Marine engine cooling systems come in several kinds. They rely in different ways on the water in which you are operating your boat to keep things cool. The raw water cooling system will only use this water while an enclosed cooling system will use water plus an additional coolant. The keel cooling system is also an option.

Raw Water or Direct Cooling Systems

Raw water refers to the water that your fishing boat is floating in. This can be a sea water cooling system or a freshwater cooling system, it doesn’t make a difference.

  • The process starts by drawing water into the engine through a seacock fitting. A seacock fitting is a valve that allows water to enter specifically for this purpose. A filter ensures no debris or weeds can enter the engine.
  • The water is pumped through the engine’s water jacket and ports by way of a mechanical water pump. A water jacket is a case that surrounds the engine, like a protective case, and allows cool water to pass through and around. 
  • The water flows through the engine where it will absorb the heat of the engine. Think of it like putting a cool cloth on your head when you have a fever. This cooler water absorbs heat from the engine to lower the temperature and keep it running within safe temperatures.
  • The heated water is then expelled out of the exhaust back into the body of water in which you’re boating. 
  • Direct cooling is most often seen in older boats these days as it is less efficient. Your engine is more likely to suffer damage from scale building up and corrosion using this method.

Enclosed or Indirect Cooling Systems

Most newer marine engines use an enclosed cooling system. 

  • In an enclosed system, there is a small tank on the 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 heat exchanger can be designed several ways but is most often a bundle of copper tubes, as copper is highly efficient at temperature transfer.
  • Heat exchangers are typically located on top of the engine and will connect to the exhaust manifold.
  • The raw 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.
  • In this system, the coolant and fresh water have direct contact with the engine and never meet the raw water from outside the boat. The raw water takes the heat by a heat exchanger which puts a barrier between the two.

Keel Cooling System

A keel cooling system uses externally mounted tubes and pipes on the outer hull of the boat, below the water line.

  • This system works a bit like a radiator. 
  • Hot water or coolant from the engine is pumped through the pipes which are underwater. The cooling water in which the boat is immersed absorbs the heat as it circulates through the pipes and returns to the engine.
  • This system is only effective if it is big enough. A boat will need a large enough surface area for the submerged pipes in order for sufficient heat exchange to happen. 
  • Keel cooling is less common for most small, recreational vessels.

Raw Water Cooling vs Enclosed Cooling Comparison

Indirect cooking systems are better than direct cooling systems which is why they are the choice of modern boats. The advantages of the enclosed, indirect system over the raw water direct system are extreme. This is especially true 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 indirect system, the water that flows through the engine’s water jacket and ports is the freshwater and coolant. 

  • The only part the raw water flows through is the heat exchanger. 
  • The coolant should contain corrosion inhibitors that will extend the life of your engine compared to one directly cooled with salt water.
  • Scaling will occur but not in direct contact with the engine. 
  • 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 worst case is that you would have to replace the heat exchanger. This would be much less expensive than replacing the engine.
  • An advantage of this over traditional raw water systems is that you get better temperature control in cold weather. Coolant has anti-freeze properties and allows your engine block to better endure cold weather and even freezing temperatures.

Your Internal Hot Water System

Most boats will recycle the heat from the engine while cooling it down. Not every vessel is built this way, but the most efficient ones are. As the hot water leaves the engine, it can be circulated around your boat’s internal hot water tank to heat the water before it’s expelled through the exhaust. This makes it more energy efficient to have hot water on your boat and ensures that the hot water is always being heated while your boat is operating. In this way, your water for bathing or washing dishes is part of your engine cooling system.

How the Parts of the Cooling System Work

Other components of the cooling system, whether it be raw water or enclosed, are:

  • Seacock
  • Sea strainer
  • Thermostat
  • Hoses
  • Clamps
  • Belts
  • Water pump impeller
  • Heat exchanger


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.

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.


  • A thermostat is necessary to ensure temperature regulation. 
  • A temperature sensitive valve will remain closed while the engine is at a lower temperature and then, as it rises to the high end of the operating threshold, it will open to allow coolant through. 
  • A functional thermostat ensures the engine will always operate at the correct temperature even if the outside temperature is scorching hot or freezing cold.
  • A direct cooled engine has to operate at a lower temperature than an indirectly cooled engine. 
  • This is because, at higher temperatures, a direct cooled engine would suffer a lot more lime and scale build up. 
  • Direct cooling systems operate at around 55°-70° C or 131°-158° F
  • Indirect cooling systems operate at around 70°-85° C or 158°-185° F
  • If your engine runs hotter than these temperatures you likely have an issue somewhere in your cooling system. Likewise, you will suffer more corrosion and scale damage until you get it fixed.
  • Keep in mind, low temperatures are also potentially dangerous. Operating at low temperatures for too long will reduce the overall lifespan of any engine. Low temperature engines burn too much fuel, stutter, stall and struggle to maintain speed.

Hoses, Clamps and Belts

  • 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 diameter, length and temperature requirements that the manufacturer suggests.

Water Pump

  • 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. 
  • The blades on the impeller are likely made of rubber and will need to be maintained. If they dry out or suffer damage they will fail quickly. The result can be clogs and inefficient cooling.

Coolant (Enclosed Only)

  • 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 a 50/50 mixture. 
  • In colder climates you may want to increase the coolant percentage. 
  • Coolant should be changed about every two years. Though it will still be able to cool an engine past this time, the anti-corrosion compounds will begin to fail around the two year mark.

Heat Exchanger (Enclosed Only)

  • The copper tubes of your heat exchanger need regular maintenance.
  • Your filter is not 100% effective so some fine debris will get through and stick to the heat exchanger, prohibiting its ability to effectively exchange heat.
  • You should remove and clean the heat exchanger stack once per year.

The Bottom Line

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.

The major difference between these two is that, in the enclosed system, raw water never directly touches the engine. This is more efficient as a result since your engine will suffer less scaling and corrosion as a result.