Did you try the -2 IMCO shortie without the spacer and the big prop?

Ext box you have pictured is a +3. You probably already know that. If the box is known to wrk good on your hull go for it, seems rare they don’t work good. Have you tried a full diameter Maximus? My Baja actually likes the long tubes, lifts the whole boat out of the water. I’d also give a bblades 4 speed a try, long tube big diameter. If your boat doesn’t like the tubes they’re easy to remove. Most say a higher pitched prop spinning slower results in a little more speed vs a lower pitch spinning faster.

Either way good stuff and keep posting your findings

Same issues with all my props (3) that had the bell cut off. I blocked off the exhaust cavity in the drive to keep air from going thru it and the props grabbed water like you wouldn't believe. Planing and staying on plane at lower speeds no issues anymore. Was like magic.

I have through hull exhaust and the exhaust tunnel is blocked. I think my prop shaft is only about 4" below the bottom and that may be some of the problem as well. I am also pushing about 700hp with a 0-60 time of about 8 seconds going easy. I had a long talk with Tom the owner of Hydromotive when I was in town. He and Steve Stepp are old buddies and Tom really understands what the Velocity's like. Hydromotive's forging source was bought out by Yamaha so he's had to find a new source and get them dialed in. I can't wait to try a QIV 30P, I should be in the mid-nineties.

SB brings up a very good point. With the water rising 1” for every 10-12” behind the boat, your prop is probably closer to 5.5” below the water line, assuming the leading edge of your prop blades are 24”or so from the edge of the pad. So in theory that should be plenty of water for a four blade to hook up good, unless the air is traveling through the exhaust passge in the drive aerating the prop.

For comparison sake the leading edge of the prop blades on my setup are 43.5” from the edge of the pad. When I mounted my go pro in the notch of the boat it appears that the water is rising about 3-4”. So my .25” below the bottom is more like 4.25” at the prop

SB - I did the same thing with my -3 IMCO shortie lower. I used a piece of lexon to seal off the exhaust passageway in the lower. This Mod made a noticeable improvement in planning.

ummm... NO.. I did NOT.. was sold to me as a "neutral" box..
I don't have room to raise the engine ANY...

Guess I need to dig up a merc inner transom plate and see where it lines up...

I'm not sure what the exhaust tunnel is. Transom assembly or at/in the drive itself ?

Y pipe ?

You can always mount the box lower on the transom as long as the hole in the transom is covered or lowered.