Sec. 71.3 



DESIGN OF MISCELLANEOUS PROPULSION DEVICES 



639 



There are many rivers in the so-called navigable 

 areas of the world in which the depth of water is 

 of the order of 2 or 3 ft only, and in which trees, 

 logs, and all manner of debris, floating and water- 

 logged, are constantly encountered. Under these 

 conditions paddlewheel or sternwheel propulsion 

 is almost a necessity [Hobson, C. A., "Sternwheel 

 Vessels for River Work," Ship and Boat Builder 

 and Naval Architect, London, May 1953, pp. 

 371-377]. 



Paddlewheels are still to be considered for 

 pleasure steamers, tugs, and other craft plying 

 on the smooth, shallow waters of lakes, estuaries, 

 and rivers, where draft restrictions prevent the 

 use of screw propellers large enough to give high 

 or even moderate efficiencies. If the paddles are 

 separately driven, as on many European tugs, the 

 maneuverability can not be approached by any 

 other type of propulsion except multiple rotating- 

 blade propellers diagrammed in Fig. 37. P. 



The low rate of rotation n of paddlewheels 

 does not match the high n of most modern pro- 

 pulsive machinery, but the current and future 

 developments in reduction gearing and flexible 

 couplings give promise of adequate means for 

 utilizing both, while retaining their individual 

 advantages. 



Although the paddlewheel does not retain the 

 prominence it once enjoyed, it is still reckoned 

 as one of the standard ship-propulsion devices, 

 for which design data should be available. For 

 this reason, and because the paddlewheel data 

 in the Uterature are rather scanty and widely 

 scattered, some space is devoted here to a more- 

 or-less systematic presentation of them. 



Judged on the basis of the rotating-blade 

 propeller and the screw propeller with a Kort 

 nozzle, both reasonably acceptable as shallow- 

 water propulsion devices, the efficiency curves of 

 Figs. 34. M and 34.N reveal the paddlewheel as 

 a rather poor third. If, however, corresponding 

 curves were added for screw propellers working 

 under tunnel sterns, as alternatives for shallow- 

 water propulsion, it would be found that they 

 too had low efficiencies. In fact, it is believed that 

 the latter will average about 0.4 at reasonable 

 thrust-load values, and that these efficiencies will 

 rarely approach or exceed 0.5 in actual service. 

 Provided, therefore, that the thrust-load factor 

 of a paddlewheel design can be kept between 

 1.0 and 0.5 or below, it need not suffer from the 

 handicap of low propulsive efficiency for its 

 particular applications. 



The action and geometry of the feathering 

 paddlewheel, a type which is almost universally 

 used when the propulsive efficiency is important, 

 are described in Sec. 32.3 and illustrated in Fig. 

 32. B. The text and drawings include definitions 

 of the various hydrodynamical and .mechanical 

 terms. Diagram 2 of Fig. 15. G indicates that the 

 effective thrust-producing area of a pair of side 

 paddlewheels, equivalent to the disc area Aq oi 

 a screw propeller, is equal to the combined 

 (transverse) length 2s of the blades of both wheels 

 times the maximum immersion of their lower 

 edges, known as the dip. The dip ratio is the dip, 

 measured to the at-rest WL, divided by the blade 

 width or height h. ' 



Based upon the principle that the most efficient 

 propulsion takes place when the least -|-A?7 value 

 is imparted to the greatest mass of hquid, the 

 blades of an efficient paddlewheel should have 

 the greatest area (transverse length times radial 

 width or height) consistent with a balanced 

 wheel-and-ship design. Plunging the blade into 

 the water and lifting it out again constitutes 

 unwanted and undesirable motion and involves 

 wasted energy in the water. The blades are 

 therefore as long, parallel to the water surface, 

 as their positions and as operating requirements 

 permit. In other words, blade area is achieved 

 preferably with blade length measured trans- 

 versely, rather than with radial width or height. 

 Theoretically, since the ships on which modern 

 paddlewheels are fitted encounter waves only on 

 rare occasions, there are apparently no Umits to 

 the transverse length of blade except those 

 imposed by mechanical and structural considera- 

 tions. When the blade length becomes excessive, 

 with undue twisting of the blade on a feathering 

 wheel, two separate paddlewheels, side by side, 

 are mounted on each side of the vessel and keyed 

 to the port and starboard ends of a single shaft, 

 with a separate feathering mechanism for each of 

 the four assemblies. However, if the blades are 

 too large and the thrust loading is too small, 

 too great a proportion of the work is expended in 

 blade friction through vertical motion, and there 

 is too much churning through plunging each 

 blade into the water and Ufting it out again. 

 Like a screw propeller, the paddlewheel can 

 have too much blade area for its own good. 



The volume swept through by an immersed 

 blade is partly boundary layer, with an average 

 U less than the ship speed V, and partly a poten- 

 tial-flow region where the average U is almost 



