484 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 66. IS 



to be determined. Unfortunately, the results of the 

 parallel-middlebody series of models developed 

 by D. W. Taylor, reported in the 1943 edition of 

 his book "The Speed and Power of Ships," 

 pages 70-72 and 257-271, have not been analyzed 

 and put in suitable design form. It is therefore 

 necessary to use an empirical design curve. Taking 

 as a basis Taylor's original 1910 diagram [the 

 same as S and P, 1943, Fig. 83, p. 71], his data 

 have been supplemented by parallel-middlebody 

 percentages (Lp/L) for ship models on the 

 SNAME RD sheets whose performance was 

 equal to or better than that of the TSS model of 

 the same proportions. When suitably extended to 

 cover higher and lower values of Cp , the new 

 plot of Fig. 66.M reveals that these ship data he 

 in a rather narrow lane running diagonally across 

 the diagram. Since Cp and T„ (or F„) are, for 

 easily driven ships, related by the lower design 

 lane of Fig. 66. A, the data recently analyzed 

 are therefore plotted in Fig. 66.M on a basis of 

 Cp only. The new design lane gives directly the 

 proper ratio of Lp to Lg^L- 



D. AV. Taylor's original diagram [S and P, 1943, 

 Fig. 83, p. 71], as well as Fig. 66.M, reveal that: 



(a) Inserting parallel middlebody of length Lp 

 is a definite advantage at low T, and F„ values. 

 For a given Cp it adds displacement amidships 

 and allows finer ends. It also gives rectangular 

 passenger and cargo spaces and may result in 

 reduced building costs. 



Percentaqe of Ship Length in Parallel Middle Body 



Fig. 66.M Design Lane fob Percentage op 

 Parallel Middlebody 



(b) The ratio Lp/L, as it affects resistance, is 

 insensitive to changes in the fatness ratio 

 V/{Q.10LY or the displacement-length quotient 

 A/(0.010L)' 



(c) For a given T, and Cp there is an optimum 

 ratio of Lp/L. This optimum may be departed 

 from materially without much increase in resist- 

 ance. However, it is to be remembered that 

 excessive amounts of parallel middlebody produce 

 undesirable shoulders in both the section-area 

 curve and the near-surface waterlines. They 

 make the entrance too blunt and thus affect the 

 resistance adversely. The design problem be- 

 comes one of balancing the return in increased 

 displacement volume and the ease of construction 

 against the price of added resistance and fuel 

 consumption. At Cp values of 0.80 or higher the 

 demand for carrying capacity on a given set of 

 overall dimensions usually outweighs any endeavor 

 to achieve the best hydrodynamic performance. 



(d) Length of entrance Le and run L^ are closely 

 related to and should be considered with Lp , 

 both as to magnitude and position of the latter. 

 The fore-and-aft position of the midlength of the 

 parall el middlebody is given by the value of 

 LMA, the same as for the longitudinal position 

 of the section of maximum area. 



(e) Practical variations in Lp/L in a given case 

 have little effect on the wetted surface and friction 

 resistance 



(f) The ratio Lp/L has a controlling effect on the 

 ship's transverse wave pattern, for the reasons 

 given in Sec. 25.10 



(g) Hard spots or shoulders at the ends of the Lp 

 are to be carefully avoided. These shoulders 

 initiate their own wave systems and increase 

 resistance. 



Comparing Fig. 66.J, for percentage of parallel 

 waterline, with Fig. 66. M for percentage of 

 parallel middlebody, the values indicated on the 

 former are always greater than those on the latter 

 for any ship or normal form with tapered ends. 

 The design lanes of these two graphs narrow in 

 width with increasing Cp . Both vanish at the 

 terminal point of Cr = 1.00, where the parallel 

 waterline and the parallel middlebody both 

 extend the entire length of the body or ship. 



For the ABC design, with its Cp of 0.62, the 

 amount of parallel middlebody given by Fig. 

 66. M is definitely zero. Assuming that in another 

 design a certain amount of parallel middlebody 

 is advantageous, this uniform section is, as indi- 



