SHIPBUILDING, as a science, has, under the term Naval Archi tecture, become so extensive a subject as to render it difficult to condense within the limits of a concise statement, sufficient to illus trate even its main principles, without undue intrusion upon the consistent limits of a Cyclopedia.
The precise, form of a ship need not here be a main consideration ; under the word STEAM-VESSEL will be given such information here upon as illustrates the question of speed; which, although belonging also to certain classes of sailing-vessels, is more immediately and more generally connected with steamers whose powers are dependent en a limited amount of their primary means of propulsion, namely, coal. We here only consider, under this word, the hull or body of a ship as a hollow shell, intended to accomplish a required displacement of water under different specific amounts of immersion. Were it merely the object to build such a fabric as is capable of floatation in a fluid at rest, the elucidation of principles already given would suffice ; but we have to illustrate certain considerations which are conducive to strength : and a difficulty arises of fairly estimating the amount of strength demanded, not at any particular part in the frame of a ship, but throughout the whole mass ; for it becomes a most intricate question to estimate in a ship even an approximate amount of relative strain, and its importance in the economical distribution of material may be said to comprise the most exacting demands on the skill of the accom plished mechanic and mathematician.
The subject here would evidently divide into two distinct branches; for while there exists in vessels of the mercantile marine a series of qualifications adapted to the general requirements of commerce, such as the carrying of heavy burdens, or of passengers, another class of ships is demanded of the naval architect which shall fulfil all the neceasities of fighting ships ; but those of the latter class will be more conveniently treated of under STEAli•VESSEL.
When a ship is to be constructed, the first consideration is the required amount of tonnage. This being once determined, a knowledge of the purpose for which the ship is intended regulates the builder in his fixing the amount of midship area, from which all the other dimen sions originate. In forming the area of the midship section, the good qualities of the ship chiefly depend on the selection of the breadth (or beam as it is called) for on this rests the important question of stability or stiffness under saiL It is customary to consider the primary form of the ship as a quadrangular prism of contents equal to the required tonnage :—of course its midship section will be a rectan gular parallelogram. Sup e, fur example, it were required to build a ship of certain tonnage TONNAGE), whose midship sectional area is to be SO5 square feet, an the beans 35 feet, then = 23 feet the depth. The primary body plan and whole breadth plan would be as under :— Fig. 1.
con n would be the midship area, cn the beam, c o the depth. Now although this represents the amount of displacement for tonnage, it requires the skill and taste of the naval architect to adapt the out line of such capacity to the requirements of the ship, having proper regard to speed, currying qualifications, rolling, stability, &c. &c. This is to be done by an interchange of areas in the following manner : Suppose a moderate amount of speed is to be blended with average carrying qualities, the dotted curve would be so drawn as to let the area E compensate for the abstracted area n; or if speed in particular be desirable, then the area r would compensate the area a. It will thus be manifest that a wide range of form is open to the shipbuilder, while his chief aim is not to disturb the amount of area of the mid ship section. And again taking the form of the ship, at the load-water line—the primary parallelogram would be A a C D, a borizontal section of the primary quadrangular prism as under,— Now in order to shape the ends so as to give sailing qualities, it, as in the former case, again taxes the skill of the builder to interchange areas, so as not to disturb the gross displacement :—in the above figure we have supposed the area marked a to be taken from the parallelogram and transferred to a' ;—that marked c to be transferred to 1/ and d to d'. Hence the outline in plan and section is determined, and drawings representing the necessary intermediate elevations and sections, generally on a scale of a quarter of an inch to a foot, are prepared, and copies of these, enlarged to the full size of the objects which they represent in the intended ship, are traced with chalk on the floor of an apartment called the "mould-loft." The
length of the loft is generally equal to half that of the greatest ship which it may be proposed to build, and of the whole height of the hull in addition ; so that there may be traced upon it a horizontal plan of half the ship in the direction of its length, and beyond one extremity of the plan a representation, in the same plane, of a transverse section of tho ship in a vertical plane at the place of its greatest breadth. Such plan and section being laid down, there are drawn with chalk, from their proper places in the plan, representations of the timber ribs or frames as they would appear in as many transverse sections of the ship : pieces of plank about three-quarters of an inch thick are then cut so as to correspond to the forms of the timbers; and these, which are called the moulds, become the patterns by which the timbers are to be cut from the tree or log of wood. But, as such a mould can only give the form of the timber in the direction of its length, and as the oblique positions in which the timbers stand in the ship may cause the angle which the faces of each timber make with one another to be acute or obtuse, and to vary in the same piece, certain marks on the surfaces of the boards are used to indicate the directions in which the sides of the timbers are to be cut. The operation of cutting the timbers agreeably to the forms of the mould-boards is called " con verting." A row of blocks of oak are placed on the building-slip (ground cut in an inclined plane descending towards the water) in the direction of the length of the intended ship, so that their upper surfaces may be in a plane making an angle of about three degrees with the horizon ; and on these blocks is laid the keel A B, fig. 3. This, which is the lowest timber of the ship, extends from one end to the other, and upon it is raised the whole fabric ; it is of elm, and for a large ship it con sista of two or more pieces scarfed together at their extremities. Timbers, called the "dead-wood," are then placed at c and D longi tudinally upon the keel from each extremity of the latter towards its middle ; the upper surface of this mass is cut in a curvilinear form b, b, and with this line the bottom of the ship's body is to coincide. At A and !I, the extremities of the keel, the stern-post and stem-post arc set Up: the latter is curved near the bottom ; and if the stern is to be what is called square, a particular frame, consisting of two transoms or horizontal timbers, and two side-pieces, is fixed above the stern-post, in order to determine the form which is to be given to this part of the ship. The sides and upper surface of the keel and dead-wood are cut to receive the floor-timbers (the lower portions, d e, fig. 4, of those timbers which are to form the ribs of the hull): these are placed across the keel perpendicularly to its length, and the other portions (called futtocks), e f, f g, &c., of which each rib is formed, are placed succes sively above them, abutting end to end, or the head of a lower one against the heel of that which is immediately above It. The ends of the futtock-pieces in every rib are made to fall near the middle of the length of those in the rib on each side of it ; and they are united together by cylindrical emits, or plugs of wood, which enter about two inches into those ends at the places of junction ; c, c, c, &e., fig. 3, represent sections of the ribs made by a plane passing longitudinally through the middle of the keel, and the interior surfaces of parts of the ribs appear at d, d, &c. The ribs were formerly placed so that their planes were perpendicular to the keel, but Dr. Inman has dis posed them so as to be in vertical positions when the ship floats npright. Except near the two extremities of the ship, their planes are perpendicular to a horizontal line drawn in the plane of floatation through the whole length ; but at the bows always, and at the stern if the latter is to be curvilinear, the vertical planes with which the ribs on each side coincide are oblique to the length of the ship, in order that, on a horizontal plane, the proper curvature of the extremities may be obtained. The rib-timbers above the surface of the water are nearly rectilinear, but below that plane they are made with various curvatures. About the middle of the ship they have at bottom nearly the form of a semicircle, while towards the head and stern they form curves of contrary flexure, uniting on the keel with their lower con cavities towards the exterior of the ship.