Comparatively few wooden steamers are now built, but 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 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 spans, 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 filling 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 placed under the wood to give the proper strength to the struc ture in different 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. lvater-tightness, and reserve stability and tloatability, 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 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 flooded. To reduce tlfis 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 by 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 bilge or higher. The inner bottom thus fitted is laid over the inside of the frames and secured in the same manner as the outside plating. The frames in the double bottoms are deep enough to give considerable space between the inner and outer plating, which is necessary to give access for cleaning and painting. Most frames are lightened by holes cut through them, but about every fourth or fifth frame is water-tight and has no holes. The space between the imperforate frames forms a double-bottom compartment, ac cess to which is had by a manhole closed by a removable water-tight cover. Most ships are fitted with a water-tight bulkhead close to the bow, called the collision bulkhead.