520 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.14 



Fig. 67. K Profile View of Transom-Stern Model for ABC Ship 



volume by hollowing the main portion of the 

 hull at and forward of the propeller position. 



The upper after corner of the rudder is rounded 

 off to provide a moderate gap below the free-water 

 surface and to avoid breakdown of the rudder at 

 large angles. 



These features are pictured in photographs 

 of TMB model 4505, representing the transom- 

 stern ABC ship, reproduced here as Figs. 67. J 

 and 67.K. 



A further modification of this shelf or transom 

 design, particularly for a high-speed craft, limits 

 the single centerline skeg to a sort of vertical 

 bossing which terminates just below the shaft. 

 Carrying this design one step further cuts the 

 bossing back to a short length of fairing where 

 the shaft emerges from the hull and supports the 

 propeller bearing by a single-arm or double-arm 

 strut, with intermediate struts as may be neces- 

 sary. 



67.14 Stern Forms for Twin- and Quadruple- 

 Screw Vessels. The Taylor Standard Series 

 stern, delineated in Fig. 51.A [S and P, 1943, 

 Figs. 185 and 186, pp. 182-183], was adapted 

 from the British twin-screw armored cruisers of 

 the 1895-1905 era but it still represents an excel- 

 lent basic form for twin-screw hulls of today. It 

 is adaptable to several types and shapes of single 

 centerline rudder, including the spade or under- 

 hung rudder, as well as to rather wide variations 

 in profile. It may be used with open struts to 

 support the twin shafts or twin bossings may be 

 superposed on it. 



Lack of space precludes the insertion of sections 

 in which a second alternative stern, with twin 



screws, could be designed for the ABC ship, as 

 an additional example of hydrodynamic design 

 procedure. However, a twin-screw stern (excluding 

 bossings) usually poses fewer shaping problems 

 and interferences, and it may be expected to 

 have less bare-hull resistance than an equally 

 good single-screw stern. 



Save for the cases where quadruple screws are 

 carried underneath a flat-bottomed or transom- 

 stern hull, with the shafts supported by struts, or 

 for the cases where two of the four quadruple 

 screws are carried by deep skegs, the design of 

 a normal form of quadruple-screw stern follows 

 that of the normal twin-screw stern rather closely. 

 The inboard propellers are placed in about the 

 same positions as for twin screws. The outboard 

 propellers are mounted farther forward and 

 farther from the centerline, usually far enough so 

 that their discs are clear of the inboard propeller 

 discs when both sets are projected on the plane 

 of the maximum section. Whether the four 

 propellers project far beyond the above water 

 sides of the hull, as on the light cruisers of the 

 (U.S.) Omaha class of the 1920's, or whether they 

 lie entirely below the hull as on a wide transom- 

 stern design, is more a matter of the general hull 

 shape than of the underwater hull design. Rarely, 

 if ever, is the hull shape modified for the out- 

 board propellers, except as may be necessary to 

 accommodate machinery parts inside. The four 

 shafts are carried in bossings, in open struts, or in 

 any desired combination of the two. 



The positioning of screw propellers relative to 

 the hull, especially the matter of tip clearances, 

 is discussed further in Sees. 67.24 and 69.3. 



