456 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 653 



is achieved, the designer adds a definite allowance 

 to that speed. This may be, say, one-third of the 

 power margin, on the basis that the shaft power 

 varies about as V^. The speed allowance is then 

 +5 per cent. He proceeds to design the ship 

 closely to this augmented speed. 



When the trials are run it is usually as easy, 

 or perhaps easier, to measure the 15 per cent 

 extra power as the 5 per cent extra speed. How- 

 ever, with a speed margin that can be reliably 

 predicted, and with a ship fashioned and built 

 for the augmented speed, the designer is in a 

 better position to promise the given sustained 

 speed than if he crowds extra power into a ship 

 built only for that speed. Other reasons for de- 

 signing-in a speed margin rather than a power 

 margin are set forth in Part 6 of Volume HI 

 under wavegoing. 



E. V. Lewis points out that with the large 

 powers and high smooth-water speeds of many 

 modern (1956) vessels it becomes increasingly 

 difficult to maintain high average speeds in 

 certain rough-water areas such as the North 

 Atlantic [SBSR, 30 Aug 1956, p. 277]. It may be 

 expected, however, that increased emphasis on 

 wavegoing characteristics and further progress in 

 wavegoing design will increase the rough-water 

 speeds so that, when it is sufficiently important, 

 a high sustained speed can be achieved in any 

 service. 



For the preliminary hydrodynamic design of 



the ABC ship, or for any other design in which a 

 speed rather than a power allowance is to be 

 incorporated, the selection or determination of 

 that allowance requires careful study, combined 

 with intelligence, judgment, and wisdom. There 

 are considerations of sea routes to be followed, 

 times of arrival and departure during the day, 

 reliability in maintaining the sailing schedule, 

 economics, and many others which need not be 

 entered into or elaborated upon here. 



On the basis that heavy weather slows the ship 

 to say 0.7 times its sustained speed for a certain 

 portion of an open-sea run, simple arithmetic 

 indicates what the augmented sea speed must be 

 to bring the average up to the sustained speed. 

 A similar procedure can be applied to the speed 

 effects of bottom roughening and fouling or to an 

 assumed compulsory slowing of the propelling 

 plant for any given length of time. The contin- 

 gency in which the delays occur unexpectedly at 

 the very end of a run is met by speeding up for 

 good measure in the early part of the run. This 

 matter is discussed by R. K. Craig, when de- 

 scribing the service performance of a passenger 

 liner with engines aft [SBMEB, Dec 1955, p. 693]. 



The speed allowance of 1.8 kt for the ABC 

 ship, from 18.7 to 20.5 kt, or roughly 10 per cent, 

 as specified tentatively in Table 64. d and as 

 incorporated in Table 65. a with other design and 

 performance allowances, is intended to serve 

 only as an example of the procedure involved. 



TABLE 65. a — ABC Ship: Design and Performance Allowances 



