Sec. 77.20 



PRELIMINARY DESIGN OF A MOTORBOAT 



841 



great amount of development by the inventor, 

 John Plum, in the period 1945-1955. From the 

 first, however, it produced amazing increases in 

 the capabilities of a planing craft as compared 

 with boat performance without trim control. 



(4) Surface propellers, such as those fitted to sea 

 sleds, and propellers on ultra-high-speed racing 

 motorboats which become surface propellers at 

 high speed, exert an upward lift at the disc 

 position which acts as a trim-control device. In 

 fact, some planing craft with surface propellers 

 are not able to get through the hump speed and 

 to plane without their help. 



(5) It is possible that some form of auxiliary 

 subsurface hydrofoil may be found useful in the 

 future for trim control but no practical device of 

 this kind is as yet developed. 



Because of its rather specialized nature, no 

 trim-control device is considered for the ABC 

 planing-form tender, and it is not discussed 

 further here. 



Lacking the trimming effect of a trim-control 

 device, of surface propellers, or of an auxihary 

 hydrofoil, and neglecting for the moment the 

 effect of buoyancy, it is apparent that the fore- 

 and-aft position of the center of gravity CG of a 

 fuU-planing craft must lie close to the center of 

 pressure CP of the dynamic lift on the wetted 

 area of the bottom. There is no direct, practical 

 method now available (1955) for determining 

 this CP position on an actual boat. It must be 

 estimated, therefore, with reference to some part 

 of the planform, on a basis of expei'imental data 

 from models. The chine planform, projected on 

 the baseplane, is the logical element to use, and 

 the center of area of this element is the logical 

 reference point in it. Unfortunately, the chine 

 area has not been in use long enough, or to a 

 sufficient extent among planing-craft designers, 

 to build up a fund of reference data indicating 

 optimum relative fore-and-aft positions of the 

 center of the chine area and of the CG. E. P. 

 Clement gives some performance data for several 

 types of motorboats with the CG abaft the center 

 of chine area ^c by from 2.1 to 11.1 per cent of 

 the chine length Lc ["Hull Form of Stepless 

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

 1955, Pis. 6-8, 10]. On Plate 9 of the reference 

 Clement gives a plot of CG positions expressed 

 in the manner described, related to the ratio 

 ■4 c/y^'^, based on data from half a dozen models 

 of large planing boats. 



In the absence of more extensive chine data it 

 has been customary to indicate the fore-and-aft 

 CG position as a fraction of the WL length from 

 Sta. at the waterline beginning or from the 

 extreme stern. The chine planforms of all single- 

 step planing craft terminate at the transom, and 

 the chine planforms approximate a shape that is 

 rather well standardized. Referring the CG to the 

 at-rest WL termination is therefore a reasonable 

 enginee ring p rocedure. 



The LCG values for the two test conditions 

 reported by Clement on Plate 6 of the reference 

 cited are O.b'ilLwL and 0.567L,i^i , respectively. 

 C. W. Spooner, Jr., in refe rence (26) of Sec. 77.41, 

 states that t he value of LCB (and presumably 

 also of LCG) varies from about 0.53Ln^z, at a T^ 

 of 1.5 to 0.58L[F£, at or above a T, of 3.0. He reports 

 that successful high-speed designs have had the 

 CB as far aft as Q.^SLwl , with engines in the 

 stern and V-drives to the propellers. 



Moving the CG forward reduces the running 

 trim and lessens the risk of porpoising but at 

 the expense of increase in wetted length and 

 wetted area. The effect on the total resistance 

 depends upon the variation of friction and 

 pressure resistance with trim angle, indicated by 

 graphs similar to those in Fig. 77.0 of Sec. 77.26. 

 At low trims the total resistance usually increases 

 but this is not to be taken for granted by any 

 means, as witness the beneficial effect of trim- 

 control devices. 



77.20 Spray Strips. The form, position, and 

 function of spray strips are well described and 

 generously illustrated by R. Ashton [ETT Tech. 

 Memo 99, Feb 1949]. These devices, probably 

 first employed on multi-engine seaplanes and 

 flying boats to keep spray out of the offset pro- 

 pellers, are excellent illustrations of the more-or- 

 less major hydrodynamic effects produced by 

 minor, if not almost insignificant physical features. 



Spray strips are most important along the 

 forward portion of the boat. However, it is 

 usually beneficial to fit them for the full length. 

 They can be used to advantage on all hulls, 

 round-bottom as well as V-bottom with chine, 

 except perhaps for forms with concave sections 

 and low chines which have extremely sharp 

 chine corners. On certain round-bottom craft 

 they can also be used as fender strakes. 



Spray strips usually result in a sUght increase 

 in resistance at low and cruising speeds. However, 

 their many beneficial effects outweigh this slight 

 disadvantage. They: 



