Sec. 77.26 



PRELIMINARY DESIGN OF A MOIORBOAT 



819 



TABLE 77.h — Calculation for CENTEn-OF-PuEssonE Location fob Fuli^Planing Hull op Fia. 77.K by 

 Murray's Planing Surface Data 



Cols. B and C — From previous calculations of Table 77. g 



Col. D— From lower part of Fig. 53.C 



Col. E — K from upper part of Fig. 53. C 



Col. F— [(Col. D)(Col. E)] 



Col. G—CP/Lws = K\"; CP = (Col. F)(Col. B). 



the magnitude of the normal force F on the 

 bottom, and as the mhior hydrodynamic forces 

 H are not well known at this time (1955), it is 

 necessary to substitute for the expression 

 [F sin d + (E + H) cos d] the equivalent expres- 

 sion found in most of the technical literature, 

 namely (L tan 6 + R^ sec 6). Here Rr = L tan 

 = W tan 6. The friction drag Rp would normally 

 be augmented by the factor sec d but since 6 is 

 usually small, probably not exceeding 6 or 7 deg, 

 with a value of sec 6 not more than about 1.0075, 

 it is customary to omit this factor. The friction 

 resistance Rp is then added directly to the 

 residuary resistance Rr to give the total resistance 

 Rt ■ 



The complete calculations, by Murray's method, 

 of the friction, residuary, and total resistances 

 of the ABC planing tender, for a series of trim 

 angles at the designed speed of 24 kt, are set 

 down in Table 77.g. Entering the lower diagram 

 of Fig. 53. C in Sec. 53.5 with the appropriate data, 

 the position of the center of pressure CP ahead of 

 Sta. 10 at the transom (or the AP) is calculated 

 by the method illustrated in Table 77.h. 



It is to be noted that in Col. E of Table 77.g 

 the mean wetted length Lws comes out as con- 

 siderably longer than the waterline length Lwl 

 for a trim angle of 2 deg. However, this seems 

 not to affect the validity of the final results. 



It is possible that the full ATTC 1947 ACp 

 allowance for full-scale roughness of 0.4(10"^), 

 indicated in Col. J of Table 77. g, is not justified 

 for a high-speed motorboat of this kind. As a 

 tender for the ABC ship it will be in the water 



and subject to fouling and deterioration of the 

 bottom coating for only a small percentage of 

 the time. Again, however, the use of the full 

 allowance in the preliminary-design stage leads 

 to a conservative estimate of the engine power. 



The various resistance and center-of-pressure 

 values derived in Tables 77. g and 77. h are plotted 

 on a base of trim angle 6 in Fig. 77.0. Murray's 

 curves of minimum (total) resistance per pound 

 of displacement for planing hulls [SNAME, 1950, 

 Fig. 21, p. 677] afford a check of sorts on the 

 values derived in the foregoing. From Table 77. f 

 the displacement-length quotient of the ABC 



Fig. 77.0 Predicted Resistance Components 

 and Trim Angle fob Planing Tender of ABC Ship 



