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Name: Ms. Magilavy
Status: teacher
Grade: other
Location: CA 
Country: N/A
Date: 1/1/2006


Question:
This from a news article in Le Monde: "The former aircraft carrier, with a mass of 24,000 tons for a displacement of 35,000 tons, will be dismantled..." What do they mean by this? I thought the mass was the same as what it displaced.


Replies:
The actual weight or mass of an empty ship is called the "displacement" tonnage. For this, the ton is the metric ton. The "light displacement" is the empty weight. The "load displacement" is the weight when fully loaded. The "deadweight" tonnage is the actual carrying capacity of the ship.

On the other hand, if you are measuring the internal volume of a ship, 100 cubic feet is considered to be a "ton." The "net" tonnage includes only the volume of the cargo spaces. The "gross" tonnage includes the total volume of the entire ship.

Bob Erck


Dear Ms Magilvay,

I am not a nautical expert and so should probably not attempt to answer this. However, the statement you quote is such nonesense (at least to the uninitiated) that I cannot resist replying.

As you correctly state (except for confusing mass and weight), the displacement of a ship is equal to its weight, which of course must be equal to the weight of the water displaced. So if the total weight of the aircraft carrier is 35,000 tons, it will displace exactly 35,000 tons of water. This means, incidentally that in fresh water, which is less dense than salt water, the carrier will float lower in the water than it does in salt water.

In researching your question, I was surprised to find no clear definition of displacement and tonnage. The clearest (which may be right) is in http://www.du.edu/~jcalvert/tech/fluids/cargo.htm. He states: "The tonnage of a ship is not a weight, but a volume. One ton is 100 cubic feet. The total internal volume of a ship is its gross tonnage, and if we subtract all the volume not used for cargo, we get the net tonnage." As for displacement, he says: "The total weight of the ship and everything in it is the displacement, measured in long tons of 2240 lb. A long ton is only a little larger than a metric ton of 1000 kg,but is considerably larger than the US short ton of 2000 lb." That sounds about right, but I can guarantee you that the displacement is the total weight of the ship and contents no matter what units you use!

Best, Dick Plano,
Professor of Physics emeritus, Rutgers University


You are correct, mass should equal displacement. However, I think the problem is more simple than that. An Aircraft Carrier does not put to sea with just its mass, it also takes along crew, fuel, spare parts, stores, and most obviously, aircraft. So the 35,000 ton displacement they mention is a reference to the ships full loadout.

Ryan Belscamper


The mass refers to what in common terms would be called the carrier's "weight". The displacement (expressed in units of tons) is the mass of THE WATER that the vessel displaces when it is placed in the water. So the carrier is buoyed up by a force of (35,000 - 24,000 = 11,000 tons). So it is capable of carrying 11,000 tons of fuel, cargo, planes, etc. The displacement depends not only on the mass of the vessel and its cargo, but on the shape of the vessel. If the vessel is flat, like a barge, it has a greater displacement. In fact that is why cargo barges are designed to be flat rather than narrow.

Vince Calder



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