Sec. 66.9 



STEPS IN PRELIMINARY DESIGN 



473 



taking first the dimensional wetted-surface coeffi- 

 cient Cws from Fig. 20 on page 22 of the reference. 

 With B/Ii = 2.92 and Cx = 0.96, the value of 

 Cws is, as nearly as can be determined, 15.02. 

 For a 515-ft ship the length-correction factor 

 Q:(alpha) is 0.997, taken from the diagram at the 

 right of Fig. 188 on page 188 of the reference. 

 Then for a A/(0.010L)'' quotient of 126.7 and 

 a T, of 0.903, the friction resistance per ton of 

 displacement for a ship having a Cws of 15.4, 

 from the plot of Fig. 188, is 6.1 lb. For the 515-ft 

 ship with a Cws of 15.02, and an a of 0.997, 



R. = 6.1 



(15.02) 

 (15.4) 



(0.997) = 5.932 lb pert. 



For the Cp of 0.62, the displacement-length 

 quotient of 126.7, the B/H ratio of 2.25, and the 

 r„ of 0.90, the value of Rr/^. is, from page 201, 

 3.55 lb. For a B/H ratio of 2.92, it is, by hnear 

 interpolation between B/H = 2.25 and B/H = 

 3.75, 3.75 lb. Similarly, for T^ = 0.95, Rr/L is 

 5.779 lb. Again interpolating linearly for T, = 

 0.903, i2«/A is 3.872 lb for the parameters given. 



The bare-hull resistance Rt is then 



(1^ + ^)a = (5.932 + 3.872)17,300 

 = 169,610 lb. 



The agreement with 171,830 lb as found by 

 the third method is within 1.3 per cent and is 

 good enough at this stage of the design. 



Another quick method for approximating the 

 total bare-hull resistance /2j. of the ship is to use 

 the graph of Fig. 56. M, comprising values of 

 iZr/A plotted on T^ over a wide range of relative 

 speeds. It applies to any type of vessel from a lake 

 freighter up to a high-speed patrol craft. Entering 

 Fig. 56.M with a T^ of 0.903, corresponding to the 

 designed speed of 20.5 kt of the ABC ship, the 

 value of flr/A is 10.2 lb per long ton. With an 

 estimated displacement of 17,300 tons, this gives 

 a bare-hull resistance 72?. of 176,400 lb. This is 

 2.66 per cent higher than the resistance estimated 

 by the third method described, but is at least 

 on the high side for the present. 



If the vessel is to be driven by a single screw, 

 the ship requirements appear to call for no 

 appendages except a single rudder and a pair of 

 roll-resisting keels. A rudder having an area of 

 0.02{LH) would have a projected blade area of 

 about 0.02(515)26 = 268 ft^, and a surface area 

 of something over 536 ft^. A pair of roll-resisting 



keels 200 ft long and 3 ft wide would have a 

 total wetted area of about 2(200)3(2) = 2,400 ft'. 

 The hull area covered up by the bases of roll-resist- 

 ing keels of triangular section would average about 

 1 ft wide by 2(200) ft long, or say 400 ft'. The 

 total added area for rudder and keels is then 

 536 + 2,400 - 400 = 2,536 ft', which is 2,536/ 

 (46,231) = 0.055, or 5.5 per cent of the bare-hull 

 area. On the basis of additional wetted area 

 alone, this is not more than 6 per cent of the 

 friction resistance, which in turn is only about 

 62 per cent of the total. The increase in total 

 drag is therefore of the order of 4 per cent. 

 However, it seems wise at this stage to double 

 this effect and allow about 8 per cent of the total 

 resistance for the final appendage drag. Since 

 the requirement of item (26) of Table 64. d states 

 that "A reasonable expenditure of weight or 

 power, or both, to secure effective roll-quenching" 

 is acceptable to the owner, it may be considered 

 advisable, at a later stage of the design, to make 

 the roll-resisting keels even larger than indicated 

 here. 



In the absence of any better information, an 

 additional 2 per cent is included to cover the drag 

 of the condenser scoop and the circulating-water 

 discharge, making 10 per cent in all. 



So far as can be determined at tliis time there 

 is sufficient allowance for fouling in the 1.8-kt 

 difference between the 18.7-kt scheduled speed for 

 the whole voyage and the 20.5-kt trial speed 

 under clean-bottom smooth-water conditions. 



The tabulated data at the end of Sec. 60.11 

 give a range of propulsive coefficient of 0.82 to 

 0.72 for clean, new, single-screw ships of modern 

 hydrodynamic design. It seems reasonable to 

 assume that a value of iji. as high as 0.74 can be 

 achieved for a single-screw ABC ship, even 

 though the design is not yet worked out. Using 

 an appendage-and-scoop factor of 0.10 for added 

 resistance, and a propulsive coefficient of 0.74, a 

 first approximation to the shaft power is (10,820) 

 (1.10)/0.74 = 16,084 horses. 



Item (22) of Table 64. d states that the sustained 

 sea speed of 20.5 kt shall be attained by the use 

 of not more than 0.95 of the maximum designed 

 power. The latter is therefore 16,084/0.95 = 

 16,930 horses. This is considerably less than the 

 first and second estimates of 19,560 and 20,550 

 horses, but aU are within the capabilities of a 

 modern single-shaft plant and a single propeller. 



Again it is emphasized that all these estimates 

 are for average performance, with generous 



