Design of Low-Resistance Hull Forms 



It can be seen that the agreement with experiment is certainly no better for 

 the quasi -bulbous form, except for speeds below 3.5 ft/sec where the water sur- 

 face does indeed remain nearly flat as assumed in the theory. The expected re- 

 duction of resistance as compared with the uniform-source form is not for the 

 most part realized. The experimental resistance curve is much less undular in 

 Fig. 10 than in Fig. 9, and in this qualitative respect the experimental results 

 resemble the theoretical ones. However, if the experimental results can be be- 

 lieved (and as mentioned above they are subject to some uncertainties) it would 

 appear that wavemaking theory applied in the Inui manner may be a very unsafe 

 guide to hull design. 



Computations are in hand to determine the wave resistance according to the 

 Michell thin-ship theory for the two head forms tested. It will be interesting to 

 see if this theory agrees better or worse with the experimental results than the 

 Inui theory, and it is hoped to present results at the meeting. 



PART 3 - AN ATTEMPT TO DESIGN A LOW- 

 RESISTANCE FORM 



If a more theoretical approach to ship design is to lead to better results 

 than current practice can achieve, its use ought to make possible the design of a 

 ship with a substantially lower power than is predicted by Eq. (10). In this sec- 

 tion a very crude attempt is made at such a design. This follows in the wake of 

 the remarkable achievements of Inui (1) and Pien (5) but lays more stress on 

 the frictional component of resistance. 



In the discussion of Part 1 we talked of regarding a ship as half an airship, 

 so modified as to reduce its wavemaking without losing the viscous-drag advan- 

 tages of the airship form. Airships, unlike ships, have their maximum-thickness 

 sections well forward of amidships. Such a form would, for a ship, usually lead 

 to high wavemaking, but if we were clever enough at reducing the latter, might 

 not this shape then have advantages due to reduced frictional drag? These 

 somewhat naive questionings prompted the design shown in Fig. 11. 



The starting point of this design was the observation that in Fig. 5 the after- 

 body of the quasi-bulbous form is roughly semieliptical in section. An airship 

 has circular cross sections, as this gives the minimum wetted area. For the 

 same reason a low -re si stance ship ought to have semicircular cross sections. 

 (We ignore here matters such as lack of stability and high costs of construction 

 which sections of such a shape might give rise to, since we are for the moment 

 only concerned with the artificial design task of achieving a purely hydrodynamic 

 optimum.) The head form of Fig. 5, stretched laterally so that it ended as a 

 semicircle rather than an ellipse, might therefore, it was thought, make a fore- 

 body of sufficiently low wave resistance to be joined onto an afterbody derived 

 from an airship form. (These ideas were conceived prior to the actual testing 

 of the quasi -bulbous model, and would have seemed less attractive in the light 

 of the unfavourable experimental results for it.) The airship form chosen was 

 as in Fig. 1, and the afterbody was basically the part of this downstream of the 

 maximum section. However, to avoid the draught becoming zero at the stern, 

 the after sections were smoothly changed from semicircles to semiellipses of 



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