G78 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 73.5 



may be studied and the merits of each may be 

 assessed for every new design as it arises. 



Whatever the advantages of bossings for any 

 particular application, there is a low limit to a 

 bossing size unless the latter is to be closed up 

 completely from the outside. The workmen who 

 have to get inside of these bossings for fabrication, 

 erection, riveting, or welding are of more-or-less 

 fixed size, as are those who must take care of 

 repairs and maintenance for the life of the vessel. 

 For these reasons bossings are little used on small 

 vessels. 



Table 73. a summarizes the advantages and 

 disadvantages inherent in the great majority of 

 strut and bossing installations. The comments 

 apply to single-screw and triple-screw ships as 

 well as to the arrangements customary on twin- 

 and quadruple-screw vessels. 



It is difficult to make any general statements 

 concerning reductions in shaft power to be 

 achieved by the use of either shaft struts or 

 bossings in any particular case, assuming that 

 alternative designs benefit from the same amount 

 of study, experimentation, and development 

 [Mandel, P., SNAME, 1953, pp. 466-468]. The 

 designer of a large or important vessel, or one 

 which is to serve as the lead ship for quantity 

 production, is believed justified in carrying alter- 

 native strut and bossing designs through to the 

 model stage at least. 



73.5 Strut Design for Exposed Rotating 

 Shafts. If the weight displacement of large 

 bulky bossings is undesirable, propeller shafts are 

 left exposed, carried by water-lubricated bearings 

 supported from the hull by double arms set in 

 the form of a Vee. For certain applications, single 

 arms have been employed, especially when they 

 are short and can be given adequate rigidity. 

 Parsons used a number of them successfully on 

 the three shafts of the Turbinia in the 1890's 

 [SNAME, 1947, Fig. 10, p. 105]. However, more 

 modern experience, \vith larger sizes and higher 

 shaft powers, indicates that when the single 

 arms are longer than the maximum strut-hub 

 diameter they suffer from: 



(a) Lack of lateral stiffness 



(b) PossibiUty of resonant lateral vibration as a 

 cantilever weighted at the outboard end 



(c) Excessive lateral loading by Magnus Effect 

 on the shaft, or hydrodynamic lift due to cross 

 flow when turning, or both. Fractures of modern 

 single-arm struts in service, due to transverse lift 

 produced by cross flow when turning, to vibration, 



and to other causes, indicate the wisdom of 

 avoiding them unless the arms are shorter than 

 the limit given. 



The proper or best shape of the strut-arm 

 section has been the subject of long and careful 

 study, based upon structural as well as hydro- 

 dynamic considerations. The differences in drag 

 between the various shapes of long-established 

 usage are small, even in proportion to the total 

 appendage resistance. It is probably more 

 important that the strut arm as installed conform 

 closely to some specified shape, worked out by a 

 long development process, than that the shape be 

 of a particular kind or have special characteristics. 

 The section delineated by D. W. Taylor, used in 

 U. S. Naval vessels for many decades past, could 

 have been shorter for the same thickness, with a 

 c/tx ratio of 6.0 instead of 7.5. This would have 

 involved cutting off only the tail; in fact, this 

 portion often disappeared anyway as a result of 

 erosion, pitting, or rusting in service. 



Excellent replacements are the: 



(a) EPH or Ellipse-Parabola-Hyperbola section 

 developed by the David Taylor Model Basin 

 during World War II. This has a trailing edge 

 sufficiently blunt to get rid of the previous 

 difficulties with fabrication and corrosion of the 

 slim Navy Standard strut. It is not so blunt as 

 to cause objectionable separation and eddy 

 buffeting of the trailing portion in the absolute 

 sizes normally used for shaft struts. 



(b) Section proposed by P. Mandel [SNAME, 

 1953, pp. 468-469], with a c/tx ratio of 4.3. 



The comparative proportions and shapes of the 

 three sections mentioned, plus an NACA sym- 

 metrical section, are shown graphically in Fig. 

 73. C. The abscissas and ordinates for constructing 

 the section outlines accurately are tabulated by 

 Mandel [SNAME, 1953, Fig. 3, p. 468]. Many 

 other characteristics are described by him, 

 together with design considerations involving 

 both structural and hydrodynamic features. 



Of far more importance than the shape of the 

 strut-arm section is the placing of this sectiori in 

 the local direction of flow so that in service it runs 

 with no yaw angle or angle of attack. It is true 

 that the local direction of flow changes with yaw 

 during steering, with rate of swing during turning, 

 and with ship position during wavegoing. It may 

 also change with displacement, draft, and trim. 

 Nevertheless, it probably remains constant within 

 a degree or so at all normal operating speeds in 



