Sec. 7 J. 7 



DESIGN OF MISCELLANEOUS PROPULSION DEVICES 



645 



length of arc of the blade circle, passing through 

 the blade trunnions, which is immersed in the 

 designed-draft condition. For a paddlewheel to 

 run at a medium rate of rotation, or perhaps 

 above average, say not more than n = 35 rpm 

 or 0.58 rps, this arc should extend for about 50 

 deg forward and aft of the lower center position, 

 or a total of 100 deg. For a fast-running paddle- 

 wheel of less than average diameter, with normal 

 dip, the arc on either side of lower center may be 

 55 deg or more, corresponding to a total of 110 deg. 



For use in selecting a final dip ratio for the 

 ABC ship, in the designed-load condition at 

 which the paddlewheels are to give their best 

 performance, Bragg's table has values of apparent 

 dip ratio, measured to the at-rest waterline, 

 varying from 1.27 to 1.52. The ABC value of 

 1.35, selected tentatively in Sec. 71.5, combined 

 with a blade height of 6.5 ft, gives a dip of 8.76 ft. 

 With a diameter of 37.53 ft and a radius of 18.77 

 ft, the wheel center lies 18.77 - 8.76 = 10.01 ft 

 above the at-rest water level. The circle passing 

 through the midheights of the blades, with a 

 radius of 18.77 - 3.25 = 15.52 ft, therefore 

 strikes the at-rest water surface at an angle ahead 

 of the lower center of cos"'(10.01/15.52) = 49.8 

 deg. This is acceptable, and the dip ratio of 1.35 

 may be considered as fixed. 



With an apparent-slip ratio of 0.16 the value 

 of V° = 41.2 ft per sec, from Sec. 71.4. Dividing 

 this value by the blade-circle circumference of 

 97.5 ft gives a rate of rotation n of 0.4225 rps or 

 25.35 rpm. 



It is now possible to sketch a layout of the 

 proposed paddlewheel alongside the transom- 

 stern ABC ship, about as indicated in Fig. 7 LB. 

 Rounding out the dimensions to get rid of the 

 small decimal fractions, the paddlewheel center 

 is placed 10.0 ft above the DWL, or at the 36-ft 

 WL. The external wheel diameter is nominally 

 37.5 ft, but for a wheel of the feathering type the 

 volume swept through by the outer edges of the 

 blades is not a true cyhnder. Nominally, the 

 outside wheel diameter is twice the distance from 

 the wheel axis to the bottom of the blade at the 

 lower center or 6 o'clock position, corresponding 

 to twice the distance AG in Fig. 71.B. 



The maximum waterline beam lies very close 

 to Sta. 11, and a mechanical clearance of 0.4 ft 

 between the hull and the inner ends of the blades 

 appears to be adequate. With blades 22.6 ft long, 

 their outer ends lie 23 ft from the widest portion of 

 the ship's side (including the shell plating). The 



feathering mechanism is to be outside the wheel, 

 with the eccentric pin carried by a fore-and-aft 

 guard forming the lower edge of the paddlewheel 

 box. If there is an outer shaft bearing carried by 

 this guard the feathering links must be pinned 

 to an eccentric strap around the shaft, and possibly 

 also around the bearing. The mechanism then 

 resembles that sketched in Figs. 32.B and 71.A. 



Although the dip appears large when drawn in 

 the elevation from aft of Fig. 71. B, the markers 

 showing the WL elevations at Sta. 11 for the 

 two variable-load conditions of Sec. 66.32 and 

 Fig. 66.T indicate that the dip is actually too 

 small for those lighter displacements. 



Some paddlewheels are designed so that both 

 the radial width or height and the radial position 

 of the blades may be changed without too much 

 difSculty and expense after the ship is built. 

 A certain measure of adjustment is desirable if 

 the draft at the wheel axis is to change more-or- 

 less permanently during the life of the vessel. 

 It would be very much better, of course, if the 

 vertical position of the paddlewheel axis could 

 be changed as well. 



71.7 Design Notes on Paddlewheel Details 

 and Mechanism. The next phase of the design 

 involves an analysis of the dimensions, ratios, 

 proportions, and other features to insure that 

 those tentatively established in Sec. 71.6 will 

 produce a good overall mechanical and hydro- 

 dynamic design. This involves a more careful 

 consideration of the features selected rather 

 arbitrarily in Sees. 71.5 and 71.6. 



Clearance between inboard paddle ends and 

 the fixed hull is generally limited to the minimum 

 permissible mechanical value, say from 0.2 ft on 

 small vessels to 0.5 ft on large ones. The value of 

 0.4 ft indicated in Fig. 71.B is rather small but 

 not too small. Every fraction of a foot added 

 here adds to the overhang of the shaft. 



The blade-spacing or pitch ratio, defined as 

 the circular-arc distance CJ over the blade width 

 FG in Fig. 71. A, should preferably be about 2.0 

 times the blade depth, to avoid interference 

 between blades. However, this calls for very 

 large wheel diameters. Practical limitations on 

 weight and space are usually such as to reduce 

 this ratio to as low as 1.6 or 1.5. If the dip ratio 

 WG/FG of that figure is made about 1.2 to 1.5 

 the combination of pitch ratio with dip ratio and 

 a reasonable blade-circle diameter gives a total 

 immersed-blade area of from 2.0 to 2.5 times the 

 area of one blade, irrespective of the angular 



