wakeboard prop

Hi i bought an old 4.3m sportsman craft with a 1976 johnson seahorse 70hp. It has a 13 1/4 x 17 prop that is just not enough to get wakeboarder out of the water. After research i asked my local shop for 13 1/4 x 15 prop they gave me a 12 1/4 x 15 and said it would be all good the diameter wouldnt make any difference. I installed it and just tested it even with no one behind the boat the prop cavitated/ airated (unsure, motor reved faster and boat crept along) when taking off. Once up to speed was good but could feel even when going fast if i gave it to it it lost traction a bit.
i also fit a hydrofoil.
Prop is a turning point hustler with the swappable hub.
could the inch of diameter difference caise slipping?

 

Just had a thought. The centre ring is slightly smaller on new prop I think the exhaust may be getting blown straight onto the blades causing the slipping.
could that be possible issue?

 

Yep that would be the problem.

 

New prop and old prop. Old one is back on and will be taking new prop back on monday. Well i can mark that down in my life lessons book