Overheating your engine is a potentially very dangerous situation. It can lead to a serious damage to (destruction of) your engine. Not speaking about the fact that sudden loss of propulsion can quickly lead to a loss of your boat. So what to do if your engine temperature's alarm starts to pip?
Act fast. Put engine in neutral (disengage gear) and check: does sea water flow out of the boat's exhaust?
If YES - increase RPM while keeping the engine in neutral! This will increase sea-water flow and increase cooling efficiency. The problem will most probably be in the "secondary" cooling circuit – see p.2
If NO – the problem is most likely in the "primary" sea-water circuit. So you need to check the water in-take, valve, raw-water pump impeller, filter.
Checking secondary coolant circuit: level of coolant, leaks of coolant – check clamps and connections of hoses, any blockage in the circuit (plugged hose or heat exchanger). Later check also pump, thermostat sensor, functioning of the thermostatic valve, heat-exchanger.
Some additional tips to help cooling the engine in emergency:
- open engine compartment and surrounding hatches to let in fresh air
- use any kind of fans to blow fresh air directly onto the engine
- place (and regularly change) wet towel on the engine's heat-exchanger
- add cold coolant into expansion tank
- if you have a boiler connected to the engine – think of using it to cool down the engine simply by releasing hot water from the boiler and re-filling it by cold water
- if you need the engine for emergency propulsion, run it on lowest possible RPM (1000-1300) and only when really needed (but consider risk of serious damage!)
For in-depth cleaning of the raw water/coolant circuits use hydrochloric acid or radiator cleaner.
PS: If you have other/better ideas or comments, let us know!