708 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 74.3 



having a diameter as large as that of the hub, and 

 following the direction of the streamlines for the 

 ship flow in the \'icinity. 



Wherever practicable, it should be possible to 

 remove propellers, dismantle propeller shafts, and 

 examine propeller bearings without disturbing or 

 removing rudders or diving planes mounted near 

 them. 



It is not necessary that the axes of offset rudders 

 be exactly vertical or that those of multiple 

 rudders be parallel, if their hydrodynamic per- 

 formance can be improved thereby. Mechanical 

 simplicity in the steering gear need not be a 

 determining factor in cases of this kind. 



With regard to the placing of rudders in regions 



Projected Rudder Area at Full Anole 



DWL 



Propeller- Disc Positions 

 Propeller Outflow- Jet Section 



\ 



On a Stern of 

 Normal Form the 

 Flow is Aft, Upward, 

 ond Inward. The 

 Propeller Outflow 

 Jets, Marked b'y 

 Broken Lines, Follow 

 This Flow 



Opposite the Tail of the Rudder 



Positive Cleorance from Hub 'as 5hown 

 Desiroble But Not Necessary 



Schematic Outlines of_-— 

 Propeller Jets, Neqiectinq 

 Presence of Ship 



Rudder Ancjle 



Broken Lines 

 Represent Sche- 

 moticQlly the Sec- 

 tions of the Pro- 

 peller Outflow Jets 

 Opposite the 

 Rudder Positions 



Rudders May Be 

 Inboard or Outboard 

 of the Shafts o 



For on Original Instal- 

 lation of this kind, 

 Propeller and Rudder 

 .Should 

 - Separated 

 b^ Greater 



FiG.|^74.B Positions ' of Multiple Rudders with 

 Respect to Outflow Jets 



of good flow, where they can do what is e.xpected 

 of them, the design rule embodied by W. J. M. 

 Rankine on page 95 of his 1866 treatise on 

 "Shipbuilding: Theoretical and Practical" is as 

 applicable today as the day it was written: 



"It is also necessary that the rudder should be im- 

 mersed, not in a mass of eddies dragging behind the ship, 

 but amongst particles of water whose motion, relatively 

 to the ship, consists in a steady flow astern. Hence the 

 same fairness and fineness of the water-lines and buttock- 

 lines of the afterbody which are essential to speed and 

 economy of power are essential to good steering also." 



74.3 Single or Multiple Rudders? In a 



normal form of twin-screw stern a single rudder 

 hung between two propellers usually angles far 

 enough so that a considerable portion of the 

 trailing area of its blade swings into the projected 

 disc area of one or the other of the propellers. 

 This is true even though account is taken of the 

 customary inward and upward shifts of the 

 twin-screw propeller outflow jets, conforming to 

 the general direction of flow in the vicinity, 

 and to the contraction in those jets. Diagram 1 

 of Fig. 74. B illustrates schematically the situa- 

 tion described. 



A single rudder is sometimes placed between 

 two screw propellers which lie so far apart trans- 

 versely that the rudder blade does not swing 

 appreciably into the outflow jet of either at its 

 extreme range of angle. Although not always 

 lacking in steering and turning action, such a 

 rudder is liable not to function adequately, no 

 matter what its area. 



It is possible that the single rudder, if lying in 

 a "strong" flow having a small positive or a 

 slight negative wake velocity, produces the 

 desired lateral force. The same rudder, Ij'ing in a 

 "weak" flow, with a large positive wake velocity 

 and a small speed of advance, is inadequate to 

 maneuver the ship. Smaller twin rudders, lying 

 close to or within the outflow jets, are indicated 

 for situations of this kind. Here again, depicted 

 by diagram 2 of Fig. 74. B, account is taken of 

 whatever lateral or vertical displacement, or 

 both, results from the fact that the propeller 

 outflow jot contracts and that it follows the 

 direction of flow under the hull. 



Efforts to achieve the same efi^ect with so-called 

 biplane or triplane rudders, when a single blade 

 is found inadequate, may be disappointing. 

 Multi-blade rudders, operated by a single stock 

 and steering gear, are usually used. These repre- 

 sent a simpler alteration than fitting twin rudders 



