Sec. r,7.7 



UNDERWATER-HULL DESIGN 



513 



should have a diameter not much greater than 

 half the draft. If this design rule is followed, 

 the area ratio /e of a bulb laid out in accordance 

 with these rules is limited to a maximum of 

 about 0.10, indicated in diagram 2 of Fig. 67. G. 

 If larger ratios are desired the upper part of the 

 bulb must approach the surface closer or the 

 lower part must assume more of a triangular or 

 platypus form, with a diminished rise of floor. 

 A bulb with an area ratio /b = 0.135, on a 

 vessel of relatively shallow draft, where B/H = 

 3.07, is shown by E. S. Dillon and E. V. Lewis 

 [SNAME, 1955, Fig. 9, p. 735]. The construction 

 circle for this bulb has a diameter of from 0.7 to 

 Q.SH, depending upon how it is used. 



To simplify construction, with the rather heavy 

 plating called for in this region, the sides of the 

 bulb are if possible made developable surfaces, 

 described in Sec. 27.1. The lower part of the 

 bulb may be the surface of a cone, not necessarily 

 a circular one, whose vertex lies well ahead of the 

 FP. The following is copied from D. W. Taylor 

 [S and P, 1943, p. 69]: 



"The ingenious naval architect will have no difficulty 

 in devising bulbous forms where little or no furnacing of 

 the structural plating is necessary." 



Sections 



u If c J- .' Shown 



HQlf-S,d,ng| ^^^^pp 



6.96 ftl- 



Fig. 67. G Layout Diagram for a Bulb-Bow Section 



Sketching in a section similar to that of the FP, 

 at about Sta. 1 or 2, corresponding in area to 

 either or both of those intercepts on the section- 

 area curve and fairing generally with the bulb 



TABLE 67.b — Predicted Improvement in Residuary Resistance Due to Bulb Bow 

 The value of T, for the ABC ship at 18.7 kt is 18.7/ VSIO = 0.828. 



flij/A 



Saving in Residuary 

 Resistance with Bulb 



At r„ = 0. 



1.703 



2.343 



= 0.113 or 11.3% 



= 0.181 or 18.1% 



Diff. 0.423 



At r„ = 0.90 



0.542 

 2.982 



0.182 or 18.2% 



= 0.142 or 14.2% 



