Tonnage
Tonnage is a measure of the size or cargo capacity of a
ship. The term derives from the taxation paid on
tuns of wine, and was later used in reference to the weight of a ship's cargo; however, in modern maritime usage, "tonnage" specifically refers to a calculation of the volume or cargo volume of a ship. The term is still sometimes incorrectly used to refer to the weight of a loaded or empty vessel.
Measurement of tonnage can be less than straightforward, not least because it is used to assess
fees on
commercial shipping.
Gross Register Tonnage represents the total internal
volume of a vessel, with some exemptions for non-productive spaces such as crew quarters; 1 gross register ton is equal to a volume of 100
cubic feet (2.83
m³). This calculation is complex; a hold can, for instance, be assessed for
grain (accounting for all the air space in the hold) or for
bales (exempting the spaces between structural frames). Gross register tonnage was replaced by
gross tonnage in 1994, under the Tonnage Measurement convention of 1969.
[CWP Handbook of Fishery Statistical Standards. Retrieved May 10, 2006.][International Convention on Tonnage Measurement of Ships, 1969, International Maritime Organisation. Retrieved May 10, 2006.]Net Register Tonnage is the volume of cargo the vessel can carry; ie. the Gross Register Tonnage less the volume of spaces that will not hold
cargo (e.g.
engine compartment,
helm station,
crew spaces, etc., again with differences depending on which
port or
country is doing the calculations). It represents the volume of the ship available for transporting
freight or
passengers. It was replaced by
net tonnage in 1994, under the Tonnage Measurement convention of 1969.
Gross Tonnage refers to the volume of all ship's enclosed spaces (from keel to funnel) measured to the outside of the hull framing. It is always larger than
gross register tonnage, though by how much depends on the vessel design.
Net tonnage is based on a calculation of the volume of all cargo spaces of the ship.
The
Panama Canal/Universal Measurement System (PC/UMS) is based on
net tonnage, modified for Panama Canal purposes. PC/UMS is based on a mathematical formula to calculate a vessel's total volume; a
PC/UMS net ton is equivalent to 100
cubic feet of capacity.
[Panama Canal Tolls, from the Panama Canal Authority. Retrieved May 10, 2006.]Thames measurement tonnage is another volumetric system, generally used for small vessels such as
yachts; it uses a formula based on the vessel's length and beam.
Many people including those professional people working in maritime industries for many years or even in their lifetime,often confuse "Tonnage" and "Ton" in many countries. Please be noted that "Tonnage" refers to the unit of a ship's volume in measurement for registration and "Ton" refers to the unit of weight. They are totally different in concept.
Weight measurements
While not "tonnage" in the proper sense, the following methods of ship measurement are often incorrectly referred to as such:
Displacement is the actual total weight of the vessel. It is often expressed in
long tons or in
metric tons, and is calculated simply by multiplying the volume of the
hull below the waterline (ie. the volume of water it is displacing) by the density of the water. (Note that the density will depend on whether the vessel is in fresh or salt water, or is in the tropics, where water is warmer and hence less dense.) For example, in sea water, first determine the volume of the submerged portion of the hull as follows: Multiply its length by its breadth and the draft, all in feet. Then multiply the product thereby obtained by the
block coefficient of the hull to get the hull volume in
cubic feet. Then multiply this figure by 64 (the weight of one cubic foot of seawater) to get the weight of the ship in pounds; or divide by 35 to calculate the weight in
long tons. Using the
SI or
metric system : displacement (in tonnes) is volume (in m
3) multiplied by the specific gravity of sea water (1.025 nominally).
The word "displacement" arises from the basic physical law, discovered by
Archimedes, that the weight of a floating object equates exactly to that of the water which would otherwise occupy the "hole in the water" displaced by the ship.
Lightship measures the actual weight of the ship with no
fuel, passengers,
cargo, water, etc. on board.
Deadweight is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo,
fuel, water, and
stores. Like Displacement, it is often expressed in long tons or in metric tons.
[Ton types, by Gregory M. Walsh, Ocean Navigator. Retrieved May 10, 2006.]Historically, tonnage was the tax on
tuns (
casks) of
wine that held approximately 252 wine gallons of wine and weighed approximately 2,240
pounds. This suggests that the unit of weight measurement,
long tons (also 2,240 lb) and tonnage both share the same etymology. The confusion between weight based terms (
deadweight and
displacement) stems from this common source and the eventual decision to assess dues based on a ship's
deadweight rather than counting the tuns of wine. In 1720 the Builders Old Measurement Rule was adopted to estimate deadweight from the length of
keel and maximum breadth or
beam of a ship. This overly simplistic system was replaced by the Moorsom System in 1854 and calculated internal volume, not weight. This system evolved into the current set of internationally accepted rules and regulations.
*
Ton
* The Oxford Companion To Ships & The Sea, by I. C. B. Dear and Peter Kemp. Oxford University Press, 1979. ISBN 0198606168
* Ship Design and Construction, Volume II; Thomas Lamb, Editor. Society of Naval Architects and Marine Engineers, 2004. ISBN 9990906203
