Skip to main content

How to Maintain Your Boat's Cooling-System

Survival of the Coolest

As Engine Cooling Systems Evolve, So Should Maintenance.

Have you ever considered the route sea water travels between your engine’s intake sea strainer and its exhaust system? Not just through the engine, but what components that water actually cools? How about antifreeze circulating through the engine? As horsepower from a given-size engine has increased over the years, so too has the load on cooling systems and their complexity. In fact, cooling system design changes have been as necessary to achieve that added horsepower as has electronic fuel delivery. Understanding the parts of an engine that raw water and circulated antifreeze cool respectively is increasingly important for engine maintenance.

Most outboards employ open cooling systems, meaning sea water or lake water is pumped into and circulates through the engine to cool internal components. It’s still widely used in gas inboards too. “Probably 95 percent of the boats here use open cooling,” says Jeff Fay, owner of Fay’s Boat Yard on New Hampshire’s Lake Winnipesaukee (, where fresh water is kind to metal. “I see engines from the ’60s and ’70s all the time.” 

The new Cummins QSB 6.7 has an aluminum heat exchanger with high-strength polymer end caps and expansion tank: light, durable replacement parts.

Open cooling systems are simple and therefore less expensive to buy initially and also to maintain, but components like circulating pumps and thermostats are exposed to sea or lake water and need to be replaced every few seasons. “The seal fails and water gets into the pump bearing,” Fay says. An occasional drip from the weep hole between the pump and engine is an early warning. “You might get the rest of the season, but it should be changed,” he continues. “Thermostats either stick open and the engine runs stone cold, or they fail and the engine overheats.”

Most gas inboards used in salt water and nearly all diesels employ closed cooling systems. Rust-preventing antifreeze circulates through the engine block and many components, instead of raw water from outside the boat. “As long as that antifreeze mixture is maintained, we typically change circulating pumps and thermostats only when we overhaul engines,” says John Strauss, owner of JAS Marine in West Palm Beach, Florida ( 

To transfer engine heat from antifreeze overboard, sea water or lake water routes through heat-exchanger tubes or plates immersed in the antifreeze. For boats in salt water, Strauss suggests tearing down gas-engine closed cooling systems about every three to five years, and diesels a bit sooner. “Scale builds up in both the antifreeze and saltwater sides,” Strauss says, which impedes heat transfer. “Have that heat exchanger cleaned and pressure-tested by a radiator shop.”

Fay warns to pull end fittings off heat exchangers every few years to ensure they’re not clogged with silt or weeds. “We’ll snake a wire through the tubes to clean them out,” Fay says. He also drains all old antifreeze and replaces it. “That’s about the most we have to do up here,” he says.

Click here to see Typical Cooling System Maintenance Schedules

Both Strauss and Fay stress checking antifreeze potency every season using a hydrometer or refractometer. Fay is concerned with engine blocks and components freezing and cracking over the winter. But the manufacturer-recommended mix of distilled water and antifreeze does more than guard against freezing, particularly in diesels. “It protects the metal from rust,” Strauss says. “It also softens the water so it doesn’t cavitate within the engine,” which can pit the walls of the water jacket surrounding the cylinders.

Transmission oil, power-steering fluid, and fuel may also be cooled within an engine or in heat exchangers within the rawwater plumbing system. Debris or scale buildup can impede water flow and overheat one of these components or the engine. Clean these components when cleaning the main heat exchanger. 

New engines, both gas and diesel, spray oil onto the undersides of pistons to keep them cool. “Tighter tolerances create more horsepower,” Strauss says. “Without that oil cooling, the piston physically expands with heat,” he says. “It sticks inside the cylinder and literally comes apart.”

“Oil coolers are just as important to clean as heat exchangers,” echoes Charlie Schloemer, President of Palm Beach Power in West Palm Beach ( “Intense heat from the oil acts like a magnet, grabbing whatever contaminants are in the coolant.” That scale impedes cooling, and overheated oil breaks down, leaving parts improperly lubricated.

On many MTU engines Schloemer works on, the charge-air coolers—the collective term for both intercoolers and aftercoolers—are cooled with circulated antifreeze. “That added heat seems to break down antifreeze faster,” he says. Many diesels pass sea water through charge-air coolers instead. “The sea water works on the metal and welds,” he says. “If they rupture you’ll get sea water in your engine.” Recommended maintenance and periodic oil samples insure against such an issue.

On gas engines, cast-iron exhaust manifolds are often cooled with raw water. In saltwater situations replace them every three to five years. Not so in fresh water, though. “I’ve got boats from the ’60s with original manifolds,” Fay says, adding that 10 years tends to be a minimum life. “If they fail, though, they can rot out and leak externally into the bilge, or they leak internally and you get water in the motor,” Fay says, but even this isn’t necessarily an engine killer in fresh water if caught early.

“With fuel-injected gas engines we often see antifreeze-cooled manifolds,” Strauss says. “But it’s an option that not all boats add. On the newest engines, the catalytic converter (a newly required component) is raw-water-cooled, but the rest of the system is cooled with antifreeze.” 

Diesel exhaust manifolds are typically cooled with antifreeze. In recent years manifolds have seen a design shift to cooling exhaust gases less, thereby transmitting less heat to cooling systems overburdened as horsepower increases. This also increases exhaust gas volume and velocity, which increases power from a given-size turbocharger and boosts horsepower but also stresses those turbos and the exhaust system. 

On marine diesels, sea water is typically injected into the exhaust just past the turbochargers and then routed overboard. Most diesels are fitted with stainless-steel exhaust elbows to accomplish this. These elbows last 10 years or more and are designed so water won’t flow back into engines if they leak. Replace them at the first sign of a leak though, or risk overheating rubber and fiberglass exhaust components and/or flooding the bilge.

Gas engines almost always have cast-iron exhaust elbows. A leak can ruin an engine. Like raw-water-cooled manifolds, such exhaust elbows are replaced every three to five years in sea water, but can last many seasons in fresh water.

Raw-water pumps are similar on almost all marine engines. Change rubber impellers every year or two at the very least. Waiting until one fails means having to work to find all the pieces, typically lodged in the seawater inlet of gas or diesel heat exchangers or in the thermostat housing of open-cooled gas engines. Since silt and sea water wear bronze like sandpaper, Strauss replaces raw water pumps at 1,500 hours—half that on boats that often run in shallow water. Worn pump-cover plates greatly reduce pump efficiency. “You can usually flip the plate over,” Strauss says, turning the painted surface outside-in over the impeller. 

Plastic coolant-overflow bottles mounted on bulkheads also require attention. “They’re the most neglected part of the cooling system,” Schloemer says. A pinhole in the hose that connects the overflow bottle to the engine will break the siphon between the two. “It has to be full when the engine is hot and then drain down as the engine cools,” he says. “That bottle can show everything is fine, but you could be gallons low on coolant. When the engine is cold, pull the cap on the expansion tank and check it there.” 

A warm cooling system might best stay sealed, however. Some electronic engines measure and interpret changes in cooling-system pressure as a leak, limiting rpm. Pressure sensors reset once engines cool completely. Knowing such cooling-system intricacies as well as understanding where raw water and coolant flow—and where they shouldn’t flow—can catch developing problems before they turn into costly repairs.

This article originally appeared in the June 2012 issue of Power & Motoryacht magazine.