Sec. 77.27 



PRELIMINARY DESIGN OF A MOTORBOAT 



851 



0.0175 of the 40-ft length. For a craft 38 ft long 

 the rise of the CG would be about 0.G7 ft. 



Another method is to lay out an elevation or 

 profile of the boat at its designed speed and in its 

 predicted position and attitude with respect to 

 the undisturbed smooth water into which it is 

 advancing. One detailed method of accomplishing 

 this is described in the paragraphs which follow. 



With the CG and the CP tentatively located by 

 the procedures of Sec. 77.26 in a transverse 

 plane 14.21 ft forward of the AP, the correspond- 

 ing running attitude is 4.9 deg by the stern. The 

 mean wetted length L„,s at this trim angle is, 

 by interpolation from Col. B of Table 77. h, 

 21.4 ft. 



According to B. V. Korvin-Kroukovsky, D. 

 Savitsky, and W. F. Lehman [ETT, Stevens, 

 Rep. 360, Aug 1939, Fig. 15, p. 36] the forward 

 edges of the wetted area of a planing craft when 

 underway are defined by the bases of the spray 

 roots under the bottom. These extend generally 

 in two straight lines from a point on the keel to 

 points abaft this on either chine. When the wetted 

 area on one side of the hull is projected on the 

 centerplane the point of intersection of the spray- 

 root base with the chine lies somewhat above the 

 level of the undisturbed water surface. This 

 situation is illustrated schematically in the small 

 figure pubhshed by A. B. Murray as a part of 

 Fig. 10 on page 665 of his paper "The Hydro- 

 dynamics of Planing Hulls" [SNAME, 1950]. 

 The mean wetted length Lws , measured generally 

 parallel to the mean buttock at the quarter-beam, 

 is the arithmetic mean of the wetted length of the 

 keel and the wetted length along the chine. The 

 difference between these two lengths, indicated 

 as Li in ETT, Stevens, Report 360, is estimated 

 by Eq. (17) on page 14 of that report, namely 



^ _ (planing surface beam) tan /3 

 If tan 9 



(77. ix) 



It is probable that this length is a function of 

 the local rather than of the average chine beam 

 and rise-of-floor angle. For the ABC planing 

 tender these are taken as the local chine beam at 

 Sta. 5, equal to 9.96 ft, and the local rise-of-floor 

 angle at midlength, equal to 18.25 deg. Then, 

 for a trim 6 of 4.9 deg. 



Li = 



(9.96)(0.3298) 



= 1.22(9.96) = 12.2 ft. 



3.1416(0.0857) 



On the basis of perfectly flat V-shaped bottom 

 surfaces, with constant angle /3 and no twist, the 

 wetted length of the keel should equal the mean 

 wetted length L^s plus half the length L^ . For 

 the ABC tender this is 21.4 ft plus 6.1 ft or 27.5 ft. 

 This wetted keel length is laid off along the keel, 

 forward from the AP, terminating at the point 

 K in Fig. 77. P. The wetted chine length is 21.4 

 ft less 6.1 ft or 15.3 ft. This distance is laid off 

 along the chine, also forward of the AP, terminat- 

 ing at the point C. The diagonal broken line KG in 

 the figure forms the locus of the spray-root bases 

 on each side. Taking for granted that the water 

 breaks off cleanly along the chines, and that the 

 transom clears to its lower edge, the wetted area 

 hes entirely under the after portion of the V- 

 bottom, indicated by the hatched area in the 

 figure. 



The point K is placed at the level of the sur- 

 rounding undisturbed water, and the craft is 

 trhnmed 4.9 deg by the stern. Its profile is then 

 drawn in, picturing the position and attitude of 

 the hull with respect to the undisturbed water 

 level. Fig. 77.P is this profile for the ABC full- 

 planing tender, with dimensions and explanatory 

 notes. 



[Proposed E'ye 

 Offset of Mean Buttock from Centerplane 15 -^-2 30 ft \ Position of 



Calculated Trim is 4,9 deo b\ the Stern 



Water Surface 



^Mean Welted Lencjth Lws 



\* — — is^ b^ calcuialion, 214 ft f^CP Position for Normal H^dronomic Pressures on Bottom, 14.21 ft Forword of FP 



Fig. 77.P Diagbam Showing Pbedicted Running Position and Attitude op Planing Tender at Full Speed 



