CHAPTER IX THE MEASUREMENT OF VESSELS AND TRAFFIC Kinds of vessel tonnage, 110. Displacement tonnage, 110. Deadweight tonnage, 112. Gross tonnage, 114. Net tonnage, 118. Cargo tonnage, 122. Long, metric, and short tons, 122. Measurement tons, 122. Relationship between various kinds of tonnage, 123. Documented and undocumented vessels, 123. References, 124. IN describing the various types of vessels, and in discussing ocean transportation, frequent use must be made of the words "ton" and "tonnage," and, in order to avoid confusion and error, it is necessary to keep clearly in mind the several meanings in which these terms are employed. Tonnage may refer either to the size of the vessel or to the amount of the ship's cargo; accordingly, there are two distinct kinds of tons: the vessel ton and the cargo ton. Each of these two kinds of tons is used with several different meanings. Vessel tonnage is of four kinds: displacement, dead-weight, gross and net. Each has a definite meaning and each has its particular uses. DISPLACEMENT TONNAGE The displacement tonnage of a vessel is its weight, and is equal to the weight of water displaced by the ship when afloat. Unless the term is especially qualified the displacement of a merchant vessel is its weight when its crew and supplies are on board, but before any fuel, cargo or passengers have been taken on. This is the vessel's displacement "light," and is to be distinguished from its displacement "loaded," which is its weight when fully loaded to its maximum draft or deep-load line. A merchant vessel's "actual" displacement during a particular voyage is its weight when loaded to any given draft and varies with the amount of fuel and cargo and the number of passengers on board. In rating war vessels the term "normal" displacement is commonly used. Its meaning varies in the different countries, for, although their rules commonly include the vessel's full complement of officers, men and their belongings, and all general equipment, armament and machinery, they differ as to the allowance of stores, fuel oil, coal and water on board when its displacement is normal.1 There is, likewise, no uniformity among the various countries as to the meaning of a war vessel's "light" displacement. Its "full-load" displacement, on the contrary, is calculated with substantial uniformity throughout the world, and corresponds to the usual meaning of a merchant vessel's displacement "loaded." naval vessel's "actual" displacement, of course, refers to its weight with everything on board when equipped for a particular voyage, and varies from day to day. The displacement of a vessel is expressed in tons of either 2,240 or 2,204.62 pounds avoirdupois, according to whether the English or metric system of measurement prevails. In the United States the displacement tonnage may be found by dividing the contents in cubic feet of the part of a vessel's hull that is below the water line by 35, because a cubic foot of sea water weighs 64 pounds or one-thirty-fifth of a ton of 2,240 pounds avoirdupois. The cubical contents is accurately determined at the time of the vessel's construction by means of special mathematical rules. Since a vessel is not a parallelopiped, i. e., not box-shaped, its cubical contents cannot be determined by a simple multiplication of length, breadth and depth. The marine architect is obliged to determine the ship's "block coefficient" or "coefficient of fineness," which is the ratio of the actual contents of the submerged portion of the vessel's hull to the contents of a parallelopiped of the same length, breadth and depth. When this coefficient is known, the 1 See E. R. Johnson, Measurement of Vessels for the Panama Canal, 104. 2 See Thomas Walton, Know Your Own Ship, chap. x. vessel's displacement may be determined by multiplying the product of its length, breadth and depth by its coefficient of fineness and dividing that product by 35. The coefficient may vary anywhere from 0.8 in case of a full-shaped slow freighter to 0.4 in case of a racing yacht. When determining a vessel's displacement "light" and "loaded," shipbuilders usually prepare a "displacement curve" and scale, such as is shown in diagram on page 113. Knowing the draft of his vessel, a glance at the ship's displacement curve and scale will tell the captain substantially what the actual displacement of his vessel is on any given voyage. The displacement tonnage of merchant vessels is chiefly used in their construction. It is also of use in their loading and operation, for the difference between displacement light and displacement loaded indicates the maximum weight of cargo, passengers and fuel that the ship may take on board; and the difference between displacement light and the tonnage of the ship's actual displacement indicates the weight of whatever the ship contains at any given time other than crew and supplies. The "normal" displacement of naval vessels has the additional use of serving as the basis for officially rating or expressing the size of war craft; and the actual displacement of war ships, other than transports, colliers, supply ships and hospital ships serves as the basis upon which such vessels pay tolls at the Panama Canal DEAD-WEIGHT TONNAGE The carrying capacity of a merchant vessel is sometimes expressed in terms of its dead-weight tonnage, which represents the maximum weight of cargo, passengers and fuel that it is able to carry when loaded to its deep-load line. It is the equivalent of the difference between the vessel's displacement "light" and its displacement "loaded" and is determined by subtraction of the one from the other. The actual deadweight on board at any given time will, of course, vary from voyage to voyage, but can be readily determined with sub stantial accuracy from the displacement curve and scale mentioned in connection with displacement tonnage. Knowing the draft to which his vessel is loaded, the captain can read. the actual dead-weight on board from the curve and scale which is prepared when the vessel is constructed. (See dia gram, p. 113.) Dead-weight tonnage is expressed in terms of either the long ton of 2,240 pounds or the metric ton of 2,204.62 pounds. In ocean navigation it serves as the usual basis for the charter rates paid when vessels are operated on time charters. It is also of use in the loading and transportation, in vessel-load lots, of certain heavy, bulky commodities, such as coal and iron ore, and in the construction of vessels designed for such services; for, knowing the amount of fuel needed to operate over a particular route, the dead-weight tonnage discloses to the master of the vessel the maximum weight of cargo that may be shipped. The term dead-weight tonnage is ordinarily not used in connection with express steamers, combination passenger and freight vessels, or vessels operated in a regular freight line service; for vessels of that type are rarely loaded to their deep-load line, and the prime consideration at the time of their construction is seldom the attainment of maximum capacity for heavy or so-called "dead-weight commodities." GROSS TONNAGE The gross tonnage of a merchant vessel is its total measured cubic contents expressed in "tons" of 100 cubic feet or 2.83 cubic metres.1 The actual cubical contents of any particu 1 This method of stating gross-register tonnage dates from 1854. To secure a uniform practice in measuring and registering vessels, the British Government, in 1852, adopted a method of measuring the cubical capacity of hulls that Mr. George Moorsom had worked out. The Admiralty, not wishing to change the statistics of the tonnage of the British marine more than was necessary, instructed Mr. Moorsom to submit a plan of applying his method in such a way as to cause a minimum change in the existing registry of ships. Mr. Moorsom found that the total registered tonnage of the British merchant marine |