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 the hull from the outside. This device has a handle that allows you to shut off the water flow if you have a problem such as a loose hose clamp or cracked hose. You should test the seacock shut-offs monthly to make sure they are operable. As a backup safety measure you should have a soft, tapered, wooden plug (called a bung) of the size of the seacock tied to the seacock. In case a hose parts and you can't operate the shut-off you can put the bung in the seacock to stop the water flow.

The next inline part of the engine cooling system is the sea strainer. This is a device through which the raw water flows and is designed to filter out debris, sand, leaves, etc. before it gets to the engine. This device works much like a swimming pool skimmer. There are several kinds of strainers but all have a removable filter or screen which should be checked and cleaned or replaced on a regular basis.

Hoses, clamps and belts are vital to the cooling system and should also be checked periodically. Every time you check the oil, which should be done before each start-up, you should visually inspect hoses, clamps and belts for wear. All hoses that are below the waterline should be double clamped. This will help prevent water from entering the bilge should one of the clamps fail. If you find a corroded clamp, a pinched or cracked hose or belt, they should be replaced immediately. Be sure to replace the hoses with the same size diameter, length and temperature requirements that the manufacturer suggests.

The raw water pump, which is driven by a belt on the engine, contains an impeller which makes the pump operate. It is usually fairly easy to access the impeller to inspect or replace it.

In the enclosed system, a commercial coolant (antifreeze) should be added. This will prevent the fresh water from freezing and damaging the engine in cold climates and also will help prevent corrosion build-up in the fresh water system. Normally you would use the coolant and fresh water in 50/50 mixture. In colder climates you may want to increase the coolant percentage.

In summary, the direct, raw water system circulates water through the engine water jacket which flows through the block, head, manifold, etc. This water absorbs the heat from the engine and is exhausted overboard.

The enclosed system circulates fresh water and coolant through the engine water jacket and through a heat exchanger. This fresh water absorbs the heat of the engine. The raw water is also pumped through the heat exchanger where it absorbs some of the heat of the fresh water and is again exhausted overboard.