besides being good construction to omit the reverse bar altogether and increase the sided flange of frame angle to give an equivalent W. T.DK. SHELL W. T. DECK FRAME REVERSE FRAME I. A saving in material, riveting and bending will thus be effected. In light vessels where weight must be cut down without encroaching on the strength, the maximum section modulus may be obtained for a given depth of web by employing two bars of such dimension of leg as will just give the requisite size of lap to take the proper riveting, as in Fig. 154. FRAME The practice in vogue for many years of placing the frame and reverse bars back to back has given place to that of fitting them bosom to bosom where deep framing is adopted, as by this method the beam knees can be fitted without linering in wake of reverse frames. FIG. 154. FLOORS. The deep plates riveted to the bottom framing of ships and known as the floors, are placed there to resist the transverse stresses to which the bottom plating is subjected, due to the great water pressure externally applied, and the inside forces created by the weight of the structure and cargo. Ordinarily in ships without an inner bottom these are of a size based upon the breadth and depth of the vessel and carried in a fair line up the bilge to a height equal to twice the centre line dimension as in Fig. 155. It will be seen that this contour at the bilge necessitates furnacing the tail ends to bend them to the required curve, a costly and therefore an objectionable feature. For this reason ordinary floors should be increased in their sided areas and carried straight across, striking the bilge at a point somewhat lower down than with the curved floor. This method permits of the floor being flanged across top in lieu of fitting a reverse bar, although some of the classification bureau penalize flanging plates to the extent of adding one-twentieth to their thickness; this need not, however, be made unless where specifically required and for that reason cheaper, lighter, and equally efficient construction will be obtained. In small freight steamers and barges a strong and inexpensive floor is obtained by using structural channel section thus eliminating the riveting to frame and reverse bar altogether. Floors in inner bottoms are almost entirely fitted as deep solid plates in one piece from centre vertical keel to margin plate, lightened with large manholes to cut out superfluous material and provide access to the various compartments into which the bottom is sub-divided by the floors and intercostal girders. Deep floors should be lapped to the bottom frames just sufficient to take the riveting. In wake of watertight bulkheads or at ends of ballast tanks where the floors are watertight, no holes whatever must be cut in them. The margin plate of inner bottom being continuous, is connected to the main frame by a large bracket plate or tail piece, and by double angles having a specified number of rivets and a gusset plate at top, or in the largest vessels a continuous stringer. The connection to the siding flange of main frame is by lap of sufficient width to take the riveting. See Figs. 157 and 158. At the ends of the vessel where the waterline at top of floor would necessarily be comparatively narrow, increased depth must be given to provide compensatory area and also ensure sufficient width to clip the centre keelson to floors. In the fore peak this additional depth is required to resist buckling and panting, and generally to give local stiffening at a part subjected to unusual stresses. It is also necessary to increase the floors considerably in depth in after-peak, owing to the severe stresses encountered when the propeller "races" and the stern is in air. INNER BOTTOM. Double bottoms are fitted in vessels to enable them to safely make voyages "in ballast" without incurring heavy expenses by loading and discharging dry ballast. For this purpose the floors are plated over, forming an inner bottom enclosing with the ship's plating a pontoon in which to carry sea water as ballast, an expeditious, inexpensive and clean method of doing so. Two or three methods of fitting water bottoms are met with in practice, but as these have given way to the cellular system, it is unnecessary to describe them. This method consists in the subdivision of the space formed by the pontoon referred to, into a great number of small compartments or cells bounded by the floors in a fore and aft direction and transversely by intercostal girder plates, making these cells approximately two feet by four feet, respectively, by the depth of water bot FIG. 157. tom. The water passes freely between these cells as the floors and intercostals are pierced with access holes unless where mentioned hereafter. The cells are arranged in separate groups or compartments enclosed by the centre vertical girder, watertight floors and the margin plate, this larger subdivision being neces sary for trimming and filling purposes, as otherwise a large surface of free water would be highly dangerous in certain conditions. As mentioned, the centre vertical plate is continuous fore and aft, fitted usually watertight and connected top and bottom to inner plating and plate keel with suitable angle bars. No holes whatever should therefore be cut through vertical keel plate, and although it is not necessary to caulk it in way of ballast tanks, the riveting should be of watertight pitch. Of course where fresh water is carried this longitudinal girder must be properly caulked. At the ends of the vessel where fore and aft subdivision is unnecessary the centre plate may have access manholes as in the floors. The butt connections are preferably formed with double butt straps, each of about two-thirds the thickness of plate. Through butts should not be used here, as besides interfering with the passage of the fore and aft angles they only give single shear value to the riveted con nection. The outboard side of the inner bottom, or margin plate, is fitted to shell by means of a continuous angle bar, the main frames of the ship being cut for that purpose. At the top this plate is flanged in board to take the inner bottom plat о FIG. 158. ing as shown in Fig. 157. The butts of margin plate are covered with single strap fitted on the inside of tank. This plate may also be fitted with advantage as shown in Fig. 158 devised by the author, which consists in flanging the plate outboard, a shape that the plate will take more naturally where there is curvature in a fore and aft direction. This outboard flange will also permit of machine riveting and connecting to the reverse flange or bar on the floor bracket, thus forming a continuous stringer; or, angle section may be substituted for the flange where facilities for bending are not obtainable. FIG. 159. Another method of fitting the margin is illustrated by Fig. 159, where the top plating is carried right out to the shell and flanged upwards to take staggered riveting. Flanging is preferable to fitting an angle bar, as in the latter case difficulty would be experienced in putting in the rivets on the horizontal flange of the bar. It is, however, a cheap method of construction, its principal objection being the broken stowage caused by the brackets connecting frame to inner bottom. The inner bottom plating will be of such thickness as the classification societies stipulate where the vessel is classed, when it will be found that increased scantling is required under engines and boilers, and of course the centre strake and margin plate will also be thicker than the rest of the plating, owing to the former being the rider plate member of the girder formed by the centre vertical keel and keel plate, and the latter being an important factor in the longitudinal strength of the ship. For this reason when arranging the access manholes, these must always be kept clear of the centre strake. A good shift of butts must be arranged for the plating, and these shifted clear of the butts of shell, margin plate and longitudinals. Where the strakes of inner bottom plating are arranged "in and out," the packing liners to outside strakes should be fitted short, the unfilled spaces acting as air holes. The practice of fitting wood ceiling on tank tops is giving way to coating the plating, with tar or bitumastic cement, as this prevents the deterioration that goes on under the wood, besides adding to the stowage capacity. Where, however, wood ceiling is required, BRASS ~174= RIVETED 1"PIPE THREADS PER INCH, BRASS it must be laid on fore and aft bearers and screwed to same and not fastened through tank top. For this reason, i.e., guarding against leakage the heels of the hold pillars are riveted to vertical flange of tee or angle lugs which are first riveted through inner bottom. In arranging the manholes care should be exercised that they are located in accessible parts of holds and clear of cargo hatches. In holds of ABOUT 12 THREADS Ordinary length one each side at each end about quarter the beam outboard will be sufficient, and in long holds an additional one about the middle of the length. In no case as previously pointed out should they go through the centre strake. The best location aft will of course be in tunnel alleyways, and in machinery spaces they should be fixed by the engineers. This arrangement will contribute to the best circulation of air when the covers are taken off for ventilating purposes. Ample room must be allowed for rim of manhole to clear landings, butts, longitudinal clips, etc. FIG. 160. The shell plating forming the bottom of tanks may be reduced |