Panel Discussion 



Fig 



0.5 10 1.5 2 2 5 



LCB POSITION AS % Lpp FORWARD OF MIDSHIPS 



. 2 - LCB position as % Lpp forward o£ mid- 

 ships; delivered horsepower coefficient 



0.398 for the model, which results in a value of 0.92 for the ratio (1 - W^)/(l - wj. 

 In ballast condition, the wake on the ship was 0.335. Unfortunately, the model test 

 did not allow the wake to be estimated in the ballast condition. 



Dr. Ing. A. Melodia (Cantieri Navali del Tirreno e Riuniti, Genoa) proposed 

 a criterion for the analysis of the performance of a propeller behind a hull. The 

 classical methods for the analysis of the performance of the propeller behind a 

 hull follow two different criteria: the thrust identity and the torque identity. Such 

 an analysis required the introduction of the relative rotative efficiency concept 

 and gives different results for both wake and propeller efficiency. In consequence, 

 the analysis gives different values for the relative rotative efficiency too. In order 

 to eliminate the ambiguity of analysis, it is usual to assume as wake value the 

 arithmetic mean of the values resulting from the application of the two different 

 criteria. 



Dr. Melodia, for many years (see Papers of the Collegio degli Ingegneri 

 Navali e Meccanici, Genoa 1954), had adopted for the study of propeller perform- 

 ance a criterion of analysis which eliminates the ambiguity of the relative rota- 

 tive efficiency by introducing the concept of the relation of tangential wake to the 

 number of propeller revolutions, a concept which is similar, physically, to the 

 one of relating axial wake to speed of advance. 



Assuming that the operating point of the propeller is the one for which the 

 same power corresponds to the same generated thrust, the comparison coeffi- 

 cient between open and behind propeller is no longer either Kj or Kq, but the 

 Kg coefficient, given from the relation 



K. 



K,3/Kq2 



[(27t/75)^ S3] /pD2p2 



As Kq is independent both of the propeller revolutions and the advance speed, 

 entering into open-water propeller diagrams with the Kg value, determined by 

 self-propulsion tests, gives the corresponding values of Jq and '?o, by which it 

 is possible to calculate the axial wake factor 



the tangential wake factor 



(1 - w„) = V./V 



(1 - ^Mq) - ng/n, 



1647 



