Sec. 74.11 



MOVABLE-APPENDy\GE DESIGN 



733 



augment the right rudder to be carried to inaintaiii 

 a straight course. 



The design of the contra-shaped movable foil 

 follows the design of the contra-shaped fixed horn 

 or skeg above it, described in Sec. 74.17. Normally, 

 the thickness ratio of the foil sections in a partly 

 underhung rudder diminish rather rapidly from 

 the lower bearing to the bottom of the rudder, 

 until at the lower edge this ratio may be of the 

 order of 0.03. The high value of about 0.115 for 

 the section at the 2-ft WL for the foil shown in 

 Fig. 74.N of Sec. 7-4.17 is due to: 



(1) The short vertical height of the foil, because 

 the fixed horn is extended below the propeller 

 shaft axis 



(2) The uncertainty involved in drastic thinning 

 of a contra-section, expecially in a rudder of this 

 type. 



Thinning the lower-edge sections represents no 

 problems, provided it is determined that the 

 contra-performance can be maintamed. If the 

 lower sections are left thick, a flow test might 

 indicate the advisability of sloping the lower edge 

 of the foil down and forward, with a baseplane 

 clearance of say 0.5 ft at the forward end and 

 1.5 ft at the after end. 



The compensating force exerted to port by the 

 contra-shaped foil is probably augmented by 

 holding the trailmg edge of the foil on the center- 

 plane; that is, by not offsetting it to port, as is 

 customary in contra-rudders. The foil then works 

 at a greater effective angle of attack m the out- 

 flow-jet water which, below the shaft axis, is 

 directed aft and to port behind a right-hand 

 wheel. Rather than to offset only the trailing edge 

 of the tail, above the shaft axis, and to introduce 

 a discontinuity in the rudder structure, the entire 

 portion of the blade abaft the stock axis is made 

 symmetrical. 



Finally, when the rudder is designed, and a 

 model is built, the flow is checked in a circulating- 

 water channel with tufts attached to various 

 parts of the rudder. One test should be made 

 with the propeller working, and another with 

 the rudder at an angle. Unfortunately, it was 

 not possible to do this with the model of the 

 transom-stern ABC design. 



Entirely aside from the increase in propulsive 

 coefficient which may be achieved by a contra- 

 rudder on any ship, which should be from 4 to 

 6 per cent under average conditions, there is a 

 decided advantage on a high-powered, moderate- 



speed ship in bending the leading edge of a rudder, 

 contra-fashion, instead of using a simple stream- 

 lined affair, with the base chords of all sections 

 parallel to each other. In the rotary cross flow 

 of the outflow jet the contra-flow shape acts to 

 reduce cavitation, pitting, erosion, buffeting, 

 vibration, and noise. An example- of a severely 

 pitted Mariner class rudder, not so shaped, is 

 given by W. G. Allen and E. K. Sullivan [SNAME, 



1954, Fig. 25, p. 541]. T. W. Bunyan illustrates 

 the pitting which occurred on a combination of 

 thin rudder post and a rudder hung directly 

 abaft it, lying in the outflow jet of a single, 

 centerline propeller. Both were streamlined but 

 were entirely symmetrical about the vertical 

 plane through the propeller axis [IME, Apr 



1955, Vol. LXVII, No. 4, p. 105]. 



74.17 Design of a Contra-Horn for the ABC 

 Transom-Stem Ship. For the transom-stern 

 ABC ship whose stern profile is shown in Fig. 

 74. K, the rudder horn or small skeg abaft the 

 propeller extends below the shaft axis so that a 

 fixed dummy fairing for the propeller hub may 

 be carried by it. For ease in removing the shaft 

 nut and the propeller on the actual ship, this 

 fairing is made removable. 



In view of the appreciable fore-and-aft length 

 of the fixed rudder horn it is found possible to 

 incorporate all of the twist and camber of a 

 contra-guide device in the portion above the 

 propeller axis. As explained in Sec. 74.16, this 

 leaves the tail sections of the movable blade 

 entirely symmetrical. Below the propeller axis, 

 all the twist and camber is incorporated in the 

 balance portion of the rudder foil, forward of the 

 rudder-stock axis, also described in that section. 



In the original layout of the propeller-hub 

 fairing at the lower end of the rudder horn it 

 appeared that the diameter of this fairing at its 

 juncture with the horn would be 3.0 ft, with a 

 radius of 1.5 ft. As a basis for calculating the 

 offsets for the twist at the leading edge of this 

 horn, the offset at O.lflMas above the propeller 

 axis was taken as 0.85 times 1.5 ft or 1.275 ft. 

 The remaming offsets, calculated by the sin^ <^ 

 rule described in Sec. 67.22, are tabulated on 

 Fig. 74. N. The offsets below the shaft axis, to be 

 embodied in the leading edge of the foil, are the 

 same as those above that axis, for the same radii. 



When TMB model propeller 2294 was selected 

 from stock to drive the transom-stern model, it 

 was necessary to reduce the hub-fairing diameter 

 at its junction with the horn from 3.0 ft to 2.67 



