My home-built Chesapeake Bay Hooper Island launch was redesigned to a 21' length, and finally completed and put in the water. We are experiencing an overheating problem that begins with a normal exhaust output of water/air at idle, to a reduced exhaust output and increase in engine temperature after about 15 minutes under power. We stop the engine, check everything, and start up again once cooled, only to see the problem recur. So far, we have replaced all the new rubber intake hoses with wire-reinforced hose, resealed the gasket on the impeller (which seemed to be wet/minor leak), and checked the impeller condition and rotation. Any suggestions about what to do next? Joy