644 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 71.6 



Blodes, Trunnions, Crank Arms, and 

 Links in Solid Lines ore^ 

 Drown for Trunnion' C' 

 at Lower Cerrter 



Line Normal to Blode Face 



Flonqes Stiffen Ends 

 of Blodes and Reduce 

 Water Spillover at 

 These Points 



I 



Center of Poddiewheei Shaft- 



Approximote /*" 

 Sweep Lines 



of Blodes I 



Broken Lines -Show Speciol 



Blode Actuated 

 DK 



el ft 875 ft 3+5 fj 



Trunnion Axis for Blade 



Fig. 71. B Layout Accompanying Example of a Feathering Paddlewheel Design for the ABC Ship 



velocity vector, indicated in the left-hand diagram 

 of Fig. 32. B and in the layout elevation of Fig. 

 71. B, for a greater part of their width. There are 

 also more blades acting simultaneously, with a 

 lighter loading on each and smaller losses from 

 induced effects. Furthermore, the submerged 

 blades move more nearly parallel to the straight- 

 aft direction in which the water is to be accelerated 

 by them. 



The correct fore-and-aft position of paddle- 

 wheels, important for efficiency if the wave 

 profile has marked crests and troughs, is governed 

 by the position of the wave crests along the ship 

 when running at the speed for which the best 

 propulsion performance is desired. This feature 

 is much more important for a vessel running in 

 shallow water than in deep water. To take 

 advantage of the wave wake the wave crest 

 should be approximately under the wheel center. 

 This position can sometimes be estimated for a 

 new design, and it may be predicted by com- 

 parison with an existing design if the hulls have 

 the same shape. Wave profiles for some paddle 

 steamers are shown in Sec. 52.2; references are 

 given there for other published profiles. The 

 wave profile is quickly and reliably determined, 

 however, from a model test. 



From the wave profile of the ABC transom- 

 stern ship, given on the SNAME RD sheet in 



Fig. 78. Ja, it appears that the best fore-and-aft 

 position for a pair of paddlewheels on that vessel 

 is at about Sta. 11. This happens to coincide 

 very nearly with the position of maximum water- 

 line beam. 



The next problem is to determine the vertical 

 position of the paddlewheel shaft center with 

 respect to the designed waterline. The selection 

 of the dip ratio, or the decision as to how far the 

 bottom of the deepest blade is to drop below the 

 at-rest waterline at the designed draft, may 

 hinge on practical rather than hydrodynamic 

 reasons. It depends upon the wheel diameter, how 

 many blades are to be in the water at any one 

 time, whether feathering is employed or not, and 

 the maximum variation in anticipated draft for 

 which propulsion is to be reasonably efficient. 

 For a shallow-water vessel, the greatest submer- 

 gence depth of the bottom of a blade should be 

 slightly less than the draft. Consideration of these 

 and other factors, for vessels of various types, 

 results in blade immersions varying from only 

 some 0.7 of the height to immersions of 1.4, 1.5, 

 or more of the blade height. D. W. Taylor points 

 out that the dip ratio may reach 2.0 for a very 

 long, narrow blade on a large wheel, without 

 loss of efficiency [S and P, 1943, p. 150]. 



Another relationship between the wheel diam- 

 eter and the blade dip involves the angular 



