van Manen and Oosterveld 



0.6 



2 O.i 0.6 



E Hor. In meter 



E Hor in meter 



Fig. 16 - Instantaneous thrust eccen- 

 tricity of cargo liner propellers 



Figs. 14 and 15 show clearly that the variations in torque and thrust of the 

 forward screw of the contrarotating propellers are about the same in magnitude 

 as those of the conventional screw, which implies that these variations, ex- 

 pressed in percentages of the mean values, are about twice as large for the for- 

 ward screw of the contrarotating propellers as those of the conventional screw. 

 The variations of the aft screw are lower than those of the forward screw of the 

 contrarotating propellers. Apparently the forward propeller smoothes the pe- 

 ripheral irregularities of the flow in the way of the aft propeller, as was also 

 evident from the cavitation tests. 



It appears from Fig. 16 that the thrust eccentricity of the forward screw of 

 the contrarotating propellers is considerably larger than that of the conventional 

 screw. However, the thrust of this forward screw is about half as large as that 

 of the conventional screw, so that the maximum bending moments due to the ec- 

 centricity of the thrust will not change very much. This implies that the stresses 

 due to these bending moments must be almost equal, if the diameter of the outer 

 shaft is the same as that of the shaft of the conventional screw. Since the inner 

 shaft diameter is smaller, possibly the loading of this shaft increases, since its 



154 



