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Tonnage

feet, cubic, gross, measurement, net, deck, vessel, ship, space and depth

TONNAGE, the carrying capacity of a ship. As this capacity is variously measured the word has several special meanings. As it comes from the shipyard the ship's capacity is rated as "dead weight tonnage." As it is measured by maritime surveyors and registered as of so many tons, its capacity is spoken of as "gross registered tonnage" and net registered ton nage? both figures being given, as in different ports dues collected variously, sometimes on the gross and sometimes on the net ton nage. War vessels, which have no ((carrying capacity" in the mercantile sense, are rated by their "displacement tonnage." Still another form is recognized among shippers as "cargo tonnage" or "measurement tonnage." Deadweight Tonnage is the amount of deadweight tons of 2,000 pounds which can be loaded upon a vessel at load-line (q.v.) draught when she has on board her full complement of stores and fuel.

Displacement Tonnage is the weight of sea water actually displaced by the vessel. It is computed usually by calculating from the drawings of the ship by the naval architect the cubic content of the immersed hull in feet and adding tb that figure the number of cubic feet in the propeller, the shafting exterior of the hull and the submerged portion of the rudder. This total is divided by 35, as 35 cubic feet of sea water weigh almost exactly one ton.

In ascertaining the carrying capacity of a ship under the old style of measurement (ab breviated 0. M.) the depth of the vessel was assumed to be the same as its breadth and the tonnage was obtained by multiplying the length by the breadth by the depth and dividing the product by 94, the quotient tons bur den. But this rule was found to be impracti cable, since shipbuilders sought to evade ton nage and harbors dues by building their ships very narrow and deep. In 1835 the British Parliament remedied these defects by new measurement laws, which were amended by the Merchant Shipping Acts of 1854 and 1894. Under this system, known as the Moorsom, actual measurements of the depth of the ves sel are made at certain intervals, the number of which depends on the length of the tonnage deck. and at these points transverse areas are computed.

Gross and Net Registered For purposes of measuring tonnage the United States practice divides vessels into six classes based upon their length (the British into five). These lengths and the number of longitudinal sections into which they are respectively di vided are as follows: I, vessels under 50 feet long, into six parts; II, between 50 feet and 100 feet, into eight parts; III, between 100 and 150 feet, into 10 parts; IV, between 150 and 200 feet, into 12 parts; V, between 200 and 250 feet, into 14 parts; VI, over 250 feet, into 16 parts. These divisions are set out on the "tonnage deck," which in a vessel with less than three decks is the upper deck; in a vessel with three or more decks is the second deck, counting from below. The length of this deck is measured on its centre line from the inside of its inner most plate or plank at the bow to the inner most side of the plate at the stern, but making allowance for rake. The stations being marked off at equal distances upon this line, measure ment of the transverse areas are made at each station, the depth being first taken. This is the distance from one-third up the round of the beam at the bottom to the top of the deck tim bers, allowing two and one-half inches for ceil ing. If this depth at midship is more than 16 feet the figure representing the depth is divided into six equal parts and a transverse meas urement is made at each division, and at the top and the bottom. Counting from above, the

second, fourth and sixth measurements are multiplied by four, and the third and fifth are multiplied by two. These products are added together, and to the sum thus obtained are added the top measurement and the bottom measurement. This total is then multiplied by one-third of the equal vertical distance into which the depth was divided, and the total is accepted as the transverse area at that station. These transverse areas are numbered consecu tively from the bow to the stern. Omitting the first and the last, the second, fourth and each succeeding area with an even number is multi plied by four; and the third, fifth and each succeeding area with the odd number is multi plied by two. These several products are then added together and the first and last also added, and the sum is then multiplied by one-third the equal longitudinal distance between the sta tions. The total is the cubic content of the ship in cubic feet. This figure is arbitrarily divided by 100 and the quotient is the underdeck tonnage. To this is added the number of cubic feet in all deckhouses and permanently cov ered-in spaces which are above the tonnage deck, and which arc or may be used to pack with cargo or use as transport for passengers. These additions are also divided by 100 and added to the underdeck tonnage to make the gross registered tonnage. The net registered tonnage is computed by deducting from the gross the cubic content of the engine-room (including the shaft tunnel), the crew space, cable lockers, coal bunkers, chartrooms and a,11 space needed in the navigation of the ship, and such part of the space in the double bottom as is not available for fuel, stores or cargo. In calculating the ship's registry tonnage it is a common practice to compare the engine space with the gross tonnage, and if it is above 13 per cent, and under 20 per cent, to make an arbitrary deduction of 32 per cent for a screw vessel and 37 per cent for a sidewheeler in making the figure for the net tonnage. These deductions from the gross tonnage differ at dif ferent ports, and for this reason the ship always carries its gross tonnage figures as well as the net figures for the calculation of port and canal dues by the local authorities.

Cargo or Measurement Tonnage is the true cubic content bf the ship in cubic feet divided by 40—on the principle that a ton (2,000 pounds) of average cargo will occupy 40 cubic feet in the ship's hold. It is on this as sumption that the freight charges for transpor tation are fixed. Where the cargo will weigh more than 2,000 pounds to the 40 cubic feet of space, as with stone, metals, cement, etc., the actual weight of the material is used as the basis of figuring the freight.

The British system of measurement was adopted by the United States in 1864, and sub with slight variations, by all mari time nations. At Suez, Panama and other canals where tolls are based on tonnage, the management have established rules of their own for measurement, as they have to deal with vessels of all nations. Under modern rules account is taken of the space in double bottoms, now commonly utilized for feed, water, oil, fuel, etc., and no allowance of more than 5 per cent of the gross tonnage for crew space is countenanced. Deckhouses, however, which are used only as lounging rooms by passengers accommodated elsewhere in the vessel are also deducted in arriving at net tonnage.