One other thing that is done on a sea trial is to actually measure the physical size of the ship. For a cruise ship, the most important measure is not its weight or the weight of its cargo, but the space available onboard. And just to confuse matters, space is measured as “gross registered tons” or “grt”. The history is interesting, at least to me.
Originally, a key measure of the carrying capacity of a ship was not how much it weighed, but how much space it had available for cargo. And a common cargo was wine, which was shipped in large casks called “tuns”. A tun held 8 barrels of wine or about 242 gallons. Soon they started measuring the area onboard based on how many of these boxes or tuns you could fit in. Thus, a ship that could transport 8,000 barrels of wine was known as a 1,000 tun ship. “Tun” evolved into “ton” and then into “Gross Registered Ton”. To give you a sense of scale, Clipper Ships in the mid-1800’s were built ranging in size from 400 grt to 4,000 grt.¹
The advent of steel hulled ships and steam engines allowed them to build larger and larger vessels with the Titanic measuring a whopping 48,000 grt. Aircraft carriers have to be particularly large, and today aircraft carriers measure as large as 100,000 grt. Imagine if all that space were filled with wine.
And then there is Oasis of the Seas. Our original designs called for Oasis to be a little over 200,000 grt. Early additions quickly increased this to about 220,000 tons but these were just early estimates. One of the duties of the classification society is to measure the actual space onboard the ship and to certify its grt. DNV performed its preliminary calculations during the sea trial and calculated that the grt of Oasis of the Seas is closer to 225,000 grt. This is slightly higher than estimated, but well within the expected margin of error for such estimates. This figure is still preliminary and isn’t final until DNV finish their measurements and calculations closer to delivery.
All this is why shipping people cringe when someone says that a particular ship “weighs this many gross tons.” There are other measures of a ship’s weight, the most relevant of which are:
Lightweight Tons The easiest measure to explain is the lightweight tonnage. This is simply the physical weight of the ship when it is light i.e. with no cargo onboard. If you could pick up the ship and put it on a scale, what would she weigh? This measurement is not particularly relevant to most ships and is mainly used to calculate how much steel the yard needs to build the ship and the value of the scrap steel at the end of a ship’s life. The lightweight of Oasis is about 88,600 lwt.
Deadweight Tons The deadweight tonnage of a ship is mainly used for cargo ships and is a measure of how much cargo the ship can carry by weight. Thus a 50,000 dwt coal carrier can carry about 50,000 tons of coal. To be precise, it’s slightly less than 50,000 tons because the ship’s fuel, water, crew, etc. are also considered cargo for this purpose and have to be deducted to calculate how much cargo the ship can carry.
On a cruise ship, the numbers are dramatically less then for cargo ships. For cruise ships, the “cargo” is mainly the fuel and the water that the ship carries. Passengers and crew as well as luggage and musical instruments are trivial. The deadweight tonnage of Oasis of the Seas is 17,100 dwt, most of which is allocated to fuel and water. Interestingly, this is much more deadweight capacity than we need but this gives us a cushion for changes. For example, if we put a heavy sculpture on the top deck, this would cause a reduction in the ship’s deadweight. By designing the ship with “surplus” deadweight, we have more room for such changes.
¹ Although a tun is simply a measure of volume (about 100 cubic feet), there is a connection to measures of weight. A tun of wine holds 242 gallons which weighs about 2,240 pounds. The English developed something called a “long ton” which is also 2,240 lbs. Thus, if you filled up a 700 tun ship with wine, the cargo would also weigh 700 long tons.