712 



mL 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 74.4 



_pwL/^"|gr~ 



Aperture 



Abreast Propeller 



Blode Tips Permits 



Ap Leokoqe 



From One Side 



of Skeg to the 



Other 



Wide Hinqe Slot 

 Permits Excessive 

 ^p Leakage from 

 One Side of 



Rudder to 

 the Other 



Large Aperture Forward of o Stern Ruddei 

 •of Adequate Area Provides Room 

 For o Short Horiiontal 

 Circulation Path and Insures 

 Rapid ffesponse to Angling 

 Rudder 



Fore-and-Aft Gap Length 



Should Be at Least 

 0.3 ond Preferobly 

 0.4 of Greatest Fore- 



Fig. 74.D Apertures and Gaps Ahead op Ship 

 Rudders 



good circulation path for the water. It is too large 

 as a leakage gap. It probably does more harm in 

 steering than the benefit it affords in reducing 

 propeller vibration. 



Diagram 1 of Fig. 74. D illustrates a design, 

 rather common in years gone by, in which the 

 provision of working clearances for removal of the 

 pintles and their bearings, and for lifting out the 

 whole rudder, was apparently more important 

 than ease of steering. The large leakage gap 

 detracts from the usefulness of the rudder and 

 diminishes the lateral force built up on the ad- 

 jacent hull by the angled rudder. 



Rounding the outline or profile corners of 



rudders is by no means a necessity, provided fair 

 flow is reasonably certain around the rudder at 

 all rudder angles and under all operating condi- 

 tions. 



For practical reasons, such as to provide bottom 

 clearance Avhen docking, it is often necessary to 

 terminate the bottom of a rudder in a horizontal 

 plane parallel to and just above the baseplane. 

 When such a rudder is mounted under the stern 

 of a ship, it may Lie in a region of flow having an 

 appreciable upward component of velocity, indi- 

 cated in Fig. 25.K. The after ends of the lower 

 rudder sections then project into the flow lines, as 

 does the af tfoot in that figure. If a reduction of area 

 at the bottom can be made up by an increase 

 elsewhere on the blade, the after lower edge of 

 the rudder may be cut up or the bottom of it may 

 be sloped to conform to the flow hues in this 

 region. These changes should save some pressure 

 drag during the greater portion of the time, when 

 the rudder is serviiig only to steer the vessel. 



It is good design to shape a close-coupled rudder 

 (at either bow or stern) as a continuation of the 

 adjacent hull surface. Nevertheless, this procedure 

 can become detrimental if the adjacent hull is 

 full and the thickness ratio of the rudder becomes 

 large or if the sides of the rudder have much slope 

 [Denny, M. E., lESS, 1934-1935, Vol. 78, p. 411]. 

 In any case, the free or swinging edge of the 

 rudder, the one that is do^vnstream when the 

 rudder is steering, should generally be tapered 

 to a reasonable thickness and not terminated 

 bluntly. 



Wherever and whenever practicable the leading 

 edge of a rudder is recessed into a groove in the 

 trailing edge of the horn, the skeg, or the keel 

 supporting it. This is partly for fairing but mostly 

 for closing the hinge gap against detrimental 

 leakage. Further steps to close the gap are de- 

 scribed in Sec. 74.14 and illustrated in Fig. 73. K 

 of Sec. 73.14. 



The fact that positive clearance above the 

 baseline for the foot of the rudder is often man- 

 datory should never deter the naval architect 

 from extending it below the baseluie if the needs 

 of the situation require it. The rudders of many 

 Chinese junks slide along a diagonal axis, parallel 

 to that of the stock, so that they extend well 

 below the baseplane when at sea. The portable 

 rudders of small sailboats almost invariably 

 project below the baseplane when they are 

 shipped. The successful use of fixed rudders pro- 

 jecting below the baseplane on many vessels, 



