In the old days, many marine engine cooling systems were of the “raw-water” variety, meaning simply that they relied on pumping whatever water the boat was floating in through the engine and pumping it out the exhaust system—salt water, polluted water, algae-infested water, whatever was available. The impact this had on the longevity of the engines was profound, especially in the extra-corrosive saltwater world.

So as time went on, engine manufacturers began supplying more and more “freshwater-cooled” or “closed-system” engines using antifreeze/coolant internal to the engine, but cooling it with raw water from outside the boat via a heat exchanger.

Think of the heat exchanger on your boat engine as the radiator on your car: They play the same role in shedding heat away from the closed side of the system.

A cutaway view of this heat exchanger shows cooling tubes, expansion tank, pressure cap, and sacrificial zinc anode on the lower right.

A cutaway view of this heat exchanger shows cooling tubes, expansion tank, pressure cap, and sacrificial zinc anode on the lower right.



Besides exposure to corrosive materials in the water, raw-water cooled engines suffered from another major drawback. They had a thermostat, just like all engines, but it was regulated at 145-150° F. This was done to minimize the possibility that salt in the salt water would separate out and crystallize inside the engine’s cooling passages, with 160°F being the critical turning point for this to occur. This comparatively cool running temperature limited the engine’s ability to reach a maximum level of thermal efficiency, which in turn had a negative impact on fuel economy and to some degree overall engine longevity. The cooler temperatures also caused varnish and gum buildup inside the engine, which ultimately contributed to other problems.

The trick with any engine is to get it running as hot as it can run without causing any damage. That’s when you know it’s running at peak efficiency, assuming proper fuel and exhaust system design. Consider a modern automobile that will typically run at 215° F. It is undoubtedly getting much better mileage than its 15-year-old counterpart.

Today almost all new marine engines use the closed cooling system design. These systems are pressurized, just like your car or truck. By increasing the pressure inside the closed part of the system, the boiling point of the coolant is enhanced. For every pound of pressure added, approximately 3° F can be added to your engine’s operating temperature, improving its thermal efficiency. Increased pressurization and advanced engine metallurgy have driven the folks who design and make what today must be referred to as engine coolant versus mere “antifreeze” to come up with some very sophisticated chemical formulas.

Inside the raw-water side of the heat exchanger, calcium-lime deposits will build up and reduce engine-cooling. Those deposits need to be flushed out regularly—once a season is best. Doug Logan photo.

Inside the raw-water side of the heat exchanger, calcium-lime deposits will build up and reduce engine-cooling. Those deposits need to be flushed out regularly—once a season is best. Doug Logan photo.



Engine coolant must be engineered to inhibit corrosion of internal engine passages, and neutralize acids and other byproducts of combustion. And the coolant itself must have an extended service life. Combine all those requirements and you can begin to see that modern engine coolant is a robust but complicated fluid. Prestone, for example, touts its product as being good for five years or 150,000 miles in automotive applications. This is comparable to Mercury Marine’s five-year or 1000-hour extended-life coolant for Mercury engines. However, some engine-makers recommend changing out the coolant every two years. Obviously, the best bet is to follow your engine manufacturer’s recommendations on maintaining the closed side of your engine’s cooling system.

The marine-unique issue


So, unlike automotive applications where air is the primary element in extracting heat from the engine, in marine applications we count on seawater (or lake water, or river water) to do the job. This seawater gets pumped through the heat exchanger on your engine and, via convection, pulls the heat away from the tubes inside the exchanger. The seawater then gets pumped overboard, usually via your engine’s exhaust system (there are closed-system variations on this system in the commercial and large-vessel worlds, but we’ll leave those for another article).

This raw-water side of your cooling system is where annual maintenance is critical. Back in the day, it wasn’t uncommon to remove the heat exchanger every couple of years and send it off to the local automotive radiator repair shop to get it acid-dipped and boiled out thoroughly to clean out all the deposits. Today these shops are going the way of starter and alternator rebuilding shops, into extinction due to economic and environmental reasons, so let’s rule that option out.

A raw-water strainer is the first line of defense; it’s only meant  to keep out relatively large debris. Photo courtesy of Groco.

A raw-water strainer is the first line of defense; it’s only meant to keep out relatively large debris. Photo courtesy of Groco.



No matter how sophisticated your raw-water strainer is, understand that it was only designed to keep weed and other relatively large debris from entering your cooling system and clogging the raw-water pump. Things like micro-organisms, calcium, and lime are still getting by the strainer and are indeed going to leave deposits inside the raw/seawater side of your cooling system. Left unattended, this stuff will eventually form a coating inside the comparatively small passages inside your heat exchanger. Think of it as cholesterol plaque in your cooling system.

That buildup will cause a couple of bad things to happen. One is physical blockage of cooling water flow, which will obviously have an extreme effect before too long. The other is not so obvious; the buildup interferes with the heat transfer system itself, which utilizes both convection and conduction to get the job done. We’re attempting to move the heat from one liquid through a metal interface and into another liquid. Convection gets the heat from your engine’s coolant into contact with the tubes inside your heat exchanger, where it’s conducted through the metal walls of these this mini-pipes. Convection then carries the heat away from the pipes, into the seawater, and back overboard. Any buildup of coating on these tubes will block this heat transfer and reduce the overall efficiency of your heat exchanger, in some cases quite significantly. There have been plenty of cases where this one simple problem has caused engine overheating at high speeds.

So, what to do? Flush the raw-water side of your cooling system each year as a part of your winter layup procedure. A couple of products I’ve used are Seaflush and Forté cooling system flush.

Before you flush, be sure to remove the sacrificial anode(s) screwed into your heat exchanger, and replace the anodes with new ones after the flushing.  This is especially important in a  saltwater environment. For more of a step-by-step approach to this, read Winterizing Your Diesel Engine: Cooling System.

Besides keeping the “plaque” buildup out of the raw-water side of your cooling system, you should of course test your engine coolant to make sure it’s at an adequate protection level temperature-wise. Inexpensive antifreeze hydrometers to verify this are available at just about any automotive parts store.

If you’re checking your antifreeze and find that your system needs topping up, make sure you add exactly what’s already in your system. There are many variations available in the market today and they don’t all play well together chemically. Again, make sure you do not deviate from specific engine manufacturer recommendations for coolant.

Also, it pays to test the pressure cap on your cooling system every three or four years. They do eventually wear out. Most automotive shops have an appropriate pressure tester they can check this with, and it only takes a minute to perform the test. Or you can just replace the cap as part of your preventive maintenance The pressure rating is typically embossed on the cap.

The raw-water pump impeller is a key component in the cooling system. Feed it clean water via your strainer, and change it every season or two, depending on how much you run your boat. Doug Logan photo.

The raw-water pump impeller is a key component in the cooling system. Feed it clean water via your strainer, and change it as necessary. Doug Logan photo.



And don’t forget your raw-water pump impeller; this should be replaced every season or two, depending on how much you use your boat.

Following the guidelines here and your engine will be one cool operator come summertime!

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