First though, how about trying the 'flap' instead of horizontal, mounted vertical.
dogmoat, Like a 3rd fixed rudder, like this?
One more thing .... did you try putting some strakes on the bottom to correct the turning situation ?
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.