Powerboating - Part Two

A few weeks ago I visited a friend on his boat. We had arranged to meet as he was encountering difficulty with the cooling of his engine, and had requested my advice. A quick inspection of the exhaust while the engine was running revealed the problem. Further investigation determined the cause as a shredded saltwater pump impeller.

My friend, who has had a long history of heart-related problems, looked at the damaged impeller and sighed: "Pumps, I should have known the problem would involve a pump. All my life, if anything is going to give trouble, I'll bet there is a damned pump involved!"

I suppose these sentiments have been echoed by thousands of boaties over the years. Pumps seem to have a penchant for finding trouble, so this month I thought it pertinent to discuss two of the most important pumps in the boat and all their associated circuitry that keeps the engine running at its correct temperature.

ENGINE COOLING
The saltwater plumbing for the engine cooling is usually constructed from first-class materials to withstand the harsh environment in which they operate. The general layout will be as described here. However, small variations can occur. Also, different engines bring their own diversities, but most installations will follow fairly closely to this article.

The engine cooling is the most important of all the marine plumbing and the start of the saltwater circuit is the skin fitting.

SKIN FITTING
The skin fitting is usually made of bronze or gunmetal. This material is highly resistant to corrosion due to electrolysis, a subject that will be discussed at a later date. Skin fittings are installed as follows:

ENGINE COOLING CIRCUIT - SALTWATER
Of all the saltwater circuits on a boat, the engine cooling is easily the most important and requires the most maintenance.

The water enters the hull through the skin fitting and seacock and passes into the sea strainer. This device has a removable top. In the event of the boat passing through weed or similar pollution, this material is prevented from entering the pump by the use of a fine screen.

This strainer should be checked at regular intervals by removing the top and lifting out the screen for cleaning. If the strainer is mounted below the waterline it will first be necessary to turn off the seacock. Remember to turn it back on after the top has been replaced before starting the engine, or serious damage to the pump will occur.

Engine saltwater pumps are powerful pieces of equipment capable of moving large quantities of water. They are almost always of the impeller type and generate large suction pressures, a fact which makes them self-priming. These pumps are remarkable pieces of engineering but do require regular servicing.

Every engine oil change, or six months, remove the front plate from the pump, after first turning off the seacock. This plate is made of soft bronze and requires close inspection for wear on its inside surface. The plate bears directly on the rubber impeller in the body of the pump and forms a seal to prevent the water passing around the impeller. If it is badly worn it can generally be reversed so that the outside of the plate now becomes the inside. A spare should always be carried.

Next to be inspected is the impeller itself. To remove, grasp one of the vanes with a pair of long-nosed pliers at the root and pull. Do not damage the impeller or it will have to be replaced. This should slide off the splines of the shaft, although at times they can be the very devil to dislodge.

After removal and in a good strong light, inspect the impeller. Spread the vanes and look at the root where they are attached to the body. Any signs of stress cracks and it should be discarded.

These impellers have a limited life due to the distortion imposed upon them each rotation and when they fail, shred into many pieces and can clog the heat exchanger. Regular inspection will prevent this, but if one should fail at sea, replacement is usually not difficult.

After impeller inspection and replacement if necessary, undo the screw on the top of the pump and remove the bronze cam from the body of the housing. This only requires inspection for unusual scarring. Reach into the pump body with a small screwdriver and remove the rear wear plate. This does the same job as the front wear plate and also can be reversed if excessive wear is noted.

This is as far as disassembly should be necessary for normal servicing. Grasp the shaft and feel for any movement, indicating wear of the back bearing. A lip seal is at the rear of the housing to prevent water entering. If this fails the pump will soon collapse as corrosion attacks the bearing, and if this has happened, the pump will have to be removed and sent to a specialist for repair - an expensive business, I'm afraid.

Replace the rear wear plate and cam, smear water-pump grease on the shaft and install the impeller. Fit a new gasket for the front wear plate if necessary and replace the plate. Sometimes a gasket cement will be required to achieve a watertight seal. Turn the seacock back on and operate the engine. Carefully inspect the serviced pump for any leaks and check the exhaust for cooling water.

The engine saltwater pump, having lifted the water through the strainer and then itself, passes the water through the engine oil cooler. Because the amount of heat which needs to be removed from the oil is small, these coolers tend to be simple in construction (merely a pipe through which the water passes and surrounding this pipe is a sealed jacket, allowing the hot oil to come into contact with the cool pipe and thus achieve heat exchange). So little maintenance is required other than a visual inspection for corrosion and leaks.

DON'T WORRY, IT'S COOL...
The saltwater, now slightly warmer due to the heat from the engine oil, enters the main engine heat exchanger. Most marine engines are directly cooled by freshwater (as in an automobile) and the heat exchanger takes the place of the radiator. By cooling the engine first with freshwater, the designer can operate the engine at a much higher temperature than would be possible if the saltwater was circulated directly through the engine. The reason for this is that the salt in the water is deposited out at roughly 60?C and would eventually clog the galleries of the motor. It would be impossible to clean these galleries and the engine would be rendered useless. As a consequence, raw-water or direct saltwater cooling of engines is not popular. The necessity to operate them below the temperature of salt precipitation results in inefficient combustion, dirtier exhaust smoke and higher fuel consumption.

Internally, the heat exchanger is constructed of bundles of thin-walled tubes through which the saltwater passes. The hot freshwater surrounds these tubes and is thus cooled. Because the saltwater will generally be elevated above the deposition temperature we discussed earlier, some fouling of the tubes will eventually take place and reduce the efficiency of the heat exchanger.

The effect of this will be noticed in higher than normal engine operating temperature, and if this occurs, descaling of the tubes is necessary. Most heat exchangers have removable ends to accommodate this. Take these caps off both ends to reveal the tube bundle. Inspect the tubes by peering through them. If severe fouling is evident they will have to be cleaned out.

It is important to realise that the walls of the tubes are extremely thin and care has to be taken. I recommend a piece of plastic rod of smaller size than the bore of the tubes. This can be purchased from a good plastics retailer. Push the rod right through each tube until clean and repeat this for every tube.

Replace the end caps when completed and operate the engine. Check for leaks and that the water is flowing correctly through the circuit. Monitor the engine water temperature which now should no longer be a problem.

Somewhere on the body of the heat exchanger, generally in one of the end caps, will be an anode. This will be a small piece of zinc attached to a nut. Unscrew this nut to inspect the anode. This is a vital piece of metal and protects the heat exchanger from electrolysis. Replace this if it is more than 75 per cent wasted away. Spares are usually available from any marine engine service workshop.

After exiting from the heat exchanger, the water flows through the transmission oil cooler. This is identical in principle to the engine oil cooler, and is of similar construction. It is not fitted to all engines, as some boats will have mechanical gearboxes, but this is not common. Most engines have a hydraulic reduction gearbox via which they transmit power to the propeller, and the action of the hydraulic oil in achieving this produces heat. The gearbox has a small pump which circulates the oil through the oil cooler and back to the gearbox.

COOLING-OFF PERIOD
Some cooling of the exhaust elbow and a small portion of the exhaust pipe may be done by double jacketing before the water is introduced directly into the exhaust pipe. The effect of this water on the extremely high temperature exhaust gases is immediate as they are rapidly cooled. This reduces the velocity of the gas, and as a consequence the noise.

The mixed gas and water arrives at a water lock, a device designed to prevent water entering the engine when it is turned off. From there the water is returned to the ocean via the exhaust pipe, normally at the stern of the vessel. It is a good practice to inspect for the presence of this exhaust water every time the engine is started.

Servicing of this end of the water circuit is really a visual procedure to check the integrity of the system. If a bad leak occurred at the point of water injection and it was not immediately noticed, then the exhaust system could catch fire. This is because wet exhaust systems are usually constructed from rubber or fibreglass tube. No water to cool the exhaust means that combustible temperatures are soon reached.

Even if this hideous event does not occur, the water that normally would flow to the exhaust is now pouring into the bilge and the boat could soon start to wallow or even sink.

ENGINE COOLING CIRCUIT - FRESHWATER
The freshwater cooling system is usually arranged. The expansion tank is normally pressurised by the cap on the top (similar to the radiator cap on a car). For this reason the same precautions must be observed when removing the cap from a hot engine. Extreme care must be exercised to ensure the pressure is reduced before unscrewing the cap.

The water level in the header tank should normally be maintained to approximately half-an-inch below the top of the tank. The purpose of operating the system at a pressure is to allow the engine to run hotter than would be possible with an open system. This means that all the hoses used in the freshwater circuit are also under the same pressure, therefore regular inspection of their condition is necessary.

Because some of the hoses used are of the moulded type, it is essential that a spare for each one be carried. To check their condition, squeeze the hose with your hand. They should feel soft and pliable. Hoses that are not in good condition will feel hard, and often a cracking feeling will be felt when they are squeezed.

HOTTING THINGS UP...
Note that in the diagram below a hot-water system is incorporated. The water can be heated by two methods. One is the traditional electric element, and this is used when the generator is running or shore power is available. The other, as shown, uses the hot freshwater from the engine run via a small heat exchanger incorporated in the hot-water tank.

When this engine is operating, hot water is always available. The hoses that run the water to the hot-water system should be checked when the engine cooling circuit is inspected.

The freshwater pump is mounted in the casting of the engine block and driven by a vee belt. Provided that the belt is correctly tensioned and in good condition, there is little that can be done by way of servicing. The bearings are normally checked when the engine is serviced. The addition of a good anticorrosive additive to the freshwater is recommended. Hopefully this article has increased your knowledge of the engine cooling circuit, and how regular servicing will prevent problems from arising.

Have fun on the water...