The Vibratory Output of Contrarotating Propellers 



draft 



absorbed power : 



forward propeller 



aft propeller 



8.839 m, 



16,000 SHP, 

 8,200 SHP, 

 7,800 SHP. 



To measure the vibratory output, a special arrangement had to be made to use 

 the existing measuring equipment. Because it was not possible to have the usual 

 hollow- shaft combination for the measurements, the two propellers were driven 

 by two synchronized separated systems. The forward propeller was driven by 

 the normal dynamometer, installed in the wooden model of the ship under inves- 

 tigation. The aft propeller was driven by a dummy dynamometer. The dummy 

 dynamometer was installed in an open-water boat mounted behind the ship model 

 in such a way that both propellers were properly positioned relative to the hull. 

 A stiff coupling shaft synchronized the combination. By exchanging the real 

 dynamometer and the dummy dynamometer the vibratory outputs of both propel- 

 lers were determined. Figure 3 shows the combination of the ship model and the 

 open-water boat. 



OPEN WATER BOAT 



MODEL 



Fig. 3 - Schematic view of the ship model and open-water boat 



Due to the vibratory interaction of the two propellers an exact synchronism 

 was required to avoid a nonsynchronous signal that could not be reduced properly 

 by the correlation equipment. Different positions of mutual blade encounters 

 relative to the hull can be characterized by the angular positions of the mutual 

 blade encounters. In Fig. 4 the angular positions and the sequence of the mutual 

 blade encounters are shown for the case when the first blade encounter is ver- 

 tically upward. The angular position of the nth encounter 9^ is then given by 



40 



(n - 1) 27T radians . 



239 



