Sec. 70.] S 



SCREW-PROPELLER DESIGN 



605 



propeller is large, and its harmful effect can be 

 reduced by skew-back, but the angle made by 

 the leading edge with the radius has to be large to 

 make much difference here. 



70.17 Design Considerations Governing Blade 

 Width. The matter of proper blade width for a 

 screw propeller, usually expressed as the mean- 

 width ratio Cm/D, follows rather closely after 

 the discussion of expanded-area ratio in Sec. 

 70.15. If the designer has no preconceived ideas 

 regarding the blade width he usually encounters 

 difficulty in this phase of the problem. A partial 

 solution is to adopt the blade width, or approxi- 

 mately that value, of the series model propeller 

 which appears to give the best performance from 

 the chart-design procedure. For example, W. P. A. 

 van Lammeren gives [RPSS, 1948, pp. 204, 214] 

 the chord length of the widest section for one 

 series in terms of the propeller diameter D. 

 J. G. Hill includes some brief instructions 

 [SNAME, 1949, p. 152]. 



Assuming that the blade width and outline are 

 selected, the next step is a check to determine 

 whether the blade area is sufficient to avoid 

 cavitation, using one of the available cavitation 



(a) L. C. Bunill, RPSS, 1948, Fig. 123a, p. 186 



(b) Wageningen Model Basin, RPSS, 1948, Fig. 123a, 



p. 186 



(c) J. D. van Manen and L. Troost, SNAME, 1952, Fig. 



14, p. 455 



(d) J. D. van Manen, Int. Shipbldg. Prog., 19.54, Vol. 1, 



No. 1, pp. 39-47 



(e) H. W. E. Lerbs' data in PNA, Vol. II, Figs. 30 and 31, 



pp. 179, 181 



(f) W. p. A, van Lammeren's chart in 6th ICSTS, 1951, 



Fig. 30, p. 94, reproduced with some modifications 

 of detail in Fig. 47.G of the present volume. 



Two examples illustrate how this is done, both 

 for the 20-ft propeller of the ABC transom-stern 

 ship: 



(1) Cavitation check with Lerbs' data, using the 

 symbols given in Figs. 30 and 31 of the reference, 

 with a rate of rotation of 110 rpm and a depth 

 to the shaft axis of 15.5 ft. 



Ae/Ao = 0.40, 



Ae = 0.40(Ao) = 0.40Tr(10)' = 125.66 ft' 



n = 110 rpm or 1.8333 rps 



Pst.tic = p. = Po - p. 



= 14.7(144) + 15.5(62.43)(1.027) - 52 



p. = 2,116 -I- 994 - 52 = 3,058 lb per ft' 



p,Af: 

 pn^d" 



384,268 

 1,070,410 



3,058(125.66) 

 1.9905(1.8333)'(20)' 



0.359 



For a speed of advance of 15.15 kt or 25.59 ft per 

 sec, 



7 = Zl = 25.59 

 nd " (1.8333)(20) 



= 0.698. 



For a p/d ratio of 1.0 this point falls well within 

 the region of no cavitation. 

 (2) Cavitation check by the van Lammeren 

 method, employing his symbols: 



Po — p. = p, = 3,058 lb per ft'^, from (1) preceding 



Po - P. 



3,058 



3,058 



= 4.69 



" 0.5pF; 0.99525(25.59)' 651.7 

 Factor = <To{FJF„)p = 4.69(0.4) (1.0) = 1.87 



J/p = '-ff = 0.698 



Using these factors, the plot of Fig. 47. G 

 indicates that they are in the region of no cavita- 

 tion. 



One featui'e concerning blade width, rarely dis- 

 cussed in the literature, is that of its effect upon 

 the periodic vibratory forces excited in or on the 

 adjacent ship structure. Assuming the same shape 

 of — Ap (and -)-Ap) chordwise pressure-distribu- 

 tion curve on both wide and narrow blades deliver- 

 ing equal thrust, the mean — Ap (or -|-Ap) on 

 the wide blade is smaller and the pressure peaks 

 are less pronounced. It follows that, for a given 

 rate of rotation, the periodic forces . exerted by 

 the wide-blade pressure fields should be smaller 

 in magnitude and longer in duration. Both 

 features favor the wide-blade propeller in lessen- 

 ing the vibratory forces on the ship carrying it. 



70.18 Selection of Type of Blade Section. 

 The selection of the proper type and proportions 

 of the blade sections is important. These may be 

 different for the inner, the intermediate, and the 

 outer radii. What is more important, perhaps, is 

 that the type, shape, and proportions of the blade 

 sections be suited to the work to be performed. 



Of the blade-section types illustrated in Fig. 

 32. K, the single-ended and double-ended sym- 

 metrical sections are employed primarily on 

 propellers intended to give good stopping and 

 backing performance, and to run astern for long 

 periods, as on a double-ended ferryboat. 



