23 



The question may be raised as to why Sretensky's analysis based on 

 the prismatic coefficient alone has failed, while the application of Taylor's 

 Standard Series results, based on the same parameter, has proved to be ex- 

 tremely successful in practice except for full forms. The answer is that 

 Taylor's forms are advantageous or reasonable; they have not been derived from 

 a narrow family which may yield extremely bad forms as some admitted by 

 Sretensky. A fortunate feature of Taylor's hull is for instance a small bulb, 

 which improves resistances properties. 



Indicative of the present state of knowledge is Figure 7 — a typical 

 set of resistance curves obtained by Wigley and reproduced by kind permission 

 of the Institution of Naval Architects, from his paper presented before the 

 INA in 1942. 



In the region of the first hump* (at high Proude numbers) calculated 

 values are less than the experimental ones (except for very low prismatic co- 

 efficients). According to Wigley this discrepancy is due to the fact that 

 models .and ships have freedom to rise and trim while calculations apply to a 

 fixed condition. 



At moderate (second and third humps) and low speeds the agreement in 

 average absolute values depends upon the prismatic coefficient <p and L/B and 

 B/H ratios. Theory overestimates the resistance of full and blunt bodies. 

 The maxima and, to a still higher degree, the minima of the calculated curve 

 are exaggerated; hollows in calculated curves generally appear flat in experi- 

 mental curves. The very pronounced interference effects predicted by theory, 

 especially those which result in extremely advantageous resistance properties, 

 are thus smoothed out under actual conditions. 



At very low Froude numbers, say F ~ 0.1 8, the measured residual 

 resistance cannot be identified completely with the actual wave resistance. 

 Presumably, for finer forms, the theoretical values are nearer to the truth 

 than values based on "residual resistance." 



There is a well known phase lag in the waviness of computed and ex- 

 perimental resistance curves: The humps of the latter appear at Froude num- 

 bers which are some 8 percent higher than predicted by theory (except the 

 first hump). This shifting has been explained by the effects of viscosity; 

 Wigley has shown that the phase lag is also slightly influenced by the size 

 of models. 



*The hiunp in the resistance coefficient curre ( © curve) occurring at F= O.5 is denoted by first 

 hump, at F = 0.5 by second hump, etc. contrary to the habit in naval architecture, by which the hump 

 at the highest speed is called "last" hump. This change appeared to be necessary since from a mathe- 

 matical viewpoint we have an infinite number of hvunps between O.5 > F > 0. 



