Investigation work

Took several hoses off the engine and flushed water through the system - the slab tank filler is unblocked from the filler to the heat exchange/thermostat and water is flushing through the engine. The expansion tank was blocked to the heat exchanger - very thin hose. Next week I will take the hoses off the slab tank and flush it through to check for blockages. I checked the engine oil and it is clean. I cleaned out the air intake and noted a very small amount of engine oil in it.  I decided to start the engine and it sounded ok with no oil coming out of the exhaust. I suspect that the piston rings had probably overheated and let oil past the cylinders. Hopefully this is the last overheat - we are not used to 30c temps. I might buy a temp gauge for the boat to monitor it - the blue overheat light on the control panel doesn't come on until it is too late!