The reason your getting black residue out of the engine is that the engine is designed for a tractor( pulling ) force on the crankshaft by the propellor . Since your using it as a pusher ( scale airboat ) configuration, the opposing force of the propellor's thrust is forward, that is , against the engine's backplate. There's just enough endplay in the engine(by design ) that it rubs. There's not much you can do except to ad an additional gasket under the engine's backplate. Good luck and when can I order an aluminum hull like yours ! Mike