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The various sections used in Admiralty shipbuilding are shown in Fig. 8.
The angle bar (a) is used to connect plates together; as a stiffener to a plate; for beams, frames, etc.
The tee bar (b) is used as a stiffener to important bulkheads as (n) and inside masts.
The channel bar (/) is used as a stiffener in certain cases.
The zed bar (e) is largely used for transverse framing, to avoid the use of two angles riveted back to back. It is also used as a bulkhead stiffener.
The I or H bar (g) is used for the main stiffening of important bulkheads.
The angle bulb (c) is used for deck beams.
The tee bulb (d) is used for deck beams.
A flange (k) is frequently put on the edge of a plate to serve the purpose of an angle bar for stiffness or connection.
The half round (I) is usually hollow, and is used as a moulding round the ship.
The segmental bar (m) is used as a finish round the top of hatch coamings, etc.
The sections shown in Fig. 8 are drawn to scale, and show the exact form of sections now used. It is worth noting that the flanges of the zed, channel, and I bars are more substantial than the webs. These sections thus illustrate very clearly the principles touched upon in Chapter I. in connection with beams.
Rivets, Forms of, etc.—Fig. 9 shows the usual form of rivets and riveted connections employed in Admiralty shipbuilding.
A is the most common form of rivet, called a pan head rivet, from the shape of its head. It will be noticed that the rivet is formed with a conical neck. All rivets J-in. diameter and above are thus formed, because the hole formed in the plate by punching has a slight taper, and it is most necessary that the rivet should completely fill the hole (see D, etc., Fig. 9).
D, E, F, Fig. 9, show various points associated with the pan head. D is the countersunk point necessary when the surface has to be flush, as for the skin plating. The hole formed by the punch has to be made conical by the countersinking drill. E is the point adopted for most of the internal work. No countersinking is done, and the point is hammered up full as shown. F is the point adopted where a finished appearance is desirable; this point is called a snap point.
B is a snap head rivet, and it is associated with a snap point, as G, when the riveting is done by the hydraulic riveter.
In some few cases it is necessary to have a flush surface on both sides, and in this case a countersunk head rivet is used, as C, and the point is also countersunk, as H.
Certain parts of the structure cannot be riveted by the ordinary means, as, for example, where outer bottom plating is connected to a stem casting. In such cases tap rivets are used, as L, M, and N (Fig. 9). In L, the most usual form, the head is countersunk, and the square projection is chipped off after screwing up. Where it is necessary to have the work portable, a recess is fitted in the head,
as N. Where a flush surface is not necessary, as for some internal work, the head is made hexagonal, as M.
In parts of the ship behind armour, the place would be dangerous in action, because when the armour is struck the rivet heads are likely to fly off. In such places, where men are likely to be, the inside of the framing is closed in with a steel lining (see Fig. 13, on the main deck). This lining is secured to the framing, etc., by screw rivets, as K, Fig. 9. Tap rivets, as L, M, or N, would be undesirable, because of the likelihood of the rivet breaking and the head flying out if the armour was struck.
Laps, Butts, etc.—When two plates are connected together,
they may either be lapped or butted. Laps are shown at F and G in Fig. 10. For thin plating a single row of rivets is sufficient, as F; for thicker plates a double row of rivets is necessary, as G. In the first case, the breadth of the lap, F, is three and a half times the diameter of the rivet used; in the second case, G, it is six times the diameter of rivet. This gives a clearance between the rows of 1J- diameters, and rather more than a diameter clear of the edge. The edges of plates are usually lapped, but in some cases, where a flush surface is necessary, the edges are connected by an edge strip. A special form of edge connection is seen when the
edge strip takes the form of a tee bar. Here the tee bar forms an edge strip and stiffener at the same time (see n, Fig. 8). When the end connection of plates has to be flush, as is usually the case, the connection is called a butt-strap. Butt-straps are single, double, treble, or quadruple riveted, according to the importance of the connection. These are shown by A, B, C, and D, Fig. 10, the breadths being respectively 6J, ll.V, 16^, and 21^ times the diameter of the rivet used. Quadruple riveted straps are used for specially important joints (see Fig. 51 for one instance). In some cases butt-straps are made double, i.e. in two halves on either side. Angle bars, etc., are connected together by a piece of angle, fitted as shown in Fig. 14, long enough to take two or three rivets each side as necessary.
The breadths given above for butt-straps and laps are somewhat exceeded when dealing with high tensile steel. In this case a clearance of 1^ diameters from the edge is considered necessary.
Spacing of Rivets.—The spacing of rivets from centre to centre along edges, butts, etc., is termed the pitch. This pitch varies according as it is necessary to have the joint watertight or not. For the former case the joint has to be caulked, and in order to do this it is necessary to have the rivets closely spaced to draw the work tightly together. The usual pitch for watertight work is from 4 to 5 diameters. Rather closer spacing, 3£ to 4 diameters, is necessary for oiltight work. For non-watertight work a pitch of 7 to 8 diameters is all that is necessary.
Caulking.—All caulking should be metal to metal, filling pieces being avoided as far as possible. For laps the caulking edge must be made square (planed for important parts, as the outer bottom plating). The edge near the joint is nicked with a sharp tool and the piece so left is driven against the adjacent plate, as in Fig. 10. For butts the edges must be planed; a split is made either side of the joint and the two edges are forced together with a hollow tool, giving the shape to the butt, as in Fig. 10. Butt caulking is not so efficient as lap caulking, because a pull on the joint, or in-and-out working of the plating, is more liable to open the caulk in the former than in the latter case.
An interesting and very efficient form of butt-strap, shown at E, Fig. 10, is being adopted in some destroyers. The strap is double; the inside portion is treble riveted, and the outside portion double riveted. The alternate rivets are omitted in the last row, so that the plate is not weakened more than at the adjacent frame. The middle row is closely spaced to allow the edge to be lap caulked. If fracture of the plate occurred through this line of rivets it would be necessary to shear also all the rivets in the last row. The rivets in the two inner rows are in double shear.
FRAMING OF VARIOUS TYPES OF SHIPS.
The British Navy, intended to operate in all parts of the world, is necessarily made up of many different types of vessel. It would be obviously impossible in a work of this character to exhaustively
Fig. 11.—Section of H.M.S. Royal Sovereign.
describe the construction of each type of vessel in the Navy. All that is possible, or desirable, is to take certain main types, and deal with the principles of their construction. It will be seen that