Mass vs Displacement Tonnage of Ship
Name: Ms. Magilavy
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.
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.
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
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.
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.
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Update: June 2012