A study of the behavior of models and of full sized ships of different designs and under dif ferent conditions has shown that for every de sign there is a certain critical speed below which wave-making resistance increases quite regu larly and moderately, but beyond which it in creases 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 ease. If two designs of equal length and displacement are tested, one having fair lengths of entrance and run and con siderable length of parallel middle body. and the other having no parallel middle body and a nmeh greater beam. hut tapering from the mid ship section to the how and stern. the latter will have the higher limiting speed. Ships, however, are built to carry cargo. The depth is kept as moderate as possible on account of the shallow ness of many harbors; and with a given depth only a certain breadth is practicable or the righting moment will be unduly great. There fore it is desirable to increase the displacement which pins may be placed. The mold of the frame is laid on the bending slab, and pins in serted along its edge. The hot iron angle bar (or channel or Z bar), which is to form the frame (or the outer part of the frame, if it, is built up of plates and angles), is then pressed 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.
The designs of the naval architect are pre pared on paper, and are occasionally supple mented by a wooden model. The three principal 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 trans vere planes). In the figures the dotted lines I. 2, 3, are water lines and are the intersections of horizontal longitudinal planes, and the inner sur face of the planking or plating of the hull; lines I, II, and Ill 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 trans verse 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 plat ing, 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. must be plans for decks, holds, bulkheads. etc.; of ventilating, drainage, lighting. and flushing sys tems: of engines. boilers, etc.; and a vast num ber 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 up against these pins and so given its proper curvature. A sufficient number of frames hav ing been prepared, the work of erection begins.
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 the keel or keel plate is a broad flat plate of extra thickness. It is in sections, riveted together, and joined to the stem and stern posts. After the keel is laid the midship frames are erected and held in place by shores and ribands until secured by the internal vertical keel, the longitudinzils, stringers, side and bottom plating, etc. The large castings or forgings forming the stein and stern posts are then erected, the remainder of the plat ing put on, and the interior of the ship partly completed. The next step is the launching, and this may take place at any time after the outside plating is on and the interior completed so far as necessary to assure sufficient strength and stiff ness. The weight of the vessel has so far been supported on the keel blocks and bilge shores. It must now be transferred to the launching ways. These consist of heavy timber ways inclined at about the same angle as the keel blocks (about five-eighths of an inch to the foot) , and are built up on each side of them. Resting on the launch ings ways are the sliding bilge ways, also of heavy timber, and on top of the bilge ways is built a framework that fits closely to the bot tom of the ship. This is called the cradle. To remove the weight of the ship to the launching ways wedges are driven under the cradle, lift ing the ship of the keel blocks. The under sur face of the bilge ways and the upper surface of the launching ways having been well lubricated, the ship is ready to slide into the water as soon as released by sawing the tie-piece, or knocking out the dog-shore, which holds her. She starts down the ways slowly, hut her velocity on reach ing the water is frequently considerable and must be checked by hawsers if there is not much room for her to range astern. As soon as she is water-borne she floats clear of the cradle. Small vessels are usually nearly completed before launching, but large ones are commonly launched when not much over half their weights are on board. As soon as the ship is in the water the boilers and engines are installed, and the interior and upper works finished. In England many ar mored vessels are built in dry docks. This saves the labor of lifting heavy weights, it being only necessary to lower them; and the cost of the launching and building ways is avoided. As an offset to these advantages the use of a dry dock is lost for a year or two.