Sec. 15.14 



HULL SMOOTHNESS AND FAIRING 



749 



mended that the shell at the leading edge of the 

 opening or recess be bulged outward solely to 

 move the stagnation point and the +Ap's out- 

 ward from the forward-facing surface at the 

 trailing edge. In any case, do not chamfer, relieve, 

 or set back the leading edge of the recess. According 

 to Hoerner, this approximately doubles the drag 

 of a recess with upstream and downstream edges 

 both flush and square. 



The effect of depth-width or length-depth 

 ratios of shallow recesses, and of setback as well, 

 is rather intricately bound up with boundary- 

 layer thickness and velocity profile in way of the 

 recess; possibly also with the position of the 

 recess on the hull. The best design rule here is the 

 common-sense one, which is to make the recess 

 as shallow as operating and service conditions 

 permit. If the recess is large and the trailing 

 edge can be set back to reduce the area exposed 

 to 4-Ap's, by all means do it. 



As for the effects of what might be termed 

 aspect ratio for the openings of recesses, Hoerner 

 states that the "flow jumps easily" across a wide, 

 short gap, lying across the stream. It "penetrates 

 deeply into the longitudinal groove" formed by 

 a gap lying with its long dimension parallel to 

 the stream, or within 10 deg of the parallel direc- 

 tion [AD, 1951, p. 56]. Because of the "much 

 higher drag" in the latter case, the designer 

 should, if practicable, place a recess with its 

 shortest dimension parallel to the flow. 



75.14 Practical Problems in Achieving Under- 

 water Smoothness and Fairness on a Ship. In 

 the matter of hull smoothness and fairing, the 

 problem of the conscientious ship designer re- 

 sembles closely that of the executive who must 

 carefully apportion his time and energy. The 

 executive's solution is not to pass over all the 

 details but to know which details are of sufficient 

 importance to justify his attention. Similarly, the 

 ship designer must know which roughnesses 

 require smoothing, how much time and trouble to 

 devote to fairing, and what will be the effect of 

 neglect to incorporate the necessary smoothing 



or fairing procedures. An excellent guide in this 

 respect is the group of data on the drag of surface 

 irregularities assembled by S. F. Hoerner [AD, 

 1951, pp. 49-54]. 



By and large, it is not difficult to achieve the 

 smoothness called for by specification require- 

 ments or to design proper fairings. Indeed, it is 

 often easier, if the job is planned properly from 

 the start, to make a ship or its parts fair, well 

 adapted to easy water flow around them, than to 

 make them abrupt or irregular. The difficulty 

 arises, first, 'in making good engineering compro- 

 mises between initial cost and maintenance on 

 the one hand and improved service performance 

 on the other hand. The second difficulty is con- 

 vincing all those concerned with the design and 

 building of the ship that the refinements appar- 

 ently justified by improved service performance 

 are really worth while. All too often, it is feared, 

 the designer and the shipbuilder look upon efforts 

 to provide smoothness and to incorporate fairings 

 as either outright compromises or trivial details 

 not worthy of their attention. In too many cases 

 the shipbuilder looks upon these efforts as nuis- 

 ances and the ship owner as additional means of 

 draining his pocketbook. 



In yachts, sleek appearance above water is 

 almost more important than smoothness under 

 water. Several centuries of experience with them 

 prove that when the designer and builder and the 

 artisans realize the importance of smoothness, it 

 is achieved at no great increase in cost and time. 

 New tools are designed, new techniques are 

 developed, and new procedures utiUzed which 

 make it relatively easy to smooth up the yacht 

 hull when it is known in advance that it must be 

 smooth. What has been done with yachts can 

 be accomplished with merchant and other vessels. 

 The designers and builders take pride in the in- 

 creased speeds of modern ships. They will take 

 pride in their smoothness as soon as they and all 

 others concerned are convinced that the necessity 

 for smoothness increases as the square of that 

 increased speed. 



