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How to Keep Your Cool


Engine coolant has come a long way—but can only do its job if things are working properly.

Sometimes called a recovery tank or overflow bottle, this container, if installed correctly, helps keep coolant out of your bilge and affords an easy way to check your engine’s coolant level.


The owner of a 48-foot trawler requested routine engine service, including fluid samples. It didn’t take long for us to notice that the coolant in the overflow tank had a cloudy, discolored appearance.

The owner had changed the coolant within the past year, but sure enough, the lab sample came back with troubling results: Salt water was finding its way into the coolant.

Why that was happening, and whether major engine work would be needed to fix the problem, is a story that starts with understanding how internal combustion engines work.

a different way to transfer the waste engine heat. Water is a good conductor of heat, so using water to surround the part of the engine that creates the most heat—the top and sides of the piston cylinder where the explosion occurs—seemed like a good idea.

Early water-cooled engines literally had their pistons surrounded by an open tub so the operator could monitor the water level and add more as it evaporated or boiled away. Then, marine engineers enclosed the tub and pumped seawater through the engine to cool it. The seawater was always cooler than the engine, and could carry away the excess heat.


It is an unfortunate fact that the internal combustion engine creates a lot of unused heat. Only a little less than half of the burning fuel gets turned into useful energy. The rest just makes things hot. Engine manufacturers have gone to great lengths to figure out different ways to keep that excess heat from damaging engine components. If there is too much heat, pistons will expand and seize, oil stops being slippery, and

The problem was that at temperatures above 150 degrees Fahrenheit, the salt in the seawater precipitated out and could stick to the walls of the internal passages. It became an unfortunate insulator. This process could start a vicious cycle of overheating and corrosion. For this reason, marine engines must have a cooling capacity to keep the raw-water flow below 150 degrees Fahrenheit.


Engineers then discovered that if they added ethylene glycol to the water, the boiling point could be raised and they could carefully crafted metals can deform. The bigger the engine, the more heat must be removed.

The automobile industry solved this problem by taking advantage of the airflow over a fast-moving car. That air can blow on a radiator filled with coolant, and fans supplement the cooling when the car isn’t moving.

Since a boat’s engine is located deep in the bilge and the cabin needs to be watertight, funneling enough air to cool the engine isn’t feasible. There have been attempts to do this with amphibious “duck boats,” but cruising boats need mitigate the corrosion issue.

Allowing the heated coolant to pressurize the cooling system at 15 psi raises the solution’s boiling point further, by another 45 degrees. Pure water boils at 212 degrees Fahrenheit, while a coolant’s boiling point is 265 degrees Fahrenheit. This extra cushion helps to deal with engine hot spots.

Coolant itself has an interesting story. It’s needed because water, while effective at removing excess engine heat, also expands when it freezes. In fact, water can easily expand with enough force to crack cast iron. Originally, methyl alcohol was added to keep water from freezing, but the alcohol accelerated

corrosion inside the engine and tended to evaporate quickly from earlier, open coolant systems. The major component in modern coolant, ethylene glycol, was first synthesized in 1856 by French chemist Charles-Adolphe Wurtz. It was first commercially produced in 1917, and first used as an engine antifreeze in 1926.

A lot of experimentation was required to come up with the correct mixture of ethylene glycol to water. Straight ethylene glycol will carry heat away 15 percent to 20 percent less effectively than a 50-50 mixture. And strangely, if ethylene glycol is undiluted, its freeze point is only 10 degrees Fahrenheit, which is not enough protection for some climates. Adding water in a proper mixture will protect an engine down to minus 34 degrees Fahrenheit.

One note here: The water must be pure distilled water. Unless you are in most dire circumstances, don’t put tap water into your engine. If you must, then at your first opportunity, flush it out with distilled water and a proper coolant mixture. Tap water contains minerals and salts that can accelerate corrosion inside the engine.

Along those lines, coolant and engine manufacturers have developed all sorts of additional corrosion-reducing additives. These additives typically work by keeping the solution slightly alkaline (as opposed to acidic). Over time, coolant tends to become more acidic, which is why you should change your coolant at recommended intervals.

Coolant colors differentiate the brand and type. Each color utilizes different corrosion additives to specifically suit the engine’s metallurgy.


Pure ethylene glycol is clear. Neon green dye was added to indicate what the product is when it is used in a vehicle.

Coolant also tastes slightly sweet, and is poisonous; never

leave open containers around that a pet or child could sample. The original formulation, which lasted from roughly the 1920s until the 1990s, used inorganic acid technology. IAT contains silicates and phosphate corrosion inhibitors that worked in the mostly cast-iron blocks of that time. Newer engines require more corrosion control. That’s where all of today’s different coolant colors come in.

Organic acid technology coolant does not contain silicates or phosphates, but rather fully neutralized organic acids and azoles (a kind of anti-fungal). OAT coolant is typically amber, orange or red, and it works better if the engine has aluminum or magnesium components. John Deere helped in the development of this coolant, which was the original long- life coolant.

Hybrid organic acid technology coolants are the newest mixtures. HOATs are a blend of IAT and OAT coolants, and can be yellow, turquoise, pink, blue or purple. They are an attempt to provide the most protection for increasingly complicated engines. Don’t mix the colors. Some coolant formulations may be compatible, but why take a chance? It is just not worth the gelled mess and the possible damage your engine might endure for a few bucks of coolant. Instead, follow your owner’s manual for coolant changes. All types of coolant have a service life dependent on the engine manufacturer’s tests.

The core of a heat exchanger is being pressure tested to look for leaks. The red fiberglass is clamped by nuts on the threaded rods to seal the ends of the tube bundle against sheet rubber. Once compressed air is applied, the bundle can be sprayed with soapy water and any leaks become evident.
Pressure caps are designed to slowly release coolant into the cap seat hose barb once they’ve reached their designed pressure—in this case, 15 pounds per square inch.


On board, the engine coolant gets piped into a set of tubes located in a tank that has seawater flowing through it. The seawater absorbs the excess heat of the coolant and then exits into the exhaust elbow, cooling the exhaust components on its way out. It’s as if the two fluids meet, but don’t touch— something we’re all familiar with these days.

The sizes of the thru-hull, strainer, hose, pump and heat exchanger are engineered to create the correct flow for the right amount of heat transfer. A constriction of flow from sea growth, a jellyfish or some other obstruction, or from a damaged pump or impeller, can reduce the flow and cause overheating.

Boats with dry exhaust, like many Nordhavns, send the coolant to a keel cooler. A keel cooler is essentially a radiator that mounts outside the hull. The coolant is pumped through the keel cooler, and the surrounding seawater dissipates the heat. The benefit of this system is that no seawater needs to be pumped into the engine room. The downside is that there is no cooling water to cool the exhaust. Dry exhaust systems have to be engineered and insulated to prevent fires inside the vessel. Exhaust temperatures can reach 1,200 degrees Fahrenheit at the engine, while the surfaces of the muffler and exhaust pipes can reach 400 to 900 degrees Fahrenheit.

At engine start-up, the efficiency of the cooling system works against us. Ideally, the engine and its internal lubricants will reach a proper operating temperature quickly, but a good cooling system will delay that process. We need a way to temporarily make the cooling system less effective.

A spring-loaded bypass mechanism, the thermostat, shortens the coolant path until the engine reaches its set temperature. This process helps to warm the engine quicker, so it can be more efficient. Once warm, the thermostat opens and our coolant flows around the engine, keeping things somewhere around 185 degrees Fahrenheit.

If there is too much coolant in the system, then at the set pressure, the radiator cap on the coolant expansion tank will open. For this reason, there should always be a hose attached under the cap; this hose can be led away from the engine to another reservoir, called an overflow or recovery bottle.

The overflow bottle is typically made of translucent plastic, so it is easy to monitor the quantity of fluid within it. This bottle is sized to handle the expansion of the coolant as it heats; when the engine cools, the radiator cap opens and allows the coolant to flow back into the engine. Since the overflow bottle does not work by gravity, it shouldn’t be mounted above the pressure cap. Plus, the lower location allows removal of the pressure cap (when the engine is cool) without the overflow bottle draining.

If the coolant in the expansion tank is full and cool, and if the expansion tank is filled to the line marked “low,” then the expansion tank will work as an easy reference for your coolant level.

Depending on the size of the engine, there can be quite a few gallons of coolant capacity. When engineers calculate what size expansion tank and overflow bottle to use, they take into account that the coolant needs 12 percent of total volume for drawdown capacity, and 6 percent for thermal expansion. Drawdown capacity is the amount of coolant that can be lost before air reaches the coolant pump. Thermal expansion is what happens as the coolant extracts heat from the engine, and the engine metal heats up.

In an average engine, this might mean a couple quarts of movement. If the system is overfilled with coolant, that expansion will push the coolant out of the fill cap of the reservoir, making a mess. Some overflow bottles are made with a hose barb under the lid, which may have a hose connected so any overflow can be directed to the bilge.

The black hose at the bottom of the overflow bottle carries expanded coolant from the engine’s heat exchanger to the bottle. The tinted hose at the top directs overfilled or overheated coolant toward the bilge.


Armed with this understanding of the cooling system, we began to diagnose the problem on the boat that had cloudy, discolored coolant in its overflow tank.We sent a sample—along with information about the engine model and brand, type of coolant, and how long it had been in service—to the lab. The results told us that salt water was finding its way into the coolant.

There are only a couple places on an engine where seawater and coolant can mix. One would be a hole in one or more of the tubes, or a damaged gasket in the heat exchanger, because the pressure differential between the two circuits

after running.

This boat also had antifreeze circulating in its exhaust elbow; that could be a path for mixture if the elbow’s gasket failed, or if the elbow was corroded through. Some engine manufacturers use coolant passages inside their exhaust elbows, as well as saltwater injection to help cool them. The exhaust elbow is the last metal component on the engine before the exhaust hose, and all the exhaust heat is firing right at it.

The transmission may have a cooler, but that would be cooling transmission oil, not coolant. The hydraulic stabilizers had a heat exchanger to cool the hydraulic oil. The turbo might have coolant passages, but again, a failure would tend to contaminate oil, not coolant.


The system was pressure tested, and no leaks were found. But, remember that hose attached to the barb under the cap of the overflow tank?

Although it was properly attached and installed neatly, this hose had one unfortunate flaw: it was too long and reached into the bottom of the bilge. When the engine cooled, the hose created suction, drawing dirty bilge water into the expansion tank.

We shortened the hose, flushed the system, and added the correct type and amount of antifreeze. The engine then ran properly, and the coolant remained clean. What appeared at first to be a looming major expense turned out to be an easily and inexpensively solved problem.

Steve Zimmerman is the president of Zimmerman Marine, which operates five boatyards in Maryland, Virginia, North Carolina and South Carolina. Zimmerman has been building and repairing boats for more than four decades.

Max Parker is the operations manager of Zimmerman Marine. Parker has spent time in most of the trades in his over 25-year career working on boats and still loves to grab a tool bag and fix something when he gets the chance.