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They may look alike to the casual observer, but cruise ships and ocean liners are really two very different types of vessel. The large cruise ships you see sailing out of Miami, Alaska, and other cruising destinations are related to Titanic, United States, and Queen Elizabeth II, but have very different missions, construction, and even crews.
Ocean liners came first. In technical jargon, any ship that runs a regular schedule on an ocean-going route is a liner, even if that ship doesn't carry passengers. In everyday usage, though, we think of an ocean liner as a passenger-carrying ship in the mode of the QEII. It is this difference in mission that distinguishes ocean liners from cruise ships: liners go from point to point; cruise ships don't have a final destination.
Ocean liners are also built for the open ocean routes their schedules require. They have storage for more food, water, and fuel than their cruise ship counterparts, and are built for the rougher waters of the open ocean. They typically have more freeboard than cruise ships, which simply means their highest open-air deck is higher off the water than that on a cruise ship. This makes an ocean liner a lot more expensive to build than a cruise ship. The Queen Mary II (pictured above), when she was completed in 2003, used 40-percent more steel than an similar-sized cruise ship would, and cost twice as much per passenger berth to build than a cruise ship. The builders also had to settle for fewer premium "balcony staterooms" than a cruise ship would to allow for the increased freeboard.
Cruise ships have become destinations in and of themselves; it's been said it doesn't matter where they sail as long as the scenery is pretty and the weather is decent. More than a third of all cruise ship sailings are out of and back to Miami via various Caribbean destinations. The cruise industry continues to be a huge part of the travel industry and the major cruise lines are building more ships (nearly ten a year since 2001) all the time. On the other hand, demand for ocean liners decreased as commercial passenger air travel became accessible. The Queen Mary II was the first true ocean liner built in more than 30 years and, with the retirement of the Queen Elizabeth II in 2008, remains the only true liner in passenger service in the world.
The Queen Mary II is the longest passenger ship in the world at 1,132 feet. It grosses more than 148,000 tons, carries more than 2,600 passengers, and has a crew of more than 1,200. The largest standard cruise ship in the world is Royal Caribbean's Freedom of the Seas. It is 1,119 feet long, but surpasses the QMII in gross tonnage, at more than 154,000. It can carry more than 3,600 passengers and 1,300 crew.