Sec. 73.9 



FIXED-APPENDAGE DESIGN 



685 



Fig. 73.H Shape of Short Bossing for ABC Arch-Stern Ship 



exposed propeller shaft where it emerges from 

 the hull, described in Sec. 75.10. The design rules 

 are essentially the same as for long bossings, 

 except that deflection or contra-guide endings are 

 never incorporated in them because the endings 

 are too far from the propellers. A short bossing is 

 often required to provide a fairing around a 

 coupling or flange which connects the section 

 of an exposed propeller shaft to the stern-tube 

 shaft just ahead of it. Were it not for this, the 

 fairing could well be limited to a projection from 

 the main hull on the inboard side only, filling in 

 the space where eddies would otherwise form and 

 leaving only mechanical clearance next to the 

 shaft. 



On either long or short bossings some pressure 

 drag can be saved, for an insignificant increase in 

 wetted surface, by "pointing" the outer or up- 

 stream surface of the bossing upon which the 

 flow impinges. To the customary circular trans- 

 verse shape or section of the outer barrel of such 

 a bossing there is added a triangular or Gothic- 

 arch portion, about as shown in the two end 

 views of Fig. 73. H, depicting the short-bossing 

 design for the arch-stern ABC ship. A moderate 

 amount of pointing results in marked fining of 

 the leading ends of the traces of the bossing 

 flowplanes lying generally parallel to the hull in 

 that vicinity, indicated by Sections B-B, C-C, 

 and D-D of the figure. The locus of the "points" 

 of the added triangle or arch, when projected on 

 the body plan, lies parallel to the flowlines in that 

 region. At a distance from the hull these lines are 



not necessarily parallel to those marked by flow 

 indicators directly on the hull. This modification 

 adds shghtly to the stabilizing-fin area but the 

 effect is small. 



An example of a fixed appendage producing 

 somewhat similar effects is a bar of pointed or 

 arch shape, mounted in an inclined position under 

 the exposed rotating shaft of a high-speed motor- 

 boat. It creates an air- or vapor-filled separation 

 zone in the form of an inclined ditch, within 

 which the propeller shaft revolves with negligible 

 liquid friction [H. B. Greening patent apphcation]. 



On most short bossings the flow crosses the 

 bossing barrel around the shaft bearing at a 

 considerable angle. A fining of the trailing ends 

 of the bossing flowplane traces is achieved by 

 continuing the bossing termination along the 

 leeward or downstream side of the barrel. This 

 modification, along with the pointed arch, is 

 incorporated in the short bossing of the ABC arch 

 stern, drawn in Fig. 73. H. 



To save weight and displacement, exposed 

 propeller shafts are sometimes run through the 

 shell plating simply by cutting a clearance opening 

 in the plating. The stern-tube bearing and the 

 fittings pertaining to it are then installed entirely 

 within the fair lines of the hull. The recess thus 

 presented, filled as it is with the shaft and with 

 partly inert water, presents no sensible inter- 

 ference to the flow. A small fairing may be fitted 

 inboard and astern of the protruding shaft, 

 mentioned in a preceding paragraph of this section 

 and illustrated in Fig. 75.1 of Sec. 75.10. Separa- 



