You are close to your hull speed at 6.5 knots - hull speed being 6.7 knots for your waterline of 25'.
The correct prop will let you achieve max rpm and ideally no more. If the revs exceed max rpm under power you are under propped. If you cannot hit max rpm you are over propped.
I would check the exhaust elbow in any case. If it hasn't been checked in recent years it could be restricted by carbon. They regularly corrode from the inside as well.
When an engine works too hard it will exhaust unburned fuel.
An accurate tach would help.
Living aboard in Victoria Harbour