If all other attempts to correct have failed, I would seriously consider a smaller diam prop. If no other 'correction' has made any diference, I would still say that the prop walk is doing this to you, or the hull is not flat (compared to the engine/prop). Although the transom has been cleared, the air is still being trapped by the gunwale. Keep in mind that prop walk is the sideways paddling of the prop(similar to a sidewheel on a riverboat - or a squirel cage fan). even though the prop pushes air behind it, it also pushes air to the sides, usually equally on top /bottom, side/side. with the bottom of the prop swinging below the gunwale, the sideways air movement at the bottom of the swing is negated, while the air at the top is uninhibited. I believe a 4 blade prop could go as small as 16 inches(with the same pitch), and should clear the top of the gunwales. the other thing is to cover the boat, gunwale to gunwale, under where the prop swings. This will allow the air to flow out the side on the down swing, rather than still being trapped by the gunwale. A smaller diam prop would probably decrease the problems, as the tips of the blades would be traveling slower(than the longer blades) so probably not give as much walk. On the other hand, there will be twice as many.

First though, how about trying the 'flap' instead of horizontal, mounted vertical. this could be set to counter the prop walk (similar setups are used on full size boats - water props - there is a small rudder 3-4" on the outdrive, to keep the outdrive from being torqued by the propwalk) The small flap/rudder is pitched to allow the craft to run straight, without the rudder being adjusted.

Keep trying. Thomas Edison had to try over 200 times to make a lightbulb that worked, give your boat a few more trys. Once you figure this out, you will be an expert, and people will ask you how to keep this from happening!
Sean

Something I noticed ....
When running the water , it seems like there is something causing drag on the left side ( rear ) . Is this possible ? Maybe uneven bottom ... or uneven weight on that side ? ( if this IS the case , this ALSO would cause the hull to pull as it is )

One more thing .... did you try putting some strakes on the bottom to correct the turning situation ? Throw on either one running the center from front to back , or two , seperated equally to each side also running the full length of the hull . ( or at least 3/4 starting from the back and going forwards )

dogmoat, Like a 3rd fixed rudder, like this?
Terbo, There are wear strips on the bottom, but the odd part of the whole thing is that it turns beautifully, left and right, just like you'd expect it to.....at half throttle. the problem is at full throttle. I should shoot some vid at half throttle to show what I mean.

As far as balance goes, 30% hull length is 14.4. (hull being 48" x .3 = 14.4 ) the CB is at 14 1/2. side to side balance seems to be right on the money too. I don't think hull is twisted: it sits flat on the garage floor without rocking.

I have a feeling that at top end, the rubber mounts are allowing the prop to walk a bit, changing the vectoring of the thrust.

Yes! 3rd rudder, whatever you want to call it. this could fix the symptoms. and allow you to run straight, but it may cut down on the top speed (it may not be noticeable). Just be warned, as I said this may fix the symptoms, but not nessicarily the problem. Chris may be on to something with the motor mounts.

I had a similar problem with one of my boats; suprisingly it was a pontoon style(they usually run really straight, hard to turn). After trying everything I could think of, I finally replaced the pontoons with new ones. Viola! Something I could not see nor detect was unbalanced. It was a radical solution, but it worked. Keep trying. You will find a solution!
Sean