Johnsson and Stfntvedt 



1. From the vibration measurements in full scale it can be concluded 

 that the introduction of afterbody fins lowered the level of the blade 

 frequency hull vibrations. No further improvement seems to have 

 been achieved by replacing the original five-bladed propeller on the 

 first ship by a six-bladed propeller of different design on the second 

 ship. For all configurations the amplitudes were somewhat larger in 

 ballast than in fully loaded condition. 



2. The conclusions drawn above were confirmed in general by the 

 results of the measurements of hull pressure fluctuations in different 

 points of the stern, made simultaneously with the vibration measure- 

 ments. In fully loaded condition the amplitudes of the pressure fluc- 

 tuations were, however, larger for the six-bladed than for the five- 

 bladed propeller over most of the speed range, the vibration levels 

 being about the same, as mentioned above. 



3. Under non-ca vita ting conditions reasonable agreement was obtain- 

 ed between cavitation tunnel, towing tank and theoretical calculations 

 for the amplitudes of the blade frequency harmonic of the pressure 

 fluctuations. The pressure fluctuations obtained were similar for the 

 different hull -propeller configurations. 



4. The amplitudes of thepressure fluctuations in full scale and those 

 obtained in the cavitation tunnel under cavitating conditions showed 

 reasonably good agreement. This applies to the maximum peak to 

 peak values, as well as the first harmonic for the fully loaded condi- 

 tion. In the ballast condition the amplitudes were smaller in model 

 scale. 



When judging the relative merits of the five- and six-bladed 

 propellers from the vibration point of view on the basis of pressure 

 fluctuations, measured in the cavitation tunnel under cavitating con- 

 ditions, the results have to be analysed very carefully. Thus, in the 

 present case, a relation between the different hull -propeller configu- 

 rations, agreeing with the tendency of the full scale experiences, was 

 obtained only when the maximum peak to peak values were used for 

 comparison. Apparently, the pressure signals, recorded for the five- 

 bladed propeller, were of pulse nature with rather unstable phase 

 shifting. Consequently they did not affect the levels of the filtered 

 signals, obtained in non-cavitating flow. 



In the cases considered the amplification of the amplitudes of 

 the pressure fluctuations, caused by cavitation, was larger for the 

 six-bladed than the five-bladed propeller. Results reported in [1 lj 

 show the same tendency. 



602 



