Cargo and Carrying Capacity of Ships
Computations and Detailed Explanations
Cargo and Carrying Capacity
Weight of world's seaborne trade.—No organism can be understood unless its functions are understood. The function of world shipping is to carry the seaborne trade of all countries. Therefore, discussing the cargo, nature, and volume is imperative.'
The first question to be answered is: What is the total volume or the aggregate tonnage of the commodities shipped from one country to another? In other words, what is the total weight of the world's seaborne trade? One does not have to be a statistician to realize the difficulties which render an accurate answer to this critical question well nigh impossible.
It is fortunate, therefore, that so eminent an authority as the "Departmental Committee appointed by the British Board of Trade to consider the position of the Shipping and Shipbuilding Industries After the War," whose chairman was Alfred Booth, the directing genius of the Cunard Line, has ventured an estimate based upon all data available.
According to this authority, the total weight carried by seagoing vessels plying between the different countries of the world averages 250 to 300 million tons a year. This approximation refers to the days when the war had not yet disarranged the mechanism of international trade.
1 We shall reserve the discussion of passenger traffic for a later chapter and confine ourselves to an analysis of the freight business.
Comparison with Other Weight Statistics
Such a figure means little to the average person and gains significance only compared with similar statistics. We shall compare it first with the total weights of the commodities produced in the United States.
The Shipping Board, through its statistical division, has compiled tables giving the weight of the most important agricultural and mineral products of the United States for the year 1917. These figures show a little less than a billion and a half tons.
Leading economists like Day, Stewart, Kemmer, and King have compiled careful production figures. Mr. Herbert Hoover, Secretary of Commerce, in an article that appeared in the Saturday Evening Post of April 10, 1920, gives the following table, which covers the great bulk of our products, namely, agricultural products, metals, coal, salt, cement, lumber and the products of quarries.
Production in Tons
Transportation returns show similar results. Recent railway statistics tell us that, after eliminating all duplications, the total weight of freight carried by the railroads of the United States is approximately 1,100 million tons.
While it is true that this country produces more than half of the world's total output of many essential products, nevertheless, all things considered, the world's total production of the principal agricultural and mineral commodities is probably a multiple of the figures given for the United States.
We see, therefore, that despite the unprecedented development of ocean navigation, which has marked the last one hundred years, and even though many necessities of life are today moved by water from their place of origin to distant lands, only a relatively small percentage of the world's total production enters into seaborne trade. Nevertheless, the seaborne trade of today is enormous.
Weight of seaborne trade by countries. The United Kingdom supplies the most satisfactory statistics of seaborne exports and imports among the commercial nations. One reason is that, in this case, total trade and seaborne trade are identical.
British foreign trade during selected years showed the following weight totals:
Weight of Foreign Trade of the United Kingdom (In Million Net Tons)
After careful consideration of all data available to us, we believe the following to represent fairly closely the weight balance of our own seaborne foreign trade:
Weight of Seaborne Foreign Trade of the United States in Million Gross Tons
For further purposes of comparison we give in the following table the amount of freight moved in American-owned ships in both coastwise and overseas service :1
Freight Shipped by Commodities and by Divisions
1 All tons of 2,000 pounds. 3 Does not include 80,048 tons of freight carried on fishing vessels.
1 Transportation by Water-1916, Bureau of the Census,. Washington, 1920.
Tonnage Required to Haul Seaborne Trade
The next question refers to the number of ships required to move these 250 or 300 million tons across the seven seas. The answer to this question depends, in the first place, upon the ship's space required to carry a unit —say a ton—of a given commodity, secondly, upon the length of the haul, that is, the distance between the points of origin of shipments and the points of destination and, finally, upon the speed of the ships.
The last two factors in the main determine the number of turnarounds a ship can accomplish within a year. However, this item is also affected at times like the present, even vitally so by the length of the average stay in port. Of course, many other influences come into play and make a mathematical solution to this problem impossible.
We mention only the most important:
- Ships are frequently not loaded to capacity.
- They often perform part of their voyage altogether in ballast.
- Passenger trade interferes in varying degrees with the full utilization of the available space.
Apart from these modifications, the main facts of the situation are these. Before the war, the merchant marines of the world aggregated approximately 50 million tons gross or about 35 million tons net. These ships, in the course of the fiscal year 1911-1912—the only year for which such an estimate has been compiled — made a sufficient number of voyages to bring the total of entrances of vessels, with cargoes and in ballast, _ at the ports of all countries, to 570 million tons net.
Unfortunately, the fact that many steamers call at intervening ports makes the reliable calculation of the average number of turnarounds, or the average haul, impossible.
Elements of Ship Measurement
More definite information is available regarding the other factor, — namely, the respective tonnage requirements of different commodities. Before approaching this subject, the terminology used in determining ship capacity and cargo measurement must be understood. For that purpose, we will begin by explaining the meaning of "ton" as a weight and measurement unit.
Early Tonnage Calculations
An exciting account of the early history of tonnage calculations appeared in a recent issue of the Nautical Gazette.
"Writing in the March number of the United States Naval Institute Proceedings, Lieutenant Commander Carl H. Hermance says that the term tonnage appears to have originated from the tun, cask of wine, the earliest system of measuring vessels being simply to count the number of casks or tuns of wine which could be carried and thus obtaining a measure of the internal capacity. In the reign of Henry V, A. D. 1422, the first act dealing with measuring vessels of which any record can be found required "Keels that carry coals at New-castle to be measured and marked."
"The 'keels' were marked by nails upon the bulkheads at each end of the cargo space or by driving nails into the stem and stern-post to indicate the corresponding load draft. In 1694, another act of the British Parliament was passed to measure keels, and a weight was then fixed as a standard instead of a measure. This act required 'keels' to be measured by putting them deadweights of iron or lead. It allowed 53 hundredweights to every chaldron of coals and a maximum load of 10 chaldrons or 261% tons. The load line was then marked on the stem, stern, and each side amidships.
"The measurement of ships, distinct from the rough estimates of tonnage found in early records, appears to date from the first part of the 17th century. "In 1720, a rule for measuring vessels, which was ultimately known as builders' tonnage, was first legalized in an act intended to prevent smuggling by prohibiting small vessels- of 30 tons burden and under from carrying spirits.
"In 1773, a general rule, which came to be known as the builders' old measurement rule, for the measurement of all merchant's vessels was made by Act 13 George III, and this, with some slight modifications, continued in force until 1835 and had a most evil effect upon naval architecture.
"As the register tonnage is that upon which a vessel has to pay dock and other dues, while the deadweight carrying capacity represents the earning power of an ordinary cargo vessel, it was evident that from a shipowner's point of view, the most profitable ship is the one which can carry the greatest amount of cargo concerning her register tonnage.
"The method of estimating the official tonnage known as the builders' old measurement rule, in which the square of the breadth entered into the calculations, while the depth was neglected, fostered forms, to produce a larger carrying power with a comparatively small official tonnage. This resulted in the construction of unhandy box vessels, which were positively dangerous because of their liability to capsize.
"The modern tonnage laws aim at accurately ascertaining a vessel's internal capacity. Hence, there is not now the same inducement to build such badly proportioned ships."
Today, there are three ways of applying the word ton to a ship: displacement ton, deadweight ton, and registered ton, subdivided into gross ton and net ton.
The displacement tonnage indicates the weight of the vessel. Therefore, according to a well-known principle of physics, it is equal to the weight of the water it displaces. We distinguish between the vessel's "light" and " loaded " displacement.
The former indicates only the weight of the vessel, a standard crew, and adequate supplies. The displacement "loaded" includes the weight of the cargo and bunkers i. e., coal or fuel oil. The ratio of this capacity to the ship's weight differs according to the hull construction.
A so-called displacement curve and scale are prepared to show the carrying capacity at each successive foot or inch that, with increasing load, the ship is further submerged. Displacement tons may number 2,240 or 2,204.62 pounds avoirdupois, according to whether the English or the metric measurement system is used.
Deadweight Carrying Capacity
The difference between the displacement "light" and the displacement "loaded" indicates the maximum carrying capacity of a ship and is also known as its deadweight tonnage.
The same unit, therefore, measures this deadweight tonnage as displacement tonnage, namely, a ton of either 2,240 or 2,204.62 pounds avoirdupois. A ship has a different deadweight capacity for coal than for cotton because of the difference in the specific weight of these commodities.
This term is not ordinarily applied to passenger or combination passenger and freight ships but usually serves as the basis for chartering pure cargo vessels engaged under a time charter.
The third class of ship tonnage is the registered tonnage, which gives the cubic contents of the space in a ship, as defined by rules adopted by the different governments of the sea-faring nations or by the rules of such companies as the Suez Canal Company.
This space is measured in tons of 100 cubic feet, a unit proposed in 1852 to the British Government by Mr. George Moorsom and first incorporated in the British measurement law of 1864 and since then adopted by the world's leading maritime nations. The United States, for instance, adopted the Moorsom rule in 1864.
According to the national gross tonnage rules of the United States, as interpreted by the Commissioner of Navigation, the following spaces are exempted from measurement :
- Sheltered places or superstructures with openings at the sides or ends. This exemption resulted from how the United States Commissioner of Navigation interpreted the rules on September 5, 1914.
- So-called shelter-deck spaces, i.e., spaces beneath a 'shelter deck' with approved 'tonnage openings.' This exemption was allowed on March 16, 1915, and is also the result of the interpretation of the national measurement rules by the Commissioner of Navigation. Both of these exemptions had, for many years, been granted under the measurement rules of Great Britain. They had also been accepted in Germany since 1895 when the endeavor to induce Great Britain to measure all enclosed superstructures and shelter-deck spaces was abandoned.
- Passenger accommodations in tiers of superstructures over the first tier above the upper deck.
- Hatchways up to one-half of 1 percent of the vessel's gross tonnage.
- Galleys, bakeries, toilets, and bath houses above decks.
- Spaces above decks occupied by the ship's machinery or for the working of the vessel.
- Light, air, and funnel space over the engine and boiler room to the extent that such space is above the upper deck or the shelter deck when the shipowner makes special requests to have the space measured.
- Domes, skylights, companionways (except the portion used as a smoking room), ladders, and stairways are in exempted spaces.
- Double bottoms for water ballast since March 2, 1895, and other spaces adapted only for water ballast since February 6, 1909.
- Open spaces occupied by deck loads."1
1 Johnson and Huebner, Principles of Ocean Transportation, pp. 115, 116.
Gross and Net Registered Tonnage
Because of these deductions, gross registered tonnage does not indicate the actual gross capacity of a vessel. The same holds for the net registered tonnage, calculated by deducting the cubic contents of specific spaces from the gross tonnage.
Under the national measurements rule of the United States, these spaces are as follows :
- Spaces occupied by the propelling machinery and fuel.
- Spaces occupied by or appropriated to the use of the crew, officers, and master, subject to the navigation laws, which specify that a minimum crew space varies from 72 to 120 cubic feet and from 12 to 16 square feet of floor space per man must be provided on American vessels.
- Spaces are used exclusively for the helm, capstan, and anchor gear working unless they are located above decks and consequently have been excluded from gross tonnage.
- Spaces used for keeping charts, signals, and other instruments of navigation.
- Spaces occupied by the donkey engine and boiler if located below decks and connected with the main pumps of the vessel.
- Spaces required for boatswain's stores.
- Galleys, bakeries, toilets, and bathrooms for the accommodation of officers and crew when situated below decks.
- Spaces on sailing vessels used for storing sails not exceeding 2% percent of the gross tonnage.1
1 Ibid, p. 119.
Various Tonnage Calculations
The fact is that neither the gross nor the net registered tonnage gives an accurate picture of the ship's capacity. When, therefore, the Suez Canal Company, and later the Administration of the Panama Canal, determined to make the net registered ton the basis for the calculation of tolls to be charged, a new set of rules was adopted which does not differ materially in the case of the two outstanding inter-oceanic canals, and it is to be hoped that someday an international agreement will be reached towards establishing a uniform system throughout the world.
The following table indicates the degree of difference existing at present between the several tonnage calculations outlined above:
Comparative Tonnage Statement 1
The Panama Canal Tonnage is the highest figure in each case, with either the Suez tonnage or American Register second.
1 Adapted from Johnson & Huebner, op. cit. p. 123.
Cargo, Weight, and Measurement Tons
Now, when we come to the measurement of cargo, we find that the same division into weight measurements and measurements of volume or cubic contents is found. The weight ton of the cargo is the same as the displacement ton. The deadweight ton in the case of a ship, but the measurement ton of cargo is only 40 cubic feet compared with 100 in the case of the vessel. The result is that 2% measurement tons of cargo will fit into one registered ton, ship measurement.
For this reason, the deadweight capacity of a ship is frequently calculated at 2% times the figure of its net registered tonnage.
For a modern freight steamer, the following relative tonnage figures would ordinarily be approximately correct:
- Net tonnage 5,250
- Gross Tonnage 6,850
- Dead-weight carrying capacity 10,000
- Displacement, loaded, about 13,350 (Note 1)
Note 1 From K N. Hurley, The New Merchant Marine, 1920, p. 276.
We can now resume our discussion of the tonnage requirements of different commodities. When the ship is about to be loaded with a particular commodity, the most important figure that the ship owner or ship agent has to know is the stowage factor, that is, the figure that represents the number of cubic feet of cargo space in which a long ton, (2,240 pounds) may be stowed.
It is customary in shipping practice to quote freight rates based on "weight or measurement, ship's option," that is to say, if the cargo measures more than it weighs, the freight charge is calculated based on cubic feet; if it weighs more than it measures—based on pounds or weight tons.
A commodity weighs more than it measures if it weighs more than 56 pounds per cubic foot, figure 56 being arrived at by dividing 2,240 (the number of pounds in the cargo weight ton) by 40 (the number of cubic feet in a cargo measurement ton).
It was the assumption that the average weight per cubic foot of all commodities was 56 pounds, which led to the adoption of a cargo measurement ton of 40 cubic feet.
Another explanation is that 40 cubic feet was made the standard because this happened to be the load factors of Russian wheat, at one time the most important staple product carried by ships.
General Classification of Commodities
From a shipping standpoint, commodities are generally divided into three main groups :
- Rough, low-price commodities, such as coal, timber, ores, stones, slates, fertilizers, etc.
- Bulky commodities of medium value include grain and other foodstuffs, textile materials, crude metals, oleaginous produce, petroleum, hides, skins, leather, and bulky manufactured goods.
- Fine goods of all kinds are highly valued in relation to their bulk.
Tables of Unit Displacement of Commodities
In 1919, the Bureau of Research and Statistics of the War Trade Board compiled a list of the most important ship cargoes, a copy of which was mailed to the American Expeditionary Force at the request of the General Staff and which, since the time of its publication, has been in constant demand as a source of current reference. 2
To show the nature of the plan and the degree of its completeness, we herewith reproduce part of the first page of the list.
1 See B. 0. Hough, Ocean Traffic and Trade, pp. 110, 111.2 Of equal interest is a Bureau of Standards, Department of Commerce circular entitled, Table of Unit Displacement of Commodities. This publication gives the weight per cubic foot, space per short ton, space per long ton, and methods of packing an extensive list of commodities.
Stowage of Ship Cargoes
Note.—In conformity with shipping practice, measurements are given in cubic feet and twelfths of a cubic foot. Thus, "8-9" signifies eight and nine-twelfths cubic feet.
Stowage represents the number of cubic feet of cargo space in which a long ton (2,240 pounds) may be shipped. Thus, 110 cubic feet is the cargo space required for a ton of abrasives packed in cases averaging 178 pounds gross and measuring 8-9 cubic feet.
Stowage factors of some essential commodities.—Furthermore, we have extracted from the bulletin, which covers sixty-nine pages, the data referring to the essential commodities.
- HURLEY, E. N. The New Merchant Marine. (1920).
- JOHNSON, E. R. Measurement of Vessels for the Panama Canal. (1913).
- JOHNSON AND HUEBNER. Principles of Ocean Transportation. Chap. IX. (1919).
- NATIONAL FOREIGN TRADE COUNCIL. Ocean Shipping. Sec., Ed. 1917.
- UNITED STATES. (War Trade Board, Bureau of Research and Statistics). Stowage of Ship Cargoes. (1919)
- (Department of Commerce, Bureau of Standards). Table of Unit Displacement of Commodities. (1919).
Source: Ocean Shipping. By Erick W. Zimmermann. New York: Prentice-Hall, Inc., 1921. Pages 207-222