The power plant concept in combination with AC electric propulsion has a number of economical and technical advantages - such as fuel efficiency, reduced exhaust emissions, low noise and vibration, improved manoeuvrability, machinery layout flexilibility, reliability and availability - all especially relevant for Cruise vessels. That is why the concept has been an industry standard since beginning of 1990’s. The benefits were further enhanced when ABB's Azipod system , the world’s first podded propulsion system was introduced. After the installation onboard cruise ship M/S Elation 1998, the benefits of Azipod propulsion system were so convincing that podded propulsion became a standard for most new cruise ships. Today more than 30 cruise ships are equipped with ABB's Azipod propulsion systems.

Electric Propulsion Configurations for Cruise Vessels

ABB provides electric propulsion system arrangements and integrated automation systems to meet individual customer requirements and the reliability and availability demands of Cruise vessels. Typical total propulsion power 30…50 MW is shared equally between two units. Electric Azipod® motors are speed controlled by a frequency converter for optimum control and performance. For higher powers 50…100 MW three or four Azipod® units would be used.

Typically a 6.6 kV or 11 kV system voltage is selected depending on the total installed power to keep load and fault currents in the system within design limits.

Minimum harmonic distortion levels ( less than 5%) are achieved by using 12-pulse or 24-pulse drive solutions. Additional filter installations are not necessary.

Why Electric Propulsion for Cruise Vessels ?

In addition to the various benefits of power plant and electric propulsion, there are several additional specific benefits that distinguish Azipod® podded propulsion as a superior alternative for Cruise ships.

Reduced noise and vibration
One of the main sources of vibration is the propeller. The Azipod unit propeller is a pulling propeller which operates in a better wake field and thus induces lower pressure pulses to the hull. The Azipod concept also eliminates the stern thrusters, external shaft supports and rudders. As a result lower vibration and noise levels can be reached onboard the ship which improves passenger comfort.

Lower fuel consumption and exhaust emissions
The fuel consumption and thus exhaust emissions are reduced due to improved hydrodynamic efficiency and the power plant concept. Improved hydrodynamic efficiency is a result of reduced hull resistance when traditional shatfline and related brackets can be eliminated and optimum hull design can be utilized. Pulling type propeller is also working in an optimum environment with highest efficiency. According to the experience from the first cruise vessels with Azipod propulsionthe Carnival Elation, savings up to 40 tons of fuel per week where achieved when compared to her sister vessels with conventional electric shaftline propulsion. A typical value of 10% can be used when estimating the fuel efficiency benefit of a an Azipod® cruise ship compared to a shaft line concept.
The power plant concept provides fuel savings by optimizing the loading of the diesel engines. The power management system connects or disconnects diesel generator sets to the network depending on the vessel power requirement, reducing both inefficient low load operation and running hours of the diesel engines.

Increased safety
Improved manoeuvrability and crash stop performance increases safety by giving better control in restricted passages and is valuable on cruise destinations with difficult port accesses or restricted anchorages.

Flexibility in the vessel design and general arrangement
With the Azipod® propulsion concept the need for long shaftlines, rudders, stern thrusters and reduction gears is eliminated, resulting in machinery space and weight savings. For the designer this gives flexibility and often possibility to fit in more cabins or other essential equipment.

Improved manoeuvrability
The manoeuvrability of the vessel is significantly improved with the two rotating Azipod® units compared to tunnel thrusters or shaft line propellers and rudders. Azipod® units are capable of producing fast response and directing the thrust of the powerful and efficient main propellers in the desired direction. Power plant loading during slow-speed manoeuvring is also reduced due to the improved thrust/power perfomance

Two Azipod® units can be controlled independently, which provides a maximum amount of thrust with a minimum amount of power on the propellers. This benefit has become increasingly important as cruise vessels have become larger and thus more difficult to manoeuvre.

Power plant concept Since typical Cruise Vessel operation profile includes large variation in power demand, electric power plant concept is especially suitable. Number of diesel or gas turbine driven generators connected to the network can be varied according to the power demand. As a result engines can run always at constant speed close to their maximum efficiency thus improving fuel efficiency, reducing exhaust emissions and maintenance costs.

Electric distribution

Air conditioning and various other systems onboard cruise ship need considerable amount of electricity, consumptions from 6000 to 12 000 kW are typical. Electric distribution network can be arranged in different ways. Typically the various distribution transformers supplying the low voltage distribution boards are connected directly to the main switchboard, but considering the cabling work and cost, it can be beneficial to arrange a medium voltage ring network through the ship. The optimum solution depends on size of the ship and amount of distributed power.

From conventional shaftline to Azipod propulsion

Until middle 90’s cruise vessels were mainly equipped with conventional inboard electric motors connected through long shaftlines to propeller.

With the Azipod podded propulsion concept the need for long shaftlines, rudders, stern thrusters and reduction gears is eliminated, resulting in machinery space and weight savings. For the designer this gives flexibility and possibility to fit in more cabins.
During operation Azipod system offers major benefits in enhanced manoeuvrability and hydrodynamic efficiency, resulting in shorter harbour times, safer operation in harsh weather and restricted passages, and reduced fuel consumption and exhaust emissions. With the Azipod propulsion system it is easier to reach lower vibration levels and thus increase passenger comfort onboard.