Sec. 75.3 



HULL SMOOTHNESS AND FAIRING 



739 



fact that many fairings are indeed not excrescences 

 but parts of the structure and important parts 

 at that. A tree is so shaped just above its point of 

 attachment to the ground that the stresses in the 

 wood of its trunk are very nearly constant with 

 height when the tree bends with the wind as a 

 cantilever beam. This is almost identical with 

 the most modern type of fairing at the roots of 

 screw-propeller blades, employing a constant- 

 stress transition shape where the blades join the 

 hub. The fairing of the afterbody of a whale or a 

 porpoise into its horizontal flukes is an admirable 

 combination of hydrodynamic streamlining, rapid 

 and effective change of cross-section area and 

 shape, arrangement of muscles for manipulating 

 the flukes as propulsion devices, and muscular 

 flexing of the flukes as control surfaces. 



Plates bounding the ship hull are intended to 

 be flat or gently curved, as the case may be, 

 when they are incorporated in the design and 

 delineated on the drawings. Only rarely is any 

 unfairness allowed for when calculating the 

 strength or the rigidity of a ship structure. Why, 

 then, does the ship not deserve equally honest 

 treatment when it is built? Even the uninitiated 

 realize instinctively that the underwater hull of a 

 vessel must be fair to insure efficient propulsion. 

 Why then should the vessel be penalized through- 

 out its life because of unfairness resulting from 

 a few days of improper work during its construc- 

 tion? Increases in friction resistance due to rough- 

 ness, of the order of 30, 40, 50, and up to 100 

 per cent of the smooth, flat-plate Rp , are being 

 encountered on large, fast, modern vessels. This 

 fact should be adequate proof that something 

 drastic needs to be done. Fairness and smooth- 

 ness are essential parts of a ship. An attitude on 

 the part of all concerned which recognizes that 

 these are not something to be applied, like a 

 coat of paint, just prior to launching will go far 

 toward solving this problem for the designer. 



75.3 Specific Smoothness Problems on the 

 Shell Plating. In the matter of the greatest 

 practicable smoothness of the shell in the finished 

 ship, at least four areas deserve attention: 



(1) The extreme bow, and a belt abaft it, up to 

 say 0.2 or 0.31/ from the FP. This is because the 

 local specific friction resistance Clf is very high 

 for the small ^-distance and the low R^, in this 

 region, indicated by Fig. 45.E. 



(2) The region directly in front of inlet scoops for 

 condensers and other heat exchangers, so that 



the relative liquid velocities in the inner portions 

 of the boundary layers will be high. This applies 

 to a belt about 2 or 3 times the inlet width and 

 perhaps 10 times its length. 

 (3) The afterbody, say from about 0.6 to 0.7L to 

 the extreme stern. Fig. 7.5. A is an adaptation of 



Rouqhness Consisted of V- Grooves 

 Runninq Tronsverselij Around Model cr- 



•^ Rouqhened Surfoce of Model, from FP a) 



I to Fore-ond-Aft Position Indicated, fe 



" in Per Cent of Totol Wetted Surfoce _ 



90 80 70 60 50 40 30 20 10 





20 18 

 AP 



IZO^ 



FP ^ 



Fig. 75. a Variation of Friction Drag Due to 

 Roughness Along the Ship Length 



model-test data pubhshed by G. Kempf [HSPA, 

 1932, Fig. 7, p. 81], indicating the variation of 

 friction-resistance augment, up to 140 per cent 

 of the smooth, flat-plate total, when a model was 

 roughened by cutting V-grooves around it for 

 various percentages of its length. 



(4) The region immediately ahead (within 1 or 2 

 diameters) of a propulsion device, when the water 

 leaving this surface flows directly into the device 



(5) The examples of Sec. 45.15 indicate that the 

 greater the absolute speed of the ship, the smaller 

 is the permissible roughness to achieve a hydro- 

 dynamically smooth surface. 



The foregoing states, in effect, that the only 

 part of the hull which need not be smooth is that 

 around amidships. However, if any favoring is 

 possible, or practicable, it is well to know where 

 attention to smoothing is most worth while. 

 These comments apply equally, if not primarily, 

 to structural roughnesses, many of which can be 

 prevented or eliminated in the design and drafting 

 stage. 



The rounded or peaked points of countersunk 

 rivets should project from the fair outer surface 

 of the shell by not more than the amounts indi- 

 cated in diagrams 1 and 2 of Fig. 75.B. This is 

 sufficient to insure tight rivets and to allow for 

 reasonable corrosion of the point in service. 

 Welding beads, regardless of their orientation 



