Can Your Cooling System Take the Heat?

Part III of our four-part series follows the flow through the vital parts that keep your engine from frying

Your auxiliary engine’s cooling system is one of the more potentially trouble-prone systems on your boat. A basic understanding of what this system does and how it works, together with routine maintenance, can go a long way toward preventing problems that could ruin a cruise, not to mention your engine.

Contrary to popular belief, the primary function of your engine’s cooling system isn’t to keep the engine cool but rather to maintain its operating temperature at a peak average that will produce optimum efficiency. How this system works depends upon whether your boat’s engine is raw-water cooled or cooled by the more desirable–and popular–closed cooling system.

A Comparison
Many boatbuilders in the 1970s and 1980s set up diesel engines with what is known as a raw-water cooling system. This approach keeps the number of components to a minimum, but it has significant shortcomings.


A raw-water system pumps seawater directly through the engine block, then discharges it through the exhaust system. The cooler seawater absorbs the engine’s heat and effectively transfers it back to the water outside your boat. However, seawater flowing through the engine can cause scale buildup and corrosion inside the engine’s water jacket (the internal passages where cooling water circulates) that can ultimately destroy the engine block and cylinder walls. Freshwater boats aren’t immune to this problem.

A more insidious drawback to the raw-water system is that the engine must run cooler than it could with a closed system. Above 160 F, sea salt begins to crystallize inside the cooling-system passages. This not only accelerates corrosion but also, in extreme cases, can actually block passages. For this reason, most raw-water-cooled diesels have a thermostat set at 145 F, at least 40 F cooler than a closed system. Running at this relatively lower temperature reduces an engine’s thermal efficiency, so a raw-water system simply is unable to produce as much usable power as its closed system counterparts.

The closed cooling system keeps seawater away from the expensive internal-engine parts. The heat exchangers and some of the exhaust-system components still get a daily saltwater bath, but these parts are much less expensive and certainly easier to replace. Also, because the closed side of the cooling system is sealed, it can be pressurized, which–as the laws of thermodynamics dictate–raises the boiling point of the coolant within the system. This allows the closed side of the cooling system to operate at 180 F to 200 F. The end result? Increased thermal efficiency, more power, and better fuel economy.


Raw-Water Checkup
Carrying out routine maintenance of your engine’s cooling system extends not only the life of the individual components in the system but also the life of your engine. The key components of the raw-water system are the inlet sea strainer, the inlet seacock, the raw-water strainer, the thermostat, the water-jacketed exhaust manifold, the exhaust elbow, connecting hoses, and related plumbing.

Some boats will have a bronze strainer that’s located on the outside of the hull and covers the raw-water inlet port. Some of these consist of a simple coarse grating; others have a screen mesh behind the outer casting. The purpose of this grating is to prevent any flotsam from clogging the seacock, inlet hose, or raw-water strainer, which removes finer debris. Many cruising sailors and sailboat builders prefer not to install inlet strainers, since they tend to become fouled with barnacles.

The outside of this strainer typically gets a coat of antifouling paint every time you paint the bottom. But barnacles and other sea growth may accumulate on the inside of the strainer and in the intake port to the seacock. If you can’t remove the strainer for painting, carefully scrape out any growth. An ice pick works well, but be careful not to stab the inlet hose inside. Then, using a coarse artist’s brush, paint the inside of the strainer grating and the inside of the seacock intake as far up as the brush can go. Next, from inside the boat, inspect the seacock for proper operation and service if needed. (For more on servicing seacocks, see “The Care and Feeding of the Seacock,” April 1998).


Carefully inspect the intake hose from the seacock to the seawater strainer. This hose should immediately be replaced if it shows any signs of cracking or collapsing. A break in this hose could sink your boat. A collapsed hose obviously restricts water flow, which could damage your water pump and ultimately cause the engine to overheat. Use only rigid-walled, preferably wire-reinforced, hose approved for this purpose, such as that made by Trident (724-745-9311, www.tridentma No automotive heater hose here–heater hose is designed to run under pressure; this hose must be able to withstand suction. Hose attachments below the waterline should be double-clamped with all-stainless-steel clamps, such as those made by ABA of America (815-332-5170, www.abao

Moving through the system, the next stop is the raw-water strainer, which should be mounted below the waterline so it can easily be refilled after servicing. Since most of these have a transparent housing, routine visual checks for debris usually suffice. Occasionally, you should remove the cover for a closer inspection; clean the corrosion that invariably accumulates on the nut and threads that hold the unit together. A light coating of waterproof grease on the fasteners will ensure easier disassembly in the future. If your boat is in the water, make sure the seacock is closed when you service the water filter.

Next in line is the heart of the system, the raw-water pump. (In many raw-water cooled engines, seawater passes through a gear-oil cooler or even a crankcase-oil cooler before it reaches the raw-water pump.) You should inspect the water pump at least once annually. Your engine’s service manual should provide step-by-step instructions for servicing and replacing key pump components; if it doesn’t, a workshop manual will. Depending on your level of skill and confidence, some jobs, like replacing bearings, might require professional assistance, since these may need to be hydraulically pressed in place.


Contributing editor Steve D’Antonio recently examined raw-water pumps in detail (“Know Your Pumps, Inside and Out,” April 2001), but a few additional inspection tips are worth mentioning. Inspect the pump’s impeller blades closely for signs of cracking. A broken blade can block the system, causing the engine to overheat. Also check the flex characteristics of the blades themselves. If they stay in the bent, flexed position when the impeller is pulled from the pump body, it’s time for a new impeller.

Also, some pumps that mount directly on the engine have a weep hole at the bottom that drips water when the seals for the pump shaft begin to fail. Signs of corrosion here indicate it’s time for new seals. With a bright light, you may also be able to check for drips while the engine is operating.

Next in line is the thermostat. This is located near the top of your engine, and you can identify its housing by following the hose from the exhaust elbow back to the engine. This hose connects to the outlet of the thermostat housing. Periodically, you need to disconnect the inlet and outlet hoses, remove this housing, and clean scale from the passages. The tube that connects the outlet hose to the exhaust elbow is another common point for salt buildup. A third place to check for crystallization is the anti-siphon valve, which is sometimes installed in-line on the hose between the exhaust elbow and the thermostat housing. (Location of the anti-siphon valve varies according to installation; some engines simply don’t have one.) This valve prevents water from flowing back into the engine. If it gets clogged so that air can’t vent into the system, it’s useless.

When cleaning the thermostat, use a soft-bristled wire brush to get rid of any scale and debris. To check the opening temperature of the thermostat, I use a small coat hanger to suspend the unit in a pan of water that I heat on a stovetop. I use a meat thermometer to check the water temperature. The thermostat valve should begin opening at approximately 145 F. A malfunctioning thermostat must be replaced.

Finally, check your owner’s manual to find out if your engine has any sacrificial zinc anodes threaded into the engine block. These protect the cooling system from galvanic corrosion caused by dissimilar metals being immersed in salt water. If your engine has zincs, you must check them regularly by unscrewing the threaded cap on which the anodes are seated. If more than 50 percent of the original anode is missing, it’s time to get a new one. Compare it for size with one in your spare-parts kit. If you don’t already have several spares on board, buy some.
You’ll need them.

Never use Teflon tape on the threaded cap that holds the anode in place. There must be good electrical continuity between the threads and the engine block. If you must seal the threads, use a liquid thread sealer instead. This still allows for adequate electrical continuity.

Closed-System Care
The closed cooling system is really two systems combined: a raw-water system that pumps seawater through a heat exchanger mounted on the engine and a freshwater system that circulates a coolant/antifreeze solution around your engine’s expensive innards. This system requires two water pumps, one to circulate the coolant inside the engine block, the other to pump seawater through the heat exchanger and out the exhaust, adding a twist to the raw-water cooled system.

The closed system will also have a coolant reservoir, with a pressure-rated cap. On modern engines, the reservoir will have an expansion, or header, tank to accommodate fluctuations in coolant volume, which, as we’ve seen, varies between 180 F and 200 F according to the engine’s design and operating temperature.

The key component in this system is the heat exchanger, in which coolant/antifreeze flows through a carefully designed chamber with tubes running through it. These tubes carry the raw water. As the raw water threads its way through the heat exchanger, it absorbs the heat from the surrounding coolant solution and sends it on its merry way out of the exhaust system.

How do you maintain this system? As with the raw-water system, intake strainers, seacocks, plumbing, and raw-water pumps will need periodic attention. The closed side of the system may or may not have sacrificial zincs installed, and the raw-water side almost always will. Review your engine manual to find out where these are located, then check them periodically. Since it lives in a much friendlier environment, the thermostat on the closed system won’t need the routine cleaning that one in the raw-water system requires. As long as the engine is operating at the normal, prescribed operating temperature, you can be certain that the thermostat is functioning properly.

Check the coolant periodically to be sure it’s at the correct level and appropriately mixed to ensure proper freeze protection to -25 F. This usually means a 1:1 mixture with fresh water, but some coolant needs no dilution. Be sure to confirm what kind you have and what your engine requires. Your coolant mixture also prevents rust and corrosion, and this protection diminishes over time. Replace it at least every two seasons. Some engine manufacturers recommend the use of supplementary additives for corrosion protection; check your engine manual.

Next is the coolant-tank cap itself. Inspect it carefully for excessive rust and check that the seal is in good condition. It’s a good idea to have an old cap checked at an automotive shop to determine that its pressure-relief mechanism opens at the prescribed pressure. As these caps age, they often cause loss of pressure, which reduces the boiling point of the coolant and drastically impairs the system’s efficiency. (See “Put a Cap on It,” November 1999).

Finally comes the heat exchanger itself. Over time, the tubes inside the heat exchanger will become restricted, diminishing the unit’s effectiveness. Depending on the design of your heat exchanger, you may be able to clean it easily yourself. If not, radiator shops often have experience cleaning heat exchangers. Check your engine manual to see how to clean yours.

One way to clean both raw-water and closed cooling systems without disassembling parts of the engine is to flush them with a solution that dissolves scale, such as Marsolve (203-834-8278, www.mar Improper use of a dangerous chemical such as muriatic acid can cause costly damage to the engine.

Some heat exchangers are quite easy to deal with. Simply remove the end cap on the unit to expose the raw-water tubes. At least annually, take off the end cap for inspection and clean out any accumulated scale, seagrass, or other foreign matter. Debris commonly builds up here. I recommend cleaning the tubes at least every three or four seasons with a small wire brush mounted on an extension handle; Mill-Rose (440 974-6730, www.clean makes one.

Following the steps outlined here will get you through basic cooling-system maintenance, which should ensure that your diesel engine always keeps its cool. In our final article in this series on diesel engines, we’ll look at common maintenance chores and troubleshooting challenges.

Ed Sherman is CW’s electronics editor.