Sec. 66.21 



STEPS IN PRELIMINARY DESIGN 



485 



cated in a preceding paragraph, placed with its 

 midle ngth at the selected fore-and-aft position 

 LMA of the section of maximum area. 



66.19 Bulb-Bow Parameters. Considering 

 next the forward end of the ship, it is determined 

 first whether a bulb bow is advisable as a means 

 of saving pressure resistance. This is governed 

 largely by the ratio of the speed to the length at 

 the point where maximum performance is desired. 

 The greatest saving is in the region of a T^ of 

 1.0; it tapers off down to a T„ of about 0.75 or 

 less and it diminishes at 1\ values up to 1.5 or 

 more. Inspection of F. H. Todd's Table 5 [IME, 

 Feb 1945, p. 18], as well as of Fig. 67.D in Sec. 

 67.6, reveals that for the speed-length quotient 

 of 0.908, corresponding to the trial speed of the 

 ABC ship, a bulb bow is indicated. 



It is to be borne in mind that this ship, for 

 probably the greater part of its time at sea, will 

 run at a speed closer to 18.7 kt than 20.5 kt. In 

 other words, the T, for the majori ty o f service 

 hours will approximate only 18.7/ VolO = 0.828, 

 F„ = 0.247. At this lower speed the bulb may 

 show up to less advantage. Furthermore, a 

 smaller /^ is called for than at the higher T, of 

 0.908, in the ratio of about 0.07 to 0.08 or more. 



The bulb parameters may be worked out by 

 D. W. Taylor's method [S and P, 1943, pp. 65-70, 

 243-254]. However, it is pointed out in some detail 

 in Sec. 67.6, where this procedure is illustrated, 

 that it is rarely possible to utilize all of the 

 optimum section area in a bulb, because of 

 interferences with bower anchors and possible 

 under-the-bulb slamnoing. 



There is one other factor to be considered. In 

 the lighter-load conditions on the ABC ship it is 

 comtemplated that liquid cargo or water ballast 

 in the tanks aft will be used to bring the stern 

 down and to give the propeller adequate tip 

 submergence. At these varied trims by the stern 

 the bulb at the bow will be nearer the surface and 

 will emerge at less angles of pitch than at full load. 



If the proposed under-the-bottom anchor 

 installation described in Sec. 68.11 does not work 

 out, it may be necessary at a later design stage 

 to fit bower anchors in the orthodox side locations. 

 This consideration alone points to the wisdom 

 of using, for the ABC design, a considerably 

 smaller bulb area Je than that indicated by the 

 full-speed, full-load conditions. The value of 

 Se = 0.02 from the broken hne of Fig. 67.D, 

 representing installations of the past, is hardly 

 enough to make a bulb worth while. An inter- 



mediate value of f E — 0.06 lies within the opti- 

 mum range, and it affords ample room for the 

 bottom anchor contemplated, although it is 

 somewhat lower than the optimum for the pro- 

 portions of this vessel. 



From the upper diagram of Fig. 67. D a value 

 of is = 0.9 is tentatively selected for the designed 

 range of T, of from 0.828 to 0.908. 



The detail design of the bulb is worked out in 

 Sec. 67.6. 



66.20 Transom-Stern Parameters. For an 

 estimate of the immersed-transom area and the 

 value of fn it is assumed first that the transom 

 is definitely to clear at the designed speed of 

 20.5 kt. In other words, at this speed the entire 

 transom area is to be exposed to the air. On the 

 assumption described in Sec. 67.20 that the 

 corresponding Froude number, using the im- 

 mersed-transom depth Hu as the length dimension, 

 is hmited to 5.0, this immersed depth works out 

 as follows: 



F, = 5.0 = V/VgHv, 



whence 



, _ [ 20.5(1.6: 

 ''^ ~ L 5.0 



g{H^) = 



6889)1' 



^t/ = 3^ (6.925)' = 1.49 ft. 



Taking the transom width previously agreed 

 upon of (0.3)73 = 21.9 ft and a constant depth 

 of 1.5 ft, the immersed-transom area at rest 

 would have a maximum value of about 33 ft'. 

 The terminal value Ju is about 33/1,815 = 0.018. 

 A tentative value of f r = 0.02 seems reasonable 

 when first sketching the section-area or A-curve. 

 Certainly it will not be larger than this. 



Further details of immersed-transom design 

 are given in Sec. 67.20. 



66.21 The Preliminary Section- Area Curve. 

 With the data thus assembled it is possible to 

 lay down, in the standard 1:4 box described in 

 Sec. 24.12, a tentative section-area curve for the 

 ABC design. The typical A-curve on S. A. Vin- 

 cent's 1930 data for a Cp of 0.60 [MESA, Mar 

 1930, Fig. 4, p. 138] is ticked in with dots on the 

 plot. This is checked from the cross curves of 

 W. P. A. van Lammeren [RPSS, Fig. 42, p. 92], 

 from those of F. H. Todd for the TMB Series 60 

 [SNAME, 1953, pp. 516-589], or from similar 

 sources, provided the curves lend themselves to 

 vessels with bulb bows. 



