Deadweight tonnage
Deadweight tonnage (also known as deadweight, abbreviated to DWT, D.W.T., d.w.t., or dwt) is a measure of how much weight a ship is carrying or can safely carry.[1][2][3] It is the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew.[1] The term is often used to specify a ship's maximum permissible deadweight, the DWT when the ship is fully loaded so that its Plimsoll line is at the point of submersion, although it may also denote the actual DWT of a ship not loaded to capacity.
Deadweight tonnage was historically expressed in long tons but is now usually given internationally in tonnes.[4] Deadweight tonnage is not a measure of the ship's displacement and should not be confused with gross register or net tonnage.
Gross Register Tonnage
Gross register tonnage is a measure of the internal volume of a ship expressed in units of 'register tons'. Gross register tonnage (GRT, grt, g.r.t.) represents the total internal volume of a vessel, while Net register tonnage accounts for certain non-productive spaces and is always lesser in value. A register ton is equal to a volume of 100 cubic feet (~2.83 m³).[1][2] Gross register tonnage is not a measure of the ship's weight or displacement and should not be confused with terms such as gross tonnage, deadweight tonnage, net tonnage, or displacement.[1][3]
Calculation of gross register tonnage 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 dead space between the ship's ribs). It has been superseded by Gross Tonnage and is not used or even calculated for modern ships.
Gross tonnage
Gross tonnage (often abbreviated as GT, G.T. or gt) is a unitless index related to a ship's overall internal volume. Gross tonnage is different from gross register tonnage.[1] Neither gross tonnage nor gross register tonnage is a measure of the ship's displacement (mass) or should be confused with terms such as deadweight tonnage, net tonnage, or displacement.
Gross tonnage, along with net tonnage, were defined by The International Convention on Tonnage Measurement of Ships, 1969, adopted by the International Maritime Organization in 1969, and came into force on July 18, 1982. These two measurements replaced gross register tonnage (GRT) and net register tonnage (NRT). Gross tonnage is calculated based on "the moulded volume of all enclosed spaces of the ship" and is used to determine things such as a ship's manning regulations, safety rules, registration fees and port dues, whereas the older gross register tonnage is a measure of the volume of certain enclosed spaces.
Gross tonnage is calculated by measuring a ship's volume (from keel to funnel, to the outside of the hull framing) and applying a mathematical formula.
Calculation
The Gross tonnage calculation is defined in Regulation 3 of Annex 1 of The International Convention on Tonnage Measurement of Ships, 1969. It is based on two variables:
The gross register ton (GRT) is a unit of volume defined as 100 cubic feet (about 2.83 m³).
• V, the ship's total volume in cubic meters (m³), and
• K, a multiplier based on the ship volume.
The value of the multiplier K varies in accordance with a ship's total volume (in cubic metres) and is applied as a kind of reduction factor in determining the gross tonnage value - which does not have a unit such as cubic metres or tons. For smaller ships, K is smaller, for larger ships, K is larger. K ranges from 0.22 to 0.32 and is calculated with a formula which uses the common or base-10 logarithm:
K = 0,2 + 0,02*log10 (V)
Once V and K are known, gross tonnage is calculated using the formula, whereby GT is a function of V:
GT = K x V
As an example, we can calculate the gross tonnage of a ship with 10,000 m³ total volume.
K = 0,2 + 0,02*log10
= 0,2 + 0,02*log10 (10.000)
= 0,2 + 0,02*4
= 0,2 + 0,8
= 0,28
Then the gross tonnage is calculated:
GT = K x V
= 0,28 x 10.000
= 2.800
Builder's Old Measurement (BOM) is the method of calculating the size or cargo capacity of a ship used in England from approximately 1720 to 1849.
The BOM estimated the tonnage of a ship based on length and maximum beam. The formula is:
Tonnage = {(length - beam * 3/5)x Beam x Beam/2}/ 94
where:
• Length is the length, in feet, from the stem to the sternpost;
• Beam is the maximum beam, in feet.[1]
Thus, BOM estimates the weight of the cargo carrying capacity of a ship measured in tons, a weight that is also termed deadweight tonnage. The Builder's Old
Measurement formula remained in effect until the advent of steam propulsion. Steamships required a different method of measuring tonnage, because the ratio of length to beam was larger and a significant volume of internal space was used for boilers and machinery.
In 1849 the Moorsom System was created in Great Britain. Instead of calculating deadweight, the Moorsom system calculates the cargo carrying capacity in cubic feet, a volumetric measurement rather than a weight measurement. The capacity in cubic feet is then divided by 100 cubic feet of capacity per gross ton, resulting in a tonnage expressed in tons.
The first tax on the hire of ships in England was levied by King Edward I in 1303 based on tons of burthen (an archaic term for burden). Later, King Edward III levied a tax of 3 shillings on each "tun" of imported wine. At that time a "tun" was a wine container of 252 gallons weighing about 2,240 lb (1,020 kg). In order to estimate the capacity of a ship in terms of 'tun' for tax purposes, an early formula used in England was :
Tonnage = (Length x Beam x Depth) / 100
where:
• Length is the length (undefined), in feet
• Beam is the beam, in feet.
• Depth is the depth of the hold, in feet below the main deck.
The numerator yields the ship's volume expressed in cubic feet If a "tun" is deemed to be equivalent to 100 cubic feet, then the tonnage is simply the number of such 100 cubic feet 'tun' units of volume.
• 100 the divisor is unitless, so tonnage would be expressed in 'ft³ of tun' [1]
In 1678, Thames shipbuilders used a deadweight method assuming that a ship's burden would be 3/5 of its displacement. Since displacement is calculated by multiplying Length x Beam x Draft x Block Coefficient, all divided by 35 ft³ per ton of seawater, the resulting formula for deadweight would be:
Deadweight = (Length x Beam x Beam/2 x 3/5 x 0,62) / 35
where:
• Draft is estimated to be ½ Beam.
• Deadweight of cargo is assumed to be 3/5 of displacement.
• Block Coefficient is based on an assumed average of 0.62
• 35 ft³ is the volume of one ton of sea water.[2]
Or by solving :
Deadweight = (Length x Beam x Beam/2) / 94
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.
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.[3]
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 in many countries, including those professional people working in maritime industries for many years or even in their lifetime, often confuse "Tonnage" and "Ton". Please note 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.
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 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.
Deadweight (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.
Historically, tonnage was the tax on tuns (casks) of wine that held approximately 252 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 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 straight-forward and transparent method of assessment.
CONTAINER SHIP
“Twenty-foot equivalent unit”
The twenty-foot equivalent unit (often TEU or teu) is an inexact unit of cargo capacity often used to describe the capacity of container ships and container terminals.[1]
It is based on the volume of a 20-foot long intermodal container, a standard-sized metal box which can be easily transferred between different modes of transportation, such as ships, trains and trucks.[1]
One TEU represents the cargo capacity of a standard intermodal container, 20 feet (6.1 m) long and 8 feet (2.4 m) wide.[1] There is a lack of standardisation in regards to height, ranging between 4.25 and 9.5 feet (1.30 and 2.9 m), with the most common height being 8.5 feet (2.6 m).[2] Also, it is common to designate 45-foot (14 m) containers as 2 TEU, rather than 2.25 TEU.[3]
A 20-foot (6.1 m) long ISO container equals ½ FEU.
“Equivalence”
TEU capacities for common container sizes
Length Width Height Volume TEU
20 ft (6.1 m) 8 ft (2.4 m) 8.5 ft (2.6 m) 1,360 cu ft (39 m3) 1
40 ft (12 m) 8 ft (2.4 m) 8.5 ft (2.6 m) 2,720 cu ft (77 m3) 2
45 ft (14 m) 8 ft (2.4 m) 8.5 ft (2.6 m) 3,060 cu ft (87 m3) 2[3] or 2.25
48 ft (15 m) 8 ft (2.4 m) 8.5 ft (2.6 m) 3,264 cu ft (92.4 m3) 2.4
53 ft (16 m) 8 ft (2.4 m) 8.5 ft (2.6 m) 3,604 cu ft (102.1 m3) 2.65
High cube
20 ft (6.1 m) 8 ft (2.4 m) 9.5 ft (2.9 m) 1,520 cu ft (43 m3) 1[2]
Half height
20 ft (6.1 m) 8 ft (2.4 m) 4.25 ft (1.30 m) 680 cu ft (19 m3) 1[2]
As noted above, the TEU is an inexact unit, and hence cannot be converted precisely into other units.
The related unit forty-foot equivalent unit (often FEU or feu) however is defined as two TEU. The most common dimensions for a 20-foot (6.1 m) container are 20 feet (6.1 m) long, 8 feet (2.4 m) wide, and 8.5 feet (2.6 m) high, for a volume of 1,360 cubic feet (39 m3).
However, both 9.5 feet (2.9 m) tall High cube and 4.25 feet (1.30 m) half height containers are also reckoned as 1 TEU.[2][3] This gives a volume range of 680 cubic feet (19 m3) to 1,520 cubic feet (43 m3) for one TEU.
While the TEU is not itself a measure of mass, some conclusions can be drawn about the maximum mass that a TEU can represent. The maximum gross mass for a 20-foot (6.1 m) dry cargo container is 24,000 kilograms (53,000 lb).[4] Subtracting the tare mass of the container itself, the maximum amount of cargo per TEU is reduced to approximately 21,600 kilograms (48,000 lb).[4]
The MV Emma Mærsk officially carries 11,000 TEU although it is believed to actually carry up to 15,000.[5][6]
Similarly, the maximum gross mass for a 40-foot (12 m) dry cargo container (including the 9.5 feet (2.9 m) high cube container) is 30,480 kilograms (67,200 lb).[4] After correcting for tare weight, this gives a cargo capacity of 26,500 kilograms (58,000 lb).[4]
Twenty-foot, "heavy tested" containers are available for heavy goods such as heavy machinery. These containers allow a maximum weight of 67,200 pounds (30,500 kg), an empty weight of 5,290 pounds (2,400 kg), and a net load of 61,910 pounds (28,080 kg).
“Construction”
Container ships are designed in a manner that optimizes space. Capacity is measured in Twenty-foot equivalent unit (TEU), the number of standard 20-foot containers measuring 20 × 8.0 × 8.5 feet (6.1 × 2.4 × 2.6 metres) a vessel can carry. This notwithstanding, most containers used today measure 40 feet (12 metres) in length. Above a certain size, container ships do not carry their own loading gear, so loading and unloading can only be done at ports with the necessary cranes.
However, smaller ships with capacities up to 2,900 TEU are often equipped with their own cranes.
Informally known as "box boats," they carry the majority of the world's dry cargo, meaning manufactured goods. Cargoes like metal ores or coal or wheat are carried in bulk carriers. There are large main line vessels that ply the deep sea routes, then many small "feeder" ships that supply the large ships at centralized hub ports.
Most container ships are propelled by diesel engines, and have crews of between 20 and 40 people. They generally have a large accommodation block at the stern, near the engine room. Container ships now carry up to 15,000 TEU (approximately equivalent to 35 100-car double-stack intermodal freight trains) on a voyage. The world's largest container ships, the M/V Emma Mærsk and her sisters, have a capacity of 15,200 containers.[2]
In 2008 the South Korean shipbuilder STX announced plans to construct a container ship capable of carrying 22,000 TEU,[3] and with a proposed length of 450 metres and a beam of 60 metres.[4] If constructed, the container ship would become the largest seagoing vessel in the world.[5]
CARGO SHIP
“Sizes of cargo ships”
Cargo ships are categorized partly by their capacity, partly by their weight, and partly by their dimensions (often with reference to the various canals and canal locks through which they can travel). Some common categories include:
• Dry Cargo
o Small Handy size, carriers of 20,000 long tons deadweight (DWT)-28,000 DWT
o Handy size, carriers of 28,000-40,000 DWT
o Seawaymax, the largest size which can traverse the St Lawrence Seaway
o Handymax, carriers of 40,000-50,000 DWT
o Panamax, the largest size which can traverse the Panama Canal (generally:
vessels with a width smaller than 32.2 m)
oCapesize, vessels larger than Panamax and Post-Panamax, which must traverse the Cape of Good Hope and Cape Horn in order to travel between oceans
•Wet Cargo
- Aframax, oil tankers between 75,000 and 115,000 DWT. This is the largest size defined by the average freight rate assessment (AFRA) scheme.
- Suezmax, the largest size which can traverse the Suez Canal
- VLCC (Very Large Crude Carrier), supertankers between 150,000 and 320,000 DWT.
- Malaccamax, the largest size which can traverse the Strait of Malacca
HOPPER BARGE
Hopper barge is a kind of non-mechanical ship or vessel that cannot move around by itself, unlike some other types of barges. Designed to carry materials, like rocks, sand, soil and rubbish, for dumping into the ocean, a river or lake for land reclamation.
Hopper barges are seen in two distinctive types; raked hopper or box hopper barges. The raked hopper barges move faster than the box hoppers; they are both designed for movement of dry bulky commodities.
There are several "hoppers" or compartments between the fore and aft bulkhead of the barge. On the bottom of the barge hull there is (are) also a large "hopper door(s)", opening downwards. The doors are closed while the vessel is moving, so she can carry the materials that are to be dumped. The door(s) open when the ship has arrived at the spot where the materials are to be dumped.
Split barges serve the same purpose, but instead of a door in the hull's bottom, the hull of the whole barge splits longitudinally between the end bulkheads.
The vessel consists of two major parts (port and starboard halves), both are mostly symmetrical in design. Both parts of the vessel are hinged at the deck and operated by hydraulic cylinders. When the vessel splits the load is dumped rapidly, which means the barge has to be very stable in order not to capsize or otherwise get damaged.
