Cooling Systems

There are two basic types of cooling systems. 

One is what is called a closed system. This is a system where fresh water or antifreeze type coolant circulates inside the engine. This system is similar to a car engine where the antifreeze in the radiator is circulated through the engine.

The second type of cooling system is called a raw water cooling system. In a raw water cooling system seawater is circulated through the engine instead of fresh water or antifreeze and that sea water cools the engine directly. When the boat is used in a fresh water lake this system is excellent but when a boat is used in salt water the salt water circulating inside the engine tends to shorten the life span of the engine.

Why are cooling systems important in boat engines? Salt water circulating inside an engine will shorten the life span of the engine. In addition overheating problems associated with boat engines are very common and often result in engine failure. 

Since the fiberglass hulls on most boats will outlast the engines, the condition of the engines and their expected life span becomes a primary concern. Replacement of a gasoline engine will commonly cost $5000 to $10,000 depending on the type of engine and the labor involved. Diesel engines will cost about three times or more that this to replace. 

     Raw water Cooled system

The cooling system has a direct correlation to engine life expectancy . The type of cooling systems becomes a factor in older boats. Estimated engine life for a fresh water cooled inboard engine is 1500 hours and for a salt water cooled engine is less than 1000 hours.

With a fresh water cooled engine, the engine is cooled within a closed system by antifreeze coolant. The coolant exchanges the engines heat to sea water within a heat exchanger. Salt water or raw water is pumped into a heat exchanger which cools the coolant which then circulates throughout the engine. This works very similar to a radiator in a car where the coolant is cooled by the car fan or by air as the car is driven. In a heat exchanger the salt water or raw water is what cools the engine coolant.

In an engine that is directly cooled by salt water, corrosion and rust build up inside the engine. This restricts the water way sometimes causing the engine to operate at higher temperatures, and may corrode the internal cooling system of the engine causing engine failure. A fresh water cooled engine does have its own problems to contend with, like maintenance of the cooling system, so there are plus and minuses to each type of installation.

Just a few words on outboard engine here. All outboard engines are raw or salt water cooled. They are subject to the type of corrosion caused by salt water. However since they sit vertically salt water drains from them naturally. While this does provide some protection to the cooling system of the engine, the engine will still deteriorates internally due to exposure to salt water.