The ocean liner S.S. United States was super sleek, extremely fast, and highly engineered…yet soon became obsolete due to air travel; cruise ships are now the royalty of the seas.

The previous sections of this article looked at the fastest and most innovative of all the ocean liners, the S.S. United States, which was soon made obsolete by air travel and especially jet airplanes. Cruise ships are the ocean vessels which carry people everywhere and nowhere, so to speak.

What is the difference between an ocean liner and a cruise ship? In brief, the ocean liner was intended transport passengers from point A to point B as quickly as possible, as a mobile middle-to-high-class hotel. In contrast, a cruise ship is a floating, ocean-going amusement park/entertainment complex, generally returning to the port from which it departed (usually with intermediate stop-offs) and in no special rush (Figure 1).

Although ocean liners and cruise ships are both sea-going vessels which are designed to carry thousands of passengers, that is where their similarity ends. Today’s cruise ships are not only huge, but they are designed to keep passengers busy and happy while essentially going nowhere. Their design has very different imperatives than those of the ocean liner and is driven by different considerations.

First, the appearance of the cruise ship is unlike an ocean liner. A cruise ship is not designed for speed in itself. having a cruising speed of about 25 knots.  Many cruise ships look like large apartment buildings that have been tipped sideways (the esthetics of this are very debatable) with considerable topside weight (Figure 2), and thus have stability issues which must be addressed to avoid excess tipping or even rollover. Features such as large top-deck swimming pools add to that weight as well as maintenance issues. The electrical power required for all the non-critical system operations is enormous and is provided by two, four, or more separate non-propulsion generators, so that main engines do not have to run while the ship is at dock.

While the passenger accommodations, entertainment, and amenities get the most attention, the design and construction of a modern cruise ship is also unlike the steam boilers, turbines, gearing, and propeller on the shaft of the ocean liner, as well as auxiliary power. Today’s cruise ship uses an engine/generator/electric-motor arrangement, similar to a diesel-electric locomotive (Figure 3). The boiler is gone, and instead, a diesel engine or other prime source turns a generator, and the generator powers huge electric motors. This form of propulsion is more efficient than using a gearbox transmission and offers much better control of power as well as advanced system monitoring.

Some cruise ships use a gas turbine instead of the diesel engine, as the choice of propulsion system must balance range without refueling, speed, fuel costs, pollution and emission restrictions, and many other objectives. Note that for ships, slower is better for fuel consumption since the power needed to the vessel through the water increases exponentially with the speed due to the huge mass of the ship and water friction.

The advantages of the motor-generator can be significant. As tangible proof, the Queen Elizabeth II underwent the largest conversion project in shipping history: a seven-month project where it was converted from steam turbine to diesel-electric plant. The turbines of the QE II are not located deep in the hull as is customary with non-electric drive, but instead are placed close to the funnels in extra-sound-proofed chambers so the large, space-wasting ducts are not needed to convey air to them down below – this is another advantage of “decoupling” the power source (turbine and generator) from the electric motors.

The other change on many cruise ships relates to the standard shaft-mounted propeller arrangement. Some cruise ships do not have the drive shaft with propeller configuration which we see in the most photos and movies. Instead, the electric motors which drive the propellers (usually just two) are mounted in pods outside the hull and can be swiveled 180⁰ or more (Figure 4). This enables the huge ship to maneuver itself in tight turns to a large without the need for external assistance, and also eliminates the need for a separate rudder. Consider it as a form of “active” steering versus “passive” steering.

To further enhance maneuvering capabilities in tight spaces, most cruise ships now have aft-mounted side-thruster propellers on their port and starboard sides. These bow-thrusters provide left and right movement, and when coupled with pod-mounted main propulsion, allow these huge ships to maneuver in the same way that a much-smaller vessel would be able to move.

Conclusion

Passenger-transport ocean liners such as the S.S. United States are largely obsolete, although some liner/cruise hybrids still do exist to a lesser extent, such as the Queen Mary 2. As ocean liners have gone and been replaced by cruise ships, ship technology has advanced tremendously with respect to passenger amenities, connectivity, navigation, and propulsion systems. The glory days of the ocean liners are gone and will likely never be restored. However, from an engineering standpoint, the cruise ships represent an amazing combination of engineering, technology, fabrication, operation, and performance attributes.

In addition to providing more information on the S.S. United States, References 6 through 14 cited below – especially References 13 and 14, which are detailed, very readable PowerPoint presentations – include a considerable amount of information on cruise ships.

EE World References

“Electric locomotives and catenary power systems – Part 1: basic functions”
“RCA & Color TV: A dominant company and standard, both now gone – Part 1”
“RCA & Color TV: A dominant company and standard, both now gone – Part 2”

 

External References

1. Steven Ujifusa, “A Man and His Ship: America’s Greatest Naval Architect and His Quest to Build the S.S. United States,” Simon and Shuster, 2012
2. Steven Ujifusa on William Francis Gibbs and His Ships (podcast and transcript)
3. SS United States Conservancy , especially the link “The Ship”
4. Vox Media, Curbed NY, “Deadline Looms for Historic Ocean Liner’s Move to Brooklyn”
5. Cruise Maven, “Crystal Cruises Hopes to Return S.S. United States to Oceangoing Service” (2016)
6. Cruise Mapper, “Cruise Ship Design Construction Building”
7. Maritime Cyprus, “Ovation of the Seas Floated Out At Meyer Werft Shipyard”
8. Cruise Mapper, “Deck Plans”
9. Cruise Mapper, “Cruise Ship Engine Power, Propulsion, Fuel”
10. The Telegraph, “13 things you didn’t know about cruise ship design”
11. Interesting Engineering, “Understanding the Incredible Engineering of Cruise Ships”
12. GCaptain, “Ship Engines – 7 Monster Engine Designs, Part 1”
13. Carnival Corp., “The Design and Construction of a Modern Cruise Vessel”
14. Martin Ottaway, “The Delightful Frustration of Cruise Ship Power Plant Design”
15. The Drive/The War Zone, May 7, 2020, “Satellite Images Show Armadas Of Vacant Cruise Ships Huddling Together Out At Sea”
16. CNN Travel, March 17, 2020, “Multiple cruise ships are left stranded as coronavirus cases increase”

 

Addendum: Cruise Ships and the Corona Virus

It’s no secret that the cruise-ship world is in a very deep “sleep” due to the worldwide Covid-19 virus. Once the thousands of passengers on the ships were allowed to disembark – after a long delay in many cases – most of these ships needed to be “parked” for an indeterminate period. But there is no place to put them all at the same time, as there aren’t enough docks and piers for them.

As a result, many are anchored in clusters in places such as in the Pacific Ocean off the coast of California and in the Caribbean, each ship with a modest but still significant and costly skeleton crew for on-ongoing maintenance (Figure-Addendum). When any of these ships, and how many, will be back on their normal tourist voyages is anyone’s guess.For now, they are idled, billion-dollar behemoths resting at sea with many of their siblings and requiring attention and upkeep. Perhaps some will eventually be re-purposed and end up in other roles as hospitals, hotels, or conference centers rather than resume their original role. Still, others may be abandoned and suffer a fate similar to the SS United States. We just don’t know, and any so-called “expert” predictions have little credibility at this time. Nonetheless, seeing these grand ships clustered and anchored at sea is a stark and eerie sight, in some ways resembling how bird flocks group for mutual protection against the cold and wind.