Sec. 73.13 



FIXED-APPENDAGE DESIGN 



689 



therefore placed so that the meanhne at the nose 

 is parallel to the streamlines of the incident flow. 

 For a leading contra-vane ahead of a stern wheel, 

 as in diagram 2 of Fig. 32. C, it is necessary to 

 determine the flow direction at the nose position 

 by some kind of ship or model test. 



M. Hart has published two diagrams which 

 show the resultant-velocity vectors, with reference 

 to a stream of undisturbed water flowing past 

 the side of the ship, of the upper and lower edges 

 of the blades in a series of immersion positions 

 [ATMA, 1906, Vol. 17, p. 179 and Pis. II, III]. 

 These do not take account of the velocities in- 

 duced by the blades, for which no comprehensive, 

 reliable data can be found. It is considered, 

 therefore, that the run or after portion of the 

 leading vane can best be shaped with its upper 

 surface parallel to a tangent to the sweep circle 

 of the outer blade edges at that point. There 

 should be enough clearance between this upper 

 surface and the outer blade edges to pass any 

 floating debris that may be drawn down below 

 the surface. 



The trailing edge of the forward contra-vane 

 should lie at about the level of the blade trunnions, 

 perhaps slightly below the midwidths of the blades 

 in their lowest positions. As for chord length fore 

 and aft, this can be of the order of 1.5 to 2.0 

 times the blade width on the paddlewheel or 

 sternwheel, for both leading and trailing vanes. 



For the after or trailing vane, the predominant 

 flow is that which forms the high, steep wave 

 just abaft the wheel. This is indicated in Fig. 

 73. J, adapted from the Siiberkriib reference 



mentioned earlier in the section. The contra-vane 

 is best placed where the flow vector at the sub- 

 surface level of the vane has its steepest slope. 

 In the layout of the figure it is possible to keep 

 the vane above the at-rest waterline, although a 

 position under other wave-slope conditions might 

 be as low as the lowest trunnion level of the blades. 

 The lower surface of the run portion of the trailing 

 contra-vane, just ahead of the trailing edge, 

 should conform generally to the flow within the 

 wave crest at that position and level. 



Since the lift of the after vane has a forward 

 thrust component, this curved-section hydrofoil 

 should have a high lift and a low drag. The mean- 

 lines of both leading and trailing vanes will have 

 a rather large camber for their chord lengths. 



The contra-vanes may be made reasonably thin 

 if supported at say two intermediate transverse 

 points as well as at the ends. Their curved section 

 shape gives them inherent stiffness, as for a blade 

 of the wheel itself. 



Were a vessel fitted with contra-vanes to run 

 in waves these devices would be subject to impact 

 or slamming. They would have to be designed to 

 withstand an impact load much larger than the 

 lift load. The equivalent static load would prob- 

 ably be of the order of 2,000 lb per sq ft or more. 

 This is about 4 times the uniform propelling load 

 applied to the blades of a paddlewheel. 



Side paddlewheels are in themselves rather 

 effective roll-quenching devices. The contra-vanes 

 are much more effective for this purpose, but even 

 without impact they may require support for 

 vertical forces far greater than those imposed 



Outline of Section of Cbntro-Vone, Lyinq Abaft 

 Entire Lanqth of Paddle Blade 



The Ahead Thrust Force Exerted by the Contra-Vane 

 Under the Conditions Pictured is T 



The Incident- Velocity Vector is Generolly Porollel to the Wave Surface 

 site or Bock Of the HydrodynatrKc Center of the Foil 



Fig. 73.J Proposed Contra-Vanb Aerangement of F. Suberkbub 



