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Old August 13th, 2006, 09:05 PM
Rev22:17 Rev22:17 is offline
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Dave,

Important to note: There is a difference between displacement when talking about air craft carriers and tankers versus the interior volume of cruise ships. For example, the Voyager class have an interior volume of 138,000 tons. Who knows what their actual displacement is??? I think is is far less than 138,000. Roland? Norm? I guess I can try to find it...

I read "Supertankers" awhile back and it was written in the 70's about how dangerous those ships were. They had drafts close to 90 feet, some of them.


Unfortunately, that seems to be cruise writers proving their ignorance again -- this time, in the form of half truths. Let's go back to an ancient Greek named Archimedes, who demonstrated that the weight of a floating object is equal to the weight of liquid that it displaces. Thus, the weight of a ship is equal to the weight of sea water that would occupy volume of the portion of the hull that's below the water line (except in the Panama Canal and most of the great lakes, where it's the weight of fresh water that would occupy the volume of the hull). Theoretically, one can compute this by multiplying the density of sea water (64 pounds per cubic foot) by the volume of the underwater portion of the hull.

Now, the catch is that the displacement of a ship is not static. Rather, it changes constantly because it depends upon the ship's load, so the ship's engineering officers have to monitor the ship's load continuously (and it's actually more complicated than that because they also have to monitor the distribution of the weight in order to maintain trim). Historically, the draft markings on the side of the hull, forward and aft, were precisely for this purpose but modern ships also have instruments inside the hull that provide similar information.

Anyway, the U. S. Navy actually publishes three displacements for each ship -- "Navy light" (minimum in port, no load-out), "standard" (normal leaving port for routine operations, with partial food and minimal ordinance) and "full load" or "combat load" (loaded to the gills for a sustained deployment). Merchant ships are registered based upon their maximum load. Thus, the published displacement is stated in "gross registered tons" (GRT). If you want to relate this figure to volume, it's the volume of the underwater portion of the hull at maximum load -- that is, with a full complement of passengers and their luggage aboard, a full load-out of stores, and full water and fuel tanks.

Many cruise ships do not operate under "full load" conditions -- which probably would include enough provisions for 90 to 180 days at sea and a goodly amount of fuel -- because they don't operate that far from resupply. By operating at less than full load, they cut drag and draft, and thus fuel consumption, considerably. You may have noticed that some ships have two-tone hulls with a horizontal break a few feet above the actual waterline. The break in color is actually at the design waterline, which probably is the maximum safe displacement.

Here is the answer to my own question. The QM2, for example, has gross tonnage of 151,000 but displacement of only 76,000 tons. It is alo longer and with a deepr draft than the Freedom so one could assume the Freedom has less displacement unless the hull bottom is more flat than QM2.

Do you know what happens when you ass-u-me???

These ships have completely different hulls that you simply cannot compare in this way. The Freedom of the Seas class has a standard cruise ship hull. If you look at it from the side, it's perfectly flat for most of the length of the ship. A few feet at the front curve into a bow and there's a more or less square stern at the back. If you were to look at the underwater portion, it would go straight down and then curve into the keel, which tapers up at the stern, forming a wide "U" for most of the length of the ship. By contrast, MV (RMS?) Queen Mary 2 has an ocean liner hull. It's curved for most of its length, with a very long bow and some taper inward toward the stern. Going down into the water, it forms a fairly sharp "v" at the that broadens somewhat as you move toward amidships, then moreso when the keel turns upward to accommodate the propellers.

There's a parameter called the "coefficient of form" that's defined to be the ratio of the underwater volume of the hull to the product of the length at the waterline, the maximum beam at the waterline, and the maximum draft. The standard cruise ship hull has a coefficient of form that's very close to one whereas the ocean liner hull has a coefficient of form that's much smaller. Unfortunately, you can't calculate the coefficient of form from the numbers provided because they are extreme length and extreme beam rather than the respective values at the design waterline.

Norm.
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