Design of Low-Resistance Hull Fornns 



doublets might be used, since although the displacement thickness of the far 

 wake will be nonzero, it will be considerably less there than it is at the stern. 



Figure 15 provides concrete evidence in favour of the general argument ad- 

 vanced above that the rapid boundary layer thickening near the stern matters 

 far more than the gradual thickening over the complete hull. However the ef- 

 fects are smaller than one would perhaps expect for the action of viscosity, so 

 it may well be equally important to model the rapid contraction of the wake just 

 downstream of the stern. This, of course, would be to return very nearly to 

 Havelock's original proposals (10). 



r NO BOUNDARY LAYER, AND 

 1(a), X-O, 6 = O'OOS 

 (b), X= 0-9L, CsO-OS 



Pn = (KoO 



-'/? 



0-4 



Fig. 15 - Computed effects of 

 (a) and (b) in Fig. 14 on wave 

 resistance 



Whatever the complete explanation may be for the discrepancy between the two 

 resistance curves in Fig. 12, the outcome of the present exercise has been that, 

 by happy accident rather than skillful design, a hull form of lower resistance 

 than Eq. (10) has been achieved. Thus, scaled to a 693-ft-long ship, of 34,300 

 tons displacement, at 20 knots, the results for the model in the bulb-aft condi- 

 tion at 5.66 ft/sec lead to an estimated horsepower of 13,800. Here a conserva- 

 tive quasi-propulsive coefficient of 70 percent has been taken rather than the 

 measured value of 74 percent. Run bulb forwards, the horsepower would be 

 16,430. Equation (10) gives P^^^^^ = 15,520 for 34,300 tons displacement and 20 

 knots. Thus it appears that with the vessel run the opposite way round to what 

 was originally envisaged the target has been beaten by a small margin. The 

 change of direction does not necessarily mean that the ideas behind the original 

 design were entirely unsound, although the low-speed results do suggest that, at 

 any rate for a ship of this slenderness, the viscous-drag advantages of having 

 the maximum thickness forward are very small, so that to utilize them one 

 would have to be far more clever than I have been in reducing wave resistance. 



727 



