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THE DESIGN OF WARSHIPS.
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 battleship. 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 attack— guns, 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.—
1. General equipment.
4. Engineer's stores.
6. Armour and protection to hull.
„ „ armament.
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 when the detail of the armament is settled. An important point in connection with this is the number of rounds taken per gun.
3. Machinery.—When the I.H.P. is provisionally settled,1 an estimate is prepared by the Engineers of the necessary weight. This will depend on several things, e.g. the type or types of boiler to be used; the revolutions and stroke of the engines and the speed of the pistons; the degree to which the boilers are to be forced. For the largest set of engines fitted up to date (15,000 I.H.P.), 120 revolutions is the maximum; as engines of lower power are reached higher revolutions are possible; thus the engine for 6250 I.H.P. has 180 revolutions, and for 4900 I.H.P. has 250 revolutions. Destroyer engines are faster still. The adoption of watertube boilers has had a great influence on design in recent years, enabling a larger power to be developed on a given weight than formerly. The employment of turbine machinery will doubtless greatly influence the conditions of design in the near future.
4. Engineer's stores.—The allowance of these stores depends on the intended service of the ship as well as on the power.
5. Coals.—It is the practice in the designed displacement of H.M. ships to include a certain weight of coal. This is called the legend weight. Thus the Royal Sovereign has 900 tons, as also Majestic, Formidable, and Duncan classes. The total capacity available for coal is considerably more than this, being over 2000 tons in the latest battle-ships. All the official steam trials to test the speed are carried out at the draught corresponding to the legend condition. Sometimes, however, trials are carried out before the completion of the ship to determine the acceptance of machinery from the contractors. These trials are simply for I.H.P., and not for speed, and then the ship is not necessarily ballasted to her normal load line, so long as proper immersion is given to the propellers.
6. Armour and deck protection.—The weight devoted to protection may be divided into—
(a) Vertical armour for the protection of the buoyancy and stability;
(b) Vertical armour for the protection of the armament; and
(c) Deck protection.
The percentage of each of these of the total weight of protection in a recent battle-ship was as follows: (a) 38 per cent.; (6) 34 per
1 See Chapter XXII.
cent.; (c) 28 per cent. This shows clearly how large a proportion is devoted to the effective protection of the armament. This includes barbettes and casemates, but not gun shields, which are taken in the armament. The barbettes especially are well protected. Thus in the Duncan the side armour is 7 in., but the barbette armour is 11 in. The reason is that each barbette represents such a large proportion of the fighting power of the ship. A single shot piercing the belt might not be a serious matter, but one shot through the barbette armour would probably cripple nearly one-half of the ship's fighting capacity.
A feature of modern battle-ship designs has been the larger area covered with armour than formerly, with a corresponding reduction of thickness. This has been fully dealt with in Chapter XIII. The improvements made in the quality of armour has also had a great influence on cruiser designs. Up to the Diadem the protection was considered to be best obtained by a thick protective deck at the waterline, as Figs. 21 and 22. Owing, however, to the introduction of Krupp armour, this system of protection was modified by the adoption of a broad patch of 6-in. armour in the Cressij, over about half the length, in association with thick decks and bow protection (see Figs. 136 and 137). Armour protection has been adopted up to the present time for first class cruisers.
7. Hull.—The weight devoted to the hull comprises—
(a) Weight of the structure ; and
(b) Weight of fittings, etc., not contributing to the structural strength.
The total weight of hull in large ships varies from 35 to 40 per cent, of the total displacement, and it is only by a most careful arrangement of the material, combined with high-class workmanship, that the weight can be brought as low as this. The corresponding weight in large merchant steamers is considerably in excess of the above figure; this is very notable when we consider that about one-half of the weight of hull in a war-ship is concerned with fittings, etc., which do not contribute primarily to the structural strength.
Economies of weight during building are important to keep weights down, and a great deal can be done in this direction with no loss of strength. Lightening holes are largely employed. Thus in Fig. 16, showing a longitudinal, we notice that the plate must be weakened by holes connecting it to the transverse frame, so that we can well afford to cut away the plate by a hole between