CHAPTER 74 



The Design of the Movable Appendages and 

 Control Surfaces 



74.1 General 706 



74.2 Positioning Rudders and Planes 706 



74 . 3 Single or Multiple Rudders? 708 



74.4 Shaping the Rudder and the Adjacent 



Portion of the Ship 709 



74.5 Design Procedure for Conflicting Steering 



Requirements 713 



74.6 First Approximation to Control-Surface 



Area 713 



74.7 Determining the Proper Areas of Various 



Control Surfaces 715 



74.8 Positioning the Stock Axis Relative to the 



Blade; Degree of Balance 720 



74.9 Selection and Proportioning of Chordwise 



Sections 722 



74.10 Structural Control-Surface Design as Af- 



fected by Hydrodynamics ....... 723 



74.11 Design Notes for Motorboat Rudders . . 724 



74.12 Design of Close-Coupled and Compound 



Rudders . 726 



Conditions Calling for Tubular Rudders . 726 

 Closures for Rudder Hinge Gaps .... 726 

 Rudder Designs for Alternative Stems of 



ABC Ship 727 



Design Notes for a Contra-Rudder . . . 729 

 Design of a Contra-Horn for the ABC 



Transom-Stern Ship 733 



Design for Rapid Response to Rudder 



Action 735 



Utilization of Automatic Flap-Tjrpe Rudders 



and Diving Planes 735 



Design Notes for Bow Rudders; Rudders 



for Maneuvering Astern 735 



General Design Rules for Bow and Stern 



Diving Planes 736 



Contra-Features for Diving Planes .... 736 

 Setting Neutral Control-Surface Angles . . 736 

 Selection of Swinging Propellers for Steer- 

 ing and Maneuvering 737 



74.1 General. This chapter undertakes to 

 set forth design notes for the movable appendages 

 whose use and effect are described in Chap. 37. 

 Most of these are control surfaces of one kind or 

 another, such as nidders and diving planes. The 

 fact that maneuvering in general, and the pre- 

 diction of the effect of control surfaces in particular 

 are discussed in Part 5 of Volume III means that 

 the design rules of this chapter lack some of the 

 numerical values to be found in other chapters of 

 this part. 



Specifically, the methods of predicting the 

 effects of given control surfaces and of estimating 

 the torques on their spindles or stocks are not 

 described here but are included in Part 5. Only 

 an outhne is given, in Sec. 74.7, of an improved 

 method for determining the proper areas for 

 control surfaces. The detailed procedure is un- 

 finished at the time of writing (1955). 



74.2 Positioning Rudders and Planes. Other 

 things being equal, rudders and diving planes are 

 best placed where the swinging and divmg (or 

 rising) moments of the forces exerted by them are 

 the greatest. This usually means placing them at 

 the greatest practicable horizontal distances from 

 the CG. 



The magnitude of the mduced velocity in the 

 outflow jet increases mth distance downstream 

 from a screw propeller, explained in Sec. 16.3. 

 Its beneficial effect upon the resultant velocity 

 past a rudder mcreases the transverse force 

 developed by the rudder when the latter is 

 mounted well abaft the propeller. This is explamed 

 in Sec. 33.21 but a shghtly different version is 

 added here. 



From the diagram of Fig. 16.A, the area of the 

 outflow jet in an ideal liquid diminishes directly 

 as (f/ -(- kUi) increases. The outflow-jet diameter 

 diminishes as A"'^ while the lift increases as 

 ([/ -f- kUif or as A~^. Therefore, the transverse 

 force on that portion of a control surface lying 

 within the jet should increase as A~^'^. This 

 means that the increased outflow-jet velocity at 

 a distance abaft the disc more than compensates 

 for the reduction in the area of the control surface 

 covered by the jet. 



For this reason, confirmed by model tests, the 

 best relative position of the rudder, when it lies 

 within an outflow jet, is with its leading edge from 

 0.75 to 1.00 times the propeller diameter D abaft 

 the plane of the disc [van Lammeren, W. P. A., 

 RPSS, 1948, p. 336]. Fig. 74.A illustrates sche- 



706 



