Johnsson and Stfntvedt 



DISCUSSION 



John P. Breslin 



Stevens Institute of Technology 



Hoboken, New Jersey s U.S.A. 



Unfortunately, I read this paper late last evening and have not 

 been able to prepare a written discussion. I wish first of all to defend 

 the unsteady lifting surface theory which is castigated by the authors 

 (onp. 599) on the unexplained comparison shown on Figure 24. Here it 

 is shown that the variation of blade bending moment with blade posi- 

 tion is seriously over predicted by the unsteady lifting surface u. 1. s. 

 theory whereas the model measurements are very well fitted by the 

 QnY. calculations. The u. 1. s. theory was found for this application to 

 yield excessively high values of the mean thrust and mean torque, al- 

 though the unsteady force and moment components were found to be 

 quite close to experimental results available to QnX (This point is not 

 at all mentioned by the authors). We subsequently furnished EtnV. with 

 an engineering correction for the mean loading distribution which to- 

 gether with the unsteady components placed the resulting bending mo- 

 ment variation much closer to the measured variation. The correc- 

 tion was based on the use of the thrust and torque coefficients from 

 Troost's charts which for the mean wake value (obtained without the 

 propeller) agreed very closely with the values provided by Qn\£ from 

 the self -propulsion test. This agreement of coefficients made it dif- 

 ficult for the discussor to believe that the propeller was seriously af- 

 fecting the wake. In view of this subsequent effort I must strenuously 

 object to this comparison which unfairly costs grave doubts on the 

 usefulness on the u. 1. s. program developed at Davidson Laboratory. 

 Had the authors only informed us of this intended comparison we would 

 have provided a still latter result obtained by application of a more 

 exact theory. The results were recently provided by Qny, and they 

 show excellent agreement of both measured and vibratory forces were 

 found to be virtually equal to those from the less exact, earlier cal- 

 culation. 



It is very gratifying to learn that Mr. Huse and researchers at 

 NSMB have realized that the influence of intermittent cavitation on 

 blade frequency pressures predominantly arises from the time varia- 

 tion of the cavity volumes. I have been espousing this mechanism for 

 some time in the USA where it has been rebutted by the idea that it is 

 due to the thickening of the blade sections in steady-state fashion. At 



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