Sec. 74.15 



MOVABLE-APPENDAGE DESIGN 



727 



For an excellent discussion of the hydrodynamic 

 and other problems associated with the closing 

 (or narrowing) of vertical and horizontal gaps the 

 reader is referred to P. Mandel [SNAME, 1953, 

 p. 489, under "Rudders — Gap and Horizontal 

 Break"]. S. F. Hoerner gives useful information 

 concerning the relative drag of various across-the- 

 flow gap configurations between a symmetrical- 

 section airfoil and its trailing-edge flap, together 

 with notes on the effect of the airfoil thickness 

 and flap thickness at the hinge [AD, 1951, pp. 

 57-58]. These data apply directly, for straight- 

 ahead running, to ship rudders hung abaft thin 

 skegs or rudder horns. 



74.15 Rudder Designs for Alternative Stems 

 of ABC Ship. The estimates of area for the 

 rudder designs of the ABC ship, described in this 

 section, are based only on the first approximation 

 of Sec. 74.6, and not upon the more logical pro- 

 cedure outlined in Sec. 74.7. Furthermore, the 

 designs of the rudders themselves are carried 

 only far enough in this section to produce control 

 devices which would have about the same re- 

 sistance (certainly no less) than could be expected 

 of the final refined rudder designs for the two 

 alternative ABC ship sterns. 



When roughing in the stern profile of the ABC 

 ship with transom, described in the first part of 

 Sec. 66.25, a first approximation for the area of 

 the movable blade of a single centerline rudder was 

 0.02(L)F, or 265.2 ft'. From Table 74.b this is a 

 high-hmit value for large, medium-speed passenger 

 and cargo ships. The selection of the maximum 

 value in the table was based upon the need for 

 excellent steering and turning qualities in the 

 Port Amalo canal and when maneuvering out of 

 the harbor at Port Bacine, indicated in Figs. 

 64.A and 64.B. 



Based upon the use of a clear-water aftfoot and 

 no rudder shoe, the rudder had to be partly or 

 completely underhung. To avoid placing the 

 single rudder in the paths of the hub vortexes 

 from the single propeller ahead, it was decided to 

 embody a partly underhung rudder and to carry 

 it by a horn which extended far enough below 

 the hull to include a fixed fairing abaft the pro- 

 peller hub. A foil of generous area placed below 

 the horn, with a short circulation path and plenty 

 of clearance for the circulatory flow, would 

 provide the quick response necessary for steering 

 the ship in the Port Amalo canal and in the river 

 below Port Correo. 



Small-scale sketches indicated that, with a 



transom submergence of 2 ft and a baseplane 

 clearance of 0.5 ft for the foot of the rudder, the 

 portion of the blade abaft the stock axis could 

 be at least 22 ft high by 10 ft long, with a pro- 

 jected area of about 220 ft'. Adding say 25 per 

 cent for the balance portion of the foil ahead of 

 the stock gave a tentative total area of 220 -|- 55 = 

 275 ft', which appeared ample. With the balance 

 portion extending from 0.5 ft above the baseplane 

 to about 1.5 ft below the shaft axis (at the 10.5-ft 

 WL), its height was 8.5 ft, giving a tentative 

 fore-and-aft length of 55/8.5 = 6.5 ft. The length 

 balance was then 6.5/(10 -|- 6.5) = 0.394. This 

 value seemed rather large but not too large for a 

 rudder which must be angled without requiring 

 an excessive torque when going astern. 



With a clear-water skeg ending and a baseplane 

 clearance of 0.5 ft for the propeller, the baseplane 

 clearance of 0.5 ft for the rudder appeared some- 

 what small to the owner and operator. It was 

 therefore increased to 1.0 ft. The upper after 

 corner of the tail of the blade was also cut away to 

 provide a greater gap and more protection against 

 air leakage below the edge of the transom. The 

 three lower corners were left square. The shape 

 and dimensions of the blade when sketched at 

 the conclusion of the preliminary design of the 

 transom-stern huU appear in Fig. 74.K. It has a 

 total area of 273 ft', or 0.0206 of the product 

 L{H) = 510(26) at the designed draft. 



The lateral area of the fixed horn as sketched 

 in the figure is about 89 ft', or roughly 24 per 

 cent of the combined area of blade and horn. 

 Because of its closeness to the blade it appears 

 that the differential pressures set up on the fixed 

 horn wUl add materially to the lateral force 

 exerted by the blade when the latter is angled. 



For the arch-stern ABC hull described in Sec. 

 67.16 and illustrated in Figs. 67.L and 67.M, 

 there was little in the way of design data for 

 guidance, by which to select a ratio of rudder- 

 blade area to the product of the waterhne length 

 and draft. The stern was laid out so that, at a 

 reasonably small rudder angle, either the port 

 or the starboard blade would swing into the 

 projection of the propeller disc. Fig. 67. P indicates 

 that the traihng edges of both rudders, at zero 

 angle, lie close to this projection. The contraction 

 in the outflow jet is assumed small because the 

 stock axes lie only about 0.33D abaft the disc 

 position. 



It was estimated that an increase of at least 25 

 per cent over the blade area of a rudder lying 



