Cooling Hot Iron

The majority of the diesel engines in launches and yachts are heat exchanger cooled. The fresh water that keeps the engine at a regulated temperature is cooled by salt water pumped through a heat exchanger. The two water flows do not mingle and the saltwater - which enters cool and exits hot - is dumped via the exhaust pipe or raw-water discharge.

The exceptions are a keel-cooled fresh water system where the fresh water is cooled via piping that is attached to the outside of the hull (an external heat exchanger) or a raw-water cooled engine which is feed saltwater directly into the water jacket. Keel cooling is normally used by commercial boats or craft operating in dirty water-ways where blockages of the salt water system can regularly occur, and raw water cooled engines are mainly lower horsepower yacht auxiliaries.

In a heat-exchanger-based cooling system, the coolant is drawn by the fresh water pump from the heat exchanger and is forced through the oil cooler, cylinder block, cylinder head(s), after-cooler (if fitted), exhaust manifolds and water-cooled turbo charger housing to the thermostat housing.

A bypass from the thermostat housing(s) to the inlet of the fresh water pump permits the re-circulation of coolant when the thermostat is closed.

Being a closed circuit, the pressure in the fresh water system is maintained by a coolant pressure control cap or radiator cap. A radiator cap is normally stamped with its pressure rating so a cap with, say, 9 stamped on it is designed to permit a pressure of approximately 9psi (62kpa) before the release valve opens. The pressure raises the boiling point of the cooling liquid to 234° Fahrenheit (112° Celsius) and permits an increased engine operating temperature without the loss of any coolant from boiling over.

To prevent the collapse of hoses and other parts which are not internally supported, a second valve in the cap opens under vacuum when the system cools. When removing a radiator cap on a warm engine use extreme care because the sudden release of pressure can result in a geyser with the potential to scald. It is safest to remove the cap slowly after the engine has cooled overnight.

Most freshwater circulating pumps are of the centrifugal-type and either belt or gear driven off the front of the engine. An impeller secured to the water pump shaft pumps the water through the cooling system. Occasionally this impeller will come loose on the shaft, causing a reduced water flow that will result in over heating. A more common problem on belt driven fresh water pumps is belt slippage or breakage. This either slows or completely stops the water flow.

Regularly check the condition of the water pump belt and tighten to the recommended level. Replace any belts that show excessive wear. Also check the fresh water pump for any water leaks through the seals or covers. If leaking, get it repaired or replaced because water pump leaks only get worse and they could eventually lead to engine overheating and main bearing failures. Both of these are precursors to terminal bank account failure.

The thermostat maintains engine temperature at between 170° and 195° F (76.6° to 90.6° C). It remains closed when the engine is cold and gradually opens as the temperature increases. At temperatures below 170° F the thermostat remains closed, and this allows all the coolant flow to be diverted back to the suction side of the fresh water pump via the bypass piping. As the temperature of the coolant rises above 170° F, the thermostat valve starts to open, restricting the bypass system and permitting a portion of the coolant to circulate through the heat exchanger. Once the temperature is above 185° F, the thermostat opens completely and the bypass system is completely blocked off. All the coolant is directed to the heat exchanger where it is cooled by the saltwater.

Should an engine run consistently below 170° F, it is likely the thermostat has failed in the fully open position. If the temperature exceeds 195° F then the thermostat could have failed in the closed position, preventing any coolant from going to the heat exchanger. Overheating will occur almost immediately, particularly if the engine is working at normal cruising speed or beyond.

Should a faulty thermostat be suspected, you could replace it on speculation or test it by immersing it in a container of water progressively brought to the boil. Place a thermometer in the container but do not allow it to touch the bottom. As the water warms the thermostat should begin to open (the opening temperature is often stamped on the thermostat) and be fully open by the time the temperature reaches 195° F. The thermostat should close again when the water cools. If it doesn't pass this test, replace it immediately.

A major percentage of engine failures (up to 40 percent say some industry sources) are due to cooling system problems. With proper cooling system maintenance these failures can be easily avoided. Coolant quality is as important as the quality of lubricants and fuel but is also the one given the least attention. The three main coolant functions provide:

* Adequate heat transfer and anti-boil protection,
* Cavitation erosion and corrosion protection,
* Anti-freeze protection.

Engine coolant normally consists of water, additives and glycol. Distilled or deionized water is recommended in cooling systems; do not use hard or salt water.

In warmer northern waters the anti-freeze component of the coolant can be excluded as freezing temperatures are unlikely in the marine environment.

Additives that prevent erosion and corrosion are marketed by the major oil companies and specialist suppliers such as PriMax Distributors (PriMax RCP Corrosion Inhibitor). These must be included in the coolant mixture as they prevent the formation of rust, scale and mineral deposits, prevent liner cavitation and contain anti-foaming agents. As additives deplete during engine operation they must be regularly tested for the required concentration levels and topped up as required by the supplier.

If the engine is operated in a region where anti-freeze protection is required, then glycol must be added to the coolant mixture. This raises the boiling point of the coolant to prevent boil-over, provides freeze protection, helps prevent water pump cavitation and reduces cylinder liner pitting.

Always top-up the coolant level with a premix of the coolant because topping up with water only will dilute the mixture in the engine and reduce its effectiveness.

Any loss of coolant should be investigated because lost coolant means lost coolant pressure and reduced boil temperatures, and because leakage into the oil, air intake system or combustion chamber can cause major wallet-wrenching damage.

The heat exchanger has a core that separates the fresh water from the salt water. Salt water is sucked from the sea (hopefully through a saltwater filter) and pumped to the heat exchanger. The saltwater cools the fresh water via the core and then passes out of the heat exchanger to the exhaust system or discharge if the exhaust system is dry stacked. Apart from water leaks and the regular cleaning of the core every two or three years, the heat exchanger requires very little maintenance. However, if the engine is running at slightly elevated temperatures, the end caps should be removed and the core inspected for salt build-up and other restrictions.

A Jabsco-type saltwater pump is a positive displacement pump. The impeller - driven by a spline or key - is lubricated by the water and should not be run dry for longer than normally required for the pump to prime itself. One of the major causes of impeller failure occurs when skippers forget to turn on the saltwater skin fitting and start up the engine. Run dry, rubber impellers overheat, shed their blades and die. When replacing a damaged impeller it is imperative to find all the small bits that have detached themselves from the central boss. They could have been forced into the heat exchanger and/or inter-cooler housings where, if not removed, they will restrict the saltwater flow once the pump is repaired .

The saltwater pump will either be a detached unit that is driven by a belt off the crankshaft pulley or a gear driven pump attached to the engine. To inspect the pump impeller, remove the six cover screws that hold on the cover plate and remove the impeller using two screw drivers as levers, or a special tool that is available from your Jabsco agent. If the rubber impeller is showing any signs of cracking at the core, discard and replace it with a new one. Check the wear plate for excessive wear and rotate or replace if required. To replace the impeller, lightly grease the internal surfaces of the pump, wear plate and cover plate and the drive spline and push the impeller into the housing. Replace the spline plug, grease both sides of a new gasket and replace the cover plate while being careful not to over tighten the retaining screws or bolts. Should water leaks be observed from the drive side of the pump, the pump should be removed and overhauled by a serviceman.

The only other maintenance required on the saltwater pump is to check the integrity of the belt (if belt driven). Replace if worn. Also check the tightness of the belt and tighten to the required level if loose.

Most diesel engines have sacrificial zinc anodes screwed into the salt water side of the heat exchanger, saltwater pump, inter-cooler and/or oil coolers (if saltwater cooled). Refer to your engine manual and check them on an initial six-month schedule, then annually once you have their measure. Clean the anodes with a wire brush, or if excessively eroded, replace with new ones. To determine the condition of an anode, strike it sharply against a hard surface; a weakened anode will break. When replacing an anode, seal the retainer thread with a sealant to prevent water leakage.

While the engine is running inspect it for water leaks. Hose clamps have a habit of quietly loosening over a period and they should be routinely tightened to prevent leaks.

Too Hot To Live
Take a relatively new and sprightly V8 diesel (five years old with less than 700 hours) that was slightly over propped and installed in a poorly ventilated engine room (so the engine was always hot and working hard).

Engine temperatures progressively rose over the years, as a result of the combination of over-propping, lack of ventilation and saline deposits in the heat exchanger core.

The elevated engine temperature caused the bore and rings to wear.

The raised exhaust temperatures caused the turbo to run hot.

The raised engine room temperatures reduced the density of the intake air and caused the turbo to over rev.

The elevated engine temperatures cracked both heads.

The combination of elevated turbo temperature and the turbo spinning too fast caused it to shed a blade. The turbo was totally destroyed and the valves damaged.

Removal, rebuild and installation expenses totalled over $50,000.

Unfortunately the prematurely deceased engine was a twin. Its engine room companion suffered virtually the same fate with cracked heads, worn bore and damaged valve seats.

The good news was the turbo on this engine was salvageable. The bad news was the total rebuild cost was $100,000.