Unusual Two-Propeller Arrangements 



Fig. 7 - Propeller horsepowers from 

 tests with outward-turning interlocking 

 propellers . •, . 



while the inward-rotating propellers give almost the same result as the normal 

 single- screw. None of the propellers were optimum propellers. For instance, 

 at 16 knots the optimum number of revolutions for the single -screw and for the 

 outward- turning interlocking propellers were 106 and 94 respectively, while the 

 actual numbers of revolutions in the self-propulsion tests were 115 and 74. 



From Fig, 11 it is seen that the mean wake is higher when the propellers 

 are turning outward than when they are turning inward, and that the mean wake 

 for each interlocking propeller is decreased by the action of the other, espe- 

 cially for the inward-rotating propellers. The differences in total efficiency 

 are caused by the differences in wake, a higher wake giving a lower propeller 

 efficiency, but a higher hull efficiency. 



A small shaft distance is not advantageous. As the propellers are moved 

 inward the wake and thrust deduction increases, but the interaction of the pro- 

 pellers will increase also and decrease the resulting wake. This additional 

 inflow velocity is dependent on the propeller load, and as the resulting wake 



1531 



