Unusual Two-Propeller Arrangements 



NORMAL SINGLE SCREW P No 6507 



— . NORMAL TWIN SCREW P No 5915 



INTERL PROPS. INW. OIST. 09 "D.PNoSOT 



0« •D. P No. 6137 

 til -O. P No. 6137 

 0£8<O.P No S91S 

 OUTW. ■ 677-D.P No J915 

 0.68 -O.P No 5915 



MAIN PROP AUX. PROP SYSTEM P No 6507 



« ■ SMALL PROP • • P No 6109 



12 13 U 15 16 17 18 

 FULLY LOADED 



12 13 U 15 16 17 18 KNOTS 

 BALLASTED 



Fig. 11 - Model torque wake coefficients 



The Auxiliary Propeller System 



In this case the auxiliary propeller system does not yield favorable results 

 from the point of view of efficiency, but it must be remembered that the small 

 propeller is too large and works at a poor value of the advance coefficient. The 

 results from the stress measurements are nevertheless interesting. 



As mentioned earlier, it was possible to find on an oscilloscope the best 

 number of revolutions for the small propeller. A good result was obtained 

 within rather wide limits, but it was also seen that it was not possible to remove 

 all of the high-stress peak for the upper position of the blade. A further in- 

 crease in the number of revolutions for the small propeller only gave a decrease 

 of the stress in front of the remaining stress peak as shown in Fig. 12d. It must 

 therefore be expected that on account of the tangential wake component a position 

 of the axis of the auxiliary propeller slightly outside the center line will be opti- 

 mum. It must also be expected that an additional auxiliary propeller below the 

 main propeller axis will almost completely smooth out the stress curve. 



1535 



