Sean , you can NOT apply water prop driven boats "rules" to those of an airboat ( when talking CG and thrust ) . TOTALLY different forces being apllied . And whilst an airplanes engine is FAR from being close to the CG and a water prop boat is right on it , how can you state what you have ?
The thrust created from an air-prop dictates MORE than JUST the angle of ride for an airboat . It also dictates LIFT ! ( if the speed is there ) JUST like strakes on a water propped boat ! And based on WHERE the air thrust is being apllied , as to HOW much lift is created as well . For an airboat , if there is any negative thrust ( prop angled down towards the front ) the hull will not plane , hence , there HAS to be created a certain amount of POSITIVE thrust angle ( front of prop angled upwards ) when the engine is mounted more center , to create LIFT to get the hull to plane out
If you honestly think this does NOT apply , the try flying one of those HYDRO foamies .... OR , over powering ( larger engine ) on an airplane .
THIS will show/explain/prove , my statements on prop thrust angle , position/location of engine and lift effects