HYDRODYNAMICS IN SHIP DESIGN 



Sec. 7S.19 



engine power by a substantial amount, of the 

 order of 15 or more per cent, or he may 

 (2) Step up the schedule, retain the engine power 

 origmally estimated, and increase the ship speeds, 

 both sustained and trial. 



The background for this decision involves 

 hydrodjTiamics in onlj^ a small way but it is 

 useful to discuss that small mfluence. Consulting 

 the speed-power curves of the transom-stern 

 design in Fig. 78.Nc, and assuming no scale effect 

 between model and ship, it is safe to assume that 

 the ship can make 21.1 kt with a shaft power of 

 16,000 horses, retaining the former limit of 0.95 

 times the maximum designed shaft power. Step- 

 ping up the sustained speed to 19.5 kt, at which 

 about 10,750 horses are required for deep-water, 

 clean-bottom conditions, means that this speed 

 can be achieved with a power expenditure of 

 only 67.2 per cent of the 0.95-maximum limit. 

 This is stUl conservative planning. 



At the 21.1-kt speed the Pe/Ps ratio drops to 

 0.75, and the hull efficiency vht to 1.14, with 

 only small changes in the t and w values. At the 

 19.2-kt speed these factors have about the same 

 values as at 18.7 kt. In other words, the ship is 

 stUl running efficiently, in the higher range, so 

 far as speed and power are concerned. The rpm's 

 increase from 110 to 114 at the higher trial speed 

 but this permits smaller and Ughter gears than 

 were originally involved. To be sure, the sustained 

 speed is increased only 0.8 kt, an increase of but 

 slightly over 4 per cent from 18.7 kt. This may 

 not be sufficient to make the increased power 

 economically worth while. 



The history of mechanically driven vessels, ex- 

 tending back for more than a century and a half, 

 is replete with instances of re-engining at greater 

 powers. Some of this may have been due to in- 

 creases in load and in displacement. Much of it 

 was undoubtedly to reduce fuel consumption by 

 the use of more efficient machinery. Not a Uttle 

 was carried out to increase the sustained speed, 

 so as to keep pace with newer ships. The economic 

 features are not discussed here, but the ship that 

 benefits the most by this re-engining is the one 

 which has a speed margin designed into the hull 

 in the first place. It is a good sign, in a way, that 

 the ABC ship is a better performer at higher 

 speeds than at the speed for which it was designed. 



78. IQ Comments on Illustrative Preliminary- 

 Design Procedures of Part 4. The illustrative 

 examples worked into Part 4 of the book, com- 



prising the large ABC ship of the early chapters 

 and the motor-driven tenders of Chap. 77, are 

 carried only far enough to demonstrate the use 

 of the various graphs, diagrams, and rules 

 embodied in the text. The Hmited treatment in 

 these chapters is by no means to be taken as an 

 indication that the preliminary hydrodynamic 

 design for an actual ship can or should be limited 

 to that set down in this volume. Problems of 

 maneuvering and of wavegoing are discussed more 

 fully in Volume III, Parts 5 and 6, respectively. 

 For example, considering first the features 

 commented upon in the present chapter, relating 

 to the ship itself: 



(1) It is apparent from the position of the wave 

 profile observed on the transom-stern model and 

 drawn on the afterbody portion of Fig. 66.R that 

 the transom did not clear at the designed speed, 

 despite previous indications to the contrary. In 

 fact, it did not clear fully until the speed was far 

 m excess of any speed that the ship could reason- 

 ably have attained. The reason appeared to be 

 the large upward component of flow in the water 

 just below the lower edge of the transom. Never- 

 theless, ^\ith a resistance some 4 per cent less 

 than that of a Taylor Standard Series model of 

 the same proportions there appeared to be no 

 reason for changing the existing transom stern 

 without the benefit of a further extended study. 

 In any further development of this design, studies 

 of the separation drag abaft the transom, men- 

 tioned in (2) of Sec. 78.6, should be pursued. 



(2) The so-called neutral positions of the two 

 lower (horizontal) strut arms in the arch-stern 

 design should have been estabhshed before the 

 design of the quadruple strut-arm and hub 

 assembly was finished and this appendage was 

 added to the model. The small-scale assembly 

 could readily have been constructed to permit 

 changing these positions on the self-propelled 

 model, or some special means could have been 

 provided whereby the flow in this region could 

 have been observed in greater detail during the 

 circulating-water-channel test. 



(3) No attempt is made in Part 4 of the book to 

 calculate or estimate the power that would be 

 required to move the necessary amount of cooling 

 water through the main condenser at the designed 

 ship speed. 



Considering next a few of the ship features 

 mentioned in the early chapters of this part, and 

 not Avorked out adequately there: 



