Tonnage
Encyclopedia
Tonnage is a measure of the size or cargo
carrying capacity of a ship
. The term derives from the taxation paid on tuns or casks 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 fee
s on commercial
shipping
.
(GT) is a function of the volume of all ship's enclosed spaces (from keel
to funnel
) measured to the outside of the hull framing. The numerical value for a ship's GT is always smaller than the numerical values for both her gross register tonnage and the GRT value expressed equivalently in cubic meters rather than cubic feet, for example: 0.5919 GT = 1 GRT = 2.8316 m³
; 200 GT = 274 GRT = 775,88 m³
; 500 GT = 665 GRT = 1,883.07 m³
; 3,000 GT = 3,776 GRT = 10,692.44 m³
), though by how much depends on the vessel design (volume). There is a sliding scale factor. So GT is a kind of capacity-derived index that is used to rank a ship for purposes of determining manning, safety and other statutory requirements and is expressed simply as GT, which is a unitless entity, even though its derivation is tied to the cubic meter unit of volumetric capacity.
Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, the correct term to use now is GT, which is a function of the moulded volume of all enclosed spaces of the ship.
It is calculated by using the formula :
, where V = total volume in m³ and K = a figure from 0.22 up to 0.32, depending on the ship’s size (calculated by : 
), so that, for a ship of 10,000 m³
total volume, the gross tonnage would be 0.28 x 10,000 = 2,800 GT or 3531.46 GRT. GT is consequently a measure of the overall size of the ship. For a ship of 80,000 m³
total volume the gross tonnage would be 0.2980617 x 80,000 = 23,844.94 GT.
Net tonnage
(NT) is based on a calculation of the volume of all cargo spaces of the ship. It indicates a vessel’s earning space and is a function of the moulded volume of all cargo spaces of the ship.
A commonly defined measurement system is important; since a ship’s registration fee, harbour dues, safety and manning rules etc., are based on its gross tonnage, GT, or net tonnage, NT.
Gross register tonnage
(GRT) represents the total internal volume
of a vessel, where a register ton is equal to a volume of 100 cubic feet
(2.83 m³
), which volume, if filled with fresh water, would weigh around 2,800 kg or 2.8 tonne
s. Calculation of GRT is complex; a ship's hold can, for instance, be assessed for bulk grain
(accounting for all the air space in the hold) or for bale
s (omitting the spaces into which bulk, but not baled cargo would spill). Gross register tonnage was replaced by gross tonnage in 1994 under the Tonnage Measurement convention of 1969, and is no longer widely used term in the industry.
Net register tonnage (NRT) is the volume of cargo the vessel can carry; i.e., 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 passenger
s. It was replaced by net tonnage in 1994, under the Tonnage Measurement convention of 1969.
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.
The Suez Canal Net Tonnage (SCNT) is derived with a number of modifications from the former net register tonnage of the Moorsom System and was established by the International Commission of Constantinople in its Protocol of 18 December 1873. It is still in use, as amended by the Rules of Navigation of the Suez Canal Authority, and is registered in the Suez Canal Tonnage Certificate.
Thames measurement tonnage
is another volumetric system, generally used for small vessels such as yacht
s; it uses a formula based on the vessel's length and beam.
Displacement
is the actual total weight of the vessel (mostly without pay load). It is often expressed in long ton
s or in metric ton
s, and is calculated simply by multiplying the volume of the hull
below the waterline (i.e. the volume of water it is displacing) by the specific gravity of the water. (Note that the specific gravity 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 ton
s. Using the SI
or metric system : displacement (in tonnes) is volume (in m³) 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 or Lightweight measures the actual weight of the ship with no fuel
, passengers, cargo
, water, etc. on board.
Deadweight tonnage
(often abbreviated as DWT for deadweight tonnes) 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.
Metric Tonnes per Centimetre Immersion (usually abbreviated to TPC or TPCMI ) is the number of Metric Tonnes (1,000KG) that need to be loaded on the ship for the salt water draft to increase by one centimetre.
Imperial Tons per Inch immersion (usually abbreviated to TPI) is the number of Imperial Long Tons (2,240 lbs) that need to be loaded on a vessel for the draft to increase by one inch.
Old imperial TPI measurements are still occasionally used within the USA and the Panama canal. As no ship has been measured by a Classification Society since the 1950’s using Imperial measures, modern TPI figures are therefore a conversion from the original Metric measurements and should not be relied upon to be accurate.
The TPCMI figure is used to calculate the draft of the vessel with a given DWT of cargo loaded.
On a typical Panamax Bulk carrier with a TPCMI of 80, it would mean that the ship will sink (Draft increase) by one centimetre for every 80 tonnes of cargo loaded.
Draft
The distance, usually measured in meters, between the lowest point of the keel and the waterline which varies dependant on the load (DWT) the vessel has on board
s) of wine
that held approximately 252 gallons of wine and weighed approximately 2,240 pounds
. This suggests that the unit of weight measurement, long ton
s (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 Builder's 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.
When steamships came into being, they could carry less cargo, size for size, than sailing ships. As well as spaces taken up by boilers and steam engines, steamships carried extra fresh water for the boilers as well as coal for the engines. Thus, to move the same volume of cargo as a sailing ship, a steamship would be considerably larger than a sailing ship.
"Harbour Dues" are based on tonnage. In order to prevent steamships operating at a disadvantage, various tonnage calculations were established to minimise the disadvantage that the extra space requirements of steamships presented. Rather than charging by length or displacement etc., charges were calculated on the viable cargo space. As commercial cargo sailing ships are now largely extinct, Gross Tonnage is becoming the universal method of calculating ships dues, and is also a more straightforward and transparent method of assessment.
Cargo
Cargo is goods or produce transported, generally for commercial gain, by ship, aircraft, train, van or truck. In modern times, containers are used in most intermodal long-haul cargo transport.-Marine:...
carrying capacity of a ship
Ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...
. The term derives from the taxation paid on tuns or casks 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 fee
Fee
A fee is the price one pays as remuneration for services. Fees usually allow for overhead, wages, costs, and markup.Traditionally, professionals in Great Britain received a fee in contradistinction to a payment, salary, or wage, and would often use guineas rather than pounds as units of account...
s on commercial
Commerce
While business refers to the value-creating activities of an organization for profit, commerce means the whole system of an economy that constitutes an environment for business. The system includes legal, economic, political, social, cultural, and technological systems that are in operation in any...
shipping
Shipping
Shipping has multiple meanings. It can be a physical process of transporting commodities and merchandise goods and cargo, by land, air, and sea. It also can describe the movement of objects by ship.Land or "ground" shipping can be by train or by truck...
.
Tonnage measurements
Gross tonnageGross tonnage
Gross tonnage is a unitless index related to a ship's overall internal volume. Gross tonnage is different from gross register tonnage...
(GT) is a function of the volume of all ship's enclosed spaces (from keel
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
to funnel
Funnel
A funnel is a pipe with a wide, often conical mouth and a narrow stem. It is used to channel liquid or fine-grained substances into containers with a small opening. Without a funnel, spillage would occur....
) measured to the outside of the hull framing. The numerical value for a ship's GT is always smaller than the numerical values for both her gross register tonnage and the GRT value expressed equivalently in cubic meters rather than cubic feet, for example: 0.5919 GT = 1 GRT = 2.8316 m³
Cubic metre
The cubic metre is the SI derived unit of volume. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère...
; 200 GT = 274 GRT = 775,88 m³
Cubic metre
The cubic metre is the SI derived unit of volume. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère...
; 500 GT = 665 GRT = 1,883.07 m³
Cubic metre
The cubic metre is the SI derived unit of volume. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère...
; 3,000 GT = 3,776 GRT = 10,692.44 m³
Cubic metre
The cubic metre is the SI derived unit of volume. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère...
), though by how much depends on the vessel design (volume). There is a sliding scale factor. So GT is a kind of capacity-derived index that is used to rank a ship for purposes of determining manning, safety and other statutory requirements and is expressed simply as GT, which is a unitless entity, even though its derivation is tied to the cubic meter unit of volumetric capacity.
Tonnage measurements are now governed by an IMO Convention (International Convention on Tonnage Measurement of Ships, 1969 (London-Rules)), which applies to all ships built after July 1982. In accordance with the Convention, the correct term to use now is GT, which is a function of the moulded volume of all enclosed spaces of the ship.
It is calculated by using the formula :
, where V = total volume in m³ and K = a figure from 0.22 up to 0.32, depending on the ship’s size (calculated by : ), so that, for a ship of 10,000 m³Cubic metre
The cubic metre is the SI derived unit of volume. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère...
total volume, the gross tonnage would be 0.28 x 10,000 = 2,800 GT or 3531.46 GRT. GT is consequently a measure of the overall size of the ship. For a ship of 80,000 m³
Cubic metre
The cubic metre is the SI derived unit of volume. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère...
total volume the gross tonnage would be 0.2980617 x 80,000 = 23,844.94 GT.
Net tonnage
Net tonnage
Net tonnage is a dimensionless index calculated from the total moulded volume of the ship's cargo spaces by using a mathematical formula...
(NT) is based on a calculation of the volume of all cargo spaces of the ship. It indicates a vessel’s earning space and is a function of the moulded volume of all cargo spaces of the ship.
A commonly defined measurement system is important; since a ship’s registration fee, harbour dues, safety and manning rules etc., are based on its gross tonnage, GT, or net tonnage, NT.
Gross register tonnage
Gross Register Tonnage
Gross register tonnage a ship's total internal volume expressed in "register tons", one of which equals to a volume of . It is calculated from the total permanently enclosed capacity of the vessel. The ship's net register tonnage is obtained by reducing the volume of non-revenue-earning spaces i.e...
(GRT) represents the total internal volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
of a vessel, where a register ton is equal to a volume of 100 cubic feet
Cubic foot
The cubic foot is an Imperial and US customary unit of volume, used in the United States and the United Kingdom. It is defined as the volume of a cube with sides of one foot in length.-Conversions:- Symbols :...
(2.83 m³
Cubic metre
The cubic metre is the SI derived unit of volume. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère...
), which volume, if filled with fresh water, would weigh around 2,800 kg or 2.8 tonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...
s. Calculation of GRT is complex; a ship's hold can, for instance, be assessed for bulk grain
Bulk cargo
Bulk cargo is commodity cargo that is transported unpackaged in large quantities. This cargo is usually dropped or poured, with a spout or shovel bucket, as a liquid or as a mass of relatively small solids , into a bulk carrier ship's hold, railroad car, or tanker truck/trailer/semi-trailer body...
(accounting for all the air space in the hold) or for bale
Bale
- Places :* Bale , a small town in Croatia* Bale, California, in Napa County* Bale, Norfolk in England* Bale, Poland* Balé Province, Burkina Faso* Basel, the Swiss city, for which the French name is Bâle-Ethiopia:* Bale Mountains...
s (omitting the spaces into which bulk, but not baled cargo would spill). Gross register tonnage was replaced by gross tonnage in 1994 under the Tonnage Measurement convention of 1969, and is no longer widely used term in the industry.
Net register tonnage (NRT) is the volume of cargo the vessel can carry; i.e., the gross register tonnage less the volume of spaces that will not hold cargo
Cargo
Cargo is goods or produce transported, generally for commercial gain, by ship, aircraft, train, van or truck. In modern times, containers are used in most intermodal long-haul cargo transport.-Marine:...
(e.g., engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
compartment, helm
Ship's wheel
A ship's wheel is the modern method of adjusting the angle of a boat or ship's rudder in order to cause the vessel to change its course. Together with the rest of the steering mechanism it forms part of the helm. It is typically connected to a mechanical, electric servo, or hydraulic system...
station, crew
Crew
A crew is a body or a class of people who work at a common activity, generally in a structured or hierarchical organization. A location in which a crew works is called a crewyard or a workyard...
spaces, etc., again with differences depending on which port
Port
A port is a location on a coast or shore containing one or more harbors where ships can dock and transfer people or cargo to or from land....
or country
Country
A country is a region legally identified as a distinct entity in political geography. A country may be an independent sovereign state or one that is occupied by another state, as a non-sovereign or formerly sovereign political division, or a geographic region associated with a previously...
is doing the calculations). It represents the volume of the ship available for transporting freight or passenger
Passenger
A passenger is a term broadly used to describe any person who travels in a vehicle, but bears little or no responsibility for the tasks required for that vehicle to arrive at its destination....
s. It was replaced by net tonnage in 1994, under the Tonnage Measurement convention of 1969.
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
Cubic foot
The cubic foot is an Imperial and US customary unit of volume, used in the United States and the United Kingdom. It is defined as the volume of a cube with sides of one foot in length.-Conversions:- Symbols :...
of capacity.
The Suez Canal Net Tonnage (SCNT) is derived with a number of modifications from the former net register tonnage of the Moorsom System and was established by the International Commission of Constantinople in its Protocol of 18 December 1873. It is still in use, as amended by the Rules of Navigation of the Suez Canal Authority, and is registered in the Suez Canal Tonnage Certificate.
Thames measurement tonnage
Thames Measurement
Thames Measurement, also known as Thames Tonnage, is a system for measuring ships and boats. It was created by the Thames Yacht Club, and was designed for small vessels, such as yachts. It was originally used for calculating the port dues for yachts; the formula was also used in some early...
is another volumetric system, generally used for small vessels such as yacht
Yacht
A yacht is a recreational boat or ship. The term originated from the Dutch Jacht meaning "hunt". It was originally defined as a light fast sailing vessel used by the Dutch navy to pursue pirates and other transgressors around and into the shallow waters of the Low Countries...
s; it uses a formula based on the vessel's length and beam.
Weight measurements
While not "tonnage" in the proper sense, the following methods of ship measurement are often incorrectly referred to as such:Displacement
Displacement (ship)
A ship's displacement is its weight at any given time, generally expressed in metric tons or long tons. The term is often used to mean the ship's weight when it is loaded to its maximum capacity. A number of synonymous terms exist for this maximum weight, such as loaded displacement, full load...
is the actual total weight of the vessel (mostly without pay load). It is often expressed in long ton
Long ton
Long ton is the name for the unit called the "ton" in the avoirdupois or Imperial system of measurements, as used in the United Kingdom and several other Commonwealth countries. It has been mostly replaced by the tonne, and in the United States by the short ton...
s or in metric ton
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...
s, and is calculated simply by multiplying the volume of the hull
Hull (watercraft)
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...
below the waterline (i.e. the volume of water it is displacing) by the specific gravity of the water. (Note that the specific gravity 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
Cubic foot
The cubic foot is an Imperial and US customary unit of volume, used in the United States and the United Kingdom. It is defined as the volume of a cube with sides of one foot in length.-Conversions:- Symbols :...
. 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 ton
Long ton
Long ton is the name for the unit called the "ton" in the avoirdupois or Imperial system of measurements, as used in the United Kingdom and several other Commonwealth countries. It has been mostly replaced by the tonne, and in the United States by the short ton...
s. Using the SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
or metric system : displacement (in tonnes) is volume (in m³) multiplied by the specific gravity of sea water (1.025 nominally).
The word "displacement" arises from the basic physical law, discovered by Archimedes
Archimedes
Archimedes of Syracuse was a Greek mathematician, physicist, engineer, inventor, and astronomer. Although few details of his life are known, he is regarded as one of the leading scientists in classical antiquity. Among his advances in physics are the foundations of hydrostatics, statics and an...
, 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 or Lightweight measures the actual weight of the ship with no fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
, passengers, cargo
Cargo
Cargo is goods or produce transported, generally for commercial gain, by ship, aircraft, train, van or truck. In modern times, containers are used in most intermodal long-haul cargo transport.-Marine:...
, water, etc. on board.
Deadweight tonnage
Deadweight tonnage
Deadweight tonnage is a measure of how much weight a ship is carrying or can safely carry. It is the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew...
(often abbreviated as DWT for deadweight tonnes) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
, water, and stores. Like Displacement, it is often expressed in long tons or in metric tons.
Metric Tonnes per Centimetre Immersion (usually abbreviated to TPC or TPCMI ) is the number of Metric Tonnes (1,000KG) that need to be loaded on the ship for the salt water draft to increase by one centimetre.
Imperial Tons per Inch immersion (usually abbreviated to TPI) is the number of Imperial Long Tons (2,240 lbs) that need to be loaded on a vessel for the draft to increase by one inch.
Old imperial TPI measurements are still occasionally used within the USA and the Panama canal. As no ship has been measured by a Classification Society since the 1950’s using Imperial measures, modern TPI figures are therefore a conversion from the original Metric measurements and should not be relied upon to be accurate.
The TPCMI figure is used to calculate the draft of the vessel with a given DWT of cargo loaded.
On a typical Panamax Bulk carrier with a TPCMI of 80, it would mean that the ship will sink (Draft increase) by one centimetre for every 80 tonnes of cargo loaded.
Draft
Draft
Draft or draught may mean:* Draught beer or other beverage, served from a bulk keg or cask rather than a bottle or can-Watercraft dimensions:* Draft , distance from waterline to keel* Draft , degree of curvature in a sail...
The distance, usually measured in meters, between the lowest point of the keel and the waterline which varies dependant on the load (DWT) the vessel has on board
Origins
Historically, tonnage was the tax on tuns (caskCASK
Peripheral plasma membrane protein CASK is a protein that in humans is encoded by the CASK gene. This gene is also known by several other names: CMG 2 , calcium/calmodulin-dependent serine protein kinase 3 and membrane-associated guanylate kinase 2.-Genomics:This gene is located on the short arm of...
s) of wine
Wine
Wine is an alcoholic beverage, made of fermented fruit juice, usually from grapes. The natural chemical balance of grapes lets them ferment without the addition of sugars, acids, enzymes, or other nutrients. Grape wine is produced by fermenting crushed grapes using various types of yeast. Yeast...
that held approximately 252 gallons of wine and weighed approximately 2,240 pounds
Pound (mass)
The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...
. This suggests that the unit of weight measurement, long ton
Long ton
Long ton is the name for the unit called the "ton" in the avoirdupois or Imperial system of measurements, as used in the United Kingdom and several other Commonwealth countries. It has been mostly replaced by the tonne, and in the United States by the short ton...
s (also 2,240 lb) and tonnage both share the same etymology. The confusion between weight based terms (deadweight
Deadweight
Deadweight may refer to:* Deadweight loss, an economics concept* Deadweight tonnage, a ship's carrying capacity with crew and supplies* "Deadweight" , a song on Beck's 1997 album A Life Less Ordinary...
and displacement
Displacement (fluid)
In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced .An object that sinks...
) stems from this common source and the eventual decision to assess dues based on a ship's deadweight
Deadweight
Deadweight may refer to:* Deadweight loss, an economics concept* Deadweight tonnage, a ship's carrying capacity with crew and supplies* "Deadweight" , a song on Beck's 1997 album A Life Less Ordinary...
rather than counting the tuns of wine. In 1720 the Builder's Old Measurement
Builder's Old Measurement
Builder's Old Measurement is the method of calculating the size or cargo capacity of a ship used in England from approximately 1720 to 1849. It estimated the tonnage of a ship based on length and maximum beam...
Rule was adopted to estimate deadweight from the length of keel
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
and maximum breadth or beam
Beam (nautical)
The beam of a ship is its width at the widest point. Generally speaking, the wider the beam of a ship , the more initial stability it has, at expense of reserve stability in the event of a capsize, where more energy is required to right the vessel from its inverted position...
of a ship. This overly simplistic system was replaced by the Moorsom System
Moorsom System
The Moorsom System is a method created in Great Britain of calculating the tonnage or cargo capacity of sailing ships as a basis for assessing harbour and other vessel fees...
in 1854 and calculated internal volume, not weight. This system evolved into the current set of internationally accepted rules and regulations.
When steamships came into being, they could carry less cargo, size for size, than sailing ships. As well as spaces taken up by boilers and steam engines, steamships carried extra fresh water for the boilers as well as coal for the engines. Thus, to move the same volume of cargo as a sailing ship, a steamship would be considerably larger than a sailing ship.
"Harbour Dues" are based on tonnage. In order to prevent steamships operating at a disadvantage, various tonnage calculations were established to minimise the disadvantage that the extra space requirements of steamships presented. Rather than charging by length or displacement etc., charges were calculated on the viable cargo space. As commercial cargo sailing ships are now largely extinct, Gross Tonnage is becoming the universal method of calculating ships dues, and is also a more straightforward and transparent method of assessment.
See also
- Builder's Old MeasurementBuilder's Old MeasurementBuilder's Old Measurement is the method of calculating the size or cargo capacity of a ship used in England from approximately 1720 to 1849. It estimated the tonnage of a ship based on length and maximum beam...
- Displacement (fluid)Displacement (fluid)In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced .An object that sinks...
- TonTonThe ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of weight, and as a unit of volume. It can also be used as a measure of energy, for truck classification, or as a colloquial term.It is derived from...
- Short tonShort tonThe short ton is a unit of mass equal to . In the United States it is often called simply ton without distinguishing it from the metric ton or the long ton ; rather, the other two are specifically noted. There are, however, some U.S...
- Long tonLong tonLong ton is the name for the unit called the "ton" in the avoirdupois or Imperial system of measurements, as used in the United Kingdom and several other Commonwealth countries. It has been mostly replaced by the tonne, and in the United States by the short ton...
- Metric tonne
- Measures of ship displacement
- List of world's largest ships by gross tonnage