Engine Cooling Systems Explained
Raw water refers to the water that the boat is floating in. It makes no difference whether it is salt or fresh, both are used to cool the engine. The process starts by drawing water into the engine through a seacock fitting and pumping it through the engine's water jacket and ports by way of a mechanical water pump.
In a raw water system the water is drawn up through the seacock by the water pump. The water flows through the engine and directly out the exhaust. This cooler water absorbs heat from the engine to help keep it cool.
Most newer marine engines use an enclosed cooling system. This means that there is a small tank on the top of the engine that uses a combination of fresh water and coolant. This fresh water is circulated through the engine and through a heat exchanger. The fresh water, in this system, absorbs the heat of the engine. Raw water is still drawn up through the seacock but only flows through the heat exchanger jacket. This cooler raw water absorbs the heat from the fresh water through the heat exchanger jacket and is then pumped out the exhaust.
The advantages of the enclosed system over the raw water system are extreme, especially if you are operating in salt water. Salt water tends to build up a corrosive scale when the engine operates above 140°. In the raw water system this scale is building up inside the engine's water jacket and ports. When the scaling builds to the point that water flow is restricted the engine starts to overheat. At this point you are probably looking at replacing the engine.
In the enclosed system, the water that flows through the engine's water jacket and ports is the fresh water and coolant. The only part the raw water flows through is the heat exchanger. The same scaling occurs however. When water flow is restricted and the engine begins to overheat you may be able to "acid boil" the scale out of the heat exchanger and continue to use it. The worse case is that you would have to replace the heat exchanger. This would be much less expensive than replacing the engine.
Other components of the cooling system, whether it be raw water or enclosed, are the seacock, sea strainer, hoses and clamps, belts and water pump impeller.
The seacock is a through-hull device that allows water to enter
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