896 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 78.18 



14 16 18 20 



Ship Speed, kt 



Desiqned Speed 



Fig. 7S.Q Ratios of Effective Powers, With and Without Appendages, for Transom-Stern and Arch- 

 Stern ABC Designs 



increase of bare-hull wetted surface of 5.8 per 

 cent over that of the bare transom-stern hull, 

 the effective power Pe at designed speed is about 

 2 per cent lower than that of a Taylor Standard 

 Series hull of the same proportions. This superi- 

 ority increases rapidly with speed above the trial 

 speed of 20.5 kt, even more so than for the tran- 

 som-stern hull. 



Although the several appendages were designed 

 with great care, the increase in resistance of 18 

 per cent due to them appears high. However, in 

 this case the wetted areas of the exposed shaft, 

 the strut hub, and the quadruple strut arms are 

 not added to the bare-hull wetted area. Con- 

 sequently, the pressure drag of these parts is 

 certain to show up as a large augment of the 

 specific residuary resistance Cr . Subsequent dis- 

 cussion with designers of tunnel-stern shallow- 

 water craft reveals that appendage resistances 

 in this type of vessel are consistently high. 



When making propeller calculations prepara- 

 tory to selecting the stock propeller, mentioned 

 in Sees. 70.6 and 78.4, values of the probable wake 

 and thrust-deduction fractions were determined 

 by the Schoenherr formulas used for the transom- 

 stern design. On this basis, admittedly inadequate 

 but the only one available, the values derived 

 for the arch stern were 0.273 for the wake fraction 

 and 0.164 for the thrust-deduction fraction. 



For the designed or trial speed of 20.5 kt, the 

 arch-stern self-propelled data from Fig. 78.1 are: 



Shaft power, 18,300 horses 

 Effective power, 12,500 horses 

 Propulsive coefficient ■np = P e/Ps = 0.686 

 Wake fraction, 0.072 

 Thrust-deduction fraction, 0.175 

 Hull efficiency rj^ = (1 - 0.175)/(1 - 0.072) 

 = 0.889. 



The expected high wake inside the arch did not 

 exist. In fact, the w-value derived from the self- 

 propulsion test was only shghtly more than one- 

 quarter of the estimated value. The thrust-deduc- 

 tion fraction was slightly higher than the antici- 

 pated value of 0.164. It was known that large 

 thrust-deduction forces would be generated by 

 the four strut arms lying in the outflow jet of the 

 propeller, but it was hoped that these would be 

 partly or wholly compensated by the contra- 

 effect on these arms. It is possible that there were 

 appreciable upward components of velocity in 

 the flow passing the two horizontal arms. This 

 would have increased the appendage resistance 

 as well as the thrust-deduction forces. 



Nevertheless, the propulsive coefficient of over 

 0.68 is extremely high for this type of stern, where 

 values of 0.50 are considered very good. 



Analysis indicates that the real or working 

 efficiency tjq of the large-diameter 24.22-ft pro- 

 peller at the designed speed of 20.5 kt is about 

 0.750. This is higher by 6.5 points, or 9.5 per 

 cent, than the tjo for the 20.51-ft wheel of TMB 

 model 4505 of the transom-stern ship. 



78.18 Proposed Changes in Final Design of 

 ABC Ship. In the normal course of a building 

 program for a multi-milhon dollar ship the design 

 and model-testing procedure described in Part 4 

 represents only the early stages of the development 

 prior to the completion of the contract plans. 

 Nevertheless, the designer who has carried the 

 ship along this far is convinced that, given a 

 little more time, he can modify and improve upon 

 it considerably. A few such improvements are 

 outhned here for the ABC ship. 



In the first place, the designed waterlines of 

 both alternative hulls have values of transverse 

 square moment of area It that are certainly on 



