Focus On...Propulsion

Wake behind the azipod-driven Norwegian Pearl.

Photo © Aaron Saunders

Many passengers can tell you exactly which night lobster will be served in the dining room, but sadly, most do not give much consideration to the amazing technologies that allow their ship to speed across the ocean at a comfortable 22.5 knots, all without causing their drink to ripple.

Modern cruise ships use two types of propulsion: a standard propeller-and-rudder combination, or a pair of advanced azimuthing pods capable of rotating 360 degrees and functioning more like a submerged outboard motor.

In 1843, Isambard Kingdom Brunel became the first to adopt screw propeller technology into his SS Great Britain.  Up until this time, most ships were still sail-based, driven purely by the wind itself.  If a ship did have a steam engine, it was likely underpowered, dangerous, and drove a pair of large paddlewheels fitted amidships on either side.  These wheels were easily damaged during rough seas, and their reliability was highly suspect.  This caused both passengers and shipping companies alike to continue to rely on sail power well into the early 1900's. 

In the intervening years, screw technology became more advanced and reliable.  The introduction of the triple-expansion steam engine resulted in ships that were smoother and faster than previous ones.  The introduction of the turbine engine further improved upon this, almost eliminating vibration.  Both speed and vibration-less passages were the two most sought-after features on the early transatlantic liners, and are directly responsible for some of the resulting technology used on today's cruise ships. 

Traditional propellers and rudders as seen on Sea Princess

during a drydock.

Photo courtesy of Princess Cruises

With the traditional propulsion system, typically two variable-pitch propellers are connected to shafts which run into the hull of the ship.  The shaft and corresponding tunnel connect to the main engines, which turn the propeller.  Maneuvering the vessel is accomplished with two rudders, one placed just aft of each propeller.  When docking, side thrusters are required to move the ship closer to, or away from, the dock, in addition to the main propellers.  Being variable-pitch, the propeller blades can actually rotate along their axis, giving the vessel better maneuverability and fuel efficiency.

 Azipods on Freedom of the Seas.  The two facing forward

are azimuthing; the center pod is fixed.

Photo courtesy of ABB Group.

With newer azimuthing pods, the propeller itself acts as the rudder.  Typically, two pods are fitted underneath the ship and are capable of turning 360 degrees in any direction.  The pod itself also contains the entire engine, which is directly connected to the propeller.  The lack of a propeller shaft allows the pod to sit lower in the water than traditional propellers, where they can have access to a less-turbulent area of water.  Their full rotation capabilities also allow the ship to make minute course adjustments for the best possible fuel economy.  Unlike traditional propellers, which face aft and "push" the ship through the water, azimuthing pods face forward and "pull" the ship through the water.  They also eliminate the need for stern thrusters, as the pods themselves can be used for this purpose when docking or undocking.

Newer azimuthing pods (also known as Azipods, despite the fact the name itself is a trademark of the ABB Group for their brand of thruster) feature excellent reliability, but in the past, teething problems have occurred.  Celebrity Cruises was the most unlucky, with the Rolls-Royce "Mermaid" pods on several of their Millennium-class ships failing repeatedly and frequently between their launch in 2001 and 2005-2007.  Premature wear on the bearings caused the majority of these faults.  Thankfully, the problems have been almost eliminated in the past few years.  Royal Caribbean, Celebrity's parent company, sued Rolls Royce for the defects.

Carnival Corporation PLC also sued Rolls Royce over problems with Queen Mary 2's three "Mermaid" pods.  Each pod should have a lifespan of 15 to 20 years, but Carnival has been forced to change pods completely every three years - again, due to prematurely failed bearings.

Despite these teething problems, ships with azimuthing technology are remarkably quiet, with very little vibration, even in heavy seas.  Almost all newbuilds feature "Azipod" propellers - with the notable exception of Princess Cruises, which has yet to feature azimuthing propulsion on any of its ships, including its most recent newbuild, Ruby Princess.

The first cruise ship to be fitted with Azipod propulsion was Carnival Elation in 1998.