Sec. 7''.P 



HULL SMOOTHNESS AND FAIRING 



745 



that this maximum diameter occurs in the assem- 

 bly the greater must be the slope of the fairing 

 abaft it, unless a blunt-ended cap is used. 



For high-speed ships, a propeller hub fairing 

 long enough to eliminate entirely the^'swirl core 

 described in Sec. 23.14 and illustrated in Fig. 

 23. K would probably have to extend for one or 

 more propeller diameters abaft the hub. This is 

 on the basis that the core is generated entirely 

 by the water set in rotation by friction around 

 the hub. In practice, such an appendage is out 

 of the question, especially as the propeller hub 

 fairing must also clear any rudder placed in the 

 outflow jet. There are two compromises available 

 here. One is to give the profile of the propeller 

 hub fairing a range of slope angles from about 

 15 deg to a maximum of about 22 deg, terminating 

 the fairing in an ogival form having a radius of 

 about 0.1 the radius of its larger end. Variations 

 of such a form are drawn in full and broken lines 

 at 2 in Fig. 75. H. The other compromise, on the 

 basis that the swirl core is in reality the combined 

 vortex of the several blade-root vortexes, is to 

 terminate the fairing in a square end at about 

 0.7 to 0.5 the propeller hub diameter d. This is 

 also shown on Fig. 75. H. 



Further comments on propeller-hub fairings 

 and caps are given in Sec. 70.14. 



Exposed sleeve-type couphngs on a propeller 

 shaft are partly faired by trimming off the ends 

 of the sleeves themselves. This has the added 

 advantage of a gradual transition in the combined 

 rigidity of the sleeve and the shaft. The result is 

 a stress concentration in the shaft of diminished 

 magnitude at the ends of the sleeve. 



A flange-type shaft coupling exposed to the 

 water all around may be enclosed in a substantial 

 casing of fair form, about as illustrated in Figs. 

 73. H and 74. L. If the fairing rotates with the 

 shaft as in the latter figure, it may be filled with 

 wax or other preservative to protect the mechan- 

 ical parts of the coupling. If it is stationary, as in 

 Fig. 73. H, it can be "pointed" slightly on the 

 upstream side and given a streamlined tail of 

 sorts on the downstream side, following the 

 general shape of the short bossing diagrammed in 

 that figure. So far as known, it makes little 

 hydrodynamic difference whether the transition 

 from the stationary to the rotating parts, and 

 vice versa, occurs in a parallel or cylindrical 

 portion of the enlargement, or at its beginning or 

 ending, next to the shaft surface. Gaps required 

 for working clearances between fixed fairings and 



rotating shafts need not be large and these dis- 

 continuities need have no detrimental effects. 



The ends of non-ferrous journal sleeves shrunk 

 onto steel shafts generally lie within other fair- 

 ings. If not, they may abut rubber or other 

 protective coverings around the portion of the 

 shaft which would otherwise be exposed to sea 

 water. In any case they are relatively thin and 

 need little or no fairing of their own. 



The fairing of intermediate strut hubs follows 

 the general lines suggested in other paragraphs of 

 this section, as does the fairing of propeller hubs 

 abaft skegs or bossings. 



Water-lubricated shaft bearings require a 

 continual longitudinal circulation of liquid through 

 the bearing when the ship is underway. An 

 opening around a rotating shaft and just inside 

 a fixed leading-edge fairing of oval or ogival shape 

 is in a region of -fAp, sufficient to force the water 

 through. Any — Ap occurring abaft a trailing 

 fairwater helps to draw water aft through the 

 bearing. The fairing ahead of a shaft-bearing hub 

 may rotate with the shaft, as does the propeller 

 hub of the arch-stern ABC ship in Fig. 74. L. A 

 beU-mouthed strip around the bearing hub, shown 

 in that figure, then serves as a scoop for the lubri- 

 cating and cooling water. A hole in the center of 

 the fixed fairing abaft the bearing hub serves to 

 draw the water out of the after end. 



75.9 The Fairing of Propeller Hubs in Front 

 of Simple or Compound Rudders. For a propeller 

 placed immediately ahead of a simple or com- 

 pound-type rudder, or ahead of a rudder hung on 

 a fixed rudder post, the propeller hub is faired 

 neatly by a swelling of the fixed portion, by a 

 projection extending forward from the post, or 

 by both. A rudder of the balanced type, with its 

 stock axis intersecting or lying close to the pro- 

 peller shaft axis, can be notched moderately on 

 its forward edge, cutting into the balance portion, 

 to provide clearance for a conical or ogival 

 propeller fairing cap of reasonable length. 



One form of fixed fairing for a propeller hub, 

 easily worked into a deep horn or into the fixed 

 portion of a compound-type rudder, is embodied 

 in the transom-stern design of the ABC ship, 

 illustrated in Figs. 66. Q, 67.U, 74.K, and 74.N. 

 Rope and cable guards to protect the opening 

 between the rotating propeller hub and the fixed 

 fairing, as well as removable sections of the latter 

 to facihtate taking off the propeller shaft nut and 

 the propeller, are readily incorporated in a fixed 

 fairing of this type. 



