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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.
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. 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, some of the most commonly used are the Mermaid unit constructed by Kamewa and Cegelec, the Dolphin unit which was produced by John Crane Lips and STN and the SSP Thruster 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 works 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 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.
There are many different types of podded propulsion systems but the most commonly used type is defined as an "electrically driven thruster with an AC motor incorporated in a streamlined azimuthing pod unit directly driving a fixed-pitch propeller. The motor is fed and controlled by a frequency converter. The pod carries the propeller and azimuths through 360o so that thrust can be created in any required direction".
Advantages and Disadvantages of using Podded Propulsion
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 and emissions; 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 electric propulsion.
The advantages of the pod drives are (in order of their significance):
More cargo space because the engine can be located more freely.
Lower noise level.
Low speeds are possible.
Suited as booster drive in order to increase the speed.
Less working expense in ship manufacturing
Power requirement can be lower for twin screw ships.
The disadvantages of the pod drives are (in order of their possible significance):
Higher capital costs
Diesel electric system required (power loss).
Power requirement higher for single screw arrangements.
Limitation in power.
Limitation in speed.
It is not easy to realistically judge the suitability of pod propulsion for different ship types
based on these advantages, disadvantages and limitations. A deeper analysis leads to the
following order of suitability for some ship types :
Very well suited for:
Cruise Liner, twin screw
RoRo-Passenger Ferry, twin screw, V < 26 kts (V < 30 kts in preparation)
Well suited for:
Bulker/Tanker, twin screw
Bulker/Tanker, single screw
Hardly suited for:
Container Vessel, single screw, < 1000 TEU
Container Vessel, single screw, 1000 to 3000 TEU
Not well suited for:
Container Vessel, twin screw, 1000 to 3000 TEU
Container Vessel, twin screw, 3000 to 6000 TEU
Not possible for:
Container Vessel, single screw, > 3000 TEU
The podded solution aboard the icebreaker niche is particularly notable, as it allows the vessel to efficiently perform other duties in the off-season, versus traditional vessels that might simply sit idle
Mermaid, a unique design which uses a minimum of mechanical parts to optimize available vessel space, ensure greater reliability, and reduce mounting time.
The manufacturer says that optimization of the shape, positioning, and angle of Mermaid in relation to the ship-allows an increase in efficiency of up to 15 percent versus conventional propulsion solutions.
"She is vibration-free and easy
to handle, even in poor weather
and difficult passages."
Captain Uwe Beck
"Azipods have extremely good fuel
efficiency. The saving is around
10-15 per cent compared with
Senior Vice President of
Royal Caribean International
"I would be very surprised if future
iceworthy vessels built in Finland
were fitted with any propulsion
system other than the
Master MSV Botnica
The following advantages have been claimed for the propulsion pods
Plant and automation simplification by employing the all Electric ship concept
Reduced exaust gas emmisions
Flexibility in machinery arrangements
Reduced number of main conmpoments in particular lack of reduction gears
Fuel saving through good hydrodynamic efficiency
Lower noise and vibration
Excelent maneuvering capabilities also at slow speeds and during berthing operations the steering capabilities of pods is far superior than that of rudder and stern trhusters
Space saving in general arrangements
Minimized crash stop distance
Easier shipyard and logistic construction
It can be concluded that only the five last points are of real importance when considering the installation of a propulsion pod. It comes as no surprise that to date cruise vessels have been by far the predominant market for propulsion pods
Power losses due to electric propulsion
Higher capital costs
Limitations in power per screw
It must also be considered that propulsion pods cannot be fitted on single screw two stroke diesel engine powered vessels since propulsion pods installation on such vessels should be disregarded as a general rule due to the lower propulsion eficincy
Failure (see pdf for more)
Propulsion pods have been affected by electrical failures, thrust and support bearing failures, shaft seal failures, lubricating oil contamination
Some of the benefits of using pods include increased efficiency and maneuverability,
and increased design flexibility . Pods increase maneuverability by being
able to rotate through 360° to provide thrust in any direction needed. Eliminating the
long shaft line associated with a conventional drive increases design flexibility. By applying
hydrodynamic pod and hull design improvements, fuel consumption can be reduced
and propulsion efficiency can be increased . Pods also simplify the shipbuilding
process. Complete podded propulsion systems can be delivered and fitted in days,
thus avoiding the lengthy installation of a traditional system . Using pods can eliminate
much of the auxiliary and support equipment associated with a traditional propulsion
system. For example, in a podded system, the rudder, steering gear, propeller, propulsion
motor, main propulsion shaft, the shaft bearings, the thrust bearing, and the shaft seals are
all integrated into a single unit . Figure 7.1 compares a podded drive with a conventional
drive. The pod in the upper half of Figure 7.1 contains all of the components of the
conventional shaft line that is shown in the lower half of Figure 7.1. Figure 7.2 provides
a detailed view of a typical pod. A commercial vessel is shown in Figure 7.3 with two
pods mounted under the aft portion of the vessel in the pulling configuration