Sec. 66.6 



STEPS IN PRELIMINARY DESIGN 



467 



point plots almost in the middle of the upper 

 lane; the 515-ft length is apparently about right. 

 At least it seems so at this stage. A ship longer 

 than 525 ft on the designed waterline need not 

 be considered until other features are investi- 

 gated for these three lengths. Although a still 

 greater length involves additional wetted surface 

 there may be other good reasons for using it. 



The significance of the special spots found on 

 this and succeeding diagrams is explained sub- 

 sequently in the chapter. 



At the bottom of Fig. 66.A there appears a 

 subdivision worked out by D. W. Taylor [S and P, 

 1943, p. 48] which indicates the position of humps 

 and hollows in the curves of residuary resistance 

 for a large range of T, values. Reference to this 

 subdivision indicates that the three TJs for the 

 tentative lengths of 500, 515, and 525 ft all he 

 in the middle of a hollow, slight but definite, for 

 vessels of normal form. Here Rr is slightly less 

 than it would be for a smooth curve of mean or 



"natural" values of Rr on V [Davidson, K. S. M., 

 PNA, 1939, Vol. II, p. 70]. This is explained in 

 Sec. 10.14. 



Fig. 66. B, adapted from Taylor's shaded 

 length-speed diagram [S and P, 1943, Fig. 55, 

 p. 48], gives for ready reference a set of dimen- 

 sional values in English units by which the 

 hollow-hump positions for any ship length and 

 any speed are found by inspection. 



66.6 The Longitudinal Prismatic Coefficient. 

 The prismatic-coefficient design lane of Fig. 

 66. A, especially at the low-speed end, does not 

 give the optimum Cp for a given T, or F^ , such 

 as may be obtained from the Taylor Standard 

 Series contours of Rr/^, because in that region 

 the friction resistance is generally the major 

 part of the total resistance. For example, the 

 original contours of Rr/A. for the Taylor Standard 

 Series [S and P, 1943, pp. 201, 227] for a T, 

 value of 0.90 and a A/(0.010L)' range of 138.4 

 for the 500-ft length to 119.6 for the 525-ft 



Taylor Quotient Tq or Speed- Lenqth Quotient V/Vu 



6.0 5.0 4.5 4.0 3.5 3,0 2.5 Z.O 



10 20 30 40 50 60 80 |60 150 ZOO 250 300 



Ship Lenqth, ft 



Fig. 66.B Diagram Illustrating Positions of Humps and Hollows in Residuabt-Resistance Curves, in 



Terms of Ship Length and Speed 



