510 



HYDROnVNAMICS IN SHIP DESIGN 



Sec. 67.7 



(c) The upper and lower limits of Cp and fatness 

 ratio for which bulb bows give beneficial results 

 are not yet determined 



(d) It is possible that the best value qUe depends 

 upon factors other than T, but if so no definite 

 trends are yet apparent. Selecting the proper 

 value oi If. appears, however, to be less important 

 than using the proper value of /e . 



A rather surprising feature of the lower or 

 f E diagram of Fig. 67. D is that, for a large number 

 of existing vessels of good propulsion performance, 

 the bow bulb-area ratio f b is rarely more than 

 half the optimum value, although in one case it 

 approaches and in another case it exceeds the 

 optimum. These low values are traceable partly 

 to conservatism, partly to lack of precise knowl- 

 edge of the behavior of bow bulbs at sea, but 

 mostly to possible damage from bower anchors, 

 dropped from the orthodox stowage positions 

 high in the vessel and close to the stem. 



The design rules laid down by W. C. S. Wigley 

 in the summary of his classic paper "The Theory 

 of the Bulbous Bow and Its Practical Application" 

 [NECI, 1935-1936, Vol. LII, p. 65], can hardly 

 be improved upon today, twenty years later. 

 They are set down here, with the present author's 

 comments in parentheses, and with only minor 

 changes to comply ^ath the nomenclature in this 

 book: 



(1) The useful speed range of a (ship with a) 

 bulb is generally from T, = V/VL of 0.8 to 1.9 

 (somewhat different from that of Fig. 67. D) 



(2) The worse the wavemaking of the hull 

 itself is, the more gain is to be expected with the 

 bulb, and vice versa 



(3) Unless the lines (forward) are extremely 

 hollow the best position of the bulb is with its 

 (longitudinal) center at the bow, that is, with its 

 nose projecting forward of the hull 



(4) The bulb should extend as low as possible 

 consistent with fairness in the lines of the hull 



(5) The bulb should be as short longitudinally 

 and as wide laterally as possible, again having 

 regard to the fairness of the lines 



(6) The top of the bulb should not approach too 

 near to the water surface. As a working rule it is 

 suggested that the submergence of the highest 

 part of the bulb should be not less than its own 

 total thickness (measured transversely). 



Moving the bulb well forward of the FP pro- 

 duced excellent results on pre-World War I 



battleships of the U.S. Navy, from the Delaware 

 class of 1910 to the Arizona class of about 1915. 

 An even greater forward projection is embodied 

 in the recent French liners Flandre and Antilles 

 [SBSR, 24 Jul 1952, pp. 115-117]. 



A bulb of extremely large f e ratio, very low, 

 very wide, and very flat on the bottom, was the 

 "platypus" forefoot of EMB model 3383, devel- 

 oped and tested by E. F. Eggert in the late 1930's 

 [SNAME, 1939, pp. 303-330]. 



67.7 Laying Out the Bulb for the ABC Ship. 

 For the ABC design, projecting the bulb forward 

 of the construction FP by the length of a cutwater 

 such as mentioned in Sec. 67.5 makes it possible 

 to provide a nearly plumb external profile below 

 water, as drawn in Fig. 67. E. There is no particular 

 virtue in this plumb profile as such, except 

 possibly as a matter of appearance. Likewise 

 there is none in the ram-bow profile that would 

 accompany a bulb bow forward of the FP, 

 except to get the bulb into that forward position, 

 relative to the waterline. 



Extending the bulb below the baseplane is 

 generally out of the question in practice, although 

 it might be done on vessels designed to meet 

 particular requirements, or it might be extended 

 below the baseplane as a special appendage. 



Taking the ABC ship as an example of the 

 design procedure, the fitting of a bulb bow on this 

 vessel was settled, at least tentatively, in Sec. 

 66.19. It was decided to use, pending a further 

 check, a section-area intercept f e of 0.06 and a 

 terminal value of Ie = 0.9 at the FP. These 

 values are indicated by special spots on Fig. 

 67. D. It remains to be seen whether a value of t 

 as small as 0.9 will fit the final section-area curve. 



Assuming for the moment that the bulb can be 

 worked physically into the ship, a brief calculation 

 is made to determine how much pressure resist- 

 ance is likely to be saved by it. Taking values of 

 iJ/j/A in lb per ton from Figs. 241 through 244 of 

 D. W. Taylor's S and P, 1943, a curve of residuary 

 resistance in pounds per ton of displacement is 

 plotted in Fig. 67. F for the fine ship of series A. 

 This has a Cp of 0.60, a displacement-length 

 quotient A/(0.010L)' of 60, and a B/H of 3.35. 

 Four Ru/L values, for T^'s of 0.559, 0.783, 

 1.006, and 1.118, enable this curve to be located 

 reasonably well, especially for the region of 

 T„ = 0.8 to 0.9. Reference to Figs. 248 through 

 252 of S and P, 1943 edition, enables a second 

 curve of residuary resistance in pounds per ton 

 displacement to be plotted for the fat ship of 



