690 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 73.14 



upon them when acting ouly to change the direc- 

 tion of flow. 



Limited hydrostatic pressure on the upper 

 surfaces of the guide vanes makes cavitation, 

 separation, and air leakage a problem. The situa- 

 tion is aggravated because the considerable 

 transverse length of the contra-vanes requires 

 for their support a series of vertical plates which 

 project up through the free-water surface. 

 Despite all that may be said to their advantage, 

 contra-vanes are in the category of the short 

 vanes of the contra-propeller described in Sec. 

 36.9, with most of the disadvantages enumerated 

 there. 



If the friction drag of both sets of contra-vanes 

 is considered too much a handicap, the forward 

 set can be omitted, since it is probably the least 

 effective of the two. 



The smaller the wheel diameter, the greater 

 the dip, the greater the immersion arc of the 

 trunnion circle, and the greater the angle (with 

 the horizontal) which the blades enter and leave 

 the waves, the more useful should be the contra- 

 vane installation. 



73.14 Design Features of Supporting Horns 

 for Rudders; Partial Skegs. Rudders of the 

 balanced, partly underhung, compound or flap 

 type require a fixed support in the form of a 

 horn or partial skeg ahead of the hinge or stock 

 axis. This support may take a great variety of 

 shapes, depending upon the single or multiple 

 functions for which it is designed. It may be a 

 structural support only, it may be intended to 

 exert large lateral forces by pressures induced 

 from a movable tail, it may have to serve as a 

 vertical stabilizing fin, and it may have contra- 

 features built in. Profiles of representative shapes 

 are sketched or illustrated in Figs. 21. B, 24. C, 

 26.E, 28.A, 33.B, 37.A, 37.D and 37.J of Volume I, 

 and in Figs. 74.K, 74. N, and 75. E of this volume. 



The profile of a horn or partial skeg is deter- 

 mined by the structural support it is intended to 

 give, the location of internal members to which it 

 can be anchored, and the amount of vertical 

 projected area desired in it. Few rules can be 

 given for laying out these profiles. It can only be 

 pointed out that structural skegs or horns usually 

 require a long base on the hull or a long extension 

 into the hull. 



Hydrodynamic considerations should never be 

 permitted to squeeze the upper and the lower 

 bearings of any rudder-and-horn assembly so 

 close together vertically that the lateral forces on 



these bearings become excessively large. Such a 

 distorted design requires, as a rule, long (high) 

 bearings, having length/diameter ratios so large 

 that it is almost impossible to obtain uniform 

 pressure over the whole bearing length because 

 of bending or deformation of the parts. As a 

 consequence, the bearings wear unevenly and 

 excessively, leaving the rudder free to vibrate. 

 Since the rudder is already in a region of disturbed 

 flow the resulting slackness may cause pounding, 

 with more wear and still more vibration. 



Partial skegs intended as restoring-moment 

 stabilizers benefit from large aspect ratios and 

 relatively narrow tips. Those intended as damping- 

 moment stabilizers have a moderate to large area 

 in combination with the attached movable rudder. 

 Skegs intended to be self -clearing when encounter- 

 ing ropes, cables, and nets, such as those on sub- 

 marines, require their leading edges to be set at 

 rather small angles to the direction of ship motion. 

 These are not necessarily small with respect to 

 the direction of local water flow. The outer or 

 lower ends of horns or partial skegs, especially 

 if they are long in a fore-and-aft direction, should 

 lie generally parallel to the adjacent flow except 

 for such appendages of this nature as are utilized 

 for docking or resting purposes. 



Structural skegs intended only for docking or 

 resting can be integral parts of the hull or addi- 

 tions on the bottom. If the latter, it is simpler to 

 omit the fairings along the edges where the skeg 

 sides join the hull; this is acceptable if the con- 

 ditions mentioned in Sec. 75.5 are satisfied. It is 

 generally the case for a partial centerline skeg. 

 If the skegs are built as integral parts of the hull 

 it is simpler and better to fair them easily into 

 the hull surfaces and to provide generous means 

 of access from the inside. Incidentally, it is 

 desirable although by no means necessary that 

 skegs which have docking functions should 

 terminate with their lower surfaces on the base- 

 plane or at the level of the bottom of the keel. 

 However, to save displacement, wetted surface, 

 and vertical fin area in the lateral plane, they can 

 be cut up from the keel plane by the heights of 

 one, two, or three tiers of docking blocks, gen- 

 erally 14, 28, and 42 inches, respectively. 



The fineness of the leading edge on any horn 

 or projecting skeg is determined by the range of 

 du'ections from which the local flow may impinge 

 upon it under normal operating conditions. In 

 other words, the horizontal section has sufficient 

 thickness abaft the leading edge so that separation 



