Sec. 67.9 



UNDERWATER-HULL DESIGN 



515 



described in the preceding paragraph were 

 reported long after the completion of the under- 

 water hull design and the construction and tests 

 of the ABC model. 



When calculating the cavitation number for 

 the ship it is well to be conservative and to omit 

 the allowances for increased head above the 

 various parts of the bulb due to the bow-wave 

 crest or to sinkage at the bow. The transient 

 cavitation which occurs along the sides of the 

 bulb during wavegoing, when the bulb rises 

 toward the surface, may be accepted. 



Incidentally, it is not possible to observe either 

 the separation or the cavitation on a routine 

 model test because the Reynolds number R^ or 

 jSj at the stem is too low to produce a flow dy- 

 namically similar to that on the ship, and because, 

 with full atmospheric pressure above the basin 

 water, the model cavitation number is much 

 higher than in the full scale. 



Assuming for the ABC ship that at some light- 

 load displacement and trim the depth of water h 

 to the axis or Avidest part of the bulb, reckoned 

 from the at-rest WL, is only 13.5 ft, that the 

 head hJ^ corresponding to the atmospheric pressure 

 is 33 ft, and that the head due to the vapor pres- 

 sure of water, hv ,is 0.5 ft, the total static head 

 at the axis of the bulb is represented by {h -\- Ha — 

 hy) = (13.5 -h 33 - 0.5) = 46.0 ft. Assuming 

 also that the speed is 20.5 kt, or 34.62 ft per sec, 

 the velocity head hu = Vy2g = (34.62)7 

 (64.348) = 18.63 ft. Then o- = 46.0/18.63 = 2.47. 

 This value is far in excess, numerically, of the 

 cavitation number o- = 0.50 at which cavitation 

 occurs on the hemispherical head of a body of 

 revolution, from diagram 1 of Fig. 47. E. Indeed, 

 it is in excess of that at which cavitation occurs 

 on a blunt head [Rouse, H., and McNown, J. S., 

 "Cavitation and Pressure Distribution: Head 

 Forms at Zero Angle of Yaw," State Univ. Iowa, 

 Studies in Eng'g., Bull. 32, 1948, pp. 54, 65]. 

 No cavitation in smooth-water running need be 

 expected around the bulb bow of the ABC ship. 



The cavitation number cr, corresponding to 

 conditions on this or any other ship, is found 

 quickly by the use of the monogram of Fig. 47.B, 

 by entering it with the total head in ft and the 

 ship speed in kt. 



67.9 Selection of Section Shapes in Entrance 

 and Run. The first decision with respect to 

 section shapes in the entrance and the run is 

 whether they shall be of predominantly U- or 

 V-shape. If the vessel is to give its best perform- 



ance under conditions which make a bulb bow 

 of advantage, the entrance sections in the forward 

 fifth- or quarter-length are necessarily of U-form. 

 They begin with an hour-glass shape at the bulb, 

 considering the abovewater sections as well, and 

 work aft into a more or less straight side, which 

 may be vertical or flared outward. 



Following this rule, the sections in the forebody 

 of the ABC ship are of predominantly U-form. 

 That portion of the entrance lying inside the 

 estimated position of the bow-wave crest is made 

 sensibly vertical so as not to accentuate the 

 crest with outward-sloping section lines above the 

 DWL, with a consequent increase in pressure 

 drag due to wavemaking. 



An easy path for the curved and twisting flow- 

 lines around the lower portion of the entrance 

 is achieved by working as large radii as possible 

 into the lower "corners" of the U-sections in 

 way of the forward quarter. This is illustrated 

 for the ABC design in Figs. 66.P and 66.R. It is 

 shown more prominently in the TSS body plan 

 in Fig. 51. A. It has been possible to reduce bare- 

 hull resistance by the order of 8 per cent in a 

 bulb-bow model solely by cutting away the 

 lower outer corners of the sections abaft the bulb 

 and relocating the displacement volume upward 

 by filling out the waterlines of the same sections. 

 At the suggestion of S. A. Vincent, this was 

 done with the first set of forebody fines of the 

 ABC ship. Following the model tests of the hull 

 shown in Fig. 66. P it is believed that the form 

 would benefit by a further change of the same 

 kind. 



If deep or shallow V-sections are to be used in 

 the entrance, the section shapes follow a fairly 

 regular pattern from the stem to the main body 

 of the hull forward of amidships, with their lower 

 outer corners well rounded. The relatively small 

 differences between V-shaped and U-shaped 

 sections are well illustrated by G. Vedeler, in a 

 diagram embodying alternative hull designs for a 

 given set of specifications [6th ICSTS, 1951, pp. 

 169-170]. 



The design of cutaway dory-type bows with 

 pronounced V-sections, as in the Maierform, in 

 icebreakers, and in vessels like the old pilot boat 

 New York, is discussed under these special forms 

 in Chap. 76. 



The shape of sections in the run is determined 

 largely by the tentative shape selected for the 

 designed waterline and by the positions selected 

 for the propulsion devices. The latter is a major 



