Performance of Partially Submerged Propellers 



considerable exchange of energy taking place which can give rise to large im- 

 pulse forces. On a partially submerged propeller the time duration of the im- 

 pulse force can be relatively large when considered in relation to the time of 

 one revolution. For the root sections of Propeller 3768 when operating in a 

 semisubmerged condition, the impulse phase lasts as long as one-fourth of a 

 revolution, and its duration is approximately one-third of the time the blade is 

 in the water. Even at the 0.7 radius the duration is more than 20% of the time 

 the blade is in the water. -. 



Open-Cavity Phase. This phase occurs when the blade proceeds beyond the 

 flow-forming phase, and an open cavity grows outward from the region of flow 

 separation, either base-vented or fully vented, depending upon the advance co- 

 efficient of the propeller. This phase is the dominant regime of the propeller 

 action and has been discussed in the previous section on performance. 



Considerably less knowledge exists on the water-exit regime of rigid bodies 

 such as missiles, which geometrically are far from similar to a rotating pro- 

 peller blade. It is known from this work that the amount of entrained water 

 exiting with the body is equivalent to 7 - 12% of the body volume. This tends to 

 reduce the vertical force on the propeller. This is also the source of much of 

 the spray generated by the propeller when operating in the base-vented regime. 



We can now examine the results of the measurements made of thrust ec- 

 centricity and transverse force on Propeller 3768 as shown in Figs. 13- 18. The 

 test results for horizontal thrust eccentricity e^^ show that the center of thrust 

 in the base- vented condition is a small distance, less than 5% of the propeller 

 diameter to the right, or starboard, for a right- hand- turning screw; whereas 

 for the fully vented condition it is almost the same amount to the left, or port. 

 This implies for the base-vented condition that more blade lift is developed on 

 the entry half of the revolution, while for the fully vented condition more blade 

 lift is developed on the exit half of the revolution. 



The vertical position of the center of thrust is obviously a function of sub- 

 mergence of the propeller, the less the submergence the larger the eccentricity 

 e^. The most marked effect is in the magnitude of the shift between base-vented 

 and fully vented operation. The implication of these results is that there is a 

 very large shift of the center of loading towards the root in the change from 

 base-vented to fully vented operation. The reason for this is not clear. 



The results of the transverse-force measurements. Figs. 16 - 18, show sig- 

 nificant vertical force as well as the expected strong horizontal force. As 

 noted before there is a marked difference in the magnitude of these forces for 

 the two flow regimes, the base-vented condition producing the larger forces. 

 The horizontal force coefficient for the fully vented condition appears to be in- 

 dependent of the test speed. Just the opposite is true for the base-vented con- 

 dition, where the force coefficient varies significantly with test speed. This is 

 not unexpected, as both the base and viscous components of the blade drag de- 

 crease with increasing speed. It would be expected that smaller differences 

 would have been obtained at the higher test speeds. This again points to the 

 necessity for proper modeling of the section a if successful predictions are to 

 be made. 



1479 



