HYDRODYNAMICS IN SHIP DESIGN 



-ae.'O Designed Woterhne 



Sec. 74.15 



2:0- 



Centerline Bultock- 



Fbrward Edqe of Horn, if a ,,' ' ^ , 



c u/ ij ». 5.5 Trp oubmerqence 



Forging or Weldment 1 "^ = 



Fig. 74.K Details of Aftfoot, Propeller Aperture, Rudder Support, and Rudder for ABC 



Transom-Stern Ship 



abaft a single propeller would be required to 

 give the arch-stern ship as good maneuvering 

 characteristics as the alternative transom-stern 

 ship. The adverse effect of the increased lateral 

 area of the twin skegs would, it was believed, be 

 offset by the increased lateral force which the 

 twin rudders would, acting as flaps, generate on 

 the portions of the fixed skegs just ahead of them. 

 Further, the addition of an underhung foil on 

 each rudder would provide the quick steering 

 and turnmg response needed for running in the 

 confined waters at each end of the ship route. 

 When finally sketched, as in Fig. 74. L, the total 

 movable-blade area of both rudders was 346.74 ft^. 

 This was 346.74/13,260 or 0.0261 of the product 

 L{H), an increase of 26 per cent over the same 

 ratio for the single, centerline-rudder design of 

 the transom-stern ship. 



These twin rudders are in effect only the 

 trailing edges of the two arch-stern skegs. Each 

 is arranged to swing about a vertical axis 8 ft 

 fonvard of what would be the skeg ending and 

 each carries an underhung foil extending below 

 the bottom of the skeg termination. The rather 

 unusual horizontal sections of the tail portions of 

 these rudders arise from a deliberate continuation 



of the inside skeg waterUnes close to the antici- 

 pated flowlines in the tunnel. The convergence of 

 the outside waterUnes is such as to terminate the 

 skegs and rudders in trailing edges that he as 

 close as possible to the outflow jet of the propeller. 

 The resulting thick and unsymmetrical sections 

 leave some uncertainty as to the hydrodynamic 

 behavior of the hinged tail portions, when acting 

 as flaps hinged to the main skegs. In view of the 

 lack of information about them, a series of maneu- 

 vering tests with a model is definitely indicated 

 before the arch-stern design is carried beyond this 

 preUminary stage. 



The selection of the proper thickness ratios and 

 the horizontal section shapes for the single, 

 streamhned rudder of the transom-stern ship 

 involves no .such hmitations as those just de- 

 scribed for the arch-stern design. To be sure, the 

 thickness ratio for the single rudder is to some 

 extent governed by the thickness of the fixed 

 horn which is necessary to afford adequate 

 rigidity to the lower rudder bearing. This is a 

 matter of structural analysis and calculation and 

 as such it is outside the scope of the book. The 

 thicknesses indicated on Fig. 74. N of Sec. 74.17 

 are estimates only, based upon similar installa- 



