Boat Ventilation

Boating Know How
Boat Cooling Systems and How they work

Boat Engines: Their Cooling Systems and Maintenance

Boat engines are different, but they all have one of two cooling systems. There is the open cooling system which uses water from the lake or ocean for cooling the engine. Then there is a closed cooling system that does not use water directly from the ... read more or ocean for cooling the engine. Then there is a closed cooling system that does not use water directly from the lake or ocean, but a combination of coolant and water input by the boater.

The cooling systems are differentiated by the words “open” and “closed”. One system takes up water from its floating point whereas the other recycles the water throughout the engine mechanism while also using some water from the sea to reduce the temperature of the recycling water and commercial coolant mixture.

Most boats use the open cooling system. Newer models in the boating industry use the closed cooling system.

Before we talk about cooling, you should have an idea of the parts that are necessary for the cooling process to be successful.

Cooling Parts of Boat Engines

The Sea Strainer

This is a filter that ensures water getting into the engine is free of debris. Water that gets into the engine needs to be just that, water. No debris should enter your engine since it will only make cooling harder by clogging up water passages that feed the cooling system.

A heated engine will fail and leave you stranded. Cooling ensures that your engine lifespan will last and will help prevent engine mishaps from occurring.

The Seacock

The seacock is the point at which water enters the hull from the outside. This design allows you to shut off water entry in case of a faulty hose or belt.

Clamps and Hoses

Hoses are waterways that are constructed of rubber. They facilitate the water inlet and outlet of the engine. They hold onto and control the water entering and exiting the pump. The clamps are metallic devices that attach hoses to different parts of the engine and prevent leakage.


The belt, which is made of rubber, is used to rotate the water pump. It is located at the water pump and is useful for machine rotation.

Water Pump Impeller

The water pump impeller is found in the open cooling systems. It ensures that raw water or water from the outside is drawn in by the pump housing when needed in order to maintain the cooling process.

The Heat Exchanger

A heat exchanger is a major component of a closed cooling system. It is responsible for the transfer of heat from the engine into the coolant system. By giving off heat, it cools the temperature. The antifreeze also laces its plates and tubes allowing heat to be cooled off as the freshwater mixture circulates the engine.

Commercial Engine Coolant

The antifreeze that is added to the freshwater is used to cool the engine in order to prevent it from freezing. When temperatures are low, it prevents corrosion from building up in the water jackets thus corroding the engine parts that come into direct contact with the cooling water.

Open Boat Engine Cooling Systems

This process is simple. Water is taken up via hoses through the seacock into another hose and fed into the pump where it is distributed to other channels and through a thermostat that checks the heat levels in the engine. If the engine is cold, the water is circulated back into the cooling system thus taking up the heat from the engine as it circulates again.

As the water temperature rises the thermostat opens a valve so that the heated water can be put back out and released overboard through the exhaust pipe, and then cooler water is pumped into the engine to continue the cooling process. This cycle repeats itself thus achieving a cool boat engine.

Closed Boat Engine Cooling Systems

The process is more complicated than an open cooling system. Water is drawn into the engine cooling system where it passes through a heat exchanger in flow tubes cooling the hot water circulating in the heat exchanger. This is the main reason it is called a heat exchanger. The water is discharged from the exhaust pipe after it has taken up heat from the heat exchanger- this is fresh water mixed with a commercial coolant. The freshwater and coolant mixture rotates around other engine parts acquiring their heat and then passing the heat through the exhaust pipe when hot. The thermostat of a closed cooling system is different in that it alters the amount of water that enters the seacock depending on the engine temperature. When the engine is too hot, more seawater enters the boat cooling jacket and when the temperatures fall. So does the amount or volume of seawater required by the heat exchanger.

How to Maintain Your Boat Engine Cooling System

If you are using the open cooling system, ensure that:

You check the belts for wear and tear. Ensure that the pump impeller is not worn and be sure to check the pump for any accumulated dirt. Clean the pump regularly to ensure that it pumps water into the system efficiently.

If you are using the closed cooling system, ensure that:

Drain your commercial coolant at least every year and replace it with a new coolant. Every three years, take your heat exchanger for scale cleaning and pressure testing by a reputable maintenance shop. You can maintain the heat exchanger and clean it yourself by opening the end fitting and removing silt and dirt build-up that may reduce the efficiency during cooling.

Check the potency of your antifreeze using a refractometer or a hydrometer every season. This will prevent engine blockages during winter. Components of the engine could be prone to cracking over the winter if your antifreeze is not potent enough.

For both systems, check all hoses and clamps, and ensure that they are replaced with the same fittings before they wear out completely. Clean the filter or screens before you run the motor to ensure water is not obstructed from entering the cooling systems through the filters. Dirt clogging the filters will lessen the filter life and require early replacement. Replace filters and screens often especially when they wear out.
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