The BBC, Costa Concordia and the Titanic

In its recent coverage of the righting of the Concordia the BBC observes that with a Gross Tonnage of 114,000 it is twice as heavy as the Titanic. I suppose they are actually comparing the Gross Tonnage of one with the Gross Registered Tonnage of the other. These are not the same thing, but neither is a measure of weight or more properly mass or displacement. Both are related (in different ways) to the volume of the ship, and different volumes of that: GRT to internal volume and GT to volume to the skin. We may also suspect that a modern cruise ship has a substantial lightweight superstructure contributing to its enclosed volume. So on any measure related to volume two ships one similar to modern cruise ship and one similar to a 1910 ocean liner but of the same displacement the modern design should come out "larger".

From its overall dimensions and a guestimate of its block coefficient we may conclude that the displacement at design draft of the Concordia is ~50-55,000 tonnes. The published displacement of the Titanic at design draft is 52,000 tons (here we are comparing metric tons with long tons but they are close enough for my purposes).

So we see that in terms of weight (displacement or mass) the two ships are very similar.