Sec. 66.23 



STEPS IN PRELIMINARY DESIGN 



487 



do this by sketching freehand on paper with 

 Hght-blue cross-section hnes, producing the result 

 depicted in Fig. 66.0. Small profiles, sections, and 

 deck plans, plus a midship section, bow and 

 stern profiles, and a stern elevation to larger 

 scale, suffice for this purpose. The first such 

 sketches for the ABC design were drawn to a 

 scale of 80 f t = 1 in. For a complete preliminary 

 design these sketches would be supplemented by 

 an inboard profile and several additional deck 

 plans, drawn to a considerably larger scale. 



The maximum-section contour of Fig. 66. G 

 and the preliminary designed waterline form the 

 bases of these sketches. A tentative freeboard 

 amidships of 23 ft is selected and then checked as 

 described in Sec. 66.30. A sheer line is drawn in, 

 more or less by eye, with the low point of the 

 deck well aft of amidships and with a sheer 

 forward that looks right, to be checked later as 

 outlined in Sec. 68.4. 



A curved raking stem and a bulb that projects 

 slightly forward of the FP complete the small- 

 scale bow profile. At the stern the immersed- 

 transom depth of 1.5 ft is laid off below the 

 designed waterline and a stern profile raking 

 sUghtly forward is added above it. The square 

 transom is indicated as fading out above the 



waterline to a rounded main-deck planform at 

 the stern. 



The wide, somewhat shallow underwater form 

 calls for rather drastic narrowing aft if proper 

 flow to both top and bottom blades of a single 

 propeller is to be achieved. A pronounced cutting 

 up of the stern might give a reasonably good 

 performance with twin screws; a twin-skeg form 

 of stern almost certainly would. However, the 

 use of two separate propelling plants for an 

 output of the order of 16,000-20,000 horses 

 involves increases in space, cost, weight, operating 

 personnel, and so on. For the reasons elaborated 

 upon in Sec. 69.2, it appears not justifiable. 



The efficient, economic transportation required 

 by item (5) of the mission, in Table 64. a, calls 

 for high propeller and propulsive efficiency. One 

 method of gaining the former is to use a screw 

 propeller of the largest practicable disc area and 

 diameter. 



For the current style (1955) in single-screw 

 merchant vessels having canoe or whaleboat 

 (cruiser) sterns, a good rule for the propeller 

 diameter D is to keep it less than O.IH at the 

 designed-load condition. This insures reasonable 

 submergence at drafts not too much smaller than 

 the maximum, and as good submergence as can 



25 5-ft Station Spacmq 



Fig. 66.0 Sketches of Outboard Profile, Main Deck and Waterline, and Sections 



