Propeller Excitation and Response of 230000 TDW Tankers 



Mean Thrust, Thrust Fluctuations and Shaft Res ponse 



Recordings of mean thrust are presented in QJ , QT] and^4~| . 

 In Figure 9 results of calculations of the natural frequency in the 

 axial mode for the two ships are shown, assuming different values of 

 the thrust bearing stiffness. As shown in the Figure, the measured 

 natural frequencies for the two cases correspond to almost the same 

 value of the thrust bearing stiffness, indicating that recorded reso- 

 nances of shaft vibration in the axial mode are predicted with fairly- 

 good accuracy by a method now in use at DnV [j>J . The vibratory 

 output from the shaft through the thrust bearings is of moderate ma- 

 gnitude for all ship - propeller configurations. 



Propeller Blade Erosion 



The results of erosion studies in full scale and model scale 

 were given in [_4J and will not be discussed in the present report. It 

 should, however, be noted that the eroded areas were similar in model 

 and full scale. Regarding the relative merits of the different configu- 

 rations, the full scale, as well as the model tests, showed that the 

 area of erosion was not reduced essentially by the introduction of fins 

 on the first ship, but was eliminated by fitting the second ship with a 

 six-bladed propeller of new design. On the propeller of the first ship 

 the erosion was, however, less rapid after the introduction of fins. 



V. MODEL TESTS 



V. 1. Test Arran g ements and Facilities 



Cavitation Tunnel 



The cavitation tests were carried out in the new, large cavita- 

 tion tunnel of SSPA. This tunnel, see \jT\ and Figure 10, is powered by 

 a 1 000 Hp motor and has two interchangeable test sections, one 

 being circular, as the remaining part of the circuit. The other test 

 section, which is of interest in this connection, is of rectangular shape 

 with a breadth of 2. 6 m , a height of 1.5 m and a length of about 

 10 m. The section is covered by a recess in which the ship model is 

 placed. This model is the one used in the towing tank for the self pro- 

 pulsion tests and it is normally made of paraffin wax. 



The vertical position of the model is adjusted in such a way 

 that the waterline, corresponding to the level of the free water surface 

 in the towing tank, is flush with the top of the test section. Individual- 

 ly cut wooden plates are then fitted to simulate the free surface, and 



591 



