Johnsson and Srfntvedt 



DISCUSSION 



Marinus Oosterveld 



Netherlands Ship Model Basin 

 Wageningen, Netherlands 



I should like to make some comments on this very interesting 

 paper. 



From model tests and structural calculations it has frequently- 

 been found that an increase of the excitation of the hull occurs when 

 the propeller RPM approaches the service RPM . Explanations for 

 the increase in excitation are : 



- from an economic point of view, most of the screw designs 

 are such that at service speed the propeller cavitates. For a proper 

 designed screw this cavitation does not give problems with respect to 

 erosion. However, as clearly shown in the paper, due to cavitation 

 the propeller generated hydrodynamic forces may increase substan- 

 tially. 



- the lowest natural frequencies of the transversal and axial 

 shaft vibrations of large tankers lie often in the blade frequency region 

 at service speed. Consequently the shaft vibrations and bearing reac- 

 tion forces are enlarged a great deal. 



- a proper analysis of the whole vibrations therefore requires 

 a determination of the propeller generated hydrodynamic hull and 

 shaft forces, and a determination of the response of the shafting. 



That brings me to my first question. Have calculations of the 

 shaft vibrations been carried out in order to determine the magnifica- 

 tion effect due to the resonance of the shafting ? These calculations 

 may have influenced the choice of the number of the blades of the 

 screw propeller. 



Secondly, the introduction of the afterbody fins lowered the 

 level of the pressure fluctuations on the ship's hull. Are large differ- 

 ences found in the wake field, and especially in the tangential compo- 

 nent of the wake field due to the introduction of these fins ? It is a pity 

 that the results of the wake field measurements are not given in the 

 paper, as these are a base for the analysis, I suggest that the authors 



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