Hub diameter is wrong. Exaust is either escapeing between the hub and lower unit or being excessivly sucked through the vent holes due to the decrease in hub diameter. Thats your cavitation problem.
I would try a 15 x 17 with the right hub diameter. That prop in the picture dosen't appear to have much cup ( Trailing edge of blade has a slight fold or "cup" to increase bite). Seacrafts like some cup in their prop. It provides stern lift. Stay away from a prop with cup on the tips. That can produce too much bow lift.
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