Understanding Boat Cooling Systems: How They Work

Boats use two main types of cooling systems: raw water cooling systems and closed loop cooling systems. Unlike car engines, boat engines use the water from below the vessel to keep cool. Raw water cooling systems suck water straight out of the ocean, filter it, and cycle it through the engine block. Closed loop systems, on the other hand, use a series of heat exchangers to separate the salt water from the coolant, which runs through the engine.

The role of the thermostat

The thermostat in a boat's cooling system plays a crucial role in maintaining the engine's temperature, usually between 170 and 180 degrees. This is achieved through a combination of freshwater (also known as coolant) and seawater (or raw water). The raw water is pumped from the sea into a heat exchanger, where it comes into contact with coils filled with freshwater. The freshwater circulates around the engine, absorbing heat, and then passes through the heat exchanger, where it is cooled by the raw water.

The thermostat ensures that the engine operates within the desired temperature range by regulating the flow of coolant. When the engine temperature reaches the upper limit, the thermostat opens, allowing coolant to flow through the engine and absorb excess heat. As the coolant circulates, it carries the heat away from the engine, preventing it from overheating. Conversely, when the engine temperature drops, the thermostat restricts the flow of coolant to maintain the desired operating temperature.

In a closed-loop or indirect cooling system, the raw water and freshwater systems are separate. The raw water passes through a strainer to filter out any debris, then through hoses and a pump, before entering the heat exchanger. After cooling the freshwater in the heat exchanger, the raw water is expelled through the boat's exhaust system, along with any exhaust gases.

On the other hand, a raw water or direct cooling system does not have a separate freshwater system. In this case, the raw water is pumped directly through the engine block to absorb heat and then discharged back into the sea. While this system is more straightforward, corrosion can become a significant issue.

Regular maintenance of the cooling system, including the thermostat, is crucial to ensure the efficient operation of the boat's engine. This includes checking for leaks, cleaning or replacing filters and strainers, and inspecting pumps and impellers for any signs of wear or damage. By maintaining the cooling system, boat owners can help prevent breakdowns and extend the lifespan of their engines.

How raw water cooling works

Raw water cooling systems are one of the two main ways to cool a boat engine, the other being closed-loop cooling systems. Raw water cooling systems are a direct cooling method that uses the water from below the vessel to cool the engine. This water can be either salt or freshwater, depending on where the boat is located.

The process starts by drawing water into the engine through a seacock fitting, which is a through-hull device that allows water to enter the hull from the outside. The water is then pumped through the engine's water jacket and ports by a mechanical water pump. The water flows through the engine and directly out through the exhaust. As the water passes through the engine, it absorbs heat, helping to keep the engine cool.

One advantage of raw water cooling systems is that they utilise an unlimited supply of cool water, which is very convenient and efficient for cooling an engine, as long as there are no blockages. However, one drawback is that over time, salt water can leave a "scale" that builds up and blocks the passageways, which can lead to engine overheating and potential replacement.

To prevent blockages, regular maintenance is required. This includes checking and cleaning or replacing the sea strainer, a device that filters out debris, sand, leaves, etc. before the water reaches the engine. Hoses, clamps, and belts should also be checked periodically to ensure they are in good condition and free from corrosion.

In summary, raw water cooling systems work by directly circulating water through the engine, absorbing heat, and then expelling it out of the boat. While this provides an efficient method of cooling, the potential for corrosion and blockages means regular maintenance is crucial to ensure the system's effectiveness and the longevity of the engine.

Enclosed cooling systems

Most newer marine engines use an enclosed cooling system. This system has a small tank on top of the engine that uses a combination of fresh water and coolant. The fresh water is circulated through the engine and through a heat exchanger. The fresh water absorbs the heat from the engine.

The enclosed cooling system is similar to a car's cooling system. It uses a combination of fresh water and antifreeze supplied by a header tank, which can be topped up when required. The main difference between a car and a marine engine is that the marine engine uses a heat exchanger instead of a radiator. The fresh water circulates around the engine and passes through the heat exchanger, which usually forms part of the header tank on smaller engines but may be separate on larger engines.

The heat exchanger is where the fresh water and raw water interact. The raw water is pumped through small-diameter cooling tubes, and the two types of water do not mix. The raw water absorbs the heat from the fresh water through the heat exchanger and is then pumped out of the exhaust.

The advantages of the enclosed system are significant, especially if the boat is operating in saltwater. Saltwater tends to build up corrosive scale when the engine operates above 140°C. In a raw water system, this scale builds up inside the engine, and when it restricts water flow, the engine starts to overheat. In the enclosed system, the water that flows through the engine is fresh water and coolant, so the scaling only occurs in the heat exchanger. When water flow is restricted, and the engine begins to overheat, it is possible to "acid boil" the scale out of the heat exchanger and continue using it. The worst-case scenario is that you would have to replace the heat exchanger, which is much less expensive than replacing the engine.

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

Air-cooled vs water-cooled outboard motors

Every engine needs a cooling system to eliminate engine heat. Engines would overheat and destroy themselves without one. The main differences between air-cooled and water-cooled outboard engines are how they work and their maintenance requirements.

Air-cooled Outboards

Air-cooled outboard engines use a system of cooling fins and deflector plates to dissipate engine heat. As air passes over the engine, heat is removed. Air-cooled outboard engines are not very efficient and prone to overheating. They are therefore usually limited to small horse-powered outboard engines.

Air-cooled outboards require little maintenance beyond routine cleaning. As long as dirt is removed from between the cooling fins, they remain healthy.

Water-cooled Outboards

There are two main types of water-cooled outboard engines: fresh water (“closed”) cooling systems and "raw water" systems.

A fresh water cooling system uses fresh water or coolant to cool the engine. Salt water loops through this closed system, but it does not come into contact with the engine. Instead, the role of the salt water is to absorb engine heat from the coolant in a 'heat exchange' unit, keeping the fresh water cool.

In a raw water system, the surrounding ocean or lake water is pumped directly through the engine to cool it. The water is then usually exited through the exhaust and out the back, or below the boat.

Water-cooled outboards are more prone to maintenance issues than air-cooled outboards. Clogged pumps and rusted parts are common issues. It is recommended that pump impellers are replaced every two years, or even earlier.

Performance and Preference

Water-cooled engines are generally considered superior in performance. Air-cooled outboards are typically louder and less powerful than water-cooled engines. They are also heavier and produce less horsepower.

However, air-cooled outboards have some advantages. They are useful for duck/goose hunters as they can access places a normal outboard would fail. They also have fewer parts and no water flowing in the block, so there is no risk of clogging a water cooling system.

How to maintain a boat cooling system

Raw Water Cooling Systems

The raw water cooling system is a simple and cheap method of cooling a boat engine. It involves sucking up water from the lake or sea and running it through the engine block to cool it down. This type of system is used on most small boats and all outboard motors.

To maintain this type of system, it is important to:

• Replace the raw water impeller periodically.
• Properly winterize the system to prevent cold weather damage.
• Backflush the engine occasionally by removing the hoses and running clean water through the system to remove loose debris.
• Regularly check the raw water strainers, especially when cruising in shallow waters. Give them a quick flush with clean water and make sure the pipework is not partially blocked.
• Check the cover seal and give it a light grease to ensure it seals properly when the cover is replaced.

Enclosed/Closed Loop Cooling Systems

Enclosed cooling systems are more complicated than raw water systems but offer better performance and are more effective at cooling the engine. This type of system uses a combination of internally circulating coolant and raw water to keep the engine cool.

To maintain this type of system, it is important to:

• Check and replace the raw water impeller if needed.
• Ensure there is no buildup or corrosion in the heat exchangers, especially if the boat operates in saltwater.
• Check antifreeze potency every season using a hydrometer or refractometer.
• Clean the oil coolers as they are just as important as heat exchangers in preventing overheating.
• Check plastic coolant-overflow bottles mounted on bulkheads, as these are the most neglected part of the cooling system.
• Test the seacock shut-offs monthly to make sure they are operable.
• Keep a soft, tapered, wooden plug (called a bung) tied to the seacock in case of a hose failure.

Frequently asked questions