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How to Check the Cooling System on Your Boat


Know what these do? You certainly should.

Temperature Check

Fall is an ideal time to inspect your engine’s cooling system.

Fall lay-up is an established and thoroughly discussed ritual, but even for those who can enjoy boating through the winter months, it’s also a good time to address maintenance issues neglected during peak season. One of those may be your engine’s cooling system, because it so rarely demands attention. Whether you own outboards or inboards, gasoline engines or diesels, if your engines are of recent vintage, you almost certainly have never been subjected to the indignities of an overheating. Modern cooling systems are about as trouble-free as anything can be on a boat.

But cooling systems do require periodic attention, although, thankfully, not much and not very often. Usually a once-over every couple of years is sufficient.

What follow are simple cooling system-related checks that you can do in a couple of hours, which will reduce the chance of overheating. Which ones you focus on depends on the kind of engines you have: Outboards have relatively simple cooling systems that don’t require much attention, but the operative word is “much.” When it comes to cooling, no engine is entirely maintenance-free.

The first order of business is coolant, one of those things that an outboard owner doesn’t have to mess with. Any engine that’s fresh-water-cooled has a closed loop of coolant that removes heat and is in turn cooled by raw water pulled from the surrounding environment and passed though a heat exchanger. 

Unlike the coolant in your car, which must be able to contend with below-freezing temperatures, the primary job of the coolant in your boat is to effectively absorb heat while preventing corrosion and deposit formation. Like any chemical compound, coolant deteriorates over time, and as it does, its effectiveness wanes. The company that manufactured your engine specifies an interval at which your coolant (and perhaps a coolant filter) should be changed, and you should ignore neither that nor the specifications for the replacement coolant. For instance, coolant is generally diluted with water (pre-diluted coolant is also available), and the proportions are critical: Too little coolant will result in corrosion; too much will degrade its ability to absorb heat. One of the most common questions I get regarding coolant is whether there’s any advantage to diluting it with distilled water. As far as I can tell, doing so has no clear advantage, although it can’t do any harm.

If your engine coolant is up to par, your next point of focus is sacrificial anodes, or as they’re mistakenly called, zincs. All types of engines have these anodes, and many have multiple ones, so it’s important to know how many you have and where they are. (Refer to your owner’s manual.) Their job is to corrode and dissolve so that other, more expensive, engine components do not. Anywhere your engine comes into contact with exterior water, there’s likely at least one sacrificial anode. It can look like anything from a narrow pencil to a large doughnut.

The general rule is to replace any anode that’s more than half gone, but I’ve always thought the logic behind that was flawed: It assumes that anodes deteriorate at a fixed rate, but often they do not. For instance, if a boat with a severe stray-current problem moves into your marina, your anodes could be gone in weeks. That’s why I replace any anode with any visible deterioration. They’re relatively cheap, so why not?

To avoid the rude surprise of overheating, the coolant must circulate properly, and to do this, inboards have an integral pump, usually on the front of the engine, and often driven by a belt. This pump typically has a long life and requires no servicing. All engines also have a second pump to circulate raw water; on outboards it’s usually in the midsection, between the power head and lower unit; on inboards it’s also typically on the front of the engine—usually, but not always, driven by a separate belt. This pump has a limited service life, especially if the boat has been operated in a sandy environment.

You can deal with the raw-water pump in one of two ways: Replace its rubber impeller annually, or leave it alone until there’s a problem. I prefer the second option, as the impeller can be difficult to install, and doing it incorrectly will almost assuredly cause overheating. Plus, before they fail, water pumps usually give some warning, such as reduced water flow out of the exhaust, or rising operating temperature.

Finally, you inboard folks, don’t forget the drive belts. It’s rare, but not unheard of, for a belt to break; a more likely problem is glazing, that shiny surface on the interior side of the belt due to slippage. A glazed belt cannot be repaired, and will continue to slip until the engine overheats—so replace it, and make sure that the tension is correct, as improper adjustment is usually what causes glazing in the first place.

This article originally appeared in the November 2016 issue of Power & Motoryacht magazine.