on the surface of the material o As long ago as I89O, Sir 

 Charles Parsons carried out calculations on the probable 

 pressures exerted by the collapse of such bubbles, while 

 today Dro Harrison has given some figures suggesting that 

 such pressures can reach e::tremely high values » It would 

 be interesting to have Mr^ Eisenberg's views on this aspect 

 of the cavitation damage problemo 



Dr. Ho Wo Lerbs, .David Taylor Model Basin 



The two papers by Mro 'Hlisenberg on the "echanics of 

 Cavitation represent a comprehensive review and excellent 

 discussion of the available literature on this subject. There 

 are, in my opinion, only a few points which may be worth \diile 

 to be added to Mro Elsenberg's representation which points 

 are related to the special case of cavitation on a, propeller <> 



Firstly, I want to make one remark relative to the lav;s 

 of similitude for propeller model tests ^ These tests are 

 usually conducted, as mentioned by Mr. Eisenberg, with the 

 cavitation number,, referred to the center of axis^ as the 

 predominant parameter and with the Reynolds' number chosen 

 greater than a certain critical nimbero Doing so, an 

 unsimllarity takes place due to the fact that an element of 

 the propeller rotates in a vertical plane o This im-ilies that 

 the static pressure at an element is a periodic function of 

 position which ret^uires a suitably defined Froude number to 

 be satisfied for the model testo However, this is usually 

 In contradiction with an overcritical Reynolds' number and, 

 hence, the Froude number is disregarded » Considering then 

 the cavitation number of a propeller element, its average 

 equals that of the full scale propeller but both the ampli= 

 tudes and the periods are different o The error which arises 

 depends' on how the cavitation performance of a section depends 

 on a periodically changing cavitation number for an equal 

 average of this number o I have formerly tried to answer 

 this question by experiments with a series of geometrically 

 similar model propellers varying only one of the significant 

 parameters and keeping the others constant. Except for a 

 small variation of the Weber number, this could be acliieved 

 when working with different combinations of water temperature 

 and absolute size of the models. These experiments showed 

 that the force coefficients of a propeller model depend 

 on Froude 's number, ioe,, on the amplitudes of the local 

 cavitation number. However^ to obtain a clear answer to the 

 effect of a periodically varying cavitation index, such tests 

 should be carried out v\/ith a hydrofoil and not with an 

 assembly of different hydrofoils as represented by a propeller, 



