Fresh water overheat

[John G]

Bavaria 30 Volvo  D 1-20. Engine running fine for 2 hours then overheat alarm. Coolant forcing its way out of expansion tank. Clearing very hot. Raw water circuit working normally with water exiting the exhaust and exhaust temperature sensor (i fitted) normal. Any help gratefully received

[SYJetzt]

Maybe one or more of the following issues:
Not enough raw water flow, raw water circuit partially blocked,
impeller ripped,
filter clogged .
Thermostat not working.
Head gasket blown.