Pair of 25’ Mirage plus in the swap shop cheap.

If those plugs are out of the prop hub those should be plugged. No reason those should be out with your setup.

What plugs? Prop vent holes? I called Bblades about this the other day. Brad-I think-said the vents could suck air thru the exhaust passage when the boat starts to climb out of the water and transom assy comes above the surface. That would cause it to blow out and slip. That is why I say he should block that exhaust passage in the drive.

Yes the prop vent holes. I've even heard of them blowing out from pressure coming in through the back of the prop hub after planing with no with no Y pipe or the Y pipe already blocked off.

You have to block the holes in the prop. They are there to ventilate the prop to add slip so engines with no low end torque can get up into the rpm / torque band.
Also putting a cap on the exhaust inlet at the front of the upper ( in the gimbal ) will help but not stop air from getting to the prop. With my boat getting on plane is not like getting on plane with a deep V. What I did was split the upper and lower and used a piece of lexon and 5200 to seal the exhaust passage to stop air from getting to the prop. There isn't any gasket between the upper and lower and even if you cap the inlet it will suck air in the gap between the two halves of the drive.

Alright.....most regular boats with regular motors will have high slip or too much slip to plane with:

Either
1) Vent holes in the prop that aren't plugged. Been there myself as have others !
2) Bellmouth cut off at end of prop. Been there myself as have others !
3) Something like a drive shower pickup that is mounted too deep below cavitation plate. Been there myself as have others !
4) Replacement drive with no or very little of a cavitation plate.

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That said I ran into a particular situation that made me discover and do almost what Tinkerer had done.

Single engine I/o.
I had some labbed props that where driving me nuts by either having super high slip everywhere in the rpm band or not planing at all. Each had the diffuser//bellmouth at end of tube taken off. I was told from ,amy this just effects stern lift. Yeh, okay.....I learned it effects more than that.

Even though I didn't have exhaust going thru drive and out the drive, I knew air was still present because of the exhaust inlet to drive being open. So, I kept asking myself 'what if ?' and finally plugged exhaust inlet to drive and wallah ! These hunk of schit mega slipping props now had as low of slip as my other props. Amazing ! :)

I’ve been chasing some high slip numbers and just figured that it was because I have an open bow, but have the CC tube removed/blocked off with no exhaust bellows to cap off the drive side. I’ll give that a try.

Again, this was with props that had the diffuser cut off. Brutal until I plugged the exhaust inlet on drive. I have not tried this with props with the diffusers still there as they worked perfectly fine slip wise.

if you have an exhaust bellows around, just put it on. otherwise, a pvc pipe cap from lowes and a buncha silicon-not 5200-should do the trick. there may be a rubber household pipe clamp-on version that is close enough also.

I’m using a Rev4 prop that has no diffuser on it and just plain straight. A 3” rubber cap from the Home Depot fit on the transom side like a gem. It should work on the outdrive as well.

SB, don’t worry - if my boat doesn’t pick up 5-10 mph I’ll get on here and blast you for giving out bad info, lol.