Shelter-deck freighters Three-deck freighters, with poop, bridge, and forecastle VALUE .65 to .70 .67 .55 to .60 .65 .70 .69 to .71 .60 to .65 .56 to .58 .56 to .58 .54 to .56. All the elements insuring the qualities that embody a wellshaped boat of the particular type contemplated and at the same time a stable ship having been thus determined, the lines may be commenced with the certainty that no unnecessary alterations will be required. The freeboard will be calculated from the legal tables given and explained herein, but in any case the limiting draught consistent with the block coefficient determined on as the maximum available for the required speed should be taken advantage of. After carefully drawing the center and other construction lines, and marking off the ten or twenty ordinates that it is proposed to (18) Bilge diagonal coefficient (see diagram) = .82. (19) Dimensions as determined = 460 × 58′ 6′′ × 33′ 6′′. (20) Displacement "D" 460' x 58.5′ × 27′ 4,670 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. |