Sec. 66.27 



STEPS IN PRELIMINARY DESIGN 



493 



about 0.7//, or 18.2 ft, its fore-and-aft length is 

 roughly 14.5 or 15 ft. With a clearance of 2 ft 

 between the leading edge of the rudder and the 

 after edges of the propeller blades, the after end 

 of the propeller hub is of the order of 19 ft forward 

 of the AP. Estimating the propeller hub as about 

 4 ft long, the plane of the propeller disc is about 

 21 ft forward of the AP. 



At this disc position the height of the tentative 

 half-siding buttock, already laid down, is about 

 23 ft above the baseline. When the aftfoot is cut 

 away to save wetted surface and improve maneu- 

 vering, the propeller should have at least 0.5 ft 

 clearance above the baseplane. With a tip clear- 

 ance at the hull of about 2.4 ft, a rather small 

 figure for a large wheel with a shelf-type stern 

 above it, the relative tip or hull clearance for a 

 20-ft propeller works out as 0.12Z). It is doubtful 

 whether a larger screw could be accommodated 

 under this type of stern, on a draft of 26 ft. 



The propeller tip circle is now drawn in on the 

 body plan, and a rough outline of the propeller 

 side projection added to the stern profile. Making 

 the aperture clearance ahead of the upper blades 

 at least 0.2D or 4 ft, rather larger than customary 

 [ME, 1942, Vol. I, p. 275], terminates the upper 

 aperture some 26.5 or 27 ft forward of the AP. 

 This is the position into which the upper part of 

 the skeg is to be faired. After the rudder is in- 

 creased in area, and other small changes are made, 

 the resulting stern profile is as delineated in 

 Fig. 66.Q. The worked-out example in Sec. 59.11, 

 combined with the design rules for propeller 

 apertures in Sec. 67.24, and with the charac- 

 teristics of the screw propellers found suitable for 

 this design, indicate that the aperture forward of 

 the upper blades is still somewhat small. 



Details affecting the bow profile are covered in 

 Sees. 67.4 and 68.7. 



Before proceeding any further with the lines 

 it is well (1) to insure that the wetted surface is 

 not becoming too large in proportion to the size 

 of the ship, and (2) to make a second check of 

 the probable shaft power for a propeller Dma^ 

 of 20 ft. These are done in the sections follow- 

 ing. 



66.26 Analysis of the Wetted Surface. The 

 wetted surface, by the estimate of Sec. 66.9, is 

 to involve an expenditure of well over half the 

 maximum designed power in overcoming friction. 

 As a check it is useful to consider D. W. Taylor's 

 broad conclusions on this subject [S and P, 1943, 

 pp. 22-23]. They are adapted here to an analysis 



of the ABC design but they apply to any usual 

 type and form of ship: 



(a) For a given volume or weight displacement 

 the wetted surface varies mainly with length, 

 very nearly as L"'^. At this stage it appears that 

 the 510-ft length of the ABC ship is not too 

 great in relation to other dimensions or with 

 respect to the ship's mission. 



(b) For a given displacement and length the 

 wetted surface varies little within the permissible 

 hmits of beam and draft in service. With a B/FI 

 ratio of 2.808 and a Cx value of 0.956 for the 

 ABC hull, reference to Fig. 45. H indicates that 

 the wetted-surface coefficient Cs is in a region 

 close to the minimum for normal vessels. 



(c) For a given displacement and dimensions, the 

 wetted surface is affected very little by minor 

 variations of hull shape. The ABC sections are 

 neither the extremely full ones which, according 

 to Taylor, are somewhat prejudicial to low S, 

 nor are they the extremely fine ones which are 

 markedly prejudicial. 



(d) After length, the most powerful controllable 

 factors affecting wetted surface are the forefoot, 

 the aftfoot or deadwood, and the appendages. 



The parts listed in (d) have large surfaces 

 compared to their volumes. For the ABC design 

 the presence of the bulb bow should more than 

 repay its extra wetted surface. It is proposed 

 to cut the aftfoot away by an undetermined 

 amount. The rudder, with its large surface in 

 proportion to its volume, is necessary. The fixed 

 horn to support it, if given a twisted or contra- 

 form to recover energy in the propeller outflow 

 jet, should likewise pay its way. The roll-resisting 

 keels, not a part of the main hull, are considered 

 in Sec. 73.18. 



There is some added surface under the transom 

 stern of the ABC ship. It is hoped that the extra 

 friction drag of this surface may be overcompen- 

 sated by the energy recovered in straightening 

 (leveling) the flowlines of the water leaving the 

 stern. 



66.27 Second Approximation to Shaft Power. 

 Making use of the thrust-load factor method for 

 powering described in Sec. 60.14, the results for 

 the ABC ship are as follows. From Sec. 66.9 

 the resistance R for the bare hull is estimated, in 

 round figures, as 172,000 lb. An increase of 10 

 per cent for appendages gives an estimated final 

 hull drag of 189,200 lb. The corresponding pro- 

 peller thrust T for an estimated thrust-deduction 



