Around the cylinders and head
cooled engines are water passages.
In a raw water cooled engine - water is drawn in from the sea, circulated through the passages then expelled overboard
- usually through the exhaust
system where is helps cool the exhaust
and muffle the engine sounds.
In a "fresh water" or "closed" cooling
system the water passages are plumbed to a cooler and back to the engine and the coolant
(coolant can be a mix of coolant
and fresh water) is circulated by a fresh water pump just like your car.
In your car the "cooler" is a radiator and the air flowing past the radiator takes the heat out of the water.
In your boat the cooler or "heat exchanger" is "pack" of cooling
pipes in a housing. Raw water is drawn in from the sea and circulated around the pack of cooling pipes, then expelled overboard
, usually through the exhaust system as above.
Corrosive elements exist in both fresh water and raw water systems. Salt
and scale are the primary drivers in raw systems. Rust is the driver in fresh systems. Keeping the right mix of fresh water and corrosion
inhibiting coolant ensures a very long life for the fresh water system on the boat.
Raw water systems can be considered more reliable - fewer parts. But closed systems have the advantage that impeller failures or other debris can't enter the engine block as they are separate systems and a properly maintained closed system should never (never say never) have internal engine corrosion