746 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 75.10 



When the propeller shaft bearing is placed 

 abaft the propeller, as is done on many high-speed 

 motorboats and on the arch-stern design of the 

 ABC ship, this bearing may be carried by a 

 partial skeg, a deep horn, or a fixed rudder post, 

 extending down abaft the wheel. The rudder is 

 usually mounted as a hinged flap along its after 

 edge. The fairing assembly is then composed of 

 the following parts, reckoned from forward: 



(1) Leading fairing, rotating with the shaft and 

 the propeller hub 



(2) Propeller hub 



(3) Propeller shaft bearing housing with rope 

 guard and water scoop on leading end 



(4) Fairing for bearing housing, which may 

 extend aft into the flap or moving portion of the 

 rudder. 



On the ABC ship, with twin rudders, a fixed 

 fairing is mounted abaft the strut barrel or hub 

 supporting the propeller bearing, indicated in 

 Fig. 74.L. 



75.10 The Fairing of Exposed Shafts at 



Mechanical Clearance 

 Around Shaft 



Emergence Points. Exposed shafts which 

 emerge, as do most of them, at small angles with 

 the adjacent hull surface represent a problem in 

 fairing. The simplest and the cheapest method is 

 to omit the fairing altogether, build a watertight 

 recess within the hull, up to the after stern-tube 

 bearing, and pass the rotating shaft out through 

 a clearance hole in the shell plating. 



A reasonable amount of fairing, with no increase 

 in displacement volume and little added cost and 

 weight, is achieved by adding a pair of removable 

 shaped plates to enclose a free-flooding space 

 between the hull and the shaft. These extend for 

 a short distance abaft the hull opening, indicated 

 at 1 in Fig. 75.1. If there is no enlargement or 

 flange on the shaft which must be drawn past the 

 fairing, the complication of bolting on a non- 

 watertight fairing which must be removed fre- 

 quently for examination of the hull underneath 

 is obviated by extending the framing locally and 

 incorporating the fairing plate into the shell 

 proper, in the manner shown at 2 in Fig. 75.1. On 

 a large or medium-size vessel the free-flooding 



Not Less Thon 4 and Preferoblu^ 

 5 Shaft Diameters — Section at A-A 



^ with Near Side FairinQ 



Plate Removed J- 



Section ot B-B 

 with Near Side 

 Plate. f?emoved c. 



Fia. 75.1 Two Types of Hull Fairing Around Exposed Propeller Shafts 



