Sec. 74.16 



MOVABLE APPENDAGE DESIGN 



26' Designed Woterlii 



729 



Fig. 74.L Vertical Centerplane Section Through Shaft, Propeller, and Strut-Bearing 

 Support of ABC Arch-Stern Design 



tions with fixed horns of somewhat less relative 

 depth. 



Before the tinie came to sketch the streamlined 

 rudder for the transom-stern ABC ship it was 

 decided to use a form of contra-rudder. A fair- 

 form rudder with a flat meanline plane, even when 

 designed as a unit with the fixed horn, involves 

 design problems which are adequately covered 

 by P. Mandel [SNAME, 1953, pp. 474-490] and 

 by others listed at the end of Sec. 74.8. They are 

 not further discussed here. The layouts of the 

 contra-rudder and the contra-guide horn, as 

 applying to the ABC transom-stern design, are 

 described in Sees. 74.16 and 74.17 which follow. 



74.16 Design Notes for a Centra-Rudder. 

 Strictly speaking, the contra-flow feature on a 

 rudder is not a functional part of the rudder as a 

 control surface but is an energy-recovering and a 



thrust-producmg device. In fact, it need not be 

 movable, as is the rudder. Typical horizontal 

 sections for a compound rudder with contra- 

 features in the fixed portion only, and in both the 

 fixed and movable portions, are shown by W. P. A. 

 van Lammeren [RPSS, 1948, Fig. 55, diagrams 

 e and f , respectively, p. 100]. However, as a rudder 

 is required with normal screw propulsion, the 

 friction drag of a separate fixed surface is elimin- 

 ated by incorporating the contra-flow feature in 

 the rudder. For the design of this feature, the 

 rudder is considered to remain at its zero or 

 neutral angle. 



The design problem consists of: 



(a) Establishing a series of horizontal planes 

 above and below the propeller axis, described for 

 the contra-guide skeg ending in Sec. 67.22, for 



