Marine engines, as well as automotive engines are, cooled by circulating water thru 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 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 thru the engine block and the water is expelled thru 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. There are hidden dangers that can accumulate over time causing you to spend big dollars on repairs. 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 with Heat Exchangers and Keel Cooled systems
Fresh water cooling systems, also known as closed cooling systems come in several varieties. The most common type utilizes 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 thru 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 aluminum extrusions on the outside of the hull where the surrounding water (lake or ocean 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 to.