596 



HYDROnVNAMICS IN SHIP DESIGN 



Sec. 70.: 



gave graphs and rules for finding the developed- 

 area ratio of a screw propeller in the preliminary- 

 design stage ["Recent Data on Cavitation 

 Criteria," Inter. Shipbldg. Progr., 1954, Vol. 1, 

 No. 1, pp. 39-47]. The following values pertaining 

 to the ABC transom-stern design were used to 

 calculate the ratio Ad/Ao by the procedure on 

 page 45 of the reference: 



Thrust to be delivered by the propeller in salt 



water, 193,476 lb 

 Rate of rotation n (assumed), 109 rpm or 



1.817 rps 

 Speed of advance Va or Ve , 15.15 kt or 25.59 



ft per sec 

 Number of blades Z, 4 

 Submergence of propeller axis below at-rest 



waterline, 15.5 ft 

 Assumed height of stern-wave crest above 



at-rest WL, 1 ft 

 Assumed vapor-pressure head e, 1 ft 

 Atmospheric-pressure head, 33 ft 

 Value of pressure head (p„, — e)/iv, 48.5 ft. 



The derived values were: 



Diameter D, 19.75 ft 



Mean pitch-diameter ratio P/D, 0.952 



Cavitation number ao at a value of R/Bm^^ = 



0.8, in the 12 o'clock position, 4.01 

 Developed-area ratio Au/Ao , 0.532. 



By one of the alternative calculation methods 

 given by van Manen the value of Ao/Aa (his 

 FJF) is 0.546. The developed-area ratio is 

 sufficiently close to the expanded-area ratio, at 

 least for not-too-wide blades, so that no distinction 

 need be made between them. 



The diameter is thus slightly smaller than that 

 assumed in the calculations preceding, and the 

 pitch-diameter ratio is some 5 or 6 per cent less, 

 but the area ratio is considerably larger. However, 

 the final design computations for the ABC pro- 

 peller, at the end of Sec. 70.31, give a r&t\o A e/A^ 

 of 0.478, indicating reasonable agreement with 

 the foregoing. 



70.7 Modification of Series-Chart Procedure 

 for Other Design Problems. For preliminary 

 designs in which the propeller is required to have 

 good backing performance, as for tugs and ferry- 

 boats, there are few propeller-series charts avail- 

 able. The stopping and backing situation is 

 discussed rather fully in Part 5 of Volume III 

 of this book. Suitable propeller-design features 

 are well set forth by W. P. A. van Lammeren 



[RPSS, 1948, pp. 259-260], based upon the follow- 

 ing references: 



(a) Kempf, G., HSPA, 1940, Vol. II, p. 46 



(b) Conn, J. F. C, lESS, 1934-1935, Vol. 78, p. 27 



(c) Gebers, F., Schiffbau, 1933, p. 235 



(d) Robertson, J. C, MESA, 1929, p. 102. 



The only comprehensive data available on the 

 characteristics of model propellers for what might 

 be termed full backing conditions are those given 

 by H. F. Nordstrom in "Screw Propeller Charac- 

 teristics" [SSPA Rep. 9, 1948]. In this report 

 Nordstrom presents the results of tests on nine 

 4-bladed model propellers, with P/D ratios 

 varying from to 1.6, over a range of advance 

 coefficient J from -|-2.0 to —2.0. 



The problem of designing screw propellers 

 intended to run normally in the partly immersed 

 condition is discussed under surface propellers in 

 Sec. 71.10. 



70.8 Preliminary Comments on Propeller- 

 Design Features. Strictly speaking, any pro- 

 peller series chart intended for analysis and 

 design purposes is valid only for other propellers 

 having the same geometric shape and physical 

 characteristics. For example, systematic data 

 based on tests of model propellers having small 

 solid hubs do not represent the expected perform- 

 ance of full-scale propellers with larger built-up 

 hubs. In the case of almost every new design it is 

 necessary to depart from the parent propeller 

 in some physical respect. The designer must have 

 at least some indication of the effect of these 

 changes upon the predicted performance of the 

 new design. 



A few of the principal propeller features have 

 to be decided upon before the preliminary design 

 is begun; they are the given quantities, so to 

 speak. Others can be determined after the principal 

 characteristics are known, such as the exact shape 

 of the blade profile or the rake. 



An alternative method is to work up a series of 

 preliminary designs with the given quantities 

 varied systematically, and then to select the best 

 of the series for further development toward a 

 final design. This is done by K. E. Schoenherr for 

 four different propellers, to determine the effect 

 of varying D and n [PNA, 1939, Vol. II, Table 18, 

 p. 172]. If series charts are used, the time involved 

 for each design is so small that this should almost 

 be considered as routine procedure. 



Rather than to burden the description of the 

 series-chart method in Sees. 70.5 and 70.6 or the 



