FIG. 27. DISPLACEMENT CURVE AND SCALE.

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Bottom of Keel

that is below the water line divided by 35 equal the vessel's displacement tonnage. If a ship were box-shaped-that is, if it were a parallelepiped-the product of its three dimensions in feet, its length, breadth, and its depth below the water line, divided by 35 would be the displacement tonnage; but, as vessel hulls are not parallelepipedons, the cubical contents of the hull of a ship have to be calculated by means of special mathematical rules, such as Simpson's rules or the trapezoidal rules.1

The ratio of the actual contents of the submerged portion of a ship's hull to the contents of a parallelepiped having length, breadth, and depth corresponding to the length, breadth, and draft of the ship is the vessel's "block coefficient" or its "coefficient of fineness." A fullshaped, slow freight steamer has a "block coefficient" of about 0.8-i. e., the submerged portion of the hull has a volume equal to 0.8 of the volume of a parallelepiped with equal dimensions. The "block coefficient" or "coefficient of fineness" of the average freight steamer varies from 0.7 to 0.75, while the coefficient of a combination freight and passenger steamer is about 0.65; that of a fast passenger steamer is about 0.6, while racing yachts may have a coefficient as low as 0.4. When the "coefficient of fineness" of a vessel is known, its displacement tonnage is determined by multiplying its length, breadth, and draft by its "coefficient of fineness" and dividing the product by 35.

In commercial practice it is desirable to know a vessel's displacement tonnage at any given draft between its "light" and "loaded" lines, for the reason that the difference between the displacement of a vessel "light" and the tonnage of its actual displacement indicates the weight of what the ship contains other than a crew and supplies. The displacement tonnage or weight of any particular ship at any given draft is shown by the vessel's "displacement curve" and scale. Figure 27 reproduces a typical displacement curve.

Figure 27 presents the displacement scale for a small vessel which draws but 7 feet of water when light, its displacement "light" being 550 tons. The vessel may load to a maximum draft of 14 feet, at which draft its displacement is 1,400 tons. The deadweight capacity of the ship is thus 850 tons. It may be noted in passing that the ship is permitted to be loaded, so that it has but 2 feet of freeboard, the freeboard being the distance between the level of the upper deck and the "deep-load line." Vessels engaged in the oversea trade would not be permitted to have such a small freeboard.

The figure also gives the ship's displacement curve. The curve is drawn as follows:

At the left the draft of the vessel and its freeboard are given in a perpendicular scale, which may be assumed to have been drawn to a scale of 1 inch to 1 foot. From the top of this vertical scale, a horizontal scale is so constructed that 1 inch equals 100 tons of displacement. By drawing horizontal lines through the points indicating the draft of the vessel at different drafts from zero to 14 feet and by drawing vertical lines through the points in the horizontal scale corresponding to the number of tons of displacement at various drafts from zero to 14 feet, and by drawing a curve through the points of the intersection of, the horizontal and vertical lines, the curve of the ship's displacement is located. With this displacement curve known, the displacement of the vessel at any given point in its draft can be read off from the displacement scale.

ADVANTAGES AND DISADVANTAGES OF DISPLACEMENT TONNAGE AS A BASIS OF PANAMA TOLLS.

If displacement were made the basis of Panama tolls the charges might be placed either upon the vessel's displacement tonnage when loaded to the deep-load line, or upon its displacement tonnage at its actual draft, when applying at the canal for passage through the waterway. If the displacement of the vessel at its actual draft when passing through the canal were made the basis of the tolls, the charges would be levied upon the weight of the ship plus the weight of the cargo, passengers, and fuel it might have on board. The tolls would thus vary with the lading of the ship.

1 Mathematical rules for the calculation of the contents of the hulls of ships are explained, among other places, in Chapter X of the book Know Your Own Ship, by Thomas Walton, London, 1909.

Displacement tonnage would have the following advantages as a basis for canal charges: 1. It would be easy to determine the tonnage upon which tolls were to be paid. The vessel's displacement scale states its displacement tonnage at any draft up to the deep-load line, which represents the vessel's maximum draft. The displacement scale would indicate the number of tons upon which the vessel applying for passage through the canal would have to pay tolls.

2. Tolls based upon the displacement of a vessel at its actual draft would vary with the ship's lading, and vessels without cargo or with a light load would pay less than the ship would pay when fully laden. In the case of low-powered cargo steamers, the weight of a vessel when "light" might be half or less than half the weight of the vessel when fully loaded, and the amount of tolls payable by such ships would be largely affected by the extent to which the vessel's cargo capacity was occupied with freight. On the contrary, high-powered passenger steamers have relatively small capacity for carrying cargo. So much machinery and fuel are required to secure high speed that the weight of the vessel "light" will probably be at least three-fourths of its weight when loaded. In the case of fast passenger steamers, there is comparatively little difference in the weight of the vessel when its passenger and freight accommodations are unoccupied and when they are filled. Such steamers, however, constitute a relatively small share of the tonnage of the world's deep-sea marine. For most ships, tolls based upon the tonnage of actual displacement would vary materially with the lading of the vessels.

3. An advantage of minor importance which displacement tonnage would have as a basis for tolls would be that merchant vessels and warships would pay charges upon the same kind of tonnage. Displacement is the only logical basis for tolls upon warships, and if merchant vessels do not pay canal levies upon displacement, the charges must be levied upon two different bases. While the inconvenience resulting from this would be relatively slight, it obviously would be better to have a single rather than a dual basis for canal charges.

The disadvantages resulting from the adoption of displacement as a basis for canal charges upon vessels of commerce outweigh the advantages, and may be briefly stated as follows:

1. Unless actual displacement were made the basis of canal charges, every vessel would be obliged to have marked upon its hull by official action of the appropriate authority its light line and its deep-load line, because the location of these lines would affect the amount of tolls payable. Freight ships under the British flag have the plimsoll mark placed upon the hull, indicating the draft to which the rules of the British Board of Trade and Lloyd's Association permit the vessel to be loaded. Passenger ships which carry comparatively little cargo, and which usually have several decks above the main deck, always have much more freeboard than the minimum requirements of the law, and thus there is no occasion for them to have a Plimsoll mark or a load line upon their hulls. If, however, the maximum load displacement were made the basis of canal charges, it would be necessary for passenger ships to have their load line officially determined, although the action taken in locating this load line would have to follow rules largely artificial in character.

Quite as much difficulty would be encountered in establishing officially any vessel's light draft, for the reason that the vessel's light line locates the ship's draft when equipped for a voyage with fittings, crew, and supplies. Vessels have their light line established without fuel on board, but an increasing number of vessels now use oil instead of coal for fuel and the oil thus used is often carried in tanks which, in the case of coal-burning steamers, would probably be used for water ballast. An oil-burning steamer when light may have less water ballast than a coalburning steamer. The draft of a vessel without cargo or passengers would not be the same at all times or for all voyages. Thus the establishment of any vessel's light line would necessarily result from the application of arbitrary rules difficult to formulate and more difficult to apply. 2. If the actual displacement tonnage of a vessel at the time of its application for passage through the canal is made the basis of tolls, shipmasters may seek to lessen the vessel's draft temporarily by reducing the amount of water ballast to a minimum limit as the entrance to the canal is approached, in order that the vessel may thereby have less draft and be required to pay less tolls. When the vessel passes from the canal to the sea, the ballast tanks could

again readily be filled, and the ship's necessary ballast at sea could in this manner be easily replaced. It might also be possible for coal companies or even steamship companies to establish stations a slight distance from each entrance to the canal for the purpose of enabling vessels to replenish their bunkers or tanks after having passed through the canal and having paid the tolls. By entering the canal with a minimum amount of coal in the bunkers, and by coaling just after departing from the canal, a vessel would avoid the payment of tolls upon the weight of fuel it would normally carry.

3. The chief and conclusive reasons for basing tolls neither upon the actual displacement nor upon the deep-load line displacement of vessels are that such tolls would be unfair as between different types of ships, and would violate the fundamental principle of giving main consideration to earning capacity in levying canal charges. Tolls upon the weight or displacement of ships would be unfair as between different types of vessels, because fast passenger steamers have maximum weight in machinery, fittings, and fuel as compared with the weight of paying load, while slow cargo steamers have a maximum capacity for freight as compared with the weight of, and space occupied by, machinery, fittings, and fuel. In the case of the passenger steamer, the paying load is relatively light as compared with the nonpaying weight or "tare," while the freight steamer has an earning load heavy in relation to "tare." Otherwise stated, the fast ship of "fine" lines has a large displacement and small dead-weight capacity, while the ship with "full" lines has large carrying space in relation to light displacement. It is manifest that injustice as among different types of ships must result from taxing them upon the basis of their weight. In order to make tolls equitable for different classes of ships, it is necessary to base the charges primarily upon either what the ship is carrying or upon its earning capacity. One method of levying tolls upon what the ship is carrying is to make "dead-weight" tonnage the basis of the charges; and, in order to determine whether that would be a desirable basis for Panama tolls, it is necessary to explain briefly what is meant by "dead-weight" tonnage and what would result from making it the basis of dues payable for the use of the canal.

DEAD-WEIGHT TONNAGE.

A vessel's dead-weight tonnage is the difference between the weight or displacement of the vessel when "light" and when loaded to its maximum authorized draft. It is the number of tons avoirdupois that the ship can carry of fuel, cargo, and passengers; it is the vessel's dead-weight capability, its carrying power.

The term dead-weight is also applied in commercial practice, to some extent, to the weight of coal and cargo actually aboard a ship at a given time. In this sense the dead-weight tonnage of a ship at any particular draft is the difference between its displacement "light" and its displacement at its actual draft.

Would it be wise to levy tolls either upon a ship's maximum dead-weight tonnage or upon the dead-weight of the fuel and lading actually aboard a vessel at the time of application for passage through the canal? As an argument in favor of tolls upon maximum dead-weight tonnage, it is urged that charges based upon the ship's carrying power are placed upon the weight from which the owners of the ship may derive traffic revenues. This argument is strengthened by the fact that the rates charged for the use of chartered vessels-i. e., charter rates are upon dead-weight tonnage and that, inasmuch as a large share of ocean freight is transported in chartered vessels, the commercial world is accustomed to charges based upon dead-weight tonnage.

The advantages to be derived from making maximum dead-weight tonnage the basis of canal tolls are, however, more than offset by the objections to making that tonnage the unit of canal charges:

1. Freight ships, especially those employed in the transportation of bulk cargoes, would be heavily taxed, because of their large carrying power, while passenger steamers having comparatively little dead-weight capability would be but lightly burdened with canal tolls. Unless the rates of toll were different for different types of ships, there would be relative injustice as among different classes of vessels.

2. Even as between freight ships carrying different kinds of cargo the charges would be inequitable. The tolls payable would be largest for vessels loaded with the heaviest, and thus ordinarily the cheapest, commodities. Minerals, nitrate, lumber, grain, and other bulk commodities have large weight in comparison with value, and the canal tolls would fall most heavily upon the classes of commodities that ought to be most favored by the tolls. If cargo were made the basis of tolls, articles which are shipped as package freight ought to be charged tolls not upon their weight but upon their measurement tonnage-40 cubic feet, instead of 2,240 pounds, being considered a ton. This would probably not be practicable, but unless it were done the discrimination against heavy bulk cargoes would be unjust to the shippers of "dead weight freight." Carriers, moreover, would find tolls upon weight of cargo less desirable than charges upon space occupied by freight.

Would it be advisable to base Panama Canal tolls upon the actual weight carried by vessels using the canal? It would seem offhand that tolls upon the actual weight borne by the vessel would be on a proper and desirable basis. Ocean carriers would thus be called upon to pay charges for the use of the canal varying with the amounts transported through the waterway. The tolls would not be placed upon the vessel, but upon what is in the ship, and would be made to vary with the weight of the vessel's burden. Moreover, the tonnage upon which tolls were payable could theoretically be obtained without difficulty. It would be necessary only to read off from the vessel's displacement or dead-weight scale the difference between the ship's "light" displacement and its actual displacement at the time of passing through the canal.

As a matter of fact, however, the objections to tolls based upon the actual weight carried by vessels are stronger than the merits of such a system of charges. There are the same practical and equitable reasons against making actual dead-weight carried the basis of canal charges as there are against the maximum dead-weight tonnage as a basis for tolls. There would be the same difficulty encountered in deciding what should be considered the "light" draft of a vessel and thus what should be taken to be its "light" displacement. Likewise there would be the same inequity of charges as among different types of ships and as between similar ships carrying different kinds of cargo.

"BLOCK DISPLACEMENT.”

A variation from the method of levying tolls upon a vessel's actual displacement at the time of passage through the canal would be to levy the charges upon the vessel's so-called "block displacement" or upon the cubical contents obtained by multiplying the length of a vessel's load water line by the vessel's breadth at the water line by its draft at any particular time. It would be the displacement of a parallelopipedon circumscribing the vessel, or of a block with dimensions equal to the length, beam, and draft of a vessel at the time of passage through the canal. The term "block displacement" is not generally used in tonnage literature, nor is the tonnage obtained by calculating the "block displacement" at present utilized for any purpose.1

"Block displacement" has never been adopted as a basis for canal tolls, dock, or other port dues, or tonnage taxes, nor has it ever been used as the basis for registering ships. Yet the idea of making "block displacement" the basis of shipping charges is a very old one. It was proposed in France as a possible basis for dock charges by the French naval architect Bouguer as early as 1746.2 It was not adopted as the basis for dock charges, and Bouguer did not propose it as a possible basis for registering vessels or for any other purpose.

The tonnage determined by calculating the "block displacement" was also proposed to the Royal Commission on Tonnage of 1881, and was considered by that commission together with other possible bases for dock charges. The majority of that commission rejected all tonnage bases, except net tonnage; one member of the commission favored dead-weight tonnage,

3

1 This system was proposed as the basis for canal tolls, and the term "block displacement" was coined by Capt. C. A. McAllister, Engineer in Chief of the United States Revenue-Cutter Service. Hearings before House Committee on Interstate and Foreign Commerce, Jan. 15, 1912, p. 436. * See White's Manual of Naval Architecture (5th Ed.), pp. 51 and 71-72.

See Appendix XVIII.