How are marine engines kept cool, and what maintenance needs to be done to the cooling systems? We’ve all lost our cool before… but when it happens at sea you can find yourself in real trouble. Unlike a car motor, boat engines use the water from below the vessel to keep cool. Most of the places people use their boats in Australia are salt-water. This means, the salt-water beneath your boat is sucked up and ran through your engine to cool it down while it’s running. 

Two Main Cooling Systems

There are two main ways engine manufacturers engineer the cooling system on a boat:

1. Raw Water Cooling System

This style sucks the salt-water straight out of the ocean, through a strainer (filter), and then cycles it through your engine block. Water is then usually exited through the exhaust and out the back, or below the boat.

2. Closed Loop Cooling System

There are a number of different ways the cooling system can be setup – closed loop half, or closed loop full. Generally this means the salt-water gets sucked up and ran through a series of heat exchangers. On one side of the heat exchanger you have salt-water, and on the other side you have “coolant” which is ran through the engine, avoiding any salt-water from running through the block.

Engine Horsepower

Another consideration to be made in relation to cooling systems is the higher horsepower an engine, the more heat it will produce. This means, the bigger and more efficient the cooling system needs to be. Having an unlimited supply of cool water (the ocean) conveniently proves to be a very efficient way to cool an engine, as long as there are no blockages.

How would you get a blockage? Over time as salt-water runs through the passages, it leaves a “scale” which blocks up the passage ways. The system needs to be pulled down and cleaned in acid to remove the scale, and seals need to be renewed.


Unfortunately, there are draw backs to this amazing supply of unlimited cool water. Because heat exchangers run at such hot temperatures, the metals are continually expanding and shrinking. This means surfaces don’t last forever, and o-rings and seals will need to be changed regularly.

Because people don’t use their boat as regularly as a car, they often think they can skip routine maintenance “because they haven’t used it”. This isn’t justifiable in the salt environment, as once you get a leaking o-ring, you have introduced salt-water to the exterior of your engine. Slowly but surely, it starts to corrode away anything in its path, and even damages the cooling component sealing surfaces. This means when you go to service the components, you will start to find throw-away items instead of just o-rings that require replacement. The last consideration is sacrificial anodes – if the engines aren’t getting serviced, the anodes definitely aren’t getting changed. Simply put, anodes are designed in the cooling system to corrode away (sacrificial) before any of the more expensive components.

Manifolds, Risers & Exhaust Elbows (Petrol Engines)

What about exhaust manifolds and risers? Manifolds and risers, are the term given to the water cooled exhaust components by engine manufacturers. The “exhaust manifolds” bolt on to the engine, with the “risers” fitted on top to increase the distance between the water level and where the exhaust exits. If the water level is above the exhaust level, you risk the salt-water back-tracking through the exhaust into your engine.

Exhaust manifolds and risers can last anywhere from 3-7 years, depending on whether they run coolant or salt-water through them. Once they become blocked, you cannot clean them out as they rust and corroded internally, making it impossible to access the areas of corrosion. The only option given from manufacturers is to replace – making them a throwaway service item.

When looking at Mercruiser branded petrol engines, another tell tale is the factory emissions sticker that is on the OEM manifolds and risers from new. If you find this sticker is still fitted and the boat is more than 5 years old – the manifolds haven’t been changed.

Original emission sticker on a Mercruiser manifold – a dead giveaway that its original

Exhaust Elbows (Diesel Engines)

Exhaust elbows are often found on diesel engines, where the elbow has the cooling water introduced before it exits overboard to cool the exhaust. This is another popular spot to “block up”, and as such, it’s recommended that exhaust elbows are checked on a regular basis.

After-Coolers (Diesel Engines)

Ever so common on newer engines, are alloy housed after-coolers. These are usually only found on engines where a higher horsepower is obtained from a small block. The idea is, the after-cooler cools the air before it enters the intake, making it more dense enabling more air and fuel to be combusted in fewer engine cycles. The downside to after-coolers is they need servicing regularly, to avoid o-ring and housing failures. Typically, you will need to remove, inspect, clean, grease and refit the coolers to keep them running optimally every 3 years. After-coolers are renowned for leaking if left untouched, even when used lightly. Once salt has been through them, they slowly corrode away. O-rings can leak or coolers can fail, and you will be unaware until it’s too late and your engine has been filled with salt-water.

blocked Cummins after cooler
Blocked Cummins After-Cooler – Photo courtesy of

So how do you avoid cooling system issues?


Remove, inspect and replace cooling system components as recommended by the engine manufacturer. Usually exhaust manifolds and risers are replaced every 3-7 years, and heat exchangers are inspected every 3-5 years.

  • Keep a log of when the exhaust manifolds and risers were done
  • Keep a log of when the heat exchangers and oil coolers were done
  • Keep on top of your anode replacement


Remove, inspect and replace cooling system components as recommended by the engine manufacturer. Usually exhaust elbows are replaced every 3-7 years, and heat exchangers, after-coolers and oil coolers are inspected every 3-5 years.

  • Keep a log of your last salt-water service
  • Keep a log of your last exhaust elbow inspection
  • Keep on top of your anode replacement

Anodes are replaced yearly, and usually a salt-water service is completed every 3-5 years. If the boat is kept on a trailer, always flush the motor with fresh water after a day out.

What To Look Out For?

When buying, there are a couple of tell tale signs. If there is no service history of the cooling system, and the boat is more than 5 years old –  you’re going to have to expect it hasn’t been done and will likely need to. Upon testing running a vessel, it is important the vessel is ran at full throttle for a period of time to ensure the temperatures aren’t creeping up. If the temperatures are increasing above 92 Degrees Celsius or more, you’ve likely got a cooling system blockage.