See. T53 



HULL SMOOrriNESS AND FAIRING 



711 



and filling of exposed edges is called for if they 

 lie within 70 deg of the normal to the flow. 



In fact, it is good design, and probably worth 

 while from the point of view of fuel saving during 

 the life of a vessel, to chamfer exposed corners 

 or to add filler along exposed edges, indicated in 

 Fig. 75. D, even though these edges lie generally 



CASE 3. WORKING LIMITS ON STRUCTURAL ROUGHNESS 

 FOR EDGES GENERALLY PARALLEL TO THE FLOW 

 APPLYING TO SHIP SPEED RANGES OF Tq <I.O 



Direction of Flow is Normal — s-Q 



to Paae, Within 30 deg Each Wo^ 1 



Fig. 75.D Working Limits on Structural Roughness, 

 Case 3 



parallel to the flow. The exact flow directions all 

 over a ship surface are not known too well, 

 despite the advances of recent years in the tech- 

 niques of observing and recording flow around a 

 model. The pitching, rolUng, and heaving motions 

 of a ship in waves, even though not violent, add 

 motion components which change the flow 

 directions relative to the ship surface. 



This is not the place to discuss the matter of 

 applying shell plates on a metal ship to produce 

 a hull surface that is fair and without waviness, 

 as contemplated by the lines drawing. It is a 

 proper design procedure, however, to emphasize 

 the necessity for accomplishing this if the con- 

 struction phases of shipbuilding are to keep pace 

 with the design phases. 



Riveted flush seams and butts, with a single 

 strap inside, lack the rigidity and the reliability 

 of lapped riveted joints, despite the lack of 

 symmetry of both, because of the stretchable butt 

 strap in between. Sad experiences with oil leakage 

 in the single-strapped butts of the bottoms of 

 numerous vessels proves that this method of 

 achieving external smoothness is structurally 

 unsound. The remedy for it in riveted construc- 

 tion, namely double butt straps, is structurally 

 good but hydrodynamically "unsmooth." Welded 

 butts are the real answer, especially for large, 

 high-powered, or important vessels which run 

 at Tj values in excess of LO, and of which a high 

 propulsive performance is demanded. 



On yachts, where glistening appearance may 



be a more important factor than easy water 

 flow, and where expense is usually not an item 

 to be considered, the outer surfaces of metal 

 hulls are often freed of their projections by 

 hand grinding. The depressions are then leveled 

 and the whole surface given a high degree of 

 fairness by troweling on a cement or filler which 

 adheres firmly to the metal for long periods 

 without repair or attention. On certain large 

 ships the coves associated with riveted lapped 

 seams and butts have been filled by applying the 

 same type of cement. The filler is tapered off to 

 zero thickness at a distance from the cove equal 

 to 4 or 5 plate thicknesses, somewhat as shown in 

 Figs. 75. C and 75. D. This filler is heavier than 

 water so that the additional displacement volume 

 of the filled coves is less than the corresponding 

 weight. In other words, the filler does not carry 

 its own weight. The extra displacement weight, 

 coupled with the extra expense, possibly may be 

 justified only in a ship running at a T„ greater 

 than about LO or LI, F„ > 0.3 or 0.33, or in 

 case there is only a small margin of power for a 

 specified minimum speed. 



Unfortunately, the smoothest metal shell 

 surface can be well-nigh ruined by the application 

 of poor anticorrosive or antif ouling coatings. When 

 the antifouling coating contains a self-leveling 

 agent like varnish or enamel, and when this dries 

 hard, the minor projections are minimized by 

 the self-smoothing action of the coating around 

 them. This thins the freely flowing material over 

 the projections and thickens it over the hollows. 



For the reasons explained in Sec. 5.21, the rough- 

 nesses which project through the laminar sub- 

 layer are primarily responsible for the roughness 

 drag. The laminar sublayer thickness 6i (delta) is, 

 as indicated by the formulas of Fig. 5.R and those 

 of Sec. 45.10 on pages 104-105 of the present 

 volume, a function of the kinematic viscosity 

 j'(nu) of the water, of the .-r-distance from the 

 bow of the ship, and of the speed V of the ship. 

 This speed, or the relative velocity [/„ of the un- 

 disturbed water, is by far the most important 

 factor. It is the reason why rough or gravelly 

 surfaces on large, fast ships generate large 

 friction resistances, even when the roughness 

 heights are minute with respect to the ship size. 



The ABC ship under design in this part of the 

 book is in what may be called the fast-speed class, 

 with a Taylor quotient r„ of 0.908 and an F^ of 

 about 0.27. It is worth while, therefore, to elimi- 

 nate all irregularities in the plating which come into 



