528 



DWU 



HYDRODYNAMICS IN SIITP DESIGN 



Corner of Transom ^-*— 5htp Centerlinb 



Bottom of Skeq 

 Cut Up b\j Heiqht of 

 Two DocNina Blocks! 



Rudders Toe Out 



to Suit Conver- 



of Tunnel 



ond Flow 



Outside of SUeos 



STERN ELEVATION, LOOKING FORWARD 



Fig. 67.P ABC Abch-Ttpe Stern Arrangement, Elevation from Aft 



taper, keys, and nut, the shaft extension must 

 have a diameter smaller than the root of the 

 threads for the nut. To obtain sufficient area in 

 the propeller bearing the journal becomes so 

 long that it is no longer stiff enough to remain 

 straight. This is aggravated by the additional 

 length required for the propeller nut. The load is 

 then far from uniform on the propeller-bearing 

 material, with unequal and excessive wear. 

 The obvious solution is to: 



(1) Attach the after end of the exposed propeller 

 shaft directly to the forward end of the propeller 

 with a pair of bolted flanges sufficiently substantial 

 to take the bending and torsion loads and to 

 withstand the torsional vibrations in the rotating 

 system. These flanges are indicated in broken 

 lines in Fig. 67.0 and in full lines in Fig. 74.L. 



(2) Fashion the propeller journal integral with 

 the propeller hub ahead of it. The journal can 

 then be made any size and shape desired. 



Structural plans for this type of stern should 

 call for a wide transverse belt of double-extra- 

 heavy plating abreast the 24-ft propeller and the 

 4-armed strut, extending approximately from 

 the bottom of the port skeg to the bottom of the 

 starboard skeg. Except for the forward end near 

 the top, this belt is cylindrical in shape, concentric 



about the propeller-shaft axis. The purpose of 

 this heavy plating is to: 



(a) Provide a permanently fair surface outboard 

 of the propeller tips, so as to hold the small tip 

 clearance constant throughout the life of the 

 vessel 



(b) Maintain a fair surface under the action of 

 the rotating pressure fields at the blade tips. With 

 correspondingly heavy transverse framing inside, 

 local forces are distributed over the whole stern 

 structure, instead of being permitted to deform 

 the structure in their immediate vicinity. The 

 thickness of this belt, at least double that of the 

 adjacent shell plating, combined with its trans- 

 verse curvature, renders it free from panting 

 without excessive local stiffening. 



(c) Serve as a strong connection for the hull 

 ends of the four strut arms. These ends, approxi- 

 mately straight fore and aft, are intended to be 

 passed through slots in the heavy belt plate 

 and welded to the internal framing as well as to 

 the belt plate. Fig. 73.F of Sec. 73.8 gives typical 

 longitudinal and transverse sections through the 

 belt plate, showing its attachment to the two 

 upper strut arms. 



67.19 Comments on Design of an Unsym- 

 metrical Single-Screw Stern. The function of a 



