Sec. 60.16 



SHIP-POWERING DATA 



387 



as before. Then for the fouled ship C^ = Ck + 

 C,. + SAC;. = (0.94 + 1.48 + 0.71)(10-') = 

 3.13(10"'). With a wetted surface of 45,420 ft^ 

 from the preceding example, the total resistance 

 of the fouled ship is 



i2i 



Ct\ £ -sf 



= 3.13(10-')(i^^)(45,420)(33.27)' 



= 156,610 lb, 

 whence 



P. =Ze.F = ^^^^«53:^ = 9,474 horses. 



Since the ship with the hot-plastic coating is 

 expected not to be as heavily fouled as with the 

 self-leveling paint in the preceding example, 

 it is estimated that the wake-fraction w is in- 

 creased only from 0.190 to 0.200, and that the 

 thrust-deduction fraction has gone up from 0.070 

 to only 0.100. For the clean ship, at 19.7 kt, 

 ii]h is 1.148 as before but for the fouled ship it is 



(wFoul — 



(1 - Ofo 



(1 - M')f„ 



(1 - 0.100) 

 (1 - 0.200) 



1.125 



The speed of advance V a for 19.7 kt, with the 

 lighter fouling on the hot-plastic paint, is 

 F[(l - i«)f„„i] = 33.27(0.80) = 26.62 ft per 

 sec. For the fouled ship at 19.7 kt, T = Rt/{1 - 

 = 156,610/(1 - 0.10) = 174,010 lb. Then 



(C ^ ^F°"' . 



^o[(FJp„„,]^ 



174,010 



(0 .99525) (20 .5 1)'(0 .7854) (26.62)' 

 = 0.747 



The thrust-load factor Ctl for the clean-bottom 

 condition is 0.639, the same as for the preceding 

 example. Similarly, TjReai for this factor is 0.717. 

 For a Cri of 0.747 on the fouled ship and a P/D 

 ratio of 1.00, the value of T/u,ai is, from Fig. 

 34.G or Fig. 70.B, 0.703. Assuming as before 

 that the relative rotative efficiency jjb is the 

 same for both clean and foul bottom, 



(^?p)i 



(^7ci)Foiil('?g)l 



(0.703)(1.125) 



= 0.961 



(0.717)(1.148) 

 Interpolating from the t/p = EHP/SHP values 



in Fig. 78.Nb, the value of nr for the clean ship 

 at 19.7 kt is 0.769 as before. For the fouled ship 

 at the same speed it is taken to be 0.769(0.961) = 

 0.739. Hence for the fouled ship with hot-plastic 

 paint, at 19.7 kt, 



9,474 



p — Ea — 



JTs — — 



r,p 0.739 



= 12,820 horses. 



This is less than the 13,250 horses required to 

 drive the clean ship at 20.5 kt. 



With a shaft power Ps of about 11,200 horses 

 to drive the clean ship at 19.7 kt, from Fig. 78. Nb, 

 an increase of 16,540 - 11,200 = 5,320 horses is 

 required to overcome 10 months' fouling on the 

 self-leveling paint, whereas an increase of only 

 12,820 - 11,200 = 1,620 horses suffices to over- 

 come both the initial roughness of the hot-plastic 

 paint and 10 months' fouling on that paint. 

 Against this advantage must be placed the addi- 

 tional shaft power that would be required to 

 drive the ship with hot-plastic paint, at all 

 speeds, when just out of dock and for a few 

 months thereafter. This and other powers can 

 be calculated by the method described. 



60.16 Increasing the Power and Speed of an 

 Existing Ship. Marine architects are often called 

 upon to increase the speed of a ship already 

 built, either by improving its form and retaining 

 its power plant, by changing its power plant and 

 not its form, or by both. 



In the matter of the power which can be de- 

 livered to and absorbed by a single screw pro- 

 peller or other propulsion device, embodying a 

 question which invariably arises whenever the 

 matter of increased power is considered, it is to 

 be remembered that shaft power is a function of 

 both torque delivered to the propeller and the 

 rate of rotation of the shaft. A given shaft can 

 often be run at a higher rate of rotation at the 

 same torque but only rarely can the same screw 

 propeller be expected to absorb the increased 

 power and to drive the ship efficiently at the 

 increased rpm and ship speed. 



It is conceivable that lengthening, fiiiing, or 

 otherwise altering an existing ship, designed for 

 slow speed, may enable the altered ship to be 

 driven at an increased speed with the same total 

 resistance Rt or effective thrust T{1 — i). The 

 increased friction drag may be more than com- 

 pensated for by the reduced pressure drag due to 

 wavemaking and separation. However, the fact 

 that the ship speed is increased, automatically 

 raises the power by a corresponding amount, 



