Unusual Two-Propeller Arrangements 



Fig. 9 - Propeller horsepowers 

 from tests with the auxiliary- 

 propeller system 



K the stress curve is looked upon as a curve of normal force, as mentioned 

 earlier, and the curve for the normal right-hand single screw is copied upside 

 down, the copy will give the normal force for a left-hand propeller. When the 

 two curves are placed together as shown in Fig. 14, their mean value represents 

 the normal force in the case of no tangential wake component, and the difference 

 between the mean curve and one of the other curves gives the additional normal 

 force caused by the tangential wake component. Figure 14 then indicates that 

 wake components are present as shown in Fig. 15. 



The stresses measured in the cases where one of the interlocking propellers 

 was removed, are compared in the same way in Fig. 16, which shows the curves 

 representing mean normal force and additional normal force. The transverse 

 wake components are indicated in Fig. 15. The normal force is at the same 

 speed, but the number of revolutions is higher for an outward- than for inward- 

 rotating propeller. When the model is self-propelled, this difference will result 

 in a lower number of revolutions and a higher wake for the outward-rotating 

 propeller, in accordance with what is well known from ordinary twin-screw ships. 



1533 



