Need prop advice, porpoising issue

[SCB.Crsby]

I'm reviving my dad's 1978 Starcraft I/O Super Sport 18ft with the original Mercruiser 140 and I'm trying to dial in the prop. Right now, I'm running a 13 3/4 x 15 aluminum prop. My main concern is that at wide open throttle, around 3900-4300 RPM, I'm getting some pretty bad porpoising. It starts around 28-30 mph and just doesn't stop, even if I play with the trim and tilt. Top speed is only about 32-33 mph, and it feels like the boat's just bouncing across the water. Am I completely off on this prop size, and could I be running out of prop at those RPMs?

 

[Starzcrafter]

At the moment I'm thinking two things. Your weight distribution is way off which is why you're proposing. I'm also thinking too much pitch which not only could cause proposing, but it's the main culprit for dropping the rpm at wide open throttle. I'm thinking 13 x 13 should get you above 4500 which is where you should be sitting at.

 

[Clieon]

Porpoising that bad on a lightweight 18-footer with a 140hp I/O isn't just about prop size, it's a sign of poor weight distribution or even hull issues