Dynamics of Hydrofoils as Applied to Naval Propellers 



The theoretical lift coefficient for this 

 foil, as calculated by integrating the velocity 

 increments along the chord, results at the 

 value of 0.2076, that is, very close to the value 

 0.208 obtainable by means of relation (6). The 

 experimental value of the lift coefficient, as 

 given by the integration of the measured pres- 

 sures diagram, closely approaches 0.151. 



The loss in lift seems to be largely at- 

 tributable to the positive pressure fall which 

 occurs all along the face of the foil, but espe- 

 cially in the vicinities of the leading and trail- 

 ing edges. On the contrary, the depression on 

 the back, for the entire width of the foil, prac- 

 tically matches that calculated theoretically; 

 a fact which could lead to the conclusion that 

 viscosity has little or no influence on this 

 particular aspect of the phenomenon. 



The verification of such a result is of 

 obvious importance from the cavitation point 

 of view. In any case the subject needs to be 

 gone into more thoroughly, and this empha- 

 sizes the usefulness mentioned earlier of ap- 

 propriate, systematic research. 



5 10 20 30 40 50 60 70 80 90 100 



It Should be noted, though, that during the Fig. 11 - NASA 16-Z06: 



tests the onset of cavitation always took place, experimental and theo- 



as far as could be seen, at values of the cavi- retical results 



tation index 20-25% higher than those evalu- 

 ated by theory. This could explain the usual 



practice of reducing the cavitation index by the same percentage both when de- 

 signing a naval propeller, and when evaluating the results of water tunnel ex- 

 periments. 



However, one should remember the difficulties of establishing the exact 

 moment in which cavitation takes place, either by natural observation or by us- 

 ing stroboscopic light, and one should also remember the error involved in sub- 

 stituting the critical value of the pressure by the vapour pressure at test tem- 

 perature. 



3. EFFECTIVE PERFORMANCE OF THE PROPELLER 



3.1 Propeller Models 



The difference between the performance of a hydrofoil in ideal fluid and its 

 performance in actual fluid has already been mentioned, and the reduction coef- 

 ficients 0.675, 0.75, and 0.75 have been suggested for the mean lines a = l, a = 0.8, 

 and NASA 65, respectively. 



1035 



