Sec. 77J2 



PRELIMINARY DESIGN OE A MOTORHOAT 



855 



This area is about the same as for the 24-kt 

 planing-type tender, the beam is about the same, 

 and the length is exactly the same. The keel 

 profile of the 24-kt boat in Fig. 77. K, at the 

 bottom of the hull proper, is therefore used as a 

 guide in drawing a similar profile for the round- 

 bottom craft. The depth at the fore-and-aft 

 position of the section of maximum area is selected 

 as 1.65 ft, about the same as for the faster boat. 



There is no general or special rule concerning 

 the longitudinal position of the maximum-area 

 section for a boat of this type. To afford a clean 

 run toward the transom it should, however, not 

 lie abaft the midlength of the waterline. It is 

 preferably placed slightly forward of that point. 

 For the design in question it is taken at 0A7Lwl 

 from the FP. 



Any small-craft designer welcomes the oppor- 

 tunity to study the hull shapes fashioned by 

 others, even though he may have little intention 

 of follomng or copying any of them. Sources of a 

 considerable amount of these data are listed here 

 for convenience: 



(a) Nordstrom, H. F., "Some Tests with Models of Small 



Vessels," SSPA Rep. 19, 1951. This pubhcation 

 gives the body plans of many round-bottom forms, 

 together with their section-area curves and tabu- 

 lated model-test data. Text is in English. 



(b) Baader, J., "Cruceros y Lanchas Veloces," Buenos 



Aires, 1951. The text is in Spanish, as yet untrans- 

 lated (1957), but the lines drawings are understood 

 by any naval architect. 



(c) Beach, D. D., "Power Boat Form," The Rudder, 



Jan 1954, pp. 38-43, 90. The author gives lines 

 drawings of seven modern hull forms, with the 

 practical and the hydrodynamic reasons for their 

 various features and characteristics. 



(d) Literally hundreds of lines drawings of modern motor- 



boats are to be found on the pages of yachting and 

 motorboating magazines, most of them mentioned 

 in the references of Sec. 77.41. 



It is extremely unfortunate that, ^vith such a 

 wealth of published data at hand, a motorboat 

 designer renders himself vulnerable by rarely 

 knowing whether the shape he is using for gui- 

 dance is a good one or not. In other words, he 

 seldom knows a fraction as much about the good 

 or bad performance of a boat as he knows about 

 its physical shape and other features from the 

 published lines, arrangement drawings, photo- 

 graphs, and descriptions. As an example of what 

 should be done with published material, D. S. 

 Simpson includes a sketchy body plan of a 

 round-bottom motorboat, but gives rather full 

 comments on the behavior of the actual craft in 



service [SNAME, 1050, pp. 081-082). Information 

 of this kind is extrem(!ly valuable but almost 

 equally rare. 



77.32 Layout of the Lines for the ABC Round- 

 Bottom Tender. Laying out the underwater lines 

 for the semi-planing motor tender being designed 

 here calls for following the general principles set 

 forth in previous sections for this operation on 

 full-planing craft. Fashioning the abovewater hull 

 is based upon considerations of wavegoing, good 

 vision from the control platform and other 

 operating requirements, convenience of the pas- 

 sengers and crew and, last but not least, appear- 

 ance. 



Specifically, the first three steps involve 

 roughing in the maximum-section contour, laying 

 out a designed waterline, and sketching a pre- 

 liminary section-area curve, much as they did for 

 the large ABC ship design in Chap. 06. Summariz- 

 ing from Sec. 77.31: 



(a) The maximum waterline beam Bwx is 10.0 ft. 

 Its fore-and-aft position is O.GOLwi, from the FP. 



(b) The half-siding at the bow may be taken 

 tentatively as 0.05 ft 



(c) The designed waterline beam at the transom 

 (or at the AP) is 7.5 ft 



(d) The maximum- sectio n area is 10.4 ft' 



(e) The value of LMA is 0.47L,^,, , reckoned 

 from the FP 



(f) The keel profile of the 24-kt full-planing 

 tender, at the bottom of the hull proper, is to be 

 used as a guide. 



The first layout of the maximum-area section 

 (actually taken at the position of Bwx) indicated 

 that the initial beam of 10 ft was too large and 

 that the keel line used for guidance lay too near 

 the at-rest waterplane. The floor lines in the 

 bottom had too smaU a rise to insure reasonable 

 freedom from pounding. The bottom slope was 

 in fact smaller than for the corresponding sections 

 of the full-planing form. Unfortunately there 

 appears to be no set of minimum values to be 

 used as a design criterion for selecting a proper 

 rise-of-fioor angle for this type of boat. 



The combination of wide beam and shallow 

 underwater body produced an extremely sharp 

 curvature at the turn of the bilge. Indeed, the 

 maximum-area section had the appearance of 

 one taken from a hard-chine hull in which the 

 chines had simply been rounded oS. For the 

 second layout the beam was decreased and the 

 keel line was lowered. 



