Online Encyclopedia

PART SHEER AND

Online Encyclopedia
Originally appearing in Volume V24, Page 969 of the 1911 Encyclopedia Britannica.
Spread the word: del.icio.us del.icio.us it!
PART SHEER AND HALF-BREADTH PLANS distances from the middle line of the half-breadth as the corresponding vertical projections are from the middle line of the body. For example, in fig. 99 pi and qi are the projections in the half-breadth of the same points of which p and q are projections in the body plan, and are found by making the ordinates of pi and qi measured from the middle line of the half-breadth plan at square station 2 equal to the perpendicular distances of p and q respectively from the middle line of the body plan. Thus points in the projections in the half-breadth of the water and diagonal lines can be found from the body plan already drawn, and in order that the surface of the ship may be fair, the series of points corresponding to any water or diagonal line must lie on a fair curve. In the case of a diagonal line the distance from the middle line of the body to the intersections of the diagonal with the square stations may be measured along the diagonal, and set off on the corresponding square stations in the half-breadth. This gives the true or rabatted form of the intersection of the diagonal plane with the ship's surface, and this, equally with the projected diagonal, must be a fair curve if the surface is fair. The diagonals are also projected into the sheer plan by measuring the height above the base-line at which each diagonal in the body plan cuts each square station, and setting up this height from the base-line of the sheer plan at the corresponding square station. The projections of the bow and buttock lines in the sheer plan are obtained in a similar manner. Thus in fig. 99 V2 is projection in the sheer plan of the The principle of contracted fairing is sometimes extended by the measurements are made to full size as before, thus making the curvature of the water and diagonal lines sharper throughout the region over which it would otherwise be somewhat flat and indefinite. As the curvature of the contracted level and diagonal lines depends upon the differences between the lengths of the ordinates of the curves and not upon their actual length, a further saving of space is effected by measuring the distances to be set up as ordinates in the half-breadth not from the middle line of the body but from a point selected arbitrarily in each water or diagonal line, generally a few inches outside the midship section. By suitably varying the distances outside the midship section of these arbitrarily chosen points in the different water and diagonal lines, it can be arranged that the curves in the half-breadth do not interfere with one another, an advantage from the point of view of clearness. With the above modifications the process of fairing by the contracted method is precisely similar to that when the ordinates are their full distance apart. In fig. 88 the diagonals iD and 2D are shown laid off by the contracted method, the spacing of the ordinates in the contracted half-breadth being -nth of that representing the spacing in the diagram of the uncontracted sheer and half-breadth. In the contracted half-breadth the ordinates 4ri, 5si, &c., are equal to the distances Or, Os, &c., measured to sections 4, 5, &c., in the body, O being a point arbitrarily selected in the diagonal sD. provision of a large drawing-board 4 or 5 ft. broad and long enough to take the whole length of the ship on a scale of .'4th full size. The ordinates of the half-breadth and sheer being set off on the board to this scale, any line in which the difference between the greatest and least ordinates does not exceed the breadth of the board can be faired thereon by this contracted method. This allows considerable lengths of the midship parts of diagonals and water-lines, and such lines as decks at middle and side, and any other lines of very flat curvature, to be faired on the board, resulting in a great saving of time and labour, owing to the convenient height at which the board can be placed, and to greater accuracy, as the fairness of the lines can be better seen and judged. At the forward and after ends of the ship the correct shapes of the water-lines are required in order to determine the shapes of the stem Fairing and stern-post, besides which the curvature of these lines is Eh' s. too great to permit of contraction of the abscissa scale. These parts are, therefore, faired by uncontracted water and other lines as already described, except that bow and buttock lines are used to less extent than in the flatter portions of the vessel. Care must be taken that at the junction of parts of the ship fairedplating, called the " middle of rabbet," marked h in the figure, are drawn in the sheer and body plans as fair lines. It should be observed that in the figure h, the middle of rabbet and b, the fore edge of rabbet of plating are shown in side elevation as coincident lines on account of the smallness of the scale; they will not be generally coincident on a full-sized projection on the floor. The middle of rabbet line is best faired in an expansion drawing. In this method a batten is bent to the curve of the-projection of the line in the sheer plan, and the position of the water-lines where sections of the stem have been shown on the drawing are marked on the batten, which is then allowed to spring straight along a straight line drawn in any convenient position on the floor, and the positions of the water-lines are transferred from the batten to the floor. The distances such as xh in the section at AA are measured from each section given in the drawing and set up in full size perpendicular to the straight line on the floor at the positions corresponding to the sections. A fair line through the ends of these perpendiculars will give the distance xh at any position in the length of the stem and enable the projections of the middle of rabbet-line to be drawn accurately in the body and half-breadth plans. To end any water-line such as AA in the half-breadth plan a perpendicular to the middle line of the half-breadth is drawn from the intersection of the line AA, with the projection of the middle of rabbet-line in the sheer plan, and the distance xh, taken from the body plan, or direct from the expansion of the middle of rabbet-line, is set out from the middle line of the half-breadth ; the point h is the ending of the water-line AA required. The water-lines having been drawn and ended in this manner, additional ordinates coinciding with the transverse frames are drawn in the half-breadth plan and their projections obtained and faired in the body plan, in order to define more closely the somewhat twisted surface of the ship in the neighbourhood of the stem. Fairing these frame sections may involve correction and adjustment of the endings of the water-lines, which corrections are made subject to the condition that the projections and expansion of the middle of rabbet-line must remain fair curves. With the middle of rabbet thus fixed in proper relation to the faired surface of the fore end of the ship, the sections of the stem by the water-planes can be reconstructed in the half-breadth plan by the help of the drawing of the stem and of any additional information contained in the specification as to the nature of the fastenings of the plank and plating to the casting and the length of the hood ends. Where the general direction of the stem is considerably out of the --vertical, sections of the frame surface by planes normal to the fore edge of the stem are obtained by the help of the closely spaced frame sections, and rabatted on the sheer plane; and sections of the stem casting constructed on them as in the case of the water-lines. In this way as many points as are required are obtained in the various lines in the surface of the stem, viz. the after edge of the casting, and the various angles of the rabbets, and these lines are faired so far as they are continuous in the three plans. The shell and protective plating and plank sheathing are also put on outside the various sections of the frame surface for a short distance in the neighbourhood of the stem, and the surface of the stem forward of the fore edge of the rabbet is faired in with the outside surface of the ship. A plain batten mould is made to the outline of the stem in the sheer plan, and the projections of the lines of rabbets and of gulleting, position and shape of webs for connecting to decks and Stem stringers and to the wood keel, lines of rabbets for con- mould. netting to keel plates at the lower end and to the tuck plate at the upper end (if the casting is not continued right up to the forecastle deck), the position of the fore perpendicular and load water-line are marked upon it. Sections of the casting taken from the floor are painted on the mould, the centre lines of the sections indicating the position where they are taken, showing more particularly the changes in shape of the casting at such positions as the upper and lower edges of the protective plating and the upper edge of the plank sheathing. The stem mould thus gives complete information for the preparation of the pattern for the casting. The positions of the fore perpendicular and load water-line marked on the mould are transferred to the casting when made, and enable the stem to be erected in its correct position at the ship. The after end of the ship is faired and the mould for the stern post and other castings prepared in a similar manner. The process of preparing the moulds for the stem and stern post is also generally similar to the above in the case of an unsheathed ship, but the castings are less complicated owing to the absence of the plank sheathing. The whole of the zI square stations which constitute the original body plan having been faired as described above, it is usual to calculate the displacement and position of centre of buoyancy of the ship from the lines laid off on the floor to ensure that in the process of fairing no departure of any consequence has been made from the original design. For this purpose the steel plating and wood sheathing, if any there be, must be put on by a process the inverse of that described as taking off the plank. If any serious departure from the original design should be discovered as the result of this calculation, the lines must be corrected and again faired. aa, Fore edge of rabbet for sheathing. bb, Fore edge of rabbet for shell and protective plating. cc, Webs for attachment of decks, breastholes,&c. d, Web for attachment of wood keel. Lower Part Upper Part, se, Line of gulleting. ti, Scarph for connect- ing the parts of the stem. -h, Middle of rabbet for shell plating. Elevation by separate processes there shall be a considerable overlap through-out which the water and other lines in the two parts are identical in order to ensure the continuity of the surface. The detailed drawings of the stem and stern castings already referred to must ensure that these castings shall form a fair continuation of the outside surface of the plating or sheathing. They are perhaps most complicated in the case of sheathed armoured warships where the surfaces of " rabbets " or recesses for housing the bottom and armour plating and the wood sheathing must also conform to the lines of the ship laid off on the floor. A sketch of the stem casting for an armoured, sheathed ship with a ram bow is given in fig. Too, the sections being shown to a greater scale than the elevations for the sake of clearness, except the section at the water-line AA, which is drawn to illustrate the method of ending the water-lines, similar sections being drawn on the floor at the other water-lines. The fore edge of the stem is drawn in full size in the sheer plan on the floor in its correct position relatively to the fore perpendicular and water-lines by measurements taken from the sheer drawing, and the projections of the line of the inner angle of the rabbet for the shell Displacement calculation. 968 The transverse frame lines are the intersections with the frame surface of transverse vertical planes passing through the lines of intersection of the two exterior surfaces of the flanges of the frame angle bars, or of the web and flange of any other type of rolled section which may be used for the frame. The distance between two adjacent frame lines, called the " frame space," is given in the specification, and the positions of the frames relatively to the ordinates are shown in the sheer plan of the sheer drawing. The frame space in a warship is commonly 4 ft. within the limits of the double bottom and 3 ft. forward and aft. In a merchant ship the spacing is usually less. The positions of the planes of the frames are set off along the middle line of the half-breadth plan, the proper scale being used in the contracted half-breadth, and ordinates are drawn to represent their traces in the half-breadth and sheer plans. The projections of the frame lines in the body are obtained from the intersections of the ordinates with the water and diagonal lines in the half-breadth and the bow and buttock lines in the sheer plan in a manner already described in the case of the more widely spaced stations used in fairing the body. These frame lines in the body should require no further fairing if the work has been accurately done when using the original square stations, and they can be at once rased in on the floor. As already stated, it is usual to dispose the transverse framing of a ship entirely in planes perpendicular to the trace of the load Cant water-plane with the longitudinal plane of symmetry frames. of the ship. This practice leads to a large and varying bevel being given to the frame bars at the ends of a vessel with a very bluff bow or stern, and it becomes a practical question whether it would not be better at such parts to dispose the frames in planes which are more nearly normal to the general surface of the ship and which need not be perpendicular to either of the three planes of reference. The disposal of frames in this way, more usually in planes perpendicular to the half-breadth planes only, when they are called " cants," is in common use in wood shipbuilding, it being of great economical importance that the timber frames shall be of square or nearly square section, but it is also adopted in iron and steel ships of unusual form or having special features, such for instance as a lifting screw propeller. To lay off a cant frame or " cant ": Let the traces of the cant be a'b', ab in fig. tot. Let LL be the projections of a level line in the three plans intersecting ab at b in the half-breadth. Then bi in the sheer is the vertical projection of b, and a curve through all such points as bi is the projection in the sheer of the shape of the frame or, as it is called, of the moulding edge of the frame. ba in the body, where a2b2 is equal to the perpendicular distance of b from the middle line of the half-breadth, is a point in the projection in the body plan; and ba where a2b3 is equal to ab is the position of the point, when the cant plane is hinged about a'b' until it is parallel with the body plane. Hence a curve drawn through all such points as b3 is the true form of the moulding edge of the cant. To obtain the angle which the surface of the ship makes with the plane of the moulding edge, a plane parallel to that of the moulding edge and distant from it the width of the bevelling board must be laid off in a suitable position in the body plan. Let g'c', gc be the traces of such a plane where af, the normal distance between it and the plane whose traces are a'b', ab, is the breadth of the bevelling board. The vertical projections of c, viz. ci and c2, in the sheer and body are found in the same way as those of b; but in order to obtain the rabatted curve of the bevelling edge in such a position relatively to the moulding edge that the perpendicular distance between the two curves measures the bevelling in the same way that the perpendicular[COURSE OF CONSTRUCTION distance between two frame lines of the square body measures their bevelling, it is necessary to first project the bevelling edge on the plane of the moulding edge before rabatting the latter. The whole operation is effected by making as c3 in the body equal to fc in the half-breadth, where af is perpendicular to ab and gc. A curve through all such points as c3 is the bevelling edge laid off in the position relative to the moulding edge required, the bevellings being taken in a similar manner to those of the ordinary transverse frames. Spots on the cant can also be obtained from diagonals as follows:—In fig. IO2 let DD be the projections of a diagonal D line in the three plans cutting the horizontal traces.. of the moulding and bevelling edges at d and e in the hglf-breadth. The projections di, el in the sheer and d2, ea in the body of the intersections of the diagonal line with the planes of the moulding and bevelling edges are obtained in the same way as in the case of the level line, and the method of obtaining the rabatted positions, when the plane of the moulding edge, with the bevelling edge projected upon it, is turned about a'b until it is parallel to the body plane, is also analogous; but in this case the corresponding points of the moulding and bevelling edges are in different level planes dad' eel. Points in the rabatted curves of the moulding and bevelling edges of the cant may also be obtained from the intersections with bow and buttock lines, as shown in fig. I03, where BB are the projections of the B i /k B bow or buttock line in the three plans. The method is analogous to that described above when using level lines and as shown by the figure, h3 and ka being rabatted positions of points in the moulding Frame lines. b' /IIi ' - " --8 B and bevelling edges respectively; where h4 ha is equal to oh and k4k3 tofk. In fig. 104 let AB, A'B' be the traces of the plane of the moulding edge of the frame in the sheer and half-breadth plans respectively. Double When, as in the figure, neither trace is perpendicular to canted the base line, the frame is said to be a double canted frame. frame, or a double cant. Let IL, 2L, 3L be the projections of level lines in the three plans, P, Q, R in the sheer plan being their point of intersection with AB. The horizontal projections of these points are found as indicated in the figure where Q' on the middle line of the half breadth is the horizontal projection of Q. The line Q'q' parallel to A'B' is the horizontal projection of the line of intersection of the double cant plane with the level plane 2L, and q, obtained by the construction shown, is the vertical projection of the point where this line of intersection cuts the surface of the ship, q' being the horizontal projection of the same point. The projections of other points in the intersection of the double cant plane with the surface of the ship are found in a similar manner by the help of other level lines; and the projections s' and s of the ending where the line of half siding of the flat keel cuts the double cant plane are found by the construction indicated. The projections of the moulding edge of the double cant frame spqr in the sheer plan and s'p'q'r' in the half-breadth are thus determined. The true form of the moulding edge is laid off in the body plan by a double process of rabatment of the double cant plane, first about the trace AB to bring it perpendicular to the sheer plan, and then about a normal to the sheer plan through A to bring it parallel to the body plan, in the following manner. Set off P2, Q2, R2 on the middle line of the body so that their distances from A2 are equal to AP, AQ, AR measured along the trace AB in the sheer plan. Draw AC in the sheer plan perpendicular to AB and measure the heights parallel to AB of the points p, q and r above AC. Draw level lines IL', 2L', 3L' in the body plan at distances above the base line equal to these heights, and from the centres P2, Q2, R2 describe circles cutting IL', 2L', 3L' in p2, q2, r2, &c., so that the radius Q2g2 is equal to Q'q', &c. The curve p2g2r2 is the true form of the moulding edge of the double cant laid off in the body plan. The plane of the bevelling edge is parallel to that of the moulding edge and at a perpendicular distance from it suitable for use as the base of a bevelling triangle similar to that which is described for the ordinary frames. The width of the bevelling board is made equal to this perpendicular distance, corresponding to the frame space in the case of the ordinary frames, and the bevelling edge969 must be laid off so that the normal distance between it and the moulding edge can be used for marking the bevelling in the same way as the normal distance between consecutive frames of the square body is used. To obtain the traces of the plane of the bevelling edge, in fig. mo4a let AB, AB' be the traces of the moulding edge plane; nm drawn perpendicular to AB and mm' perpendicular to the axis are the traces of a plane perpendicular to the plane of the moulding edge and to the vertical or sheer plane. If mM be drawn perpendicular to nm and equal to mm', nM is the intersection of the planes BAB' and nmm' rabatted on to the sheer plane, and mH perpendicular to nM is the rabatted position of a line perpendicular to the plane of the moulding edge. Make HK equal to the chosen distance of the bevelling edge plane from the moulding edge plane; draw Kk parallel to Mn cutting nm in k; through k draw DkE parallel to AB and through D, where DE meets the base line, draw DE' parallel to AB'; then DE, DE' are the traces of the plane of the bevelling edge arranged at the required perpendicular distance from the plane of the moulding edge. In laying off the bevelling edge it is first projected on to the plane of the moulding edge, and the latter then rabatted into the body plane. To effect this operation the horizontal trace Am'i, of a plane perpendicular to the double cant plane and intersecting it in the vertical trace AB must be drawn, which is done by the construction shown in fig. io4a, where nm is, as before, perpendicular to AB through any point n in it other than A, and n'm'1, drawn through n', the horizontal projection of n, is perpendicular to AB'. The projections of the traces with the several level planes of the plane of the bevelling edge, such as U'w' and the projections of the bevelling edge ltwv in the sheer plan and l't'w'v' in the half-breadth are obtained in exactly the same way as in the case of the moulding edge. The projections such as Q'w'1, of the traces with the several level planes of the plane whose traces are AB and Am'1, in fig. io4a are also drawn parallel to Am'1, through the horizontal projections of P, Q, R, &c. The vertical projection wl of the point w'1, in which Q1w'1 meets U'w' produced, is found and A2Ua set up on the middle line of the body equal to the perpendicular distance of wl from AC. A level line 2L in the body plan is drawn at a distance from the base line equal to the perpendicular distance of w from AC and a point w2 found in it such that the radius U2w2 is equal to w'lw' in the half-breadth. w2 is then the rabatted position of the projection on the plane of the moulding edge of the point in the bevelling edge whose projections are w and w'. Points 12, t2 and v2 corresponding to the projections 1 and 1', t and 1', v and v' are found in a similar manner and a curve drawn through 12t2wsv2 is the bevelling edge laid off in the body plan in the correct relation to the laid off position of the moulding edge for the bevellings to be taken. Additional points in the rabatted shape of the double canted frame may be obtained by the use of diagonals when desired. In fig. to5 AB, A'B' are the traces of the double canted plane; cd, cads are the projections of a diagonal line in the body and SHIPBUILDING
End of Article: PART SHEER AND
[back]
PART IV
[next]
PART V

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.