Sec. 77.17 



PRELIMINARY DESIGN OF A MOTORBOAT 



837 



small camber. Excessive slamming and pounding 

 in waves are accepted in the effort to rcat'li the 

 ultiinate in speed. At Froude numbers of 4.5 or 

 4.8, Taylor quotients of 15 or IG, the minimum 

 pressure and friction drag are achieved with the 

 minimiuii of bottom surface in contact with the 

 water. The spray, like the pounding, is taken for 

 granted as an unavoidable characteristic of ultra- 

 high-speed performance. If the craft jumps com- 

 pletely out of water because of impact with the 

 crests of waves, this too is accepted providing 

 the stabilizmg fin, the lower blades of the propeller, 

 and the rudder can be kept reasonably well in the 

 water. 



Obviously, a planing boat with a flat bottom 

 does not make a satisfactory boat for all-around 

 service. High rise of floor is necessary forward for 

 wavegoing and for low resistance at speeds below 

 planmg. Some rise-of-fioor angle is necessary in 

 the bottom aft to provide dynamic stability of 

 rovite and proper steering qualities, although the 

 angle required at the extreme stern is small. 

 Thus most planing boats are built with what is 

 known as a twisted or warped bottom, with a 

 decreasing rise-of-floor angle from forward aft. 

 D. Phillips-Birt gives the following typical values 

 for these angles: 



Amidships, 14 to 18 deg, flattening along the run 

 to 2.5 to 4 deg at the stern [The Motor Boat and 

 Yachting, Jan 1954, p. 28; "Motor Yacht and 

 Boat Design," London, 1953, p. 148]. 



G. A. Guins states, in the paper listed as 

 reference (16) of Sec. 77.41, that the minimum 

 rise-of-floor angle found necessary for satisfactory 

 longitudinal stability is 7 deg, together with a 

 long tapering skeg. 



Some planmg boats are built with an ^un- 

 twisted bottom, embodying a constant rise-of- 

 floor angle from amidships to the stern. Compared 

 to the warped bottom, the constant-slope bottom 

 usually gives less rise of floor amidships and more 

 at the stern. The untwisted bottom has the lower 

 smooth-water resistance of the two, but its use 

 may lead easily to other unfavorable character- 

 istics such as poor location of the center of 

 buoyancy or bad steering qualities. Most small- 

 boat designers seem to find it easier to obtain 

 good planing-boat performance with the twisted 

 bottom than with the constant- slope type. 



77.17 Chine Shape, Proportions, and Dimen- 

 sions. Inasmuch as the sensibly flat bottom of a 

 planing craft, within the boundaries of the chine, 

 produces practically all the dynamic lift, there 



are valid hydrodynamic reasons for devoting to 

 the chine the attention it has received sinc(; the 

 early 1950's at the hands of E. P. Clement ["The 

 Analysis of Stepless Planing Hulls," SNAME, 

 Ches. Sect., .3 May 1951; "Hull Form of Stepless 

 Planing Boats," SNAME, Ches. Sect., 12 Jan 

 1955]. In his 1955 paper Clement approa(!hes the 

 planing-boat design problem by using the chine 

 length Lc , the chine beam Be , the chine pro- 

 jected area Ac , the chine planform shape, and 

 certain loading factors on the chine area as the 

 fundamental design parameters. However, the 

 lack of comprehensive and reliable full-scale per- 

 formance data renders this scheme something less 

 than adequate at the present time (1956). For 

 convenience, Clement has embodied the data of 

 the referenced 1951 and 1955 reports in a new 

 report entitled "Analyzing the Stepless Planing 

 Boat" [TMB Rep. 1093, Nov 1956]. 



Nevertheless, the maximum chine beam Bc(Mai) 

 serves as an excellent starting point for deter- 

 mining the proper transverse dimensions of a 

 planing craft. Good design graphs of what appears 

 to be a maximum chine beam on a basis of 

 waterline length, although the author does not 

 mention either of them specifically, are given by 

 D. Phillips-Birt ["Motor Yacht and Boat Design," 

 London, 1953, Fig. 51, p. 147]. The graphs of 

 Fig. 77. H are adapted from curves 1 and 2 of 

 the reference cited. Ratios of Lw^/BcfM^^) be- 

 tween 3 and 4 are typical of modern planing craft. 

 In fact, many of the smaller boats, 20 to 25 ft in 

 length, have ratios less than 3. For the ABC 

 tender of 35-ft waterUne length a maximum beam 

 at the chine of about 10 ft is indicated. The 

 fore-and-aft position of the point of maximum 

 chine beam should lie in the range of 0.55 to 

 0.65L„,i abaft the FP. 



If good planing is desired the chine beam is 

 kept full to the stern, with a value at the transom 

 ranging from 0.80 to 0.90 of Bcdnao • A wide 

 stern is also beneficial to steering when planing. 

 However, in rough weather, especially in a 

 following sea, too wide a stern may cause broach- 

 ing or sudden sheering from one side to the other. 

 This condition is aggravated if the craft is running 

 below planing speed. To improve wavegoing 

 behavior and steering under these conditions, it is 

 advisable to tuck in the sides at the stern to say 

 0.65 to 0.75 of the maximum beam at the chine. 

 Acceptable chine planforms, at least for larger 

 planing craft, are shown by E. P. Clement ["Hull 

 Form of Stepless Planing Craft," SNAME, Ches. 



