Propeller -Hull Interaction 



the advance speed 



i Vq = 75 T7P/S , 



and the effective revolutions 



' "0 = Vq/Jo D . 



The fundamental efficiencies equation is thus satisfied without the necessity of 

 any corrective factor: 



1 - t _ 75RV _ RV/75 ehp 



~ 1 - Wq "^^ 75 S Vq ^° ~ SVo/75 Vq dhp 



Two typical examples of the practical application of simultaneous thrust and 

 power identity criteria are presented in Fig. 3. In the first example, the case 

 of a central propeller, the axial wake factor obtained has a lower value than that 

 obtained by applying either the thrust identity or the torque identity criteria. 

 The tangential wake factor is slightly less than unity. 



In the second example, which refers to the case of propulsion with two 

 lateral propellers (outward- rotating), the axial wake factor, on the contrary, is 

 higher than those determined by the two classical identity criteria, while the 

 tangential wake factor is slightly higher than unity. 



As this result recurs qualitatively for all the cases examined, it seems 

 possible to reach the conclusion that the overall inflow to the propeller is en- 

 dowed with a rotating component in the same direction as the propeller in the 

 central-propeller case and with a rotating component in the opposite direction 

 to the propeller in the outward- rotating lateral-propeller case. In this regard, 

 it may be observed that, independent of the identity criterion adopted, it is es- 

 sential that the available open-water tests are carried out at a Reynolds number 

 high enough to ensure that the performance of the propeller is surely turbulent 

 and, therefore, comparable with the behind flow. K this is not so, it is advis- 

 able to correct the open results by the well-known Lerbs method (J.A.S,N.E. 

 1951). As the application of this method implies, at equal advance coefficient, 

 a reduction of the torque coefficient, while the thrust coefficient remains 

 practically unchanged, the Kq coefficient will be too high. 



To sum up, both the Jg and J^ advance coefficients would tend to approach 

 to the intermediate J^ value in the central-propeller case, while in the lateral- 

 propeller case they would both tend to move away from the Jt value. 



Therefore it seems that if the analysis is carried out with open-water dia- 

 grams deduced from turbulent-flow tests (or corrected for turbulent flow), 

 instead of with diagrams deduced from experiments in flow which is not en- 

 tirely turbulent, the rotating component in the inflow would be of a lower in- 

 tensity in the central-propeller case and of a higher intensity in the lateral, 

 outward-turning-propeller case. A possible explanation of this different be- 

 havior may be the following: For a central propeller it is possible to consider 

 the inflow as divided into two equal symmetrical fields; for a lateral propeller, 



1648 



