Ships are in general built of mild steel, which is a ductile material, easily worked, differing entirely from the hard and brittle steels from which tools are made. Its ultimate strength is about 28 tons per square inch. High tensile steels are used largely for important parts of the structure of warships and liners; their strength varies from 33 to approximately 4o tons per square inch. The stem, stern frame, rudder frame, hawse pipes, etc., are generally made of cast steel (26 tons per square inch or less), cables, davits and similar fittings of wrought iron (22 tons), Whilst naval brass, gunmetal or phosphor bronze are used for many internal fittings. All materials used are subjected to a va riety of tests before being worked into the ship in order to ensure that the quality of material is up to the standard required.
The steel used in ship construc tion is worked in the form of plates or rolled sections which are generally angle bars or channels ( I). They are connected to gether by rivets, hammered or pressed hot ; and the joints are designed so that the strength of the connection is as nearly as practicable equal to that of the material it connects. In addition a large number of joints have to be made watertight or oiltight. This is ensured by closely spacing the rivets so as to draw the plates or bars well together, and by caulking the edges. The latter operation consists of splitting the material at or near the edge, and, using a tool like a cold chisel, hammering the split portion hard against the adjoining plate or bar. This effectively prevents the passage of oil or water between the two surfaces; but with thin plates it is sometimes supplemented by inserting be tween the bearing surfaces of the plates a "stopwater" consisting of tape steeped in composition.
The structural arrangement of a typical warship is illustrated by the perspective drawing of a cruiser (fig. 7). The various portions of the structure are lettered and indicated on the diagram. The vessel has two distinct skins or bottoms, viz.: the outer bottom which forms the outside of the hull and trans mits the water pressure to the general structure of the ship, and the inner bottom which is also watertight and whose function is to save the ship if the outer bottom is damaged or pierced. In this vessel the thickness of the outer bottom is one inch at or near the keels and behind the outer protective plating at the sides, and slightly less on the bilge. The inner bottom, though thinner, is
also worked so as to assist the longitudinal strength of the struc ture. The space between the two bottoms, termed the double bot tom, is utilized as far as possible for the stowage of oil fuel and reserve feed water.
The inner and outer bottom are connected together by two sys tems of frames (shown in the diagram)—longitudinal and trans verse. The former are worked continuously throughout the cen tral part of the ship in order to contribute as effectively as possible to the longitudinal strength ;
system of framing, with the longitudinals predominant, is an important feature of the structure of all large warships. The frame at the middle line—termed the vertical keel, is specially strengthened in order to resist the stresses undergone when the vessel is in dock. The transverse frames (spaced about 4 feet apart) are of minor importance and are worked in short lengths between the longitudinals. Those shown in the figure consist of angle bars and bracket plates stiffened at the edges and lightened by holes ; in another system of construc tion frequently adopted the two brackets are replaced by a single plate lightened by holes which are long enough to permit access through them. Certain of the longitudinal and transverse frames are made watertight or oiltight so as to subdivide the double bottom into a number of cells, which are useful for stowage pur poses, and, moreover, limit the ingress of seawater in event of the outer bottom being damaged.
The upper deck is formed of steel plating sheathed with wood and supported by beams worked transversely and slotted through the longitudinal girders shown. These girders with the deck plating (which is thickened amidships) are of great importance in con nection with the longitudinal strength ; and in some ships they have been increased in number and the beams correspondingly reduced. An interesting feature shown on the diagram is the bilge keel which is fitted in order to reduce the ship's rolling in a seaway. At the ends of the vessel the system of construction is considerably modified, the principal framing being transverse and the double bottom being no longer worked. Adequate watertight subdivision is still provided by the watertight bulkheads and decks, of which the former are more closely spaced than amidships.