CHAPTER II. DESIGN. In the foregoing pages we have treated with the various calculations which confront the naval architect, but the relation of these to one another and to the particular qualities that the projected ship shall possess belong to Design. In designing the ship, nothing should be left to chance, or what is the same thing — trial and error. The vessel must first be designed with figures. Before a single line is run on paper, the various element coefficients should be carefully selected and their functions worked out in consonance with the results desiderated in the finished ship. The relation of these coefficients to one another must be firstly mastered for all types of vessels and conditions of draught and trade, when with the aid of the tables given an unerring selection will be possible and a definite result attained. When the way is prepared for the drawing part of the design to be taken in hand, it will be found advantageous to have a definite routine in which to prepare the various views comprised under the general term "Lines." Each step should be taken in its proper time and order. Much time will thus be gained, and a clearer conception of the art of designing obtained. To this end we submit the following method as one fulfilling these propositions, dividing the task broadly into two parts, viz. : (a) Figures and (b) Lines, the first embracing the moulded dimensions, draught, element coefficients, and their functions, and the latter, the sheer draught, half-breadth, and body plans. The shipowner will specify the trade for which the ship is intended and the limit of draught on the particular service proposed. It will generally be found economical to take advantage of the maximum draught permissible. When the dimensions are solved to meet the requirements stipulated, the grade numerals should be worked out, for the Classification Society's Rules in which it is proposed to class the ship, and if it be found that a grade can be saved either in plating, framing or equipment numerals, or the requirements for extreme proportions evaded by a slight alteration or adjustment of the dimensions, this of course should be done. As an example we shall postulate that the shipowner requires a 3-deck freighter with complete shelter deck to carry 10,000 tons dead weight, exclusive of coal for 12 days' steaming, fresh water and stores, on a mean draught of 27 feet with a B.T. Freeboard and a sea speed of 12 knots. The ship to be classed in American Record and to conform to the U.S. Inspection Laws. To these lemands of the owner the naval architect should add the G.M. when fully loaded with a homogeneous cargo. Let us call this 1.5 ft. The first point to determine is the amount of displacement we shall require to provide for over and above the specified dead weight of 10,000 tons, to allow for weight of finished ship and nachinery, coal, fresh water, and stores. At this stage we cannot calculate these items, as we are uninformed as to the dimensions of the ship, so that the remaining method to solve this is to estimate a weight embracing all of these items based on a percentage of the dead weight. This percentage of course is determined from vessels of similar type and trade duly worked out and tabulated by the naval architect. We shall take, then, each step in its proper order: (1) Displacement = dead weight × 1.64 = = 16,400 tons. (2) Block coefficient "8"= a.ß.e. = .79. 66 δ α.β .945.* δ = = .97. a.e =-= .814. β (6) Area of L. W.L. coefficient "a = p = :.861. (7) Moment of inertia coefficient "i" (see table) = .0638. (10) Depth "H" to upper deck per Freeboard Tables = 33.5 ft. (11) Depth "Hi" to shelter deck = H+7.5 ft. 41 ft. (12) Center of gravity above base = H1g=41.559=22.90 ft. (13) Metacenter above base = C.G. + G.M. 22.90+ 1.50 24.40 ft. = (14) Breadth "B" to give M.C. of 24.4 ft. = (18) Bilge diagonal coefficient (see diagram) = .82. 460 × 58.5′ × 27′ x.79 = 16,400 tons. Should it be found, however, that the weights calculated for the dimensions as worked out are lighter than anticipated when we started with the 64 per cent of the dead weight, the length should be reduced accordingly. On the other hand, if the weights be excessive, the length must be increased. The length is the only dimension that should be adjusted, as it is the one factor which has no vital relationship to the element coefficients, as it will have been noticed that the primary quality aimed at was the G.M. as a measure of the ship's initial stability; and as the center of gravity varies with the depth, so the metacentric height is dependent on the breadth and draught. For the preliminary design it will be sufficiently close to estimate the machinery weights on the I.H.P. required, and for ordinary merchant practice the power may be calculated fairly accurately by the Admiralty constant with the formula: We then have for the present example, with constant=267, speed 12 knots, and displacement 16,400, an indicated horse-power =4000. By referring to the table given elsewhere, it will be found that for twin screw freight steamers with this speed that the I.H.P. per ton of engine boilers and water equals about 5.5, so that we get for a total machinery weight 4000 The displacement and coefficients should, in all cases of steel steamers, be calculated to the moulded line of frames, the excess water displaced by the shell plating, amounting to about 1%, being retained in hand as a margin against contingencies. In this case its value is 164 tons, representing 3 inches of draught. * See Table of Constants, and chapter on Resistance. |