Karl Townsend wrote:

My apple water bin dump uses a .5 horse 3 phase 220 VAC motor attached by
pulleys and belt to an outboard motor prop in a tube to pump water. Maybe
1"
head at 100+ GPM. I just measured the current draw on this motor at 1.5
amp,
2.0 amp, and 2.2 amp with a clamp A-meter.

Na, I'm not totally re doing something that has run 20 years. But I'd like
to try a different prop - it might be better. If not, I'll go back. My only
question is if this motor will have enough power at 1/2 speed. Do you know
how to figure that?

It likely will be fine. You could also change the pulleys to get the
ratio you need. Indeed, you could find the correct speed using a VFD and
then replace the VFD with the correct pulleys to get the prop speed you
need at the full motor speed and put the VFD on the shelf for the next
project.