CHAPTER 75 



The Problem of Hull Smoothness arid Fairing 



75.1 General Considerations; Definitions .... 738 



75. 2 The Importance of Smoothness and Fairing . 738 



75.3 Specific Smoothness Problems on the Shell 



Plating 739 



75.4 The Utilization of Casting or Welding 



Fillets 742 



75 . 5 Inside Comers Requiring Negligible Fillets . 742 



75.6 The Fairing of Appendages in General . . 742 



75.7 Recessed Lifting and Mooring Fittings . . 743 



75 . 8 Fairing the Enlargements Around Exposed 



Propeller Shafts 744 



75.9 The Fairing of Propeller Hubs in Front of 



Simple or Compound Rudders 745 



75 . 10 The Fairing of Exposed Shafts at Emergence 



Points 746 



75 . 1 1 The Termination of Skegs and Bossings . . 747 



75.12 Precautions Against Air Entrainment . . . 747 



75.13 Design Notes for Shallow Recesses .... 748 

 75 . 14 Practical Problems in Achieving Under- 

 water Smoothness and Fairness on a Ship 749 



75.1 General Considerations; Definitions. 



Smoothness as related to hydrodynamics is a phys- 

 ical characteristic of the underwater surfaces 

 of a vessel, regardless of the size, configuration, 

 or location of those surfaces. It applies to the hull, 

 the propulsion devices, the control surfaces, and 

 all appendages, hence is properly considered as a 

 general item applicable to all the external ship 

 elements. Fairing, fillets, transition pieces, fillers, 

 and their equivalents likewise apply to all the 

 external ship elements, hence it is appropriate to 

 consider them here as a class. 



Smoothness, as used in this chapter, denotes the 

 absence of small irregularities of the kind asso- 

 ciated with rough or fiaked paint coatings, rust, 

 pits, rivet points, and welding beads. It indicates 

 also that a surface has the proper curvature and 

 that it is free of waviness such as is often en- 

 countered at and between internal frames or 

 stiffening members. It takes for granted the 

 structural discontinuities inherent in the use of 

 lapped seams and butts, raised strakes, doublers, 

 and other irregularities involved in applying the 

 shell plating in relatively small pieces. Fairing, 

 aside from its frequent use as a general term, 

 applies principally to portions of generous radius, 

 expressed in multiples of the shell-plating thick- 

 ness, worked into or apphed to various parts to 

 insure easy water fiow around them. Fillets are 

 defined as the roundings worked into coves or 

 internal corners or castings, weldments, and other 

 structural members, as well as the transition 

 pieces added to bridge gaps and discontinuities in 

 the hull and its appendages. 



In general, smoothness is sought and required 

 in an effort to avoid unnecessary power losses 



due to excess friction resistance. Its possible 

 effect in reducing underwater noise is not dis- 

 cussed here, although that may become a factor 

 in fishing and other operations of the future. 



Fairings, fillets, and fillers are applied primarily 

 to avoid high dynamic pressures, cavitation, or 

 separation. In some cases these discontinuities 

 in the flow create undesirable disturbances ahead 

 of propulsion devices and control surfaces. In 

 almost every case the occurrence of high Ap's, 

 separation, or cavitation increases the drag of 

 the appendage. It is pointed out elsewhere that 

 the added drag at each appendage may be 

 insignificant compared to the total drag of the 

 ship. Nevertheless, the cumulative effect may be 

 considerable, sufficient to neutraUze the improve- 

 ment gained from some special design of hull or 

 propulsion device. 



The increased drags due to dynamic pressure, 

 cavitation, and separation are pressure effects and 

 as such vary as the square of the relative velocity 

 of water flow. The effects of inadequate fairing 

 and filleting therefore increase rapidly with the 

 speed of the vessel. What may be accepted as a 

 negUgible increment of pressure drag on a slow 

 or medium-speed cargo vessel for the sake of 

 economy of construction becomes a matter of 

 inefficient propulsion on a high-speed finer, 

 where an increment of first cost is easily justified 

 by a saving in fuel over many years of operation. 



75.2 The Importance of Smoothness and 

 Fairing. Nature goes to considerable pains to 

 work fairings into many of her creatures. Man 

 can hardly do less if he is far-sighted and looking 

 for improvements. A close study of many natural 

 structures reveals the hitherto little-recognized 



738 



