Dick, we're on the same wavelength but perhaps you and I tested different props.

Way back when a couple of buddy's and I were just getting into the whole electric thing we made up a rig to test thrust vs amp draw and played with a wide range of motors and props. At the time these were brushed motors and gearboxes as well as direct drive.

For lower pitch props our results were the same as you found and for a given motor the thrust and current climbed in step up to a point. As we used props with higher pitches things suddenly changed at around the 6 to 8 inch pitch range. At that point the thrust would still go up with the higher pitch but the current went up far more than the prop change would indicate. The thrust/amp at that point didn't climb in a linear fashion anymore. We all noticed a marked change in the sound at this point also. Hence we chalked it up to the idea that the prop was running in a stalled condition. The sudden drop in the "efficiency" as shown by the thrust to current ratio seemed to confirm this. The key here was that the motor was suddenly drawing a LOT more watts of power but the thrust went up only a small % or in a couple of cases didn't go up at all.

Since you were after hovering props I'm going to guess that you didn't try any that were higher in pitch and would be considered as more a higher speed prop?