A Hunter 270? 9 hp?
Was that a special model in the late 70's, early 80's? You should probably post this question for Torreson, in the "Diesel" forum. But he'll want to know the year and model # I think.If it is a late 70's Yanmar it is probably seawater cooled(no heat exchanger). My own 1979 2QM20 has a heat exchanger but I am certain it was an upgrade. It is fairly easy to discern from looking at the motor. There will ususally be an external waterpump, belt-driven, with the hose going to the raw water strainer. The internal waterpump circulates the water/anti-freeze mix in the closed system.