Sec. 67.29 



UNDERWATER-HULL DESIGN 



543 



throughout the ship length, a revised section-area 

 curve is drawn. The areas are determined for at 

 least 20 sections, the A /Ax values calculated, 

 and a fair curve passed through the 21 ordinates. 

 A bulb bow calls for fairing to a designated /c 

 value at the FP; a transom stern to an /« value 

 at the AP. A discontinuity of sorts is to be expected 

 opposite a skeg ending, especially at the forward 

 end of a propeller aperture. The final ^-curves 

 for the ABC ship, covering both types of stern, 

 are drawn in Fig. 67. W. 



Except for the regions known to be discontin- 

 uous the eye should detect no unevenness in the 

 curve, nor should it appear when using a batten. 

 However, the eye is often deceived by the pre- 

 sence of other lines in the vicinity, even those in 

 the coordinate network. Hence the 0-diml 

 curvature for at least 20 and preferably 40 

 stations along its length is determined by one or 

 more of the methods described in Chap. 49 and 

 is plotted to the same base as the section-area 

 curve. Fig. 67. X shows a 0-diml curvature plot 

 of the A-curve of Fig. 67.W for the ABC design 

 with the transom type of stern, as well as similar 

 plots for the Taylor Standard Series model, 

 EMB 632 (modified), and for a merchant ship 

 of good performance. 



Integrating the section-area curve, by whatever 

 procedure may be appropriate [PNA, 1939, Vol. I, 

 pp. 13-27], gives: 



(a) By its fullness coefficient, the corresponding 

 prismatic coefficient Cp . For the ABC ship this 

 should be within 0.01 or less of the selected value 

 of 0.62. 



(b) The molded underwater volume, without 

 appendages, up to the designed waterfine. This 

 should correspond to the ABC ship displacement 

 weight of 16,400 t for standard salt water . 



(c) An accurate determination of LCB, with 

 respect to the FP, for reference and comparison 

 purposes. 



This revised A-curve may be considered as 

 final if the analysis brings out no objectionable 

 features or irregularities in it, if the values of Cp 

 and V are reasonably close to those selected, 

 and if final fairing of the lines to a large scale 

 indicates no appreciable changes in the amount 

 or distribution of volume. 



67.29 Modification of Normal Design Pro- 

 cedure for a Hull with Keel Drag. It often 

 becomes necessary, for reasons of propulsive 

 coefficient, rotative speed of the propelling plant, 



