Sec. 55.<? 



DATA ON DYNAMIC LIFT AND PLANING 



269 



The method of computing the friction resistance 

 is essentially the same as for any other type of 

 surface craft, described in Sec. 45.22. The mean 

 wetted length L^s^s used as the length dimension 

 to determine the Reynolds number R„ for the 

 craft in any specified running condition. Through- 

 out the whole speed range the wetted length, 

 Reynolds number, and wetted area all change 

 with speed but at and near the designed speed 

 they are practically constant. 



Sec. 77.26 embodies an example in which the 

 wetted area and the friction resistance of a full- 

 planing type of motorboat ai'e calculated, follow- 

 ing the methods described by A. B. Murray in a 

 reference cited earlier in this section. 



If there is wetting of the sides as well as the 

 bottom at full speed it can be taken care of as an 

 augment of the wetted area. Normally, however, 

 consistent wetting of the sides of a full-planing 

 craft is evidence of poor design somewhere. Rather 

 than to calculate the effect, the cause should be 

 eliminated. 



A few words are in order here relative to rough- 

 ness of the bottom surface. Although the mean 

 wetted length of modern (1955) planing craft is 

 usually low, well under 100 ft or say 30 meters, 

 the rubbing speed of the water is high. By the 

 reasoning of Sec. 45.10, this means a very thin 

 laminar sublayer under the boat and a large 

 increase in drag if the bottom surface is rough. 

 The permissible roughness height is small, even 

 though the overall R„ may likewise be small. 



53.7 Variation of Total and Residuary Resist- 

 ances with Speed. It is most interesting to note, 

 from the diagrams in Figs. 20 and 21 on pages 676 

 and 677, respectively, of A. B. Murray's paper 

 [SNAME, 1950], that the total-resistance-to- 

 weight ratios of many planing craft, when plotted 

 on a base of speed-length quotient T, , lie remark- 

 ably close to a meanline for a rather wide range 

 of speed. The corresponding values for both 

 V-bottom and round-bottom motorboats and 

 sailing craft given by H. M. Barkla exhibit the 

 same characteristic [INA, 1951, Vol. 93, p. 237], 

 as do the data for many types of large vessels 

 plotted in Fig. 56.M of Sec. 56.10. However, the 

 ordinates of Fig. 56. M have values that are 2,240 

 times the ordinate values of the Murray and 

 Barkla graphs. 



Murray's planing-craft data cover ranges of 

 displacement-length quotient A/(0.010L)^ of from 

 100 to 180, yet it is only above a T, of about 3.5 

 to 4.0 that much dispersion is found. These data 



are, as stated by Murray, most useful for pre- 

 liminary resistance estimates, when the shape 

 and proportions of a new design of hull have not 

 yet been determined. 



Considering only residuary resistances Rr , 

 the few available data indicate a greater degree 

 of irregularity than that described in the fore- 

 going. Fig. 53. D illustrates variations in the 



Fig. 53.D Variation of Speed Exponent for the 

 Derived Rbsiduart Resistances of Two Planing 



Craft 

 To keep the presentation simple the T, values for the 



two planing vessels were calculated on the basis of the 



waterline length at rest 



exponent n of the expression Rr = fcF", for two 

 typical planing craft, over a considerable range 

 of speed-length quotient. However, despite the 

 large variation in A/(0.010L)*, the two graphs 

 resemble each other closely. Of great interest 

 here are the low values of n for the designed- 

 speed points, actually negative for the PT boat. 

 53.8 Selected Bibliography on Planing Sur- 

 faces, Dynamic Lift, and Planing Craft. There is 

 given here a rather full but by no means complete 

 list of references for the reader who wishes to 

 delve further into the matters presented in this 

 chapter on planing phenomena. Included in the 

 list are pertinent references on seaplanes and 

 fljdng boats. For references on planing-craft 

 design the reader is referred to the partial bibli- 

 ography on motorboats in Sec. 77.41. 



(1) Greer, J. F., "First Flight of an American Aeroplane 



from the Water," Scientific American, 11 Feb 1911, 

 p. 132. Gives a drawing and some illustrations of 

 the tandem planing-float scheme once used by 

 Glenn H. Curtiss. 



(2) Fauber, W. H., U. S. Patent 1,024,682 of 30 Apr 1912 



for boats or ships with planing steps 



(3) Baker, G. S., and Millar, G. H., "Some Experiments 



in Connection with the Design of Floats for Hydro- 

 Aeroplanes," Adv. Comm. Aero. (England), 1912- 

 1913, R and M 70, pp. 239-245 



