Sec. 75.12 



HULL SMOOTHNESS AND FAU^ING 



747 



space is inspected by entering the hole through 

 which the shaft is withdrawn. 



The best method of fairing at this point, and 

 one which may eliminate an intermediate strut 

 otherwise required, is to build a short bossing out 

 from the hull, terminating in a single-arm strut 

 member carrying a shaft bearing. An excellent 

 fairing of this type was incorporated in the Bath- 

 designed World War I destroyers of the U. S. 

 Navy. Despite the small clearances around the 

 shaft tube within the ship, it is preferable that a 

 short bossing of this type be made completely 

 watertight and an integral part of the hull. 



Design notes and rules covering these short 

 bossings are given in Sec. 73.9. 



75. 1 1 The Termination of Skegs and Bossings. 

 The endings of short skegs and short bossings are 

 subject to separation drag if the slopes of the 

 surfaces of these appendages exceed certain 

 angles with the flowlines. So far as known, the 

 critical values of these angles depend primarily 

 upon the hydrostatic pressure. They are of the 

 order of 13 to 14 deg at the surface and perhaps 

 20 deg at a submergence depth of 20 ft or more; 

 see also Sec. 46.2. 



The terminations of deep skegs on single-screw 

 vessels, of large skegs on multiple-screw vessels, 

 and of long bossings usually lie immediately ahead 

 of the propellers. Their terminations must be 

 fine else the eddying and other disturbances 

 created behind them are carried directly into the 

 propeller discs without an opportunity for smooth- 

 ing out the flow. The square, blunt sternposts of 

 cheaply built cargo vessels are particular offenders 

 in this respect, despite the slow speed of both the 

 ship and the propeller. Indeed, it is only because 

 of this slow speed that unfair surfaces of this 

 kind can be tolerated. 



The flexibility afforded by modern knowledge 

 and techniques in the casting of structural 

 members or in the assembly of these members as 

 weldments makes available to the ship designer 

 a ready means of fining the terminations of skegs 

 and bossings. Indeed, a shining example of 

 excellent bossing terminations, even by modern 

 standards, was designed and built into the S.S. 

 Talamanca class by the Newport News shipyard 

 in about 1930. Fig. 73. G is traced from some of the 

 Newport News drawings, with the permission of 

 the Newport News Shipbuilding and Dry Dock 

 Company. 



Sufficient rigidity can rarely be incorporated in 

 a heavy terminal member to prevent lateral 



deflection or vibration of a large skeg or bossing. 

 Since the adjacent shell plating and the framing 

 contribute a large portion of the actual rigidity 

 there is no reason why their scantlings can not 

 be increased to permit fining of the skeg or bossing 

 ending as required for easy flow into the propeller 

 position. Actually, the optimum solution of this 

 particular design problem is not necessarily to 

 stiffen the structure unduly but to shape it in 

 such a manner that the periodic and transient 

 forces acting upon it are diminished. 



The matter of shaping the endings of skegs and 

 bossings in profile to provide the necessary aper- 

 ture clearances is discussed in Sees. 67.23 and 

 67.24. 



75.12 Precautions Against Air Entrainment. 

 The phenomenon of air entrainment and its 

 detrimental effects are discussed in Sec. 20.10. 

 This section mentions methods of eliminating the 

 formation and the trapping of air bubbles around 

 the underwater hull, as well as they are known in 

 the present state of the art. 



The most direct cause of trouble on a merchant 

 ship is the multitude of air bubbles which pass 

 over the shell diaphragm of a fathometer or 

 echo-sounder, usually installed in a horizontal 

 position under the bottom. The bubbles interfere 

 with the sonic pressure waves emanating from 

 and impinging upon such a diaphragm so that 

 depth readings are not satisfactory. For smooth- 

 water operation at deep or load draft the best 

 position for such a diaphragm is well forward, say 

 in the first 0.1 or 0.15 of the length. This is ahead 

 of the point where the flowlines from the stem, in 

 the vicinity of the bow-wave crest, pass down 

 under the ship. The flow diagrams of Chap. 52 

 illustrate this feature for a great variety of hull 

 shapes. For operation in waves, especially when 

 the forefoot emerges, or for operation in ballast 

 or light-load condition, there is practically no 

 diaphragm position on the sides or under the 

 bottom which is entirely free of air interference 

 or air blanketing. 



Design rules for guarding against problems of 

 air entrainment are limited by present knowledge 

 to the following: 



(a) Avoid projections on the hull which face 

 downward and which can trap air when the bow 

 drops during pitching. A downward-facing ledge 

 as narrow as the thickness of a projecting shell 

 plate is sufficient to take some air bubbles down 

 with it. Projecting edges of fenders are worse in 



