Cavitating Prop Design and Screw Prop Development 



operates in oblique flow, however, the circumferential nonuniformity, especially 

 in the way of the root sections, is so substantial that it will inevitably give rise 

 to outbreaks of cavitation on the pressure face, should the propeller be designed 

 in the manner mentioned above. These outbreaks of cavitation may cause cavi- 

 tation erosion. To avoid this phenomenon, the design of a propeller suitable for 

 operation in oblique flow is carried out as follows. The propeller is first de- 

 signed as described above, with the design speed range determined by the value 

 of the mean pitch, and the coefficient K^ is made larger to allow for its subse- 

 quent decrease in the oblique flow. Then, using the known relation 



kp coscp 



max \ ' 



1 _ 11^ sin0 



the maximum instantaneous advance is calculated for the sections at different 

 relative radii. After this the curvature is determined by assuming that pres- 

 sure-face cavitation does not occur with this value of advance. 



For the curvature so adopted the element pitch is defined so that with the 

 mean advance of the propeller a prescribed thrust coefficient should be pro- 

 vided. As an illustration, Fig. 6 shows a comparison of curvature and pitch dis- 

 tributions for two propellers with equivalent thrust, one being designed for axial 

 flow and the other for oblique flow inclined by 12°. From this comparison it can 

 be seen that the greatest difference in the elements is observed in the way of the 

 root sections. The reduced curvature and increased pitch will obviously result 

 in the deterioration of propeller efficiency. Performance tests show that with 

 the inclination of flow by 10 to 12° the loss of efficiency due to nonoptimal pro- 

 peller elements ranges from 6 to 8%. 



Fig. 6 - Comparison of curvature and 

 pitch distributions for two propellers, 

 one designed for axial flow and the other 

 for oblique flow inclined by 1Z° 



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