SOURCE FOR BELOW: http://www.marine-power-solutions.com/how.html

P.S. My three boats, over decades, have never had a pressure cap on their expansion tanks. All had expansion tanks where the tank was above the engine. Our current cars have the pressure caps on the expansion tanks, but the older ones didn't. I'm not sure what changed?]

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Heat Exchanger and Marine Cooling Systems

Marine engines, as well as automotive engines are, cooled by circulating water through the engine block. Marine engines are unique in that there are two different types of cooling systems. The standard raw water system, and the fresh water (commonly known as the closed) cooling system.

Raw Water Cooling Systems

Raw water cooling systems draw water from outside the boat (seawater or lake water). Water is pumped from the source to the engine block then the engine circulation pump forces the raw water through the engine block and the water is expelled through the exhaust. Raw water cooling systems are relatively simple and the standard cooling system on most Marine Engines. The raw water pump in most cases is inside the outdrive. On larger engines and inboard engines the raw water pump is located inside the boat and is driven by a v-belt or directly off of the crankshaft. The danger is using salt water as a coolant in your engine. Salt water can be highly corrosive. Running salt water through your engine block and exhaust manifolds will lead to destructive corrosion that is unseen until your engine or exhaust manifolds fail.

Generally speaking, marine engines cooled with raw water, especially ones that use salt water, have a shorter life span than marine engines cooled with a closed cooling system.

Fresh Water Systems Heat Exchanger and Keel Cooled systems

Fresh water cooling systems, also known as a closed cooling systems, come in several varieties. The most common type utilises a Heat Exchanger which functions similarly to the radiator in your car. Coolant (antifreeze) is circulated through one side of the heat exchanger where it is cooled by raw water that passes through the other side of the heat exchanger. The engine coolant is then circulated back into the engine. The raw water is expelled out of the boat through the exhaust. Another common type of closed cooling systems is known as a Keel Cooler. This is done by eliminating the use of a heat exchanger. Instead of pumping raw water into the vessel’s heat exchanger where it cools the coolant, the coolant is pumped through pipes or aluminium extrusions on the outside of the hull where the surrounding water (lake, river or sea water) cools the coolant before it is pumped back into the engine. The use of keel coolers removes the need for a heat exchanger, raw water pump and the other components necessary for pumping raw water into the heat exchanger.

Fresh Water Cooling examples

Closed cooling systems are more complex than raw water cooling systems, but have distinct advantages:

Minimal internal engine corrosion.

More effective at cooling the engine and allows you to run your engine at higher temperatures, resulting in better engine performance and fuel economy.

How A Heat Exchanger Functions

A heat exchanger transfers, or “exchanges,” heat from your boat engine’s coolant to raw water pumped from the water outside of your boat. The raw water is pumped through a bundle of small tubes in a chamber filled with the hot engine coolant. The tubes are cooled by the colder raw water the allowing the tubes to absorb the heat of the engine coolant.
To function correctly, a heat exchanger must be carefully matched to your boat’s engine.

Expansion Tanks

Expansion tanks are an often overlooked but very important part of a closed cooling system. As the engine coolant gets hot it expands, increasing in volume. The expansion tank is a small tank that simply provides room for this increase in volume. In some systems, the heat exchanger is a piggy back style. This is when the expansion tank is built on top of the Heat Exchanger. On some Heat Exchanger systems, the expansion tank is a separate tank remotely mounted. Another important component to the heat exchanger is the cap on expansion tank similar to the cap on your cars radiator. It is an important component in, maintaining your cooling system’s pressure. It should, be checked regularly for leakage and corrosion. We recommend that you replace the cap on your expansion tank every two years.