In running progressive trials it is necessary to have the ship as near to the normal load draught as possible. When a ship is deep or light the horse-power required for a given speed will vary approximately as the displacement.

From the information obtained on such a series of trials a curve of I.H.P. on base of speed can be constructed, as Fig 205, and then it can be at once determined what I.H.P. is required for a given speed, or what speed can be got for a given I.H.P under trial conditions. The curve of revolutions on base of speed can also be drawn as shown, and such a curve, showing revolutions necessary for any desired speed, is found very useful in the subsequent history of the ship. Records of all trials are kept in the ship's book for the information of the officers commanding.

Power Trials. The following series of trials are now carried out in ships of the Royal Navy.

1. Battle-ships, first class cruisers, second class cruisers.
(a) Preliminary trial at sea.

(b) 30-hour trial at about one-fifth full power.

(c) 30-hour trial at about 70 to 75 per cent. full power. (d) 8-hour trial at full power.

(e) Trial after opening up (24 hours).

2. Third class cruisers.

(a) Preliminary trial at sea.

(b) 30-hour trial at half forced draught power.

(c) 8-hour trial at authorized natural draught power.
(d) 4-hour trial at full forced draught power.

(e) Trial after opening up (24 hours).

It is important to note the conditions under which the machinery and boilers are designed to exert the maximum power. It is for a certain limited time only in each case. The Drake, for instance, is not a 234-knot ship in the same sense as the Atlantic liner Kaiser Wilhelm der Grosse is a 224-knot ship. The I.H.P. in each case is about the same, viz. 30,000, but in the latter case the machinery is designed to maintain the speed across the Atlantic, and the weight involved is nearly double that for the Drake of the same power. In fact, such fast liners are practically only able to carry themselves and the necessary coals across, the amount of deadweight cargo they can carry being very small. The Drake, on the other hand, carries a large weight of armour and armament, but the machinery is only intended to attain 30,000 I.H.P. for a period not exceeding 8 hours. In a vessel like Pioneer, for instance,

the full power is 7000, but this is obtained by forcing the boilers, and cannot be maintained for a longer period than 4 hours; the authorized natural draught power is 5000, and this is intended for a period not exceeding 8 hours.

In dealing with ships of the Royal Navy the following terms are in use, viz.—

(a) The authorized natural draught power is taken as the unit. (b) "With all despatch," four-fifths the unit, for 30 hours (this is when great urgency is necessary).

(c) "With despatch," three-fifths the unit; this should not be materially exceeded when the period of steaming exceeds 30 hours. (d) "With moderate despatch," two-fifths the unit.

(e) Ordinary speed, one-fifth the unit.

The maximum speed at which the Drake, for instance, could proceed so long as her coal lasted would be about 21 knots, corresponding to 18,000 I.H.P. The speed at starting, with bunkers full, would be something less than this, but it would get greater as the ship lightened.

CHAPTER XXIII.

THE DESIGN OF WAR-SHIPS.

THE service for which a ship is intended to be employed has manifestly the predominating influence on her design. The duties which ships of a navy like the British Navy have to perform are so varied, that no single ship could possibly combine all the qualities needed in war-ships. Thus very high speed, heavy armour protection and powerful armament cannot all be embodied in one design. A compromise is necessarily effected, and if we sacrifice some protection and guns to obtain high speed and large coal capacity we have a cruiser; if we have less speed and pay most attention to protection and armament we have a battle-ship. If we want a ship that shall be able to keep the sea for long periods without docking, we must have a vessel sheathed with wood and copper, and in doing so we have to accept some increase of cost and decrease in measured-mile speed, as compared with a vessel with an ordinary steel skin. Again, a vessel intended for coast defence would need only a moderate coal capacity and a small draught of water. There are navies in which such a type of ship would be valuable; the construction, however, of battle-ships of small size has been discontinued in the British Navy for some years.

The design of a war-ship would be an almost impossible task apart from experience and data obtained from previous ships. When the main features which it is desired to embody in a new design are given by the authorities, it is the function of the naval architect to work out such a design as shall satisfactorily embody those features. Experience in the specialities of war-ship design is a necessary qualification, as the conditions to be satisfied are altogether different to those in the case of merchant steamers. There are many qualities which to a greater or less extent must be found in any war-ship design. Some of these are

1. Strength, both structural and local. We have already discussed this at some length.

2. Stability. This is a vital quality. A war-ship must have sufficient stability left after sustaining a reasonable amount of damage. It is on this account that the metacentric heights given to war-ships are greater than obtain in merchant steamers. The stability at large angles also requires careful consideration, because of the high position of the C.G. of ship. The question of the most economical propulsion frequently has to go into the second place in order to obtain a proper amount of stability.

3. Speed. This depends on the intended service of the ship. Speeds have considerably increased during recent years, this having been rendered possible by the use of watertube boilers, with high steam pressures and high revolutions.

4. Handiness.-The influence of the shape of the stern and the rudder on turning have already been discussed. A short ship also is handier than a long ship, other things being the same.

5. Habitability. This is important because of the necessity of keeping the crew in a good state of health. A high freeboard ship has a great advantage over a low freeboard ship in this respect, the living spaces being much more airy and light.

6. Convenient transport of coal and ammunition.

7. Economy of first cost and maintenance.-These two things are sometimes opposed. Thus a steel ship will be cheaper than a sheathed ship because of the cost of the sheathing and the metal stem, etc., necessary. The cost of maintenance of the sheathed ship, however, will be considerably less than the steel ship, because it will not foul so quickly or require such frequent docking.

8. Length of vitality.-The amount of coal, ammunition, etc., carried by a ship will determine how long she can remain efficient as a fighting machine. The coal will determine the radius of action. 9. Slowness of destruction.-This includes protection by armour and decks, and the provision of minute subdivision.

10. Armament.-Being the available provision for attackguns, torpedo equipment, ram.

Of the first stages of a design, Sir William White says ("Manual of Naval Architecture")—

"In the preliminary stages the processes are necessarily tentative and subject to correction. The various features of the design are, to a large extent, interdependent. At the outset the dimensions, form, and displacement are undetermined. Yet upon them depend the power which the engines must develop to give the desired speed, the weight of the hull, and the weight of certain parts of the equipment. In the finished ship the sum of the weights of the hull structure, propelling apparatus, equipment, coals, and load must equal the displacement to

the specified load-line. Apart from experience, a problem involving so many unknown yet related quantities could scarcely be solved. On the basis of experience, recorded data, and model experiments it is dealt with readily. Approximate dimensions and forms are first assumed. The weight of hull is then approximated to for the system of construction adopted and the type of ship. An estimate of the probable engine power is made, either on data obtained from the steam trials of previous ships or from model experiments. The weight of the engines and boilers is then ascertained for the horse-power, and the rate of coal consumption per hour calculated on the same basis, while the total weight of coal for the intended steaming distance at the desired speed is readily deduced. Adding together these first approximations to the weights of hull, equipment, machinery, and coal, and to the total adding the load stipulated to be carried, a grand total is reached which should equal the displacement provisionally assumed. If the sum total is in excess or defect of the provisional displacement, corrections must be made on the dimensions originally assumed, with a view of obtaining a balance. For these corrected dimensions a fresh series of approximations is made to the weights of hull, equipment, machinery, and coal. A balance between the grand total of weights, and the displacement corresponding to the form and dimensions, is ultimately obtained. When no large departure from previous experience or precedent is made, this preliminary work is rapidly performed. Under other circumstances, the selection of the most suitable dimensions and form may involve the consideration of many alternatives."

The total displacement of a completed design is made up of the following items, viz.-

7. Hull, including structure and fittings. 8. Board margin.

1. General equipment. This includes fresh water; provisions (including bread and spirits); officers' stores (including ward-room and gun-room stores and paymaster's slops); officers, men, and effects; anchors; cables; masts, rigging, etc.; boats; warrantofficer's stores. These weights depend largely on the type of ship and on the complement. The intended service of the ship has an influence, as the weight of stores allowed would be greater or less according as the vessel is intended for distant, isolated service, or with a fleet not far from a dockyard.

2. Armament. The weight of this can be very closely estimated