Page:The New International Encyclopædia 1st ed. v. 18.djvu/133

SHIPBUILDING. 101 SHIPBUILDING. 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 liiid tile midshij) frames are erected and held in place by shores and ribands until secured by the internal vertical keel, the longitudinals, stringers, side and bottom plating, etc. The large castings or forgings forming the stem 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 sutlicient strength and stiiY- 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 hea'j' 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 otf 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-pieee, or knocking out the dog-shore, which holds her. She starts down the ways slowly, but 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 connnonly 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 ad^•antages the use of a dry dock is lost for a year or two. Comparatively few wooden steamers are now built, init wooden sailing vessels are still pro- duced in considerable numbers. The general fea- tures of wooden shipbuilding resemble those of shipbuilding in iron and steel, but there are of course differences. The keel blocks are laid in the same way. On them are laid the heavy tim- bers forming the keel, which are sometimes nearly two feet square. The different lengths of tim- bers are scarfed and bolted together ; over and across the keel are laid the floor timbers of the ribs or frames and the frames are thence built up. being held in place by shores, braces, cross- spalls, beams, and ribands. Between the floor timbers and extending up usually to the princi- pal deck (sometimes to the rail) the space is closely packed with fllling timbers forming a structure which is nearly tight without planking. The beams in wooden ships are of wood and they may be attached to the frames by wooden or iron knees. The former are considered to give the best fastening, but the iron knees save much room. The advantage of having a copper bottom has caused a few composite vessels to be built. These are mostly yachts, gunboats, and small sailing vessels. Composite vessels are framed much like those of iron or steel. Over the frames, wood- planking is used instead of metal plating, though a good many plates of metal are phiced under the wood to give the proper strength to the struc- ture in dilVcreiit parts. The wood planking is attached to the frames with bolts setting up with nuts on the inside and is covered with copper to a short distance above water. The topsides of many composite vessels are plated with steel or iron above the level where coppering is neces- sary, as the metal is stronger and more durable than the wood. The safety of a ship depends upon its stability, strength, water-tightness, and reserve stability and iloatability, if injured. The stability of ships has already been considered. The strength is due to the framing and plating or planking. Water-tightness is effected by calking the seams between plates or planks. The seams of iron plates are calked by hammering the edge of the uppermost plate against the one underneath it. The seams between planks are partly filled with Fig. 10. CELLULAR DOUBLE BOTTOM OF A MERCHANT STEAMER. oakum, which is forced in, and the remainder of the seam filled with pitch, marine glue, or putty. The reserve stability and floatability when in- jured depend upon the position and volume of the interior space which is Hooded. To reduce this volume to a safe point, vessels are divided into compartments by water-tight bulkheads which extend across the ship at intervals. In merchant vessels the bulkheads usually have no passages through them, but in men-of-war many of the bulkheads have openings closed b.y water-tight doors. In addition to transverse water-tight bulkheads many ships have longitudinal ones- such, for instance, as the one separating the engine rooms in a twin-screw vessel. As a fur- ther protection against flooding due to striking ground, large vessels usually have a double bot- tom extending the whole or part of the length and rising at the sides to about the turn of the I'.ilge or higher. The inner bottom tluis fitted is laiil over the inside of the frames and secured