2. By means of equidistant planes in horizontal sections parallel to the load water-plane. (In the ship, Fig. 155, it has been found necessary to introduce a "half" waterline at the bottom.)

In calculating out the underwater volume of a ship both these methods are adopted. An excellent check is thus obtained on the accuracy of the work, because the volume as found by either method should be the same. The following example will illustrate this

The underwater body of a yacht is divided by transverse planes 10 ft. apart, and the following are the areas, viz.—

0.3, 22.7, 48.8, 73-2, 88·4, 82-8, 58·7, 26·2, 3·9 square ft.

The same body is divided by horizontal planes, 1 ft. 6 in. apart, having the following areas, viz.—

944, 795, 605, 396, 231, 120, 68, 25, 8 square ft.

To find the volume

The following is the calculation using the areas of the transverse sections, the the curve of areas being given in Fig. 153

A similar calculation is made to find the volume of a coalbunker.

EXAMPLE.-A coal-bunker has sections 173 ft. apart, and the areas of these sections are 98, 123, 137, 135, and 122 square ft. respectively. The following calculation will determine the volume

8902

Reckoning 43 cubic ft. of coal to the ton, the capacity of the bunker = 43

= 207 tons.

The capacity of each coal-bunker on board ship is calculated. in this way, the height being taken to the under side of beams, and the capacity in cubic feet thus found is marked in some conspicuous place on the bunker bulkhead.

For Welsh coal 40 cubic ft. per ton is taken, for North Country coal 43 cubic ft. per ton.

CHAPTER XV.

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

In one case the paint removed from the crew space of a destroyer weighed over 2 lbs. per square foot,