Sec. 77.2-f 



PRELIMINARY DESIGN OF A MO'IORBOAT 



843 



Rudders for motorboats are discussed in Sec. 

 74.11. Comments upon the fairing of external 

 pads for the attachment of strut arms to wooden 

 motorboat hulls are given in Sec. 75.6. 



77.23 Interdependence of Hull-Design Fea- 

 tures. The principal hull-design features of a 

 planing craft are mvich more dependent upon each 

 other than are those of a displacement-type 

 vessel, just as the latter are much more intimately 

 tied together than are the corresponding features 

 of an airplane. For a planing craft the beam, the 

 chine height, the rise of floor, and the type of 

 sections are all related and must go hand in 

 hand in fashioning the boat. 



A proper design procedure involves selecting 

 the desirable features pertaining to each individual 

 parameter, and then working out a compromise 

 to produce the nearest approach to the desired 

 overall performance that can be estimated during 

 the preliminary design. Often several different 

 types of section may be used to advantage along 

 the length of a boat. Yoke sections forward easily 

 transform into convex or straight sections aft. 

 Concave sections are often employed forward to 

 obtain good planing characteristics, transforming 

 into convex sections amidships and aft where 

 space is needed for the machinery plant and for 

 tanks to carry liquids. Convex forward sections 

 lend themselves to an easy transformation to 

 straight sections where the rise of floor is small. 



Still greater compromises are made when 

 selecting features favorable to both seakindliness 

 and planing. In many boats the seakindly round- 

 hull form is used in the entrance, transforming 

 into the hard-chine efficient planing hull in the 

 run. The reverse of this is sometimes encountered, 

 but there appears to be no advantage in such an 

 arrangement [Phillips-Birt, D., The Motor Boat 

 and Yachting, Jan 1954, p. 27]. 



77.24 Layout of the Lines for the ABC Planing- 

 Type Tender. With the comments of the pre- 

 ceding sections as a background the designer is 

 now ready to lay down a tentative set of lines. 

 He must anticipate that this set may be only the 

 first of a half dozen or more, in his search for a 

 shape that best meets the design requirements. 

 For this reason it is well to start with a scale just 

 large enough to permit measuring lengths, areas, 

 angles, and slopes with reasonable accuracy. 



The first line to draw is a waterUne for the 

 profile and the bow and stern elevations. The 

 designer is advised to work to this waterUne as a 

 sort of fixed reference plane, the same as for a 



larger ship. Even though, as described presently, 

 it may be necessary later on to change slightly 

 the water-surface level on the hull to effect a 

 balance between weight and buoyancy rather 

 than to draw a new set of lines, the original 

 waterline (or waterplane) serves a definite purpose 

 in establishing dimensions, proportions, and 

 parameters while the size and shape of the hull 

 are being worked out. 



Using the planing-type ABC tender as an 

 example, it is assumed as a starter that all the 

 requirements can be met on a WL length of 

 35 ft. A horizontal waterline of this dimension is 

 drawn to a convenient scale, with Sta. at the 

 FP or waterline beginning and Sta. 10 at the 

 AP or waterhne ending. The latter is also the 

 transom position on the centerline. The shape of 

 the hull is to be based, but only generally at this 

 stage, upon the arrangement sketch of Fig. 77. B. 



The next step is to fix the shape of the chine 

 and to position it vertically with respect to the 

 DWL. A tentative chine line is drawn according 

 to the rules previously discussed: 



(a) Chine height at the FP. Because of the good 

 wavegoing performance desired the chine height 

 at the FP is made greater than the value of 

 O.OQLwL previously mentioned in item (1) of 

 Sec. 77.17. This allows finer sections forward with 

 less probabifity of slamming and pounding. A 

 chine height of 0.07ZLwl appears adequate. 



(b) Chine height at the AP. A tentative value is 

 -0.02lL,^z, . 



(c) Straight chine fine from Sta. 7 aft, for the 

 aftermost 0.3 of the length 



(d) Chine, as projected on the centerplane, 

 crosses the DWL between Stas. 4 and 5. 



The plan view of the chine is then laid out on 

 the basis of the preliminary arrangement sketch 

 of Fig. 77.B. As a rule, the maximum chine beam 

 should fie within the range of 0.55 to 0.65Lwl 

 abaft the FP. For the ABC tender it is placed at 

 Sta. 6, or at O.QOLwt, abaft the FP. The maximum 

 chine beam is tentatively selected as 10 ft, which 

 gives a ratio Lwl/Bc of 3.5. This is typical for a 

 modern high-speed planing craft, for which the 

 ratio Lwl/Bc varies from 3.0 to 3.6. The beam 

 at the transom ending is made about 0.9 the 

 maximum beam, or 9 ft. Subsequent fairing of 

 the lines and working over the design produces a 

 5c(Max) of 10.04 ft and a chine beam at the 

 transom of 8.96 ft. 



In this design a wide stern is chosen because 



