Sec. 77 J2 



PRELIMINARY DESIGN OF A MOTORBOAT 



857 



Skot.ching a icMitativc designed watcrliiie 

 through the three points given in the summary 

 presented no problem, but there is no way of 

 knowing whether it is a good one until the sections 

 are drawn. Before this can be done, there has to 

 be a tentative section-area curve from which to 

 work. 



Before sketching such a curve for a normal 

 form of motorboat it is necessary to fix its termi- 

 nation at the AP. Here again there are few design 

 rules for selecting an immersed-transom area Au 

 but the immersed draft Hu should not exceed 

 the values given in Table 67. d. For a speed of 

 18 kt, it is about 1.15 ft; for 14 kt, about 0.695 ft. 

 Probably it should not exceed 1.0 ft in the present 

 case. With a transom beam Bu of 7.5 ft and an 

 average draft of say 0.6 ft, the value of Au is 

 4.5 ft^ and A^/Ax is 0.43. A section-area curve 

 is sketched in roughly through the three known 

 points and a check on the probable volume is 

 obtained. It should be about 6.07 (35) or 212.5 ft'. 



Many of the descriptive articles hsted in Sees. 

 77.31 and 77.41 include section-area curves with 

 the hull lines. These may be used by the designer 

 as guides. 



Starting again with a new and deeper keel 

 profile, an assumed designed waterline, and a 

 tentative section-area curve, sketching of the 

 sections at the various stations may proceed. 

 When drawing these sections a distinct effort is 

 njade to keep the buttocks in the run as straight 

 as possible. Even though the craft is not intended 

 to plane at the designed load, this shape may be 

 relied upon to encourage planing at the higher 

 speeds, under loads (total Aveights) that are 

 somewhat lighter than those specified. 



When the section areas correspond roughly to 

 the section-area curve ordinatcs, and the rise of 

 floor of each appears to be adequate to prevent 

 pounding, with convex sections in the entrance 

 and not-too-sharp transverse bilge curvature, the 

 designer may proceed to add the abovewater body. 



The main deck edge at the side is drawn to 

 give a moderate flare in the forward sections, a 

 slight flare in the midship sections, and some 

 tumble home in the stern sections. Unless deck 

 space is required in service, the extremely wide 

 decks seen in many motorboat designs do little 

 but add weight and require exaggerated flare in 

 the forward sections. The latter may, in turn, 

 easily lead to dangerous slamming when waves 

 strike under this overhang. 



For the ABC round-bottom tender the stem 

 is made more nearly plumb than that of the 

 planing tender. This provides a greater waterline 

 length on a given overall length, with lower 

 resistance at the speeds below planing. 



Upon completion of the shaping and fairing 

 for this craft, involving the preparation of two 

 successive sets of lines, it was found that the 

 volume under the tentative waterUne at rest was 

 slightly greater than that required by the heavy- 

 load W of 16,100 lb of salt water. Rather than to 

 draw a third set of lines the designed waterline 

 was lowered to give the correct volume. The final 

 faired lines are reproduced in Fig. 77.R. 



All the revised characteristics and parameters 

 were checked to make sure that they were satis- 

 factory. They are listed in Table 77. i. 



A final section-area curve of the usual 1:4 pro- 

 portions, and a curve of B/B^x ratio, are laid 

 down in Fig. 77. S, together with the fore-and-aft 



TABLE 77.i — Hull Characteristics and Other Features of Proposed Round-Bottom Tender for ABC Ship 



The principal dimensions, characteristics, and other data Hsted here are for the craft whose lines are shown in Fig. 77. R. 



LoA = 37.07 ft Bwx = 9.3 ft 



LwL = 35.0 ft Bu of transom = 7.3 ft 



Bx = 9.04 ft y = 251.4 ft' in standard salt water 



W = 16,100 lb Ax = 10.6 ft^ 



A = 7.188 long tons Aw of at-rest waterline = 250.9 ft^ 



A/(0.010L)3 = 167.6 Hu of transom = 0.87 ft at rest 



Speed Data 



V = 14 kt at full load; 



V = 18 kt at light load; 



T, = V/Vl = 2.366 

 r, = V/Vl = 3.042 



Hull Parameters 



LCB = 0.5S0LwL from the FP 



Cp = 251.4/(35)(10.6) = 0.678 Cx 



L/B^x = 35/9.30 = 3.763 C„ 



Bfj = 0.785 B^x 



lE of entrance = 21 deg; ir of run, at transom corner, = 5.75 deg. LMA = 0.47Z/ 



10.6/9.04(1.7) = 0.690 

 250.9/35(9.04) = 0.793 



