Unusual Two-Propeller Arrangements 



out to examine the possibility of reducing the high wake peak just behind the 

 stern, and the results indicate that this may be done. K so, the developed 

 blade area may be reduced, and a higher propeller efficiency for the main pro- 

 peller will be obtained. This, together with a specially designed auxiliary pro- 

 peller, should give a higher total efficiency. Yet the main reason for using this 

 arrangement should still be the reduction of the risk of vibrations and cavitation 

 on the main propeller. 



The stress measurements give no direct information on the reduced risk of 

 cavitation. It must be expected that a smooth stress curve is an expression of a 

 smooth wake field, for which it is easy to design a cavitation-free propeller, but 

 only cavitation tunnel tests will give the final information. 



At zero speed, stress measurements were taken at full power and showed 

 for both propulsion systems greater variations than normally found on single- 

 screw ships. In the case of interlocking propellers, it is expected that these 

 stresses will be reduced considerably for greater shaft distances. This problem 

 will be studied carefully in coming experiments. For the auxiliary propeller 

 system, it is obvious that the remedy will be to make arrangements permitting 

 this type of propeller to be coupled in only when a certain forward speed has 

 been obtained. 



NOMENCLATURE . - , 



D Propeller diameter 



X Nondimensional radius ■ . .: . . 



W Wake L . / 



W Resultant wake 



r 



W Tangential wake component : .r . 



W Axial wake component 



n Number of revolutions per second 



V Ship speed 



V Inflow velocity 



V. Velocity induced by the propeller itself 



V. Velocity induced by the other interlocking propeller 



N Resultant force normal to the blade-root section 



EHP Effective horsepower 



1541 



