282 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 54.10 



Anqle of Relative Wind from Ahead, deq 

 10 20 30 40 50 60 10 80 90, 



iota for a Typical 

 Heavv^ Cruiser 

 PENSACOLA (CLZ4) 



.6 



^ *• 

 1.5 -^ 



O 



1.4 -s 



1.3 "? 

 > 



I.Z Q^ 



o 



'•' ^^ 



1.0 "^ 



<D 



0.9 -S 

 0.8 1 

 01 az 

 0.6 8 



o 



0.5:? 



tn 

 W 



0.4 '^ 



-a 



0.3 .E 

 3: 



0.2 'S 



0.1 ~ 

 a 



10 20 30 40 50 60 70 60 90 

 /'^ngle of Relative Wind from Ahead, deg 



Fig. 54. E Gr.\ph of kg for a Heavy Cruiser 



Three additional graphs are given by G. Hughes 

 for a tanker, a cargo vessel, and a transatlantic 

 liner [INA, 1930, pp. 321-324 and PL XXXVI]. 



These graphs are reproduced in SNAME, 1932, 

 Fig. 14, p. 41. Additional graphs for an express 

 cargo hner are given by G. Kempf in Fig. 7 on 

 page 51 of this reference. A graph for the U. S. 

 Maritime Administration Mariner class is pub- 

 lished by V. L. Russo and E. K. Sullivan in 

 SNAME, 1953, Fig. 45, page 212. 



54.10 Prediction of Wind Resistance for 

 ABC Ship of Part 4. As examples of the method 

 by which the formulas and data of the preceding 

 sections are employed in practice, the probable 

 wind resistances of the ABC ship, designed in 

 Part 4, are calculated for several design stages 

 and conditions. 



It is assumed first, that at an early stage in the 

 preliminary design, before the abovewater body 

 is dra%vn and the upper works are laid out, the 



stUl-air resistance is to be approximated for the 

 trial speed of 20.5 kt. The transverse abovewater 

 area Aa\s taken, by Eggert's rule of thumb, as 

 (0.5)Bx • With a beam of 73 ft, A a becomes 

 0.5(73)' = 2,665 ft'. The dimensional Eq. 

 (54.vii) is used for a first estimate, and k is taken 

 as 0.004. Also, since there is no true or natural 

 wind blomng, W^ is equal to V, and there is no 

 variation of wind velocity with height, due to 

 boundary-layer effect. Then 



RsA = DjfT for this case 



= kA^Ws = 0.004(2,665) (20.5)' = 4,4801b. 



Assuming a total hydrodynamic resistance Rt for 

 the bare hull, from Chap. 66, as about 170,000 

 lb, the still-air resistance ratio is 4,480/170,000 = 

 0.0263. The bare-hull Rr value is used so that 

 the stUl-air drag may be added as a percentage, 

 Kke the overall appendage resistance, to predict 

 the probable total trial resistance. 



Next, consider the relative-Avind resistance 

 when = deg (^\'ind ahead), the ship speed at 

 sea is 18.5 kt, and the true wind velocity is 23 kt. 

 The relative-mnd speed is then (18.5 + 23) = 41.5 

 kt. Hence 



i2wiod = kA^W^ = 0.004(2,665)(41.5)' 



= 18,360 lb. 



This represents a resistance augment, over that 

 estimated for the bare hull at the trial speed of 

 20.5 kt, of (100) (18,360/170,000) or 10.8 per cent. 

 It would be a much larger proportion of the total 

 resistance at the 18.5-kt smooth-water ship speed 

 of the problem given. 



As a third approximation it is desired to esti- 

 mate the \\dnd resistance of the ABC ship, having 

 abovewater hull and upper works of the general 

 form shown in Figs. 66.0, 66. S, and 68. M, when 

 conducting a full-speed run during standardization 

 over the measured mile. Assume that the meas- 

 ured speed is 20.4 kt for a particular run, that the 

 amemometer on top of the after pair of kingposts 

 reads 41.5 kt, and that the Avind direction indicator 

 gives an angle of 22 deg on the port bow. The 

 latter two readings are both for the relative wind, 

 so it is not really necessary to know how fast the 

 ship is gouig through the water to predict the 

 wind resistance to be encountered. 



From the dimensions on Fig. 54. F, correspond- 

 ing to those on the three dra^vings mentioned, 

 the silhouette area, looking from ahead, is esti- 

 mated to be about 3,880 ft'. This is nearly half 



