Sec. 70.33 



SCREW-PROPELLER DESIGN 



625 



Fig. 70. M Camber-Ratio Chart for Circular-Arc 

 Blade Sections 



The faired chord lengths at 0.5R and O.QR are 

 greater than required to avoid cavitation. The 

 chord lengths from the O.IR to the tip are exactly 

 as determined by the cavitation criteria. The 

 final chord lengths for all radii are tabulated in 

 Col. B of Table 70. i. The corresponding values of 

 tx/c and Cl are shown in Col. C and Col. D of 

 that table. The final maximum-camber ratios 

 nix/c are obtained from Fig. 70.K. For those 

 sections which do not fall within the limits of this 

 graph, Fig. 70.M is used. As explained earlier in 

 this section for Fig. 70.K, the graphs of Fig. 

 70.M were prepared for use with the Karman- 

 Trefftz blade sections and the circular-arc mean- 

 line. Fig. 70. M also can be used to give an approxi- 

 mate solution for the circular-arc meanline in 

 combination with other thickness distributions. 

 The maximum camber ratios for all radii are 

 shown in Col. E of Table 70.i. With the faired 

 chord lengths from Fig. 70. L, the expanded-area 

 ratio Ae/Ao is calculated to be 0.478. Multiplying 

 the disc area Ao, 314.16 ft^, by the fraction 0.478 

 gives the absolute expanded area, 150.17 ft^. 

 Dividing this area by the number of blades Z, then 

 by the blade length, 8.2 ft, and then by the diam- 

 eter D, gives the mean-width ratio c^,/D = 0.229. 



70.32 Procedure When Cavitation is Not 



Involved. In many cases, cavitation is of no 

 concern for the propeller under design and the 

 cavitation number o-, or 0.85(r, is completely off 

 the chart of Fig. 70. K. It is then necessary to 

 use some other means to arrive at the blade 

 outline and chord lengths. The usual procedure 

 is to select these features from one of the standard 

 screw-propeller series such as the Wageningen or 

 TMB series. With this procedure a suitable ex- 

 panded-area ratio Ae/Ao is needed. A good first 

 estimate of the ratio Ae/Ao is made by using one 

 of the cavitation diagrams of L. C. Burrill, of 

 the Wageningen Model Basin, or of J. D. van 

 Manen and L. Troost [RPSS, Fig. 123a, p. 186; 

 SNAME, 1952, Vol. 60, Fig. 14, p. 455]. The 

 referenced diagrams give the projected blade 

 area Ap . This can be converted to the expanded 

 area Ae with sufficient accuracy by using the 

 following formula [RPSS, 1948, Formula 167, 

 p. 177]: 



Ap/Ae = 1.067 - 0.229(P/Z)) (70.xv) 



where P/D is the pitch-diameter ratio at 0.7/^. 



The value of the developed-area ratio Ao/Ao 

 may be estimated by a procedure devised by 

 J. D. van Manen [Int. Shipbldg. Progr., 1954, 

 Vol. 1, No. 1, pp. 39-47] and described in Sec. 70.6. 



Using the expanded-area ratio thus obtained, 

 and one of the standard-screw series, calculate 

 the values of c, tx/c, and d, . These values are 

 then checked to determine whether the limiting 

 conditions explained previously in Sec. 70.31 are 

 met, i.e., whether Cl < 0.60 at all blade sections, 

 and whether tx/c at the hub is < 0.16 to 0.20, 

 depending upon the type of propeller. If these 

 conditions are not met, a new expanded-area 

 ratio is assumed and new calculations are made. 

 By trial and error a suitable blade area and width 

 are finally achieved. 



70.33 Corrections for Flow Curvature and 

 Viscous Flow. Returning to the ABC propeller 

 design, with the maximum camber ratio mx/c 

 determined from the faired blade outline, it is 

 possible to apply the curvature-correction factor 

 discussed in Sec. 70.29. This factor is obtained 

 from the curves of Fig. 70.N, following the steps 

 outlined on that figure. It is used in the following 

 equation: 



(70.xvi) 



where nixo/c is the corrected maximum camber 

 ratio 



