496 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 6629 



shown in Fig. 66. R, maj^ be expected to diverge 

 slightly with distance as they move aft. Under 

 a run that is roughly flat, or of a shallow V-shape, 

 the flowlines lie generally along the buttocks, 

 parallel to the centerplane of the vessel. When 

 the buttocks terminate at a knuckle under water, 

 as at Stas. 18 to 20 in Fig. 66. R, the flowlines 

 lying somewhat parallel to the buttocks may be 

 expected to leave the ship surface at the knuckle. 

 The predicted flowlines are indicated in light 

 broken Hues on the body plan of Fig. 66.R. The 

 actual flowlines, determined from a test of a 

 20-ft model, using chemicals on the model surface, 

 are shown in heavy full lines in the figure. The 

 wave profile marked along the side of the model 

 is indicated also by a heavy full line. 



66.29 Comparison with a Ship Form of Good 

 Performance. It is still possible, with a given set 

 of principal proportions and form coefficients, to 

 vary the underwater shape within rather wide 

 limits, and to obtain perhaps wider variations in 

 the resistance for a given F„ . Existing forms, 

 often several of them, are therefore wisely used 

 as guidance or as a means of keeping one from 

 getting too far afield. Certainly a well-tried 

 parent form or a ship form which has a high 

 merit coefficient and which has proved itself in 

 service can be employed as: 



(1) A starter for laying down a set of lines 



(2) A sort of running comparison as the hull 

 shaping proceeds 



(3) A reference, after model tests have been made, 

 for judging the performance of the new form. 



If future progress is to be made, however, past 

 or existing forms should not be too slavishly 

 copied unless one knows rather accurately just 

 what features are responsible for their good 

 (or bad) performance. The designers of these 

 forms would be the first to admit that they could 

 unquestionably be improved with further thought 

 and effort. 



Following a series of model tests, comparisons 

 may be made of the effective powers of a new 

 design with the effective powers of the TSS ship 

 of the same proportions. This comparison for 

 the ABC hull, comprising the transom stern 

 designed in this chapter and an alternative arch 

 stern described in Sec. 67.16, is to be found in 

 Sec. 78.16 and Figs. 78. J and 78.K. 



66.30 Abovewater Hull Proportions for 

 Strength and Wavegoing. The ABC ship re- 

 quires, as do many others, rather large internal 



volumes for accommodating the passengers and 

 crew and for carrying the machinery and cargo. 

 The ratio of total hull and superstructure volume 

 to underwater hull volume of 2.77, derived 

 previously in Sec. 66.8, is therefore somewhat 

 large. It appears that the abovewater hull will 

 stand rather high out of the water. A flush-deck 

 type of ship is indicated, as in Fig. 66.0, possibly 

 with a short forecastle to give added freeboard 

 and hull depth at the stem, and with so-called 

 tonnage openings below the main deck near the 

 stern. 



Taking for a starter a minimum freeboard of 

 23 ft at the lowest point of the deck at the side, 

 the hull depth Z) is 26 -f 23 = 49 ft. By the 

 criteria of C. R. Nevitt [ASNE, May 1950, pp. 

 318-319] and others, it appears that the 49-ft 

 depth and the L/D ratio of 10.4 he within a good 

 design range for a length L of 510 ft and a draft 

 R of 26 ft. The ratio of draft R to depth D is 

 26/49 = 0.531. The depth from the keel to the 

 top of the highest superstructure, when related 

 to the beam, is approximately 



[26 -1-23-1- 3(9)]/73 = 76/73 = 1.041. 



Based upon Atlantic-liner practice [de Vito, E., 

 INA, 1952; partial abstract in SBSR, 13 Nov 

 1952, pp. 642-643] this ratio could be as high as 

 1.16 or 1.20. In any case, it is assumed that the 

 necessary preliminary strength calculations, not 

 gone into here, show the assumed hull depth of 

 49 ft to be adequate for a static wave whose 

 height is 1.1 VL, or 1.1 V510 = 24.84 ft [Nieder- 

 mair, J. C, "Ship Motions," ASNE, Feb 1952, 

 p. 14]. Fig. 48. E in Sec. 48.7 embodies a graph 

 of these heights for various wave lengths. 



The ship appears to have adequate freeboard 

 throughout, of the order of 0.045L or more, when 

 the abovewater hull is made large enough for the 

 volumetric capacity and for the required depth 

 of ship girder. A detailed study of the abovewater 

 hull, taking all necessary factors into considera- 

 tion, is given in Chap. 68. A further study of its 

 ability to meet all wavegoing service requirements 

 is deferred to Part 6 in Volume III. 



However, it is possible at this stage to make the 

 first estimate of its natural rolling period. For 

 this estimate the added mass of the surrounding 

 water is not taken into account, partly because 

 it is not known, and partly because the actual 

 period is then longer than the estimated one. As 

 a result, the ship should be somewhat more 

 comfortable than predicted. 



