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general. In the reign of Henry VII. ship construction was much improved and ships began to take on much of the form which they have preserved to the present day. During the next four centuries improvements of design and construction were continuously made until the wooden sailing ship reached its culminating point in the clipper ship of the nineteenth century.

So long as ships depended upon sails for propulsion shipbuilding remained a mechanical art bound by rules, traditions, and dogmas which were the result of centuries of experience. But with the advent of steam came the general scientific awakening and shipbuilding received its due share of attention. Its theoretical side has been given the name of naval architecture.

For convenience we may divide the subject into three principal parts, viz.: (1) Design as it affects the buoyancy, stability, steadiness, seaworthiness, etc. (2) Design as it affects the efficient propulsion and manœuvring powers. (3) Design as regards the strength, habitability, and general structural arrangement. The various qualities of a ship here mentioned are more or less interdependent, but it is possible to consider each separately and examine the effects of variation of form or structure which different requirements entail.

A vessel floating freely in still water displaces a volume of water equal in weight to its own, and the weight is called the vessel's displacement. This weight is supported by the pressure of water which acts at all points per pendicular to the surface of the ship's bot tom; but the sum of the vertical components of the water-pressure at all points must balance the weight of the ship, and this sum is termed the buoyancy. The total weight of a fully loaded ship may be divided into the weight of hull and weight of lading. The latter represents her carrying power or useful displacement, and it is of course desirable to make this as large as possible (compared to the weight of the hull), being consistent with other necessary requirements. The reduction in hull weight is the principal cause of the substitution of iron for wood in shipbuilding, and, in turn, the displacing of iron by steel.

In considering ships of different forms it is useful to know something definite concerning their shapes without exhaustive examination, and this is arrived at by comparing them with the parallelepipedon, which has dimensions equal to the length (L), breadth (B), and mean draught (M) of the ship. If v = the volume of the ship, and V the volume of the parallel

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In modern steamships the midship section closely approaches a rectangle, and the ordinary coefficient of fineness suffices. For steamers of exceptionally fine form (particularly those with no parallel midship body), the coefficient is from 40 to 50 per cent.; in large fast steamers, 45 to 55 per cent.; in recent battleships, 55 to 65 per cent.; in low-speed cargo steamers, 65 to 78 per cent. The coefficient of water-lines is greater and varies from about 55 to 83 per cent. in value.

In referring to the displacement of a ship it is necessary to specify some particular condition, as, of course, the displacement varies with the loading. With men-of-war the condition commonly used is that of normal, or mean load draught. That is supposed to be the average cruising condition, but is usually somewhat less. The deep load condition for a man-of-war is when her full supply of stores are on board and her coal bunkers are full. For merchant ships, displacement is only beginning to be used, and it is generally given for a light load condition-when the ship is practically empty-or when she is immersed to her Plimsoll mark (see LOAD-LINE); it may also be given for a specific mean draught of water. The tonnage of ships is a measure of capacity for cargo, and is fully treated in the article on the MEASURE

MENT OF SHIPS.

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If the ship is made to roll, the position of the centre of buoyancy will be displaced, as shown in Fig. 3. We have then a force acting vertically upward at B', and a force working vertically downward at G', producing a couple

M GAH

B

FIG. 3.

DH

B

FIG. 4.

tending to turn the ship back to her upright position. Similarly, if the ship pitches, the centre of buoyancy is displaced longitudinally and the couple acts as before. In either case, if W is the weight of the ship in tons the moment of this couple is equal to WX G'H, or WXG"H'. If a vessel rolls and pitches at the same time the centre of buoyancy will be displaced both laterally and longitudinally, and the couple will then tend to act in a plane, making an angle with the keel which is greater than 0 and less than 90 degrees. If a ship is pressed over by a constant force, such as the wind or the action of the rudder, and the surface of the water is smooth, the righting moment is simply that of the couple. But if the surface of the water is broken by waves the shape of the submerged body is constantly changing, thereby moving the centre of buoyancy and adding to or subtracting from the righting force due to the couple.

When a ship is forcibly inclined in still water the point M (Fig. 3), called the transverse metacentre, is the point in which

changes for every change in the position of the centre of buoyancy, but for angles not exceeding 15 degrees the change is slight. The value of the metacentric height usually given in tables is, therefore, that obtained by inclining the ship through a very small angle.

position

at

The rolling of a ship when forcibly inclined in still water and then allowed to right herself is like that of a pendulum which has been drawn to one side and then permitted to vibrate until it comes to rest. Acted upon by the couple (the moment of which in this case is called the moment of statical stability), she rapidly reaches the upright a constantly varying angular velocity. As soon as this position is reached the couple ceases to act, while her momentum causes the roll to continue; but beyond the upright position a couple in the opposite direction is formed and this (together with. friction and wave-making) gradually checks he roll until it ends, whereupon the new couple sets up a roll in the opposite direction just as a pendulum returns in its vibration. The rolling continues, though the arcs are smaller and smaller each time, until the vessel comes to rest in stable equilibrium in the upright position. The oscillations of a pendulum in vibrating are performed in equal periods of time, irrespective of their amplitude; and this is practically true

of the ship, though the wave-making due to the high angular velocity of deep rolls and the increased friction due to greater area of immersed surface cause some variation. The mean length of time required for a ship to make a complete double roll through a moderate angle in smooth water is called the still-water period. In rough water this period is modified by the action of the waves, which gives a constantly varying value to the total righting moment. If the waves pass under a ship in such a way as to add to this moment when the ship is rolling toward the vertical and reduce it when she is rolling away from the vertical, a dangerous condition of affairs is produced which may result in her capsizing. This condition can only exist when the wave period (time between waves) is practically the same as the ship's still-water period; when

FIG. 5. DIAGRAM SHOWING USE OF WATER-BALLAST TANKS IN A MERCHANT STEAMER.

the vertical line through B' cuts the line G'M, which is vertical when the ship is upright; and the distance G'M is called the transverse metacentric height. Similarly in Fig. 4, M' is the longitudinal metacentre, and G"M' is the longitudinal metacentric height. In vessels of ordinary type G"M' is so large that there is practically no danger of their turning end over end unless they are very small. G'M, however, is often very small, and its value must be very carefully considered. Being so much used, it is commonly referred to as the metacentric height. The determination of it is effected by inclining the ship in still water. It

The shaded portion indicates ballast tanks.

it does exist the course of the ship with reference to the waves should be materially changed.

Since the righting moment is the force which makes a ship roll, it is evident that if this force is powerful the ship will roll quickly and perhaps deeply, neither of which is desirable. To reduce the time of rolling (i.e. the still-water period) the metacentric height may be reduced as much as is consistent with safety, or the weights in the ship may be moved away from the midship plane if practicable, at the same time preserving the same height of centre of gravity. To reduce the amplitude of the roll, and therefore its angular velocity, the best means so far devised is the bilge keel (q.v.), or 'rolling chock.' Horizontal, thwartship water chambers with a central dam, or several dams, and partly filled with water, are useful to reduce small angles of roll, but the noise and shock of the moving water is so objectionable that they have not been adopted. Vessels are designed to have a certain metacentric height under various conditions of loading; and the stowage of cargo should, as far as possible, be so arranged as to give proper value to the righting moment. Vessels with double bottoms may, within small limits, vary their righting moments by filling or emptying double-bottom compartments.

To secure seaworthiness, vessels must not only be sufficiently stable at all moderate angles of roll, but they must be stable at all possible angles. The range of stability is independent of the force of the righting moment and varies in different classes of ships. In battleships and large vessels it usually reaches 70 degrees of inclination on each side of the vertical; for small vessels, such as torpedo boats, the range is usually greater. Seaworthiness further requires a reserve of buoyancy-that is, only part of the hull below the upper deck must be submerged, and the openings in the hull must be capable of being closed in rough seas. Comfort and health require that the sides of the ship, and particularly the bow, should be high above the water; without high sides a vessel can be kept at sea for a short period only.

The second part of the subject relates to efficient propulsion and manœuvring power.. In this we must consider the shape and smoothness of the hull as regards resistance to its movement through the water. The total resistance is made up of three parts: (a) Frictional resistance; (b) eddy-making resistance; and (c) wave-making resistance.

Frictional resistance is due to friction between the water and the hull of the ship. It does not depend upon the shape of the hull to any appreciable extent, but upon its smoothness, the area of the wetted surface, the length of the ship, and the speed. It forms the greater part of the total resistance of a ship moving at low speeds and an important part of it at all speeds, particularly if the bottom is rough or foul. For any given ship it varies about as the square of the speed.. The difference in resistance between a smooth and a rough bottom is very great. A smoothly painted bottom has only half that of one of the roughness of fine sandpaper, and only about a third of that of coarse sandpaper. The difference in the power required to drive a ship when her bottom is foul and when her bottom is clean is then very easily appreciated.

Eddy-making resistance is not usually important in well-designed ships, and ought not to exceed about 8 per cent. of the frictional resistance. Eddies are chiefly to be found at the stern, where the water rushes in behind the ship. If the run is long and fine, the speed moderate, and the propeller struts, rudder, etc., well designed, they are scarcely noticeable; but a ship with too short a run, badly designed rudder, propeller struts, etc., leaves at full speed a boiling, troubled, eddying wake behind her.

Wave-making is in many respects the most im

portant part of the resistance of ships, for it is the one over which we have the most control, and which is the greatest impediment to high speed. The laws which govern it are not yet fully understood, but the character of the waves and the losses entailed by them have been very carefully examined. A ship moving through undisturbed water puts certain particles of it in motion, carrying some along with her by friction and giving motion to others in such a way as to cause them to rise in waves. All the energy taken up by the water must come from the propelling machinery, and if it is not returned to the ship in pushing her ahead it is wasted.

The 'entrance' of a ship is the tapered forebody which extends from the stem to the point where her hull has obtained the full dimensions of the midship (or maximum) section; and the run is the corresponding tapered portion of the after body. These two parts of a vessel are termed the wave-making features, because the movements of the particles of water forming waves depend upon their lengths and shapes. A vessel passing through undisturbed water forms a double series of wayes at the bow and at the stern. The former are most readily seen, largely because the action of the screw tends to degrade and confuse those at the stern. One set of waves are called divergent because their crests make an angle of 40 to 50 degrees with the keel; the other waves are called transverse because their crests are perpendicular to the keel line of the ship. Both sets increase in height with the speed, and this height is a measure of the energy absorbed by them, and indicates the speed with which they are traveling. The divergent waves are thrown off, and, leaving the ship, no longer influence it; but the transverse waves move at the same speed as the ship and keep their crests and hollows at about the same points on her sides so long as the speed is constant. Furthermore, the length between crests is the same as between the crests of ocean waves moving at the same rate of speed. It is found that if a wave crest is maintained at about the middle of the run the wavemaking is decreased, but if a wave hollow exists there the wave-making resistance is increased. Some of the variations in power required to drive vessels at different speeds may be due to this cause.

A study of the behavior of models and of fullsized ships of different designs and under different conditions has shown that for every design there is a certain critical speed below which wave-making resistance increases quite regularly and moderately, but beyond which it increases with great rapidity. It is further shown that the greater the length of the entrance and the run the higher is this critical limiting speed. It was at one time supposed that of two designs of equal length and displacement that with the least midship section would give the least resistance, but experiment has shown that this is not necessarily the case. If two designs of equal length and displacement are tested, one having fair lengths of entrance and run and considerable length of parallel middle body, and the other having no parallel middle body and a much greater beam, but tapering from the midship section to the bow and stern, the latter will have the higher limiting speed. Ships, however,

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only by increasing the
length; this means that,
after allowing a suitable
entrance and run, the re-
mainder of the length is
applied to extending the
parallel middle body.

up against these pins and so given its proper curvature. A sufficient number of frames having been prepared, the work of erection begins.

Moulding

Skid Beams

Hammock Berthing

Lightening Holes

Stanchion--

Main Deck

Wood Decking

The designs of the naval architect are pre- Sheer Moulding pared on paper, and are occasionally supple- Sheer mented by a wooden model. The three principal Strake plans are the sheer plan (showing sections of the ship made by vertical longitudinal planes), the half-breadth plan (showing sections made by horizontal longitudinal planes), and the body plan (showing sections made by vertical transverse planes). In the figures the dotted lines 1, 2, 3, are water lines and are the intersections of horizontal longitudinal planes, and the inner surface of the planking or plating of the hull; lines I, II, and III are bow (forward) and buttock (aft) lines, made by vertical longitudinal planes; the full lines in the body plan are sections A, B, C, etc., and A', B', C', etc., made by vertical transverse planes, which are passed at equal.distances from each other, X being at the point of greatest breadth and called the midship section. In the body plan the right half shows half-sections forward of the midship section and the left half the half-sections abaft it.

In actual plans many more water lines, bow and buttock lines, etc., are shown, for the full plans are of large size. The planking or plating, positions of frames, decks, and much other detail are also shown. The three principal plans are only a small part of the drawings furnished by the architect to the builder. There must be plans for decks, holds, bulkheads, etc.; of ventilating, drainage, lighting, and flushing systems; of engines, boilers, etc.; and a vast number of plans showing details of construction of parts and fittings.

The drawings being completed, the work is taken up by the constructive force. The plans are laid off on the mold loft floor in full size. Wooden molds are then prepared for the frames or else the shapes of the frames are cut (or scrived) into a great piece of flooring called the scrive board. The frames are heated and bent on the bending slab. This is a large floor of thick metal with a great number of holes in

over Steel Deck

Main Deck Beam

Deck Stanchion

Deck Plating

Gun Deck

Passage Bulkhead

Coffer Dam

(for Cellulose) Berth

10

Binuter strokes. Armor Belt (tapered),--Framing behind Armor

Outside Platingb, outer strekes.

Wing Passage

Side Armor (vertical)

b

a

b

Bilge Keel

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Main Frame
Longitudinal

Bulkhead

Deck Bean

Stanchion---

Armor Deck

Deck

Stiffener Bar

Boiler or Engine
Compartment

Stringer or Tie Plate

Flat Keelson
Vertical Keel

Inner Bottom Plating、

Double Bottom

-"Strake Outside Plating Limber Hole Garboard Strake Flat Keel

FIG. 9. MIDSHIP SECTION OF BATTLESHIP.

Fore and Aft Midship Bulkhead

The building way is prepared by setting up the keel blocks. These are short heavy timbers a foot or more square built up in piles two or three feet apart and having the upper surface shaped to the keel line of the vessel. On these the keel is laid. In nearly all modern steamers

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THE CONSTRUCTION OF AN OCEAN LINER (THE "KAISER WILHELM II.").

1. Keel and bottom framing, showing double bottom.

2. View of the Stern, showing framing and rudder post.

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