ON PRACTICAL CONSTRUCTION.* When a draught of a ship is formed upon the prin ciples which have been laid down, adapting them as much as possible to the nature or the service on which she is designed to be employed, and to the seas or rivers which it is intended that she shall navigate, it becomes the duty of the practical builder to carry into effect the views of the constructer; and the first operation which he has to perform, is to delineate correctly, or as it is usually called, lay off the plane upon the mould This practice has usually been considered as coming within the province of the practical builder, and may certainly be correctly per formed, without the aid of mathematical learning; but in this, as well as in many other of the useful arts, a knowledge of geometry will correct old methods, or strike out new to forward the architect in his views and operations.
The naval engineer, after hat ing formed his plan of the ship according to certain prescribe,i dimensions, furnishes the practical architect with the slicer draught, Fig. 1. Plate CCCCXCVIII. a plan of the form of the body at various sections, (Fig. 2.) a half breadth plan, (Fig. 3.) and a scheme of the principal dimensions of the ship, and the scantlings of the tim bers, planking, &c. The sheer draught or plane of elevation, is chiefly useful for the purpose of giving the length of the ship, the depth in the hold, the hang ing of the decks and their height from each other, the longitudinal sheer and height of the topsides at the various sections, the rake of the stem and stern post, with the overhanging of the stern. Upon this plan also is delineated the cutting down line of the floors, the draught of water at which it is intended the ship shall sail; the situation of the wales, the spacing of the timbers, the number, position and di mensions of the parts; the situation of the masts and channels, and the form of the gripe and headrails. The body plan or vertical plane of the timbers, gives the form of the ship transversely at different sections, corresponding with those similarly marked on the sheer draught, and with which they agree in height; and the half breadth plan or horizontal plane, shows the form and breadth of the frame of the ship at parts cut by horizontal lines, usually denominated water lines, or by diagonal ones, which either show the length of the several timbers which form the frames or the stations of the ribbands or harpens, which serve to keep the frames in their places before the planking is brought on; these lines, but more particularly the diagonal lines, ale used to prove that the sections on the body plan are fair. The obliquity of the cant
timbers in the fore and after bodies is also shown on this plan.
To prevent the inaccuracy likely to take place in transferring the several dimensions of a ship from a drawing on a small scale to the actual size intended, the builder is furnished with a scheme of scantlings, which gives in words at length, or by figures, the principal dimensions, the breadth and thickness of the several timbers, (technically called their siding and moulding,) the nature and sizes of the fastenings, the rounding and dimensions of the beams, the thickness of the external and internal planking, decks, clamps, and shelf-pieces.
Being furnished with these plans and information, it becomes the duty of the practical builder to lay down the ship. The drawing furnished by the engi neer being usually made on a scale of one-fourth of an inch to a foot, it is necessary to place the projection on the floor forty-eight times their size, in order to make mouldings for providing the timbers of the frame. This laying off the ship is performed chiefly by horizontal and diagonal lines cutting the body in various directions; the greater number of these em ployed, the more likely to be the fairness of the lines of the body, and the accuracy of the work.
Vertical lines are also used to lay down the after body; the variety of positions of the timbers, particu larly in ships with square sterns, is such as to require in this part of the body the greatest nicety. When the builder is satisfied with the accuracy of the lines. they are slightly raised on the floor to prevent oblite ration. It has sometimes been considered that great nicety is not required with the square frames; but care should be taken to conform as nearly as possible to the form and dimensions given, or difficulty will be experienced in adapting the cant frames to the body. In fine, except a ship be accurately laid down, the labour of the engineer has been employed in vain, as the best-constructed plan may be rendered nugatory for a want of due attention in the performance of this duty, and will also give eventually considerable trou ble to the practical constructer in adapting the frames to their positions. To give the methods in use for laying clown the several parts of a ship would far ex ceed our limits; and those of our readers who may wish to he informed on all these points are referred to Stalkart's Natal .drehitecture; to The Elements and Practice of .drehitecture, third edition, or to Fincham's Directions for Laying Ships on the Mould Loft Poor.