Best Practices for Small Boat Cooling

System for Operation and Maintenance

Powerboat heading out to the fishing grounds.

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There are two common engine cooling schemes in small vessels. Raw water cooling circulates seawater through the engine block directly, while closed-loop cooling utilizes a heat exchanger to isolate the engine coolant from seawater which carries the excess heat out of a vessel.

Both systems have similar components and operation. The more complex of the two systems are actually two simple cooling loops in series. The concepts are easy to understand and so are the fixes to common problems.

Raw Water or Open Cooling

We will follow the path of the water from the sea into the intake which is fitted with a valve called a seacock to close the opening if a coolant line fails. These connections are large and will put several hundred gallons per minute into your hull if they fail.

The cooling water passes through a strainer which should be checked each day. Emptying this little basket of garbage is very important since it will impede the flow to the engine which could cause damage.

Next, the seawater travels through a hard piped line or sometimes flexible hose to the cold side of the engine cooling system. Any soft lines should be secured with double band clamps on each connection, they should be checked very often for failure or wear.

On its journey through the engine, the cool seawater absorbs heat by passing through small channels cast into the engine components. These channels give plenty of surface areas where heat can be absorbed but they do have drawbacks like clogging and freezing in cold weather.

As the seawater exits, it passes through a thermostat which can be an automotive spring type device or a sensor connected to an automatic gate valve. If the water is below the ideal temperature threshold for the engine cooling water bypasses the engine until heat removal is required. A cold running engine is bad for the machinery and the efficiency of an engine.

The cooling water and exhaust gasses are combined in a wet exhaust system where they exit the vessel. If exhaust is aerial then cooling water passes through another seacock to exit the hull.

Closed-Loop Cooling

This type of cooling is very similar to raw water cooling except in place of an engine there's a heat exchanger. Basically a tube within a tube that transfers heat without allowing liquids to mix.

The coolant circulates on the engine side while raw seawater circulates on the heat exchanger side. Other than this important point all operations are similar.

Pros and Cons of Open and Closed Systems


Pros: simple and well known, no chemicals, if hard piped the only maintenance is cleaning the strainer.

Cons: Prone to clogging with debris, pure water allowed to freeze in engine passages will crack the engine block, in some environments, the inside of the system can become home to mussels and barnacles.


Pros: Much less time to bring a engine to a stable operating temperature, less temperature fluctuation increases fuel and power efficiency, winterizing tasks and cold damage are minimized, if a clog appears it will be in the heat exchanger side which can be easily serviced; a clog in an engine passage requires disassembly excess heat can be used for space heating.

Cons: Marine coolant is expensive and many systems have a high capacity, potential to leak coolant into the surrounding water, additional anodes must be placed and monitored for signs of corrosion.

What's the Best Marine Cooling System?

The answer depends on your location and operations. Fouling and clogs are the biggest issues for most operators and local knowledge works best for these situations. If you must choose one type of system over another and everything else seems equal, then take a look at the anti-fouling paint used in your area. If it is meant to aggressively prohibit the growth of marine life, then you should consider a closed system to reduce the risk of damage.

How to Flush Your Workboat Cooling System

While there are a couple of thousand large ships in the global merchant fleet, there are perhaps a couple hundred thousand smaller workboats. Operators of these boats are often also owners and to keep costs down some go without professional maintenance services.

If you choose this approach it will save money, although it does increase the risk of damage due to human error. Working carefully and understanding some of the underlying concepts of your equipment will assure the job is done correctly while still saving money.

Many have entered this profession through the world of small boats. Those long days spent at the marina washing recreational boats for extra spending money turned into more complex jobs. Soon, those little electrical and plumbing jobs earned a few dollars, and hopefully a good reputation.

Formal education is available for these jobs and many excellent schools will give you a comprehensive understanding of the systems of any size vessel.