Nowadays the vast majority of engines are ‘intercooled’ – or cooled by an internal fresh water supply
With an inter-cooled engine, the role of the seawater is to take away the heat from the freshwater.
The seawater, or ‘raw water’, is a separate flow to the fresh water and never comes into contact with the engine. In fact it doesn’t come into contact with the fresh water either, but comes very close in the heat exchanger or ‘intercooler’, which is enclosed in an expansion tank.
The heat exchanger is a stack of thermally conductive tubes immersed in an outer jacket containing freshwater coolant.
The fresh water, which is mixed with antifreeze, is pumped around the engine, taking away the heat from the moving parts. When it reaches the heat exchanger it’s cooled down by the constantly replenished seawater, then returns to the engine block to continue with its work.
The now-hot seawater, on the other hand, is pumped into the exhaust system and used to cool the exhaust gases on its way out. That’s why we check for water in the exhaust, and it also explains why you can hear the ‘bloop, bloop, woosh’ noise of a boat going past; what’s happening is that the seawater is building up in the silencer and the pressure of the exhaust is collecting it and blowing it out.
To find out more about the marine diesel engine and how to service it, read how we serviced the Volvo MD20200 saildrive engine on our Project Boat Maximus.
For an in-depth guide to marine diesel engines, see the RYA Diesel Engine handbook