboard icebreakers such as the 16.000dwt M/T Uikku and M/T Lunni tankers. These type of vessels used the podded propulsion system in order to navigate through the North-East Passage in extremely harsh conditions by moving backwards with the propeller first, thus breaking the ice with a considerably higher efficiency than before.

Today there are several types of podded propulsion systems available, some of the most commonly used are the Mermaid unit (Appendix 3) constructed by Kamewa and Cegelec, the Dolphin unit (Appendix 4) which was produced by John Crane Lips as well as STN and the SSP Thruster (Appendix 5) which is a product of Siemens and Schottel companies. The Mermaid unit uses a low emissions gas turbine in order to power the propeller which acts as a tractor unit located in front of the pod, this type of propulsion configuration allows an optimum undisturbed water inflow to the propeller thus increasing propulsion efficiency and decreasing vibration and noise. The Dolphin unit operates by integrating a powerful electric drive into a hydro-dynamically optimized pod below the stern of the vessel resulting in a directly driven propeller. The pod is capable of rotating 360 degrees thus ensuring maximum maneuverability while the power of this unit ranges from 3 MW to more than 19 MW, thus this type of propulsion configuration is suitable for a wide variety of commercial merchant vessels. The SSP Thruster uses two twin propellers one in front of the unit and one in the stern, rotating in the same direction powered by a permanent magnet motor. The propeller located at the stern of the pod uses the rotational energy produced in order to provide thrust by using a pair of hydrofoil fins angled away from the pod thus increasing efficiency of the unit while allowing the same maneuverability as the other pod units since the whole system is capable of rotating in a 360 degrees angle.

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Conventional Propulsion Systems

Since the beginning of the development of merchant vessels, for the transportation of different types of products the propulsion systems created played a significant roll in the advancement of the maritime industry. Over the past two centuries many propulsion designs were created in order to further improve the speed, fuel consumption and efficiency of merchant and navy vessels.

Today the most conventional design propulsion systems for merchant vessels consist from a shaft, which directly drives different types of propellers while it is connected to a gearbox. Power is generated mostly from a high torque diesel engine while a rudder usually located in front of the propeller at the stern of the vessel provides the steering. Research has shown that the use of different types of propellers can produce different results depending on the type of the vessel they are used.

Fixed pitch propellers (Appendix 6) created the basis of propeller production in either its mono-block or build-up form. Mono-block propellers are most commonly used today while they cover a broad spectrum of design types and sizes. They can range from those weighing only a few kilograms to those, which can weight around 130 tonnes. Their designs can vary since the blade number of the propeller can be anywhere between two and seven blades. These kinds of propellers are used to provide propulsion for container vessel, bulk carriers and even high-speed patrol crafts. They offer several advantages in order to solve a variety of propulsion problems such as cavitation effects and excessive noise regeneration.

In contrast to the fixed pitch propellers the controllable pitch propellers provide an extra degree of freedom since they can change the blade pitch. Even thought this type of propulsion system configuration is sensitive to cavitation problems it can provide significant advantages of maneuverability since fine thrust control can be achieved without the need to accelerate the propulsion machinery. Controllable pitch propellers are mostly used in passenger ships, ferries and general cargo vessels, which require frequent berthing maneuvers.

Furthermore to the propulsion configurations we discuss above the use of contra rotating propellers (Appendix 7) can provide several advantages to the merchant marine industry. Contra rotating propellers compromise two coaxial propellers sited one behind the other while they rotate in the opposite direction. The contra rotating propulsion system provides the hydrodynamic advantage of recovering a part of the slipstream rotational energy, which would otherwise be lost. Additionally contra rotating propellers are able to balance the torque reaction from the propulsor. This kind of propulsion configuration is most commonly used in small high-speed outboard units while the use of this kind of propulsion system is not efficient for large merchant vessels since large vessels equip a large shaft line.

Finally the last conventional propulsion system we will discuss involves the use of overlapping propellers (Appendix 8). This system uses again two propellers although in this case the propellers are not mounted coaxially but are each located on separate shaft line systems with the distance between the shaft centerlines being less than the diameters of the propellers. The advantage gained from this type of configurations is to increase propulsion efficiency by achieving as mush benefit as possible from the low-velocity portion of the wake field. Despite the fact of improved propulsion efficiency this kind of configuration may increase the levels of fluctuating thrust and torque thus more research and development must be done before it can be used on large merchant vessels.

Advantages and Disadvantages of using Podded Propulsion

Since we have briefly examined the mechanical characteristics behind podded propulsion and conventional propulsion systems it is time to discuss several advantages and disadvantages of using podded propulsion in the operations of merchant vessels.

As yet, studies have not shown whether pods are more efficient than conventional shaft lines. There has been much research on the subject, but most studies have been aimed at a specific aspect of pod performance instead of an overall efficiency review. Several advantages have been attributed to pod propulsion systems, such as: reduced emissions, lower noise and vibration levels, improved steering maneuvering, and braking capabilities. The reduced number of component parts also allows for more flexibility in arranging system machinery, more efficient construction and improved shipyard logistics. On the opposite end of the argument, pods require a greater capital investment, have a 30MW power limitation (per screw), and have been known to suffer losses in power due to the electric propulsion.

Specifically by using podded propulsion merchant vessels gain a significant advantage due to the fact that they can achieve an increased space capacity in order to fit more cargo mainly because the engine can be allocated more freely thus decreasing the size of the engine room. This advantage plays a crucial role for todays shipping companies, which depend mainly in increasing their deadweight tonnage in order to reduce their costs by achieving economies of scale. Moreover another crucial advantage of using podded propulsion seems to be the environmental benefits realized not only from reduced gas emissions but also due to the decreased fuel consumption thought good hydrodynamic efficiency. Since fuel consumption is a significant factor when calculating the vessels costs, it is of high importance that the vessel is capable of covering great distances at competitive speeds with as less fuel as possible. Even though today most berthing terminals and major ports invest in increasing the size of their facilities in order to accommodate the larger merchant vessels, maneuverability plays an important role in order to avoid delays and unnecessary costs due to the use of tug assistance. Research has shown that using podded propulsion can significantly increase the maneuverability of even the largest vessels, since the pod is able to rotate in a 360 degrees angle and thus provide thrust in every direction.

Despite the advantages we mention above podded propulsion suffers from several disadvantages. The most crucial disadvantage seems to be that the capital costs associated with the production and use of pods, are a lot higher than the capital costs associated with the use of conventional propulsion configuration. This is one of the main reasons most merchant vessels are not equipped with podded propulsion. Furthermore another important disadvantage seems to be associated with the power production of podded propulsors. Due to the fact that pods have a 30MW power limitation and they depend on electric propulsion, sufficient speed may not be achieved while power losses due to the electric propulsion may be experienced.

Finally it is not easy to realistically judge the suitability of pod propulsion for different ship types based on these advantages, disadvantages and limitations. However if we examine more closely reports based on the efficiency of podded propulsion we can derive that pods would be more efficient, fitted in cruise and liner vessels using twin screw configurations and even some small bulk carriers and general cargo vessels, while on the other hand they would not be suitable for large container vessels and tankers which require high power outputs.

Conclusion

In this essay we provided the reader with a comprehensive analysis of podded propulsion systems as well as conventional propulsion configurations. We examined several benefits created from the use of both types of propulsion systems, while we furthered analyzed the podded propulsion configuration by also providing several disadvantages experienced in merchant vessels, in order to conclude whether or not podded propulsion systems will be well suited in the operations of todays merchant ships. Finally reports has shown that even thought there is still a lot of research and development to be done on podded propulsion systems their use on merchant vessels would be more extensive in the future.

Appendix

Appendix 1

Source: “Azipod® XO The new generation Azipod® takes podded propulsion to a new level”

www05.abb.com/global/scot/scot293.nsf/…/azipod%20xo_2010.pdf

Appendix 2

Source: “Electrical systems in pod propulsion”, Master of Science Thesis of Electric Power Engineering, Lena Bergh Ulrika Helldén,Department of Energy and Environment, Division of Electric Power Engineering, CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden, 2007

Appendix 3

Source: “Electrical systems in pod propulsion”, Master of Science Thesis of Electric Power Engineering, Lena Bergh Ulrika Helldén,Department of Energy and Environment, Division of Electric Power Engineering, CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden, 2007

Appendix 4

Source: “Podded propulsion drive for cruise liner Seven Seas Voyager”

www.sam-electronics.de/dateien/pad/broschueren/1.083.pdf

Appendix 5

Source: “Electrical systems in pod propulsion”, Master of Science Thesis of Electric Power Engineering, Lena Bergh Ulrika Helldén,Department of Energy and Environment, Division of Electric Power Engineering, CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden, 2007

Appendix 6

Source: ” Marine Propellers and Propulsion”, John Carlton, second edition 2007, Published by Elsevier Ltd

Appendix 7

Source: ” Marine Propellers and Propulsion”, John Carlton, second edition 2007, Published by Elsevier Ltd

Appendix 8

Source: ” Marine Propellers and Propulsion”, John Carlton, second edition 2007, Published by Elsevier Ltd