That worried me too but I was focussing on the mixture issues.

I have built maybe 3-4 airboats like this over the years, in .15-.25 size, and they have always been great fun. However I ran all of them in tractor mode with the tank in the engine pod. CG has never troubled me, they all bank into the turns. I make the pylon flush with the rear of the engine/tank pod, and use that as the vertical fin, with a single large rudder hinged off that and no water rudder. They all had great rudder authority, and could be driven on water, grass or snow. Good cheap fun. Most were made from foam flutterboards covered in either polyurethane or epoxy finish, and a square of 1/8" plywood bonded to the deck where the pylon fit through for support. I inset the pylon into the foam with liberal amounts of epoxy (though now that it is availableI would much sooner use Gorilla Glue or Probond), and glassed or V-stocked the corners to the plywood base. Never ripped a pylon out, though I never drove one into anything at full speed either. Hold the engine pod for starting and launching though.

The smaller all foam flutterboards I used to get at our erstwhile Canadian Tire chain worked great for .049 free running airboats, up to .25 powered 2 channel affairs. A little nose weight was needed with the .25, but the performance was really good, and decent with a .15FP. I haven't looked for those board for a while, I worry about their availability now. Of course, cutting one from 2" foam is hardly a major foam cutting challenge.

I still have the equipmentless hull of the .25 version, I should snap a picture. These are "great fun for the kids" too, simple and worry free RC giggles. Other than starting and launching of course, but that's why Dad is there! Here boys, better let me test drive it some more..

MJD