I've got a 1994 Alpha 1 gen II with 190 hrs on it thats overheating - 1994 Se Ray 25' Express Cruiser - Its a raw water system - Bought boat used - When test run, no problems, got to 140 held temp at idle or WOT - Prop on it had some nicks so had the dealer put a 17p on it - Ran it first time out and temp got to 160 and then held - Replaced impeller and housing & thermostat - Replaced 17p prop with a 15p (per Sea Ray recomm)and took her out - Temp went to 180 and rising when underway - At idle or in neutral making 2500 - 3000 rpms, stays on 140 with no problem and is moving lots of water - Could this be an exhaust in the water intake / ventilation problem?
Sounds like an exhaust restriction to me. The prop size has nothing to do with it. Now is the time to take off the exhaust risers and check for blockage/ restrictions, especially @ the water dump ports. If these are original they may need to be replaced, but I would try to clean them up first......J