Cavitation around the center of the blade. Try to get the prop sharper. It will help water stay in contact with the blade more.
I know that prop is aluminum, so you can't make it thinner, but you can carefully sharpen the leading edge.
Also shorten your water pickup. It does not have to be nearly so deep. I'd say cut at least half of the length off. Maybe a little more.
It could be cut flush to the bottom of the boat if you radius the front of the tube and bottom together going upward into the hole.