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NAVY LIST DISPLACEMENT, TONNAGE, ETC.
Length.—The length of ships of the Royal Navy stated in all official documents is always the length between perpendiculars. The forward perpendicular is a vertical through the intersection of the fore side of stem with the normal load waterline, the ship being supposed to be floating at this line (see Figs. 67 and 68). The after perpendicular (a) in ships with the rudder hinged at the fore side, is taken as the after side of the sternpost, as in Figs. 69 and 75 ; (b) in ships with balanced rudders, it is taken as the centre line of the rudder-head (see Figs. 71, 72, and 79).
The length on the load waterline includes the overhang of the stern at this line. In the United States Navy, for instance, this is the length used in stating a ship's dimensions. Comparisons between ships are apt to be misleading if the lengths are not taken on the same basis. It is believed that the usual French practice is similar to that in the Royal Navy.
The length of ship over all includes the overhang of the stern and the projection of the ram; the length for docking purposes includes also the overhang of the stern walk, if any.
Breadth,—The breadth stated is the breadth of the hull at the broadest part as designed. It sometimes happens that the actual breadth as built is slightly greater than this. For docking purposes the projection of casemates, bilge keels, etc., must be considered, as also the shape of the dock and dock entrance.
Navy List Displacement.—This is always used in official documents, and is a figure which attaches to the ship so long as she remains in the Navy. It is the total designed weight, including the estimated weight of hull, machinery, armour, and armament, legend weights of water, stores, and coal, and a weight appropriated to a Board Margin. The bunkers in this condition are assumed about half full of coal; it thus represents a mean condition of the ship.
Draught.—The draught of water corresponding to this Navy List displacement is the normal load draught.
It does not follow that, when the ship is finished, she will, with legend coal, etc., exactly float at the designed load waterline. For instance, the Board Margin may not be appropriated, or only a portion of it. The weights of hull, machinery, armour, or armament may turn out greater or less than estimated at the time of the design. Ships of the same class, built at different yards, from the same drawings and specification, sometimes differ among themselves by considerable amounts.
The draught marks are not usually set up on the ship at the perpendiculars, but at the points where the keel cuts up at the bow and stern. In the special case of destroyers and vessels with propellers below the line of keel, a set of draught marks are set up on the shaft brackets, showing the draught at the bottom of propeller sweep. The draught marks at the after cut up of the keel are used for docking purposes only.
The draughts of water of each ship in the Navy, as completed, in three conditions of the ship, are given in the stability statement furnished to the ship's book.
(a) The normal load draught, the ship being fully equipped, with reserve feed-tanks empty, and with the legend weights of coal, etc., on board.
(b) The deep load draught, the ship being fully equipped with fresh-water and reserve feed-tanks full and bunkers full.
(c) The light draught being an extreme light condition of the ship. All coals, water (including reserve feed), provisions, officers' stores and slops, and one-half the carpenter's, boatswain's, and engineer's stores are assumed to be consumed. No expenditure of ammunition or shell is assumed for this condition.
For vessels with considerable sail, like the sloops, which are likely to proceed under sail alone, the light condition is taken as above, but with the boilers quite full, and the engine condensers and feed-tanks empty.
A specimen stability statement is given at the end of Chapter XIX.
It should be stated that ships generally increase in draught somewhat as time goes on, owing to the alterations and additions carried out. Large weights of paint are often worked into a ship, one coat succeeding another, until the weight of the whole is very considerable.1 The question of the draught is looked into occasionally as required, and when necessary a new stability statement is issued. This is specially done when a ship has undergone an extensive refit.
The position of the deep load line is indicated inside the ship by label plates and a broken paint line for the information of the ship's officers.
Trials.—The steam trials of a ship, carried out to ascertain how far the speed estimated at the time of the design has been realized, are run at the normal load draught corresponding to the Navy List displacement. Ships are sometimes tried for this purpose when in an incomplete condition, and they are then ballasted to give the required draught of water. If the trial is, however, simply to determine the power developed, and for the acceptance of the machinery, it is not necessary to bring the ship to the load line, provided that sufficient immersion is obtained for the propellers. Except in special ships, as in destroyers, etc., contractors are not responsible for speed, but only for the power developed under the given conditions. It has been found a great convenience, in some cases, to finish the contractor's trials as early as possible, so that the opening up of the machinery, to determine the final acceptance, may proceed while the ship is being completed in other respects.
Tonnage.—We have seen that war-ships are known by their displacement tonnage, this being the total weight as designed with legend weights on board. This tonnage is specially suitable for war-ships, as these ships have to carry a fixed load of armour, guns, etc. For merchant ships, however, a different system is adopted; here the tonnage is a measure of the internal capacity of the ship.
Gross tonnage is the total closed in capacity of the ship, excluding double bottoms, reckoned in tons of 100 cubic ft. The nett or register tonnage makes certain deductions from the gross for the space occupied by the crew, etc., machinery and coal. The intention is that the nett or register tonnage shall give a measure of the earning capacity of the ship for carrying cargo and passengers.
It is necessary that ships of the Royal Navy should be measured for their tonnage, in order to form a basis for the payment of dues and other charges at foreign ports. A tonnage certificate is issued to all the ships of the Navy from measurements made by the officers of the Board of Trade. There are two systems, viz. the British system and the Danube system. They differ in the allowances made for deck erections and machinery spaces. The following shows how these compare with one another, and how the gross tonnage compares with the displacement tonnage in several cases. The Danube rule is the one used when passing through the Suez Canal.
1 In one case the paint removed from the crew space of a destroyer weighed over 2 lbs. per square foot.
It is seen that the gross tonnage in all cases is considerably less than the displacement tonnage.1
The following extract from the King's regulations gives the instructions concerning the question of tonnage.
The register tonnage according to British rule is to be inserted in all pilotage certificates, and is to be the basis of all tonnage payments at foreign ports by H.M. ships, except when entering Port Said or the Suez Canal, in which case the tonnage according to the Danube rule is to be issued.
The Board of Trade tonnage certificate, which shows the registered tonnage according to both rules, is furnished to all ships as they are commissioned at home ports.
The weight in tons shown in the Navy List is in no case to be used for the payment of pilotage, nor to he mentioned in pilotage certificates.
1 In the Atlantic liner Campania the displacement tonnage is 18,000 tons, the gross tonnage 12,950, and the register tonnage 4973. The new Cunarders are stated to be designed for a displacement of 32,000 to 33,000 tons, or twice that of King Edward VII. The different systems of reckoning tonnage (viz. war-ships by their displacement and merchant ships by their capacity in tons of 100 cubic feet), has obscured the great increase in size of the large liners as compared with the largest war-ships.
BUOYANCY, DISPLACEMENT, TONS PER INCH, ETC.
Buoyancy.—At every point of the surface of a body immersed in water there is a pressure which acts normally to the surface. The amount of this pressure will depend on the depth of the point below the surface. If d be the depth of the point below the surface in feet, and w the weight of a cubic foot of water, then the pressure per square foot is w X d lbs. Thus if a hole of 1 square ft. is made in a ship's bottom 17J ft. below the surface of the water, a piece of wood would have to be held against the
hole with a force of —^ttt^— = 0-5 ton to keep the water out. 2240
It is because of this pressure that diving operations beyond a certain depth are rendered impossible.
In the case of a floating body like a ship, these normal pressures all over the surface act in many different directions. In each case, however, the normal pressure may be resolved into its three components at right angles, viz. (i.) horizontal in a fore-and-aft direction, (ii.) horizontal in a transverse direction, and (iii.) vertical. If the ship is floating at rest, all these horizontal components must balance between themselves, since there is no bodily movement of the ship in any direction. It is the combined effect of all the vertical components which exactly balances the weight of the ship. The single vertical force, which is the resultant of an infinite number of small vertical forces acting on the ship, is termed the buoyancy. In the same way the forces due to the weights composing the ship have a single resultant, which we term the weight of the ship. The vertical forces acting on the ship at rest are therefore—
(a) The weight of the body acting vertically down.
(b) The buoyancy acting vertically up.