Sec. 60.15 



SHIP POWERING DATA 



385 



P., = ^ = 



12,551 horses. 



This compares with the value of 109.7 (or 1.82G 

 rps) derived from the self-propelled model test; 

 it indicates that the real or working efficiency of 

 the propeller was somewhat less than 0.8 of its 

 ideal efficiency. 



Taking a Jj-value of 0.748, as determined by 

 thrust identity from Fig. 78. Nb, the real efficiency 

 J/Reai or the open-water efficiency tjo from Fig. 

 78. Me is only 0.686. Assuming a value 1.02 

 for the relative rotative efficiency, 



riP = ria{vH)riB = 0.686(1. 148)(1.02) = 0.803 



Since this is in excess of the -qp = 0.761 derived 

 from Fig. 78. Nc, it indicates that the assumed 

 relative rotative efficiency is too large. The last 

 example of Sec. 60.10 shows that actually tjr 

 was only 0.968. Using an T/p-value of 0.803 would 

 have given a shaft power of 



5,543,350 

 T)P (550) (0.803) 



This is less than the predicted shaft power of 

 13,243 horses from Fig. 78. Nb. It emphasizes the 

 statement of D. W. Taylor, made several decades 

 ago, that: 



"It is better to underestimate relative rotative efficiency 

 tlian to overestimate it. Underestimation results in a 

 slightly larger propeller tiian overestimation" [SNAME, 

 1923, pp. 69-70]. 



60.1S Estimating Shaft Power for a Fouled- 

 or Rough-Hull Condition. A recommended de- 

 sign procedure for building into a ship a sufficient 

 speed margin to enable it to maintain an estab- 

 lished schedule despite the handicaps of winds, 

 waves, fouling, and other factors is described in 

 Sees. 64.3, 65.3, and 69.9, and in Table 64.d, all 

 in Part 4 of this volume. It should be possible 

 eventually to predict the effect of each of these 

 handicaps in quantitative terms, provided the 

 conditions to be met are specified in some detail. 



Considering the problem of fouling, or of 

 serious deterioration of the paint coating on the 

 underwater hull, the situation is presumably 

 worst just before the end of a dry docking interval. 

 As a check on the speed and power margins 

 incorporated in the design, which are intended 

 to be adequate all through this interval, it should 

 be possible to estimate the propulsion perform- 

 ance in the foul-bottom as well as the clean- 

 bottom condition. 



One acceptable method is worked out here for 

 the transom-stern hull of the ABC ship, designed 

 in Part 4. The ship is assumed to be painted with 



a self-leveling (commercial) type of anti-fouling 

 paint, to be 10 months out of dock, and to have 

 an increase in specific resistance due to fouling of 

 Af.CF(10^) = 1.25, corresponding to the long-dash 

 predicted ABC ship curve of Fig. 45. L. It is 

 assumed that for half of the open-sea portion of 

 a voyage under these circumstances, heavy 

 weather has slowed the ship to an average of 

 17.7 kt. For the remaining half, therefore, in 

 order to meet the sustained speed of 18.7 kt, the 

 ship is called upon to average 19.7 kt. Can the 

 ship do it, when fouled, with 95 per cent of its 

 maximum designed power? 



From Table 45.f of Sec. 45.18: 



^pCf for the plating is taken as 0.0 since any 

 roughness here is obscured by the deterio- 

 ration of the anti-fouling paint coating 

 and the presence of the fouling itself 



AsCf for structural roughness is assumed as 

 0.1(10"') 



AcCi? for coating roughness is assumed as 

 0.1(10"'), covering deterioration of the 

 paint 



AfCf for fouling, from the long-dash line of Fig. 

 45.L, is taken as 1.25(10^') 



Then SAC^ is (0.0 -|- 0.1 -|- 0.1 + 1.25) (10"') = 



1.45(10"'). 



For the "make-up time" speed j)f^ 19.7 kt or 

 33.27 ft per sec, T, = 19.7/ V510 = 0.872, 

 F„ = 0.26. At this T, , the specific residuary 

 resistance coefficient Cs is, from Fig. 78. Jc, 

 0.94(10^'). The Reynolds number R„ for this ship 

 speed, in standard salt water, from Table 45. b of 

 Sec. 45.4, is 1,324 million, for which the specific 

 friction resistance coefficient Cp from Table 45. d 

 is 1.48(10"'). Then Ct + Cp + XACp = (0.94 + 

 1.48 + 1.45)(10"') = 3.87(10"'). 



The wetted surface of the ship is, from Fig. 

 78.Ja, 69.85 times X^(lambda) for the ship or 

 (69.85) (650.25) = 45,420 ft'. Then for the fouled 

 ship. 



Rr = Cr[~)SV' 



= (3.87)(10~')f— |^V45,420)(33.27)' 



= 193,640 lb, 



whence 



p „„ (193,640)(33.27) n-,,, 



Re = KtV = ^/^ = 11, /13 horses. 



550 



For the fouled ship, at 19.7 kt, it is estimated 



