I happened to catch the Disney Magic* sailing down the Hudson and out of New York earlier this week, apparently on a cruise to St. John, New Brunswick.** Since I put up a photo of RMS Titanic not long ago, it’s interesting to compare the two in terms of their design as machines.
Actually, not just the Titanic, because it’s the wrong size. It’s too small. The Titanic was 883 feet long, 92 feet wide (at its widest, obviously) and weighed 46,000 tons.*** The Magic is substantially bigger, at 965 feet long, 106 feet wide, and 83,000 tons. A good comparison for the Magic among the old transatlantic liners would be the SS Bremen, a German liner in service 1929 to 1939, which was 939 feet long, 102 feet wide, and 52,000 tons. The Bremen and Titanic seem to be about the same ratio of hull size to weight, and they make the Magic seem bloated. What’s going on?
Engineering of any kind is about balancing various criteria. The most famous trade-off is basic functionality versus cost****, but there are many criteria on any project. We regularly look at first (construction) cost versus maintenance cost versus disruption during construction versus saving original material***** and so on. The transatlantic ships were built emphasizing speed and safety****** at the expense of comfort. They were literally the only way for passengers to cross from Europe to North America and transportation for the sake of transportation drove their design. Cruise ships are nearly the opposite: they typically take passengers in a big circle that is completely unnecessary, for the fun of it.
If you compare the appearance of the Bremen and Magic, the older ship had much more of its interior space in the hull rather than the superstructure, meaning that a lot of the passenger cabins had portholes rather than bigger windows. If your goal is simply to cross the Atlantic, that’s fine. If your goal is to see the sights, it’s not so good. I assume that the cruise ships also have proportionally more space devoted to amenities (movie theaters, large pools, gyms, spas) that don’t necessarily require windows and can be put down in the hull. The extra weight of the cruise ships is in their superstructure. It makes the ships heavier and slower*******, which is a good design trade-off for their purpose. And the difference in design goals is visible.
* Since the ship has diesel-electric engines, the old designation would have been MS Disney Magic. I guess if you’re Disney you can skip such details.
** St. John’s a nice destination, but I’m mildly surprised at a cruise north this late in the year.
*** Anyone who knows anything about ships just winced in pain at the bad terminology I’m using to simplify things. For this who care, length overall, maximum beam, and gross tonnage.
**** “An engineer can do for a dollar what any fool can do for two.”
***** Something that matters for historic preservation and for green goals.
****** Yes, even Titanic.
******* Bremen‘s top speed was 27 knots, or around 31 mph; Magic‘s is 21.5 knots or 25 mph. Titanic was about the same speed as Magic, but it was also using circa-1900 engine and propeller technology.
