Sec. 74.2 



MOVABLE-APPENDAGE DESIGN 



707 



Twin I Rudders 



Jet Diometer ' 



Less Thon D But Relotiv 



Velocity Greater Thon VX 



V Schemotic Inflow" 



Velocity VftOt 



Disc Position 



Fig. 74. a Diagram Illusteating Augment op 

 OtJTPLOW-jET Velocity at a Rudder Position 



matically an installation of this kind. This rule 

 also applies to the fore-and-aft positioning of a 

 single rudder between the outflow jets of two 

 screw propellers, provided the jets are close 

 enough so that they impinge upon it at reasonable 

 rudder angles, say 20 deg or more. Indeed, for 

 single-screw ships, where rudder effect alone is 

 considered, it is well to keep the leading edge of 

 the rudder well abaft the propeller if this can 

 conveniently be done. 



Some useful information relative to flow at the 

 centerline rudder position on a model, as affected 

 by the boundary layer, by the outflow jets from 

 adjacent screw propellers, and by the lateral 

 motion of the stern during a turn, is given by 

 W. G. Surber, Jr. ["An Investigation of the Flow 

 in the Region of the Rudder of a Free-Turning 

 Model of a Multiple-Screw Ship," TMB Rep. 

 998, Oct 1955]. It has not been possible to unearth 

 corresponding data from tests of single- and 

 twin-screw models. 



On vessels where maneuverability and rudder 

 effect is an important requirement, the movable 

 blades of rudders are placed well below the surface 

 or their upper portions are protected in some other 

 effective way from partial breakdown due to 

 leakage of air from the surface. The blade lengths 

 at the top, near the surface, may be made shorter 

 than at the bottom. The upper after corner of a 

 rudder may be cut back, as was done for the 

 transom-stern ABC ship; see Fig. 74. K of Sec. 

 74.15. The effects of heel, wave action, change of 

 trim, and other factors which obtain during 

 turning are not to be lost sight of in checking for 

 possible air leakage to the reduced-pressure side 

 of the rudder during that maneuver. 



The trailing edge of a steering rudder should 

 not be placed too near the lower corner of an 

 immersed transom. When the speed is high enough 

 to expose the whole after surface of the transom, 



down to the lower corner, air may "jump" to the 

 reduced-pressure side of the rudder. This occurs 

 when the gap between the transom corner and 

 rudder is small enough or the pressure differential 

 large enough. The resulting air leakage greatly 

 reduces the rudder force on a turn. For the 

 "lifting" rudders described in Sec. 37.18, air 

 leakage of this kind is necessary to the success 

 of the arrangement, but for a ship not carrying 

 them, air finding its way to a rudder is definitely 

 detrimental. Testing techniques are now available 

 in the larger model basins whereby this air 

 leakage can be photographed and detected on a 

 free-running model during a turn. The air leakage 

 could be prevented in a transom-stern design by 

 extending the bottom beyond the transom plane 

 as a thin horizontal lip. However, this might 

 introduce difficulties when backing or when 

 running in an overtaking sea. 



Rudders and planes hung wholly or partly on 

 the after ends of horns, skegs, fins, and keels are 

 most effective as lateral-force-producing devices 

 when coupled closely to those fixed members. This 

 does not necessarily mean that they give rapid 

 response when angled quickly, as discussed in 

 Sec. 74.18. Special devices to prevent leakage 

 of differential pressure through the hinge are 

 very much worth while; some of them are de- 

 scribed in Sees. 73.14 and 74.14 and illustrated 

 in Fig. 73.K. 



A rudder or plane should definitely be kept clear 

 of a swirl core or hub-vortex cavity, described in 

 Sec. 23.14 and illustrated in Figs. 23.K and 23.L. 

 This usually forms abaft the hub or fairing cap 

 of a screw propeller on a fast or high-speed ship 

 but there are evidences that it may appear at a 

 moderate speed. The cavity is likely to be so 

 large that the portion of a rudder against which 

 it strikes is totally ineffective. The rudder struc- 

 ture in its wake is subject to pitting, erosion, and 

 hammering, which may result in fracture of the 

 parts and tearing off of the portion under attack. 

 A rudder on a fast but not high-speed ship, 

 subject to damage of this kind, is shown by 

 V. L. Russo and E. K. Sullivan [SNAME, 1953, 

 pp. 124-125]. Another such rudder, on a slower 

 ship, is illustrated in Marine Engineering, New 

 York, October 1954, page 44. It is known that in 

 some cases, such as following a sharp turn, the 

 swirl core or hub vortex shifts around on the 

 fairing cap. In other words, it does not always trail 

 from the exact point or tail of the cap. A good rule, 

 therefore, is to keep clear of a possible swirl core 



