514 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. r,7.fi 



EquoM\j 5poced Ordinotes or Radii 

 Fig. 67.H Construction of a 3-Diml, Single-Ended Rankinb Ovoid foe a Bulb Bow 



^Axis of Ovoid 



sections at the FP, gives an approximation of the 

 slenderness of the bulb cone and the position of 

 its apex forward of the FP. 



If some variation from a developable conical 

 shape is acceptable, the waterlines through the 

 maximum transverse thickness of the bulb may 

 approximate the shape of a single-ended Rankine 

 ovoid generated by a single 3-diml source placed 

 in a uniform stream. Fig. 67. H illustrates the 

 method of graphic construction, described in 

 Chap. 43, and shows the shape of an axial section 

 through one-half of such a form. The proportions 

 are varied by changing the source strength 

 relative to that of the uniform stream. 



The lower profile of an ovoid bulb could be 

 delineated in the same way except for the practical 

 requirement of a flat keel extending well forward 

 for docking support. Having this in mind it is 

 convenient to terminate the lower profile in a 

 radius tangent to the base plane, say of the order 

 of O.lOff [SNAME, 1930, PI. 41], or it may be 

 about equal to the mean radius of the extreme 

 nose of the Rankine form. For the ABC design 

 the dimension adopted is 3. .5 ft; this means that the 

 straight keel for docking extends forward to the 

 FP. 



The matter of shaping the bottom of the bulb 

 to avoid pounding and slamming during wave- 

 going is discussed further in Part 6 of Volume III. 



If the bulb remains normally well submerged 

 in service, as it might in inland waters, where 

 pounding or slamming is rarely if ever encount- 

 ered, the bulb sections may be made rather 

 definitely triangular, with a fairly flat bottom 

 [Eggert, E. F., SNAME, 1939, pp. 303-330; 

 Lindblad, A. F., "Experiments with Bulbous 

 Bows," SSPA Rep. 3, 1944, p. 7]. This puts the 



centroid of the bulb area even farther below the 

 DWL, with its accompanying advantages. It is 

 still possible to work developable surfaces into the 

 bottom and the sides of this triangular bulb. 



67.8 Check on Bulb Cavitation. In the past 

 there has been no definite low limit of draft, in 

 smooth water at least, at which it is not advisable 

 to fit a bulb at the bow. Even for high-speed 

 light-draft vessels intended to run in large waves, 

 bulb bows have been used to advantage. An 

 example is the pre-World War II Itahan cruiser 

 Pola, having a standard displacement of 10,000 t, 

 a length of approximately 600 ft, a beam of 

 67.7 ft, a draft of 19.5 ft, and a B/H ratio of 3.47. 

 At a speed of 33.9 kt, T<, for this vessel is 1.38; 

 F„ is about 0.41. A close-up bow view of the Pola 

 is published in Schiffbau [1 Mar 1933, p. 89]. 

 Other examples are the heavy cruisers U.S.S. 

 Pensacola and U.S.S. Salt Lake City, described 

 on SNAME RD sheet 121, with /b values of 

 0.083; see also Fig. 52.Kc. 



For the first time, so far as known, cavitation 

 was recently (1954) observed on each side of the 

 bulb of another light-draft high-speed vessel 

 somewhat resembling the Pola. In smooth water 

 the two cavities were plainly visible from the 

 forecastle head. 



This new development definitely calls for 

 elliptic or pointed rather than circular beginnings 

 of the waterlines well below the DWL. The shape 

 of the waterline at each draft, and possibly also 

 the shape of some other characteristic line, must 

 therefore be one for wliich cavitation will not 

 occur at the cavitation number o-(sigma) actually 

 encountered on the ship at designed draft in 

 smooth water. This check was not made for the 

 bulb design of the ABC ship since the observations 



