476 



HYnROnVNAMICS IN SHIP DESTCN 



Ser. 66.12 



principal dimensions and to calculate the form 

 coefRcients to the molded form, as explained in 

 Sec. 66.21, it is possible to consider about 90 t 

 of the displacement as helping to support the 

 weight of the shell plating and appendages. The 

 volume of the molded hull, to the outside of the 

 frames, is therefore smaller by about 90(35) = 

 3,150 ft'. Specifically, it is 577,150 ft' less 3,150 ft', 

 or 574,000 ft'. 



Considering for the moment the middle tenta- 

 tive length of 515 ft, with its T„ of 0.903, the 

 displacement-length quotient for a weight W of 

 16,490 - 90 = 16,400 t is 16,400/136.591 or 

 120.1. The 0-diml fatness ratio is 574,000/136,591 

 or 4.202. This is just below the middle of the upper 

 lane of Fig. 66. A, so the 515-ft length still appears 

 appropriate. 



The maximum section area Ax for a Cp of 0.62 is 



A^ = 



¥ 



574,000 



L{Cp) 515(0.62) 

 Reducing the beam to 74 ft gives 

 Ax 1,798 



= 1,798 ft' 



Cx = 



Bx{Hx) 74(26) 



= 0.9345 



This maximum-section coefficient appears, from 

 Fig. 66. D, to be somewhat on the low side. Using 

 a beam of 73 ft, , 



Cx = 



Ax 



1,798 



BxiHx) 73(26) 



= 0.947 



This is still somewhat low. It is possible that, 

 with some 800 t off the original displacement, the 

 length is a little longer than need be. Taking a 

 reduced length of 510 ft, and retaining Cp = 0.62, 



Ax = 



574,000 



L{Cp) 510(0.62) 



= 1,815 ft' 



whence 



Cx = 



1,815 



73(26) 



0.9563, 



which is satisfactory at tliis stage. 



These new dimensions give an L/B ratio of 

 510/73 or 6.986 and a B/H ratio of 73/26 or 2.808. 

 The block coefficient Cb is (0.62)0.956 or about 

 0.593. The Taylor quotient T, is 20.5/22.583 = 

 0.908, the displacement-length quotient is 

 16,400/132.651 = 123.63, and the 0-diml fatness 

 ratio is 574,000/132,651 = 4.327. As a convenient 

 check at this point the graphs of Fig. 66. E 

 indicate a mean L/B ratio of about 7.4 for a 

 vessel 510 ft long. 



For convenient reference the data derived in 

 the foregoing for the tentative lengths of 500,515, 

 525, and 510 ft, plus a few additional items to be 

 derived, are Usted in Table 66. e. 



66.12 Selection of Hull Shape. Up to this 

 point the preliminary design has involved only 

 principal dimensions and proportions. It is now 

 necessary to think of the shape which the vessel's 

 hull is to take. While it is admitted that the 

 Taylor Standard Series shape, derived from a 

 twin-screw cruiser of the early 1900's, is not 

 necessarily adaptable to any vessel designed in 

 subsequent years, especially one with a single 

 screw, it is without question a good shape from 

 the standpoint of easy driving. 



Other good shapes, excellent ones, have been 

 developed through the years, shapes which no 

 designer need hesitate to copy if they serve his 

 purpose. He is cautioned, however, not to attempt 

 "breeding" better ship fines by averaging good 

 existing lines; this has been tried and definitely 

 found wanting. 



It is extremely difficult in the present state of 

 the art, especially without benefit of model tests, 

 to predict the effect of shape changes in a parent 

 form. Nevertheless, it is considered far preferable 

 to modify a given good shape to meet the de- 

 signer's needs than to make up a new shape by 

 adding a good stern to a good but unrelated bow, 

 or by any process of averaging. These matters 

 are discussed in greater detail in Sec. 66.24. 



66.13 Layout of Maximum-Section Contour. 

 The tentative value of Cx as selected from Fig. 

 66. D determines whether the maximum-section 

 contour is to be rectangular, follomng closely the 

 lines for hmiting beam and draft, whether it is to 

 be well cut away, as in a keel type of saifing yacht, 

 or whether it is to take some intermediate form. 

 If it is desired, in fashioning the form, to place 

 as much displacement as possible amidships, the 

 use of a hard bilge and a relatively "square" 

 section need not interfere materially with the 

 flow except to increase the transverse velocity 

 gradient and the local friction resistance around 

 the sharp bilge. 



In a vessel which is to run at not more than 

 medium or fast speed in deep water, there is no 

 reason why the bottom can not be perfectly flat 

 over a considerable area. The floor fines at the 

 midsection need not be raised unless this is 

 required for drainage of the tanks and spaces 

 lying just above this bottom, or for some other 

 practical purpose. 



