Propeller -Induced Appendage Forces 



force with increasing propeller spacing. This is important for optimizing the 

 rudder location. 



Their statement that the forces are significant only at the blade rate and its 

 harmonics is valid only for a noncavitation condition. When cavitation occurs, 

 appreciable nondeterministic phenomena can be expected. Therefore these 

 measurements should be extended to cavitation conditions to gain more knowledge 

 about these effects. 



DISCUSSION 



Jerome H. Milgram '"? 



Massachusetts Institute of Technology 



Cambridge, Massachusetts - •' - ' ■■ " 



Two effects related to this paper require further examination. The first is 

 that the spatial rate of decay of a hydrodynamic disturbance due to a body should 

 follow an inverse power law in the far field. The authors stress the prediction 

 and observation of an exponential decay for their studies. As regards theoretical 

 studies, exponential decay could be obtained for certain configurations of infinite 

 cascades of hydrofoils. However, in interpreting such results for an application 

 in which infinite cascades do not exist, an interpretation that leads to power- law 

 decay would be correct. The authors interpretation of their experimental obser- 

 vations is not justified on the basis of the limited data available. It is easy to fit 

 an exponential curve closely to a few data points. K more data were available, a 

 more accurate fit to the spatial decay rate would be obtained by an inverse power 

 law. 



The second effect is that of force measurements on a foil spanning a tunnel 

 of rectangular cross section. Although the geometry appears two-dimensional at 

 first glance, the flow is not two-dimensional, due to the boundary layers on the 

 side walls of the tunnel. These boundary layers reduce the lift on the foil and 

 increase the drag on the foil. The change in lift is usually very small, but the 

 induced drag due to the boundary layer has about the same magnitude as the fric- 

 tion drag of the foil. 



231 



