Sec. 73.9 



FIXED-APPENDAGE DESIGN 



683 



the rate of propeller rotation is known and the 

 propeller power is fixed within rather narrow 

 limits, the size of shafting, bearings, and other 

 naechanical parts to be housed by the bossings 

 are determined within equally close limits. In 

 combination with the room needed for internal 

 access, these features fix the minimum size of 

 certain parts. 



The second step is to settle upon the exact 

 nature and function of the bossing. Is it to be of 

 the short or the long type? Is it solely for fau'ing 

 and for protection of the shafting and bearings? 

 Is it to be of the deflection type, possibly mth 

 an auxihary support strut for the propeller bear- 

 ing, as in diagram 3 of Fig. 36. H? It may be that, 

 under a broad, flat stern, the bossing is to be called 

 upon to serve as a docking keel as well as a bossing. 



The third step is to find the direction of flow 

 at and near the hull surface in the region to be 

 occupied by the bossing. If of the long type, the 

 bossing is in reahty an extension of the water- 

 tight hull. It is not an external appendage which 

 can be modified later, if and when desired. 



Because of the length and the appreciable 

 volume of a long-type bossing, the flow around it 

 when in place may be significantly different than 

 that indicated around the bare model when the 

 initial flow tests are made. For instance, suppose 

 that the principal plane of the bossing has been 

 placed over the trace of a flowline observed on 

 the model hull without the bossing. The water 

 which formerly flowed easily along the hull 

 surface is now displaced by the bossing volume. 

 It has to flow somewhere else, and in so doing it 

 may take a route that is not conducive to good 

 flow or good propulsion. A second flow check on 

 the model is therefore indicated, to be followed 

 by wake observations in the propeller-disc 

 position abaft the bossing, including records of 

 the directions of the velocity vectors. A further 

 check is called for on the uniformity of flow as it 

 leaves the upper and lower surfaces of the bossing. 



If a long bossing is not properly shaped and 

 positioned, the flow around it may contain large 

 corkscrew vortexes. It may develop a combina- 

 tion of longitudinal and transverse eddies which 

 cause the flow at a given point abaft the bossing 

 to fluctuate with time. An unsteady flow of this 

 kind is detected on a model only with instruments 

 which are sensitive to these variations. 



On high-speed vessels, especially those of light 

 draft, where the head of water over the bossmg is 

 necessarily small, model tests of proposed installa- 



tions have even revealed an extensive uncovering 

 of the upper surface of the bossing, exposing parts 

 of the upper blades of a propeller carried by it. 



That portion of a bossing of any type which 

 encloses a propeller shaft is fixed at its after end 

 by the position of the propeller bearing. At its 

 forward end some latitude in position with respect 

 to the emerging shaft is permissible. This depends 

 upon the size of the bossing at that end, the 

 position of the propelling machinery, and other 

 internal arrangements. If the bossing is for fairing 

 and shaft protection only the bossing termination 

 slope /3(beta) and the traces of the bossing body 

 along the hull are established in such manner that 

 flow takes place around it with the least possible 

 interference. It is often difficult to estimate the 

 local directions and traces of this flow along the 

 hull. Lines of flow taken on a model, especially 

 with a temporary rod in place to represent the 

 bare propeller shaft, are most helpful at this stage. 

 Better still are flow directions, indicated by flags 

 or vanes at a distance from the hull equal to the 

 mean projection of the bossing at each station. 



It is customary first to sketch the bossing shape 

 on the body plan by stations, sections, or frames. 

 These are supplemented by the traces of bossing 

 flowplanes, flat or slightly curved, passed through 

 the bossing at varying distances from the adjacent 

 main hull. The traces correspond generally to 

 intersections of the stream surfaces in the bound- 

 ary and adjacent layers, diagrammed in Figs. 

 36. D and 36. H. The transverse slope of a bossing, 

 intended for fairing only, is approximately 

 normal to the slopes of the section lines in the 

 vicinity. This reduces the wetted area to a mini- 

 mum and avoids reentrant angles less than 90 deg. 

 Proposals have been made in the past, requiring 

 such rigid adherence to this rule that the bossing 

 plane is curved in transversely, so as to remain 

 normal to the section lines as the latter become 

 steeper with distance aft, toward the propeller 

 [Volker, WRH, 1 Jun 1934, pp. 131-132]. 



When selecting the transverse slope it is well 

 to make sure that no part of the bossing lies close 

 to the free surface of the water in any operating 

 condition, if efficient propulsion and freedom 

 from vibration is desired. 



Section shapes for the bossings of twin-screw 

 vessels are to be found in the following: 



(a) Sadler, H. C, "The Effect of Bossing Upon Re- 

 sistance," lESS, 1908-1909, Vol. LII, pp. 147-159 

 and PI. IX. Discusses effect of large and small 

 termination angle /3. 



