Adept at Depth 



Fig. 1 - Ducted thruster with ring tail 



diameter are unlimited. The two designs shown represent reasonable optimiza- 

 tions within the constraints imposed. 



We will show some of the hydrodynamic test results which, when considered 

 along with such other criteria as mission power consumption, permitted a clear 

 choice to be made. Figure 3 compares large-scale-model test data, extrapo- 

 lated to full scale, in the sway or heave mode. The measure of excellence, or 

 performance factor, is chosen as developed pounds of thrust per horsepower 

 plotted against velocity in the applied direction. Performance in pitch or yaw is 

 easily derived from these data. Surge performance is derived in quite a differ- 

 ent way and posed, perhaps, the most difficult decision criteria. On the one 

 hand, an independent system is provided for surge, and, therefore, it and the 

 body shape may be optimized for this mode without undue penalty to the other 

 degrees of freedom. On the other hand, in the tandem propeller, surge is pro- 

 vided by the same system which provides the other degrees of freedom. While 

 it enjoys a redundancy factor in that either propeller can provide surge motions, 

 it suffers from not providing the ultimate in cruising efficiency. A comparison 

 of the two systems' energy consumption per mile over a range of to 5 knots is 

 shown in Fig. 4. This imposes no penalty for power which may be required for 

 thruster control below 1-1/2 knots, where shroud control force reversals occur. 

 The sixth degree of freedom, roll, is hydrodynamically possible only with the 

 tandem propeller system. 



307 



