Performance of Partially Submerged Propellers 



2. The propeller has inherently a higher efficiency in the base- vented 

 regime because of the higher L/D ratio of the blades. 



3. The advance ratio at which the propeller changes from fully vented to 

 base-vented flow is subject to scale effects. 



4. During the period that the propeller blade is entering the water, impact 

 forces are developed which contribute to the vertical force generated by the 

 propeller and, in the fully vented regime, contribute significantly to the genera- 

 tion of the spray. 



5. During the period that the propeller blade is exiting, entrained water is 

 carried into the atmosphere, creating much of the spray, particularly for the 

 base- vented regime and contributing a downward component to the vertical 

 force. . .; , :. 



6. Propulsive efficiencies in partially submerged operation comparable to 

 fully submerged noncavitating operation can be achieved, in spite of the losses 

 mentioned in 4 and 5 above. 



7. The partially submerged propeller has a narrower range of thrust load- 

 ings over which it can operate efficiently than it does when operating fully 

 submerged. 



8. It appears for the base-vented condition, at least within engineering 

 needs, that the results of tests at various depths of submergence can be nor- 

 malized on a thrust coefficient, c^', which is based upon submerged area. 



9. The center of thrust is near the vertical center plane of the propeller 

 shifting from the starboard side to the port side for a right-hand-turning pro- 

 peller when the flow changes from base-vented to fully vented. 



10. The vertical center of thrust is reduced significantly when the flow 

 changes from base-vented to fully vented. 



11. The vertical component of the transverse force is upward for the base- 

 vented regime shifting to downward in the fully vented regime. 



12. The horizontal component of the transverse force is to starboard for a 

 right- hand -turning propeller and the force coefficient is much larger in magni- 

 tude for the base-vented than for the fully vented regime. 



13. The appearance and intensity of the spray pattern changes significantly 

 when the flow changes from base-vented to fully vented. 



14. The supercavitating-type section shows a much smaller drop in thrust 

 than does a propeller with airfoil section when operating fully vented. 



15. The condition for similarity of cavity flow pattern on ventilated partially 

 submerged propellers requires that the speed of advance of the model full-scale 

 propeller should be in accordance with Froude's law of comparison. 



1483 



