Sec. 66.1? 



STEPS IN PRELIMINARY DESIGN 



477 



Excessive transverse metacentric stability de- 

 veloping ill the course of the design is reheved by 

 narrowing the surface waterline and working 

 tumble home into the midsection or into the 

 section of maximum area for a considerable dis- 

 tance below the DWL, possibly half way down 

 to the baseplane. Increased displacement volume 

 and carrying capacity is achieved by working an 

 underwater bulge into this section below the 

 designed waterplane. If the vessel already has 

 adecjuate metacentric stability, this bulge need 

 not increase the waterline beam. 



As a check on the tentative value of Cx for the 

 ABC design at this stage a maximum-section 

 contour is drawn. This requires that the rise of 

 floor, if any, be established. It gives an idea of 

 the roll-resisting characteristics of the section, 

 leading in turn to an estimate of the bilge-keel 

 width which is required and that which can be 

 allowed. 



To meet the requirements of the ABC design 

 it is decided tentatively that: 



(a) There is to be some rise of floor, to provide 

 for internal tank drainage and to give more room 

 for water moving aft under the ship in the shallow 

 waters to be traversed 



(b) The side of the ship in way of the designed 

 waterline is to be given a shght tumble home if 

 possible but in any case is not to have an out- 

 ward flare in that region 



(c) There should be room to fit roll-resisting keels 

 which are at least 3 ft wide amidships, to help 

 counteract the effect of the shallow draft and 

 the wide beam. 



A rise of floor of 1.0 ft is tentatively selected. 

 With a half-beam amidships of 36.5 ft and a 

 half-siding of say 1.5 ft, this gives a floor slope of 

 1.0/35.0 = 0.0286, corresponding to slightly over 

 1.6 deg. The rise-of -floor to beam ratio isKF/fi^ = 

 1.0/73 = 0.0137. 



To calculate 6R with no Rise of Floor To colculoti 



5R-V?.5299(I-C,)Bx-H 



R-l/(l-Cx)B«H-0.5Bx-RF 



I Half-Siding H5 neglected y^ ^^^ 



Assuming for a starter that the floor line and 

 the ship's side are both straight, and that the 

 bilge contour is a circular arc, the appropriate 

 formula of Fig. 66. F enables the bilge radius BR 

 to be readily calculated. For the ABC ship this 

 comes out as 10.76 ft, equal to 0.14745^ , for an 

 Ax value of 1,815 ft' and a Cx of 0.9563. Taking 

 a half-siding of 1.5 ft, a molded half-beam of 

 36.5 ft, and a molded draft of 26 ft, one-half of 

 the maximum section is laid out as in Fig. 66. G. 



Fig. 66. P Formulas for Computing Bilge 

 Radius BR 



Fig. 66.G Half of Maximum-Section Contour 

 FOB ABC Design 



A heavy bed hue is drawn in at h = 29 ft. The 

 rise of floor and the slack bilge appear to provide 

 ample room for backflow under and around the 

 ship in the shallow and restricted areas of the 

 river and the canal but of course the large corner 

 radius detracts from the inherent roll-resisting 

 characteristics of the hull. However, there is room 

 enough for roll-resisting keels at least 4 ft wide 

 amidships, if desired, without having them 

 project below the floor line extended or beyond 

 the extreme beam. 



The first conflict between requirements now 

 appears in grapliic form. With the fairly large 

 B/H ratio of 2.8, the large ratio of BR/Bx = 

 0.1474, and a value of BM that is certain to be 

 large, there are indications of heavy rolhng ahead, 

 hence the need for deep roll-resisting keels. The 

 compromise thus indicated between restricted- 

 water and wavegoing needs may not be the best 

 one but it will be allowed to stand for the time 

 being. At least the restricted waters must be 

 traversed twice every voyage while waves that 

 produce deep rolling may or may not be en- 

 countered on every trip. 



