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The engine cooling system is only there for one thing - to stop the engine melting. If we could afford space technology there is no reason why our engines would need to be cooled at all - they would just glow when running.

Because cost is important in the choice of engine, we are forced to construct them of iron, steel, & aluminium so they have to be cooled.

Every horsepower produced is worth 3/4kW and as a diesel engine is only about 30% efficient you will be getting rid of an awful lot of heat.

The first choice to make is between air cooling (Lister) or water cooling and depending upon that choice a number of other choices have to be made.



There are three main advantages:-

There is no coolant to freeze, leak, or rust the engine.

The engine can run hotter (air cannot boil) and is thus slightly more efficient.

Superficially, there is less maintenance, but on a covered boat cleaning fins and maintaining ducts tends to nullify this.


There are a number of disadvantages:-

You need to shift vast amounts of air to carry away the same heat as half a pint of water.

There is no water to deaden the sound of combustion so they are noisier. The fan is also noisy. The need for large "water safe" cooling vents can lead to excess noise travelling to the steering position.

A build up of dust, grass stems etc. on the cooling fins can catch fire. Unless you have a bad oil or diesel leak no harm is normally done, but the clouds of smoke scare the hell out of you - especially as it does it under periods of high power, like pushing a flood stream.

The large amounts of air virtually ensure ducting leaks into the engine room. This can raise the ambient air temperature to such a degree that mechanical failure takes place inside the engine (SL series).

Common practice of placing the cooling air intake and outlet vents in the side of the hull reduces the water worthiness of the boat. This may not be considered a problem on canals, but it reduces the time you have to react to incidents that cause the boat to sit lower in the water. Once the vents go under there is little one can do apart from summons the salvage team.

The "tin" air ducts and baffles used on muti-cylinder engines tend to vibrate and crack.

Even if the above has frightened you, those with air-cooled engines still need to know about the cooling system maintenance.


Fan Type

The cooling air fan will either be part of the flywheel or a separate, belt driven unit, bolted onto the engine.

Flywheel fans are simple and require virtually no maintenance, apart from cleaning when possible, but they do have the habit of throwing oily engine-tray water all over the cylinders causing smoke and smell. Initial boat design should try to place the engine high enough to minimise this. Essential maintenance is to ensure the water level is kept down so this cannot happen.

Belt driven fans are often driven by a matched pair of belts and some also have grease nipples for their bearings. Obviously any grease nipples require greasing at the required intervals. Twin drive belts MUST always be changed as a matched pair (although modern production methods lead some manufacturers to supply two separate belts of the same size). In the absence of manufacturer's recommendations a typical private boat should have its belts changed every two years if they show no signs of wear before that time.


ROUTINE (Say every month)

Check belts for wear/damage

Check ducts for leaks and cracks

Sparingly grease and nipples.



Clean cylinder fins

Inspect belts (Change every second year).

Change air filter (for combustion air)

Grease nipples.

Repair cracks in baffles and ducts.

Other Considerations

Try to arrange for cool air to be ducted from a point very close to the cooling intake vent to the engine air intake. Make sure whatever you use for ducting has an adequate diameter (say 4" minimum). Plastic drainpipe is easy to use.

Make sure the cooling intake and outlet ducts are large enough - check with the engine manufacturer and then add 20 to 30%. You will need the cooling when you are in difficult conditions like floods and tide races, so do not make the mistake of thinking just because the engine has not overheated, it never will.




The main advantages are:-

Water jacket quietens the engine noise.

Readily available source of base engines for conversion.

Ability to use waste heat for heating domestic water, hot locker etc.

A small quantity of water can carry away a lot of heat (Specific heat capacity).

Most spares are available from "non-marine" sources and thus tend to be less costly.

The disadvantages are:-

More complex system requires more maintenance.

Water corrodes metal and freezes.

Hoses are made from rubber that perishes and hardens.

The systems require holes through the hull (not tank cooled) which give more opportunity for hull leaks.


There are three main type of water cooling system:-

Direct Cooling - Water is drawn through the hull of the boat and pumped through the engine to cool it, and then overboard.

Indirect/Heat Exchanger Cooling - The engine has its own water system which is pumped through a heat exchanger (see gearbox oil cooler section). Another pump takes water through the hull, through the heat exchanger - cooling the engine water, and then overboard.

Keel/Skin Tank Cooled - The engine's own water is circulated through either pipes under the hull, or a tank built onto a metal hull, so the external water cools the engine water.


One must now consider the type of exhaust system to be used:-

Wet exhaust - The water drawn through the hull is injected into the exhaust system just as the exhaust leaves the engine. This system tends to quieten the exhaust note, gives cool exhaust pipes and silencers that allow rubber/plastic components, and gives a visible indication that the cooling pump is working by observing the water from the exhaust.

Dry Exhaust - No water is introduced to the exhaust so the opposite of the above applies.


WARNING - Pump failure on a wet exhaust system with plastic/rubber pipes informs you by smell - eventually - but it is not often appreciated that the rubber/plastic components can burn through so when the water supply is returned the boat fills up!

Pump failure can also lead to delamination of rubber exhaust hoses, this can cause the inner skin to rupture and form a "valve" that blocks the exhaust, especially at higher speed. So loss of power, a hissing sound, and lots of steam after a cooling pump failure may well indicate new exhaust hose is required.



The cooling pump circulating engine water on Indirect or Keel/Tank cooled engines will be of the centrifugal type. This is illustrated on the next page.

This pump uses centrifugal force to "throw" the water away from the impeller. The water leaving the impeller draws new water into the impeller centre, which in turn is spun and thrown outwards.



















This pump is NON-POSITIVE DISPLACEMENT, which means that if its outlet is blocked it will only build up a small amount  of pressure before it stops pumping.

It can run dry without damage.

Some old examples have an oil plug or grease nipple for the bearing. If so very occasionally put a FEW drops of oil into the plug hole or a very small amount of grease through the nipple. Otherwise these pumps are maintenance free, although its drive belt needs checking and adjusting.


Wearing out & leaks

When these pumps wear they start to leak from the tell tale. If your cooling system is pressurised (see later) they will first leak when the engine is hot and RUNNING. So it can be difficult to spot the leak.

Rubbing a clean finger around the tell tale will then often show dry rust powder.

The other regular checks are to:-

Check the drive belt for perishing, wear, condition, and tension (about 20mm or 1", depending on your measuring system, total movement in the centre of the longest run).

With the belt off or slack, try to lift the pulley - you are checking for wear in the bearing. If the pulley moves enough for you to see the indication is that the pump is nearing the end of its useful life and it is time for daily checks for leaking from the tell tale.

Put a stick or long screwdriver between your ear and the body of the pump just behind pulley with the engine running. Learn what it sounds like, then, when the bearing starts to wear, the change in sound will alert you well before the pump fails.

Electric pumps working on this principle are also found in some fuel systems, bilge pumps, and some domestic water pumps that require priming if you run out. This type of pump has a slightly different impeller that can clog with lime scale, rust and dirt and just stop pumping, although the motor is running.



This type of pump is found all over the boat as shower pumps, bilge pumps, domestic water pumps, and raw water cooling pumps. In this section we concentrate on the mechanically driven raw water pump, but the others work on the same principle, requiring the same maintenance, except they are driven by an electric motor..

The basic type of pump is illustrated on the next page.

The shaft drives the impeller by means of either:

  1. A pin screwed through the impeller and through the shaft.
  2. A key, which is a little block of metal half let into a slot in the shaft. The centre of the impeller has a matching slot for the other half of the metal block to fit into. Beware, sometimes the key is fitted in such a way that the whole shaft has to be pulled from the pump to remove the key before the impeller will slide off.
  3. Splines, a series of grooves machined into the drive shaft with matching groves in the centre of the impeller. The impeller will slide off these splines, but is often very stiff. Using screwdrivers to try and lever the impeller out can damage the pump case so it will not seal.


The diagram shows how this type of pump works but there are a number of points to watch:-

Matches and similar shaped pieces of debris can enter the pump and wedge between the impeller and wear plate. This wears the impeller and forms a barrier for other debris to build up on. This wrecks the impeller and stops the pump working.

These pumps MUST NOT RUN DRY, if they do the friction between the ends of the impeller and the housing creates enough heat to melt the impeller ends (on plastic bodied pumps of this type it also wears the body). Always try and pipe these pumps so a "reservoir" of water is retained in the pump. This is difficult on engine pumps. AIR LEAKS INTO SUCTION PIPES create similar problems.

The cover plate, cam plate, and pump body or wear plate develop deep grooves where the impeller is constantly rubbing. These will eventually cause problems with priming and low output. Replacing these parts is the only real cure, although a new impeller and turning the cover plate over (first rub the burrs on the edge of the lettering off with some fine abrasive paper held on a window) will often effect a TEMPORARY repair.

Over-tightening drive belts will cause excess body wear around the shaft. The drive belt should be quite slack, especially if the shaft runs directly in the pump body, rather than through a bearing.

A water thrower actually throwing water shows the pump is leaking on its pressure side, it will soon leak on its suction side and then fail, so replace it or send it for overhaul at once.

There is a vast variety of designs, so it is not possible to give exact details of check lists, however some kind of check list is given below.



Remember the screws that hold the cover and cam plate in place are made out of brass and are very small. So if you drop one you might well loose it. You are advised to carry spares.

Overhaul kits consisting of wearing parts, gaskets and seals are available.

Check drive belt (if any) condition and tension, replace as required or every two years.

Check impeller for wear, replace if required. Replace every two years.

With impeller out of pump try to lift and rock the shaft. Anything more than the merest movement indicates bearing or body wear.

If the pump has water or oil seals, check them for wear and condition, especially the lip which fits around the shaft.

ALWAYS FIT A NEW GASKET when replacing the cover.

Remove the impeller for winter layup. If you are going to refit it then mark it to ensure you replace it so it revolves the same way. Failure to do this is likely to result in one or more legs braking off. Store the impeller in water - it will not matter if the water freezes.

If you are unsure of your own pump's construction, you would be well advised to watch a professional overhaul it the first time, just so you know how it all comes apart.



These diagrams are for "typical" systems. There is much variation within the basic types.

The diagrams assume:-

Wet Exhaust where appropriate, if a dry exhaust is used the raw water exits through a skin fitting.

A gearbox oil cooler (heat exchanger) is in use. If one is not fitted, it is simply replaced with a pipe.

An engine oil cooler is NOT used. If one is, just place it after the gearbox oil cooler.

Normal design which appear to favour long runs of suction pipe (see later).



The raw water is drawn through the oil cooler by a Jabsco type pump and passed into the engine, having passed through the engine the water is discharged via the exhaust manifold jacket and into the exhaust system for discharge.

These systems are not normally pressurised which might cause problems on automotive based engines.

The maximum operating temperature will be limited to about 70C to prevent "furring up". (as in kettle).

It is not possible to use antifreeze in these engines so it is vital to FULLY drain them for winter storage. Blocked and corroded taps and plugs can make this difficult.

Antifreeze also prevents internal corrosion, so some other form of corrosion protection   is required on these engines. On genuine marine units this is likely to take   the form of a zinc anode screwed into the water jacket - so check with the manufacturer. Industrial and automotive conversions often replace light alloy components with cast iron ones - typically the thermostat housing. This is an area which is often missed by amateur marinisers.



Visual inspection of all hoses for perishing & chaffing.

Check sea inlet strainer for debris.

Ensure adequate water is coming from exhaust



Check all joints for security and leaks

Check Jabsco impeller. (Change every 2nd year).

Check anode & replace if required.

Clean gearbox oil cooler tubes (reverse flush).





This diagram shows the heat exchanger as part of the manifold, this is modern practice. An alternative is to place it across the front of the engine.

The inside of the heat exchanger is just like a large version of the gearbox oil cooler (see earlier section) with the engine water surrounding a bundle of tubes through which the raw water is pumped.


The calorifier connections are shown at the preferable points, but the outlet to the calorifier could be out of the top hose. This might require a restrictor to be fitted to the hose and would not give such a quick water warm up.

This system requires an engine water pump to pump the engine coolant round the system and a Jabsco type pump to pump the raw water.

The system can be pressurised and as the water stays in the engine there is little furring so high and therefore efficient operating temperatures can be used.

Antifreeze should (must) be used to combat internal corrosion so light alloy parts can be used. Internal anodes are not required.

The heat exchanger raw water tubes can block so keep an eye open for less water and more steam than normal coming from the exhaust on a wet systems - a bit like a delaminated exhaust hose, only it will not hiss.




Visual inspection of all hoses for perishing & chaffing.

Check sea inlet strainer for debris.

Ensure adequate water is coming from exhaust

Check "fan" belt for tension.



Check all joints for security and leaks.

Check Jabsco impeller. (Change every 2nd year).

Check antifreeze strength and top up as required to 50% mixture.

Clean gearbox oil cooler tubes (reverse flush).

Clean heat exchanger tubes (reverse flush).

Inspect fan belt for condition and replace/tension as required.



Change antifreeze.

Change Jabsco impeller.







This diagram shows the manifold acting as the cooling system header tank. Older engines might well use a remote tank (and on heat exchanger systems).

See comments above about calorifier connections.

A Jabsco pump is NOT required if a dry exhaust is to be used. In that case engine water would be circulated through the manifold and the keel cooler/tank size would be increased to deal with the extra heat.

With six year's hire fleet experience to back me up, I am convinced that this is the preferable system. The only keel cooler problem was when someone tied a tarpaulin to a keel cooler pipe and dislodged it without anyone's knowledge.

Note all the comments for the heat exchanger system and ignore the comments on blocking heat exchanger tubes.

The engine water is circulated, by the normal engine water pump, through the keel cooler pipes or skin tank. The Jabsco pump only provides water for the wet exhaust.




Visual inspection of all hoses for perishing & chaffing.

Check sea inlet strainer for debris.

Ensure adequate water is coming from exhaust

Check "fan" belt for tension.



Check all joints for security and leaks.

Check Jabsco impeller. (Change every 2nd year).

Check antifreeze strength and top up as required to 50% mixture.

Clean gearbox oil cooler tubes (reverse flush).

Inspect fan belt for condition and replace/tension as required.



Change antifreeze.

Change Jabsco impeller.


It is not possible to cover every individual system, but understanding these three systems should enable you to work out exactly what your system consists of.



You will see I have shown the two parts of the raw water circuit with different type of dash. This is deliberate. Remember the Jabsco pump cannot run dry.

Slight leaks in suction lines draw air into the pipe, this cannot be seen, but it reduces the efficiency of the cooling (not keel/tank cooling).

Slight leaks in pressure lines cause drips, these fill your bilge or engine tray and ensure you do either rectify them or sink :-).

As a matter of policy your lecturer ALWAYS altered the basic system to run in this manner:

Sea inlet ----- Jabsco -----Gearbox oil cooler ----- Engine

This minimises the suction line length and maximises the pressure line length, thus faults are seen and repaired.




Be aware of the problems with heat exchanger tubes when selecting the mesh in the strainer. (Not that you get much choice unless you are heavily into DIY production). Just think about the matches that block and damage the Jabsco pumps and the polystyrene beads that block the heat exchanger.



Poor design of through hull fittings.

Those with steel hulls with welded in pipe as the inlet can ignore this section.

A typical sea inlet & strainer is illustrated on the next page. Note how everything is screwed together and there is nothing to stop the skin fitting from turning in the hull.

If the cap is over tightened or sticks over the winter, efforts to undo it can result in the skin fitting turning in the hull, this ensures a significant leak that is difficult to stop, especially if the nut has been glassed into a GRP boat.












A "good" sea inlet will either be bolted into the hull with three bolts through clamping rings or the skin fitting will be modified .

The modification could be making the centre hexagon shaped to accept a key to hold it in position (not a lot of good if you have to take the cock off and water is pouring in), or a modification to the skin fitting flange to cause it to "bite" into the hull and prevent turning.

Your lecturer has modified standard fittings by brazing a brass pin onto the flange and drilling the hull to match.




The majority of engines should have a thermostat fitted. This is usually in a housing where, or very close to where, the hot cooling water leaves the engine. A new gasket (or ring seal on a very modern engine) is required if the housing is to be removed.

The purpose of the thermostat is to give a quick warm up of the engine and to keep the engine at running temperature. For most of its working life any engine has the potential to be vastly overcooled, it is the thermostat that prevents this by restricting cooling water flow when the water is below operating temperature.

The thermostat also allows the calorifier to warm up quickly.

The thermostat can fail:-

OPEN - in which case the engine will not reach its normal operating temperature. The calorifier will take a long time to warm up and will deliver cooler than normal water. The diesel injector pump will ensure the engine idles well enough, but combustion will be inefficient leading to pollution and poor fuel consumption.

CLOSED - in which case the engine will overheat and boil. Before suspecting the thermostat ensure the propeller is not fouled, that there is no coolant leak, that any heat exchangers are not blocked, and the coolant is at the correct level.

An OPEN thermostat of modern design is shown below.

Older engines will have a thermostat with a brass bellows where the spring is in this picture, and the valve disc opens by moving UP, rather than down.


There are other thermostats of similar design, but with different features known as bypass thermostats.

Always replace thermostats "like for like", although the normal bellows type can be replaced by this type which is known as a waxstat.



Checking the thermostat

A failed open thermostat is easy to spot - just find the valve disc and see if there is a gap around it, if so, it has failed.

A failed closed or a faulty thermostat is more difficult to spot. It must be removed from the engine, put in a saucepan together with a (sugar) thermometer and water and heated. Watch the valve disc and thermometer. Note the temperature at which the disc starts to open. This should be within a few degrees of the number stamped on the thermostat that is, hopefully, the same as that recommended by the engine manufacturer.

Always replace a faulty thermostat with one that opens at the temperature recommended by the engine manufacturer or mariniser. This is VITAL for direct cooled engines which will have colder thermostats that their indirect cooled equivalents (remember the 60-70C fur up point).

Fitting a colder thermostat than that recommended does NOT CURE A PROBLEM, it only masks it. Remember you need your power and therefore maximum cooling in bad conditions. Not a time you want to find the engine boiling.



PRESSURISED SYSTEMS - using a "radiator" cap

Certain parts of an engine get far hotter than other parts, like exhaust valve seats. These parts heat the water around them to a higher temperature than the apparent operating temperature. Again, this effect is at a maximum when under full power.

The more power per litre the engine produces the greater this effect, so that on road vehicles the water would actually boil on these HOT SPOTS although the temperature gauge was showing normal. If this happens, the steam forces coolant out of the system, and eventually the engine seizes up.

This problem is overcome, among other ways, by pressurising the cooling water so it boils at a higher temperature. A typical vehicle will pressurise its cooling system to between 7 and 21 psi.

With boat engines being drawn from a variety of sources no specific advice can be given on which engines need pressurised cooling systems and which do not. All that can be done is to give general advice.



Marine based units

These almost certainly will not need pressurisation, although checking with the manufacturer is advisable.



Industrial based units

Again, these will probably not require pressurisation unless they are developing high powers per litre or are modern, but check with the manufacturer.



Automotive based units

The more modern, the more likely they will DEMAND pressurisation. Vehicles started being pressurised in about 1954, however, as an example, the BMC1.5 unit will operate without pressurisation for inland installations.

A 6 litre engine which nominally requires pressurisation installed in a narrow boat is unlikely to suffer if it is not pressurised, because it will never be asked to develop more than about 20% of its maximum power, so maximum heat will never be generated.




The majority of boats with pressurised cooling systems will be fitted with a metal pressure cap that most adults would refer to as a radiator cap. Some very modern units might have a plastic expansion bottle fitted with a plastic pressure cap.

The plastic caps cannot have their valves inspected so if one is suspect it must be changed - just try and find out what car/van it is off because a motor factor (CAFCO, PARTCO, Express Factors. etc) will be cheaper than most marine suppliers - you must know its pressure setting though.

The metal radiator caps with a spring loaded rubber seal beneath come in two lengths, the long one will not fit a short filler neck, but a short one will appear to fit a long neck - except it will not seal and pressurise. Make sure you have the right length cap for your filler neck.


The cap valves

The cap has TWO valves:

The large outer one, loaded by the large spring, is the pressure valve. If this valve does not seal the system will not pressurise. It is normally easy to inspect the rubber for this valve.

There is also a smaller, inner one. This is the vacuum valve that lets air back into the system as the coolant contracts when cooling. It is not easy to inspect this valve. Failure normally shows as an inability to seal, so the system will not pressurise. Occasionally they jam shut. In marine use this results in the larger diameter hoses flattening as the engine cools.

Pressure cap maintenance

Inspect the seals every time you top the coolant up and replace every 5 years or so.




Every indirect or keel/tank cooled system will have a coolant header tank. This may well be part of the exhaust manifold, heat exchanger, or a separate tank.

Both pressurised and unpressurised systems need somewhere for the coolant to expand into when the coolant heats up, thus the header tank.

The coolant will expand by 0.00045 litres per litre of coolant, per degree of temperature increase. A typical car system holds about 6 litres and its temperature rises through about 80 degrees so it expands by just under half a pint. This means you must leave a "half pint gap" above the water in the header tank when the engine is cold.

A tank cooled marine unit, with calorifier, probably holds about 20 litres or more and its temperature rise is about 70 degrees. This coolant expands by over a pint.

Now a header tank built into a heat exchanger or exhaust manifold will look empty with over a pint of coolant "missing" so most owners top the system up. The engine warms up, the water expands and "blows" out into the bilge or drip tray, the engine cools down, and the cycle starts again. In these circumstances it is easy to believe you are losing water and have a fault, when in fact there is no fault.

This is why steel boats often have large, fabricated steel, expansion tanks that give room for the coolant to expand.

This problem can be overcome by fitting a large expansion bottle from a truck.




At first sight plastic's better resistance to oil, when compared to rubber, makes it an attractive choice for coolant hoses. NEVER USE PLASTIC ON ANYTHING BUT COLD RAW WATER PIPES.

Plastic has a bad habit of softening with heat, this allows the plastic to extrude from under the hose clip, which then becomes loose. You tighten the hose clip to stop the leak and the cycle repeats. Eventually there is no plastic under the clip so the hose just falls off.

Use hose that has been designed for use on engines, making any long runs from well-supported copper pipe.

If you can afford a "Rolls Royce" job, hydraulic hose of suitable diameter is very long lasting and resists oil.

Marine hose clips tend to spend time in damp atmospheres and are often sprayed or covered in dirty water. Unless you have deliberately built up a comprehensive parts store on your boat you are likely to have difficulty getting a new clip at the moment you require it. Zinc plated clips rust, plastic clips soften, and wire clips cut into hoses. You are advised to gradually change all hose clips to stainless steel worm drive clips because they do not rust and do not cut into hoses.



Air can get trapped in heat exchanger or keel/tank cooling systems, especially if a calorifier is involved. Placing any hose of the calorifier connection above the header tank virtually guarantees an air lock. So does looping a hose in an inverted U.

Trapping air can be minimised by filling the system slowly and then running the engine on a fast idle with the expansion cap off. When the water starts to steam/move fast rev the engine for a minute or two and replace the cap.

Air in high hose runs or inverted U sections can be removed in the last straw by obtaining cooling pipes with bleed screw attached from scarp French cars (mainly) and fit these in the high points. They will usually bleed by revving a hot engine with the screw loose, tighten the screw when pure coolant is dribbling out. A hot pressurised system may bleed simply by loosening the screw. If the worst comes to the worst you might have to borrow a cooling system pressure tester and use that to force the air from the high point.

Air trapped in the calorifier is removed by revving the engine and removing the calorifier return hose from the engine. Block the engine connection with a thumb and after a while water should gush from the hose. At this time, with the engine still revving, replace the hose. Top up the cooling system.

Good installation practices, where all hoses and pipes slope downwards away from the engine/header tank with no inverted U sections and keeping the calorifier low avoid most air lock problems.



As stated before, the most important use of antifreeze is to prevent corrosion, stopping freezing is of secondary importance.

Use a 50% mixture and replace it ever two years (it will still stop freezing, but it probably will no longer stop corrosion).

Make sure the antifreeze contains NO METHANOL (Methyl Alcohol) - this evaporates quickly and then can no longer prevent freezing.

When changing antifreeze it is advisable to reverse flush the system. That is to remove the thermostat and replace the housing. Connect a water hose to the thermostat housing and run water through the system until it runs clear.

This process can be simplified by disconnecting the bottom hose and running water into the filler neck until it runs clear from the bottom hose. Tank cooled systems will now require the lowest tank connection to be connected to a hose and water run from the upper connection until it runs clear.

All connections are replaced, checked for leaks (hot & cold) and the system refilled with 50% antifreeze. Warning - you might now have to bleed the system.



If you follow the advice to change Jabsco impellers, keep heat exchanger cores free from blockage, and keep the cooling system topped up you should have little to fear from overheating.

Overheating can be caused by internal cracks in the engine or a faulty gasket allowing red-hot gases to leak into the coolant. Black oil in the header tank is one indication that this is the fault, but oil may well not be present although this fault exists.

Rectifying this fault requires professional help, so to aid the preliminary diagnosis you could find it cheaper to get the boat close to vehicle access and get your local "Home Tune" man to put his exhaust gas analyser pipe just into the filler neck (he will go ballistic if you put it actually into the coolant!) with the engine running under load. If his gauge shows Hydrocarbons and Carbon monoxide present then you need professional help. If no HC & CO are present then start looking for something else.


If the engine only overheats under prolonged, hard use and no obvious fault is present, you might find that your heat exchanger/keel cooler/skin tank is not large enough to cope with full power operation.


Calculation of Swim tank/Keel Cooler Area

Since the late 1960s I have been aware that many narrow boats have problems with overheating. In his opinion this is because the skin tanks tend to be too small for the engine power.

The "trade" tends to use 1/3 sq. ft. per hp but that figure takes no account of the fact that most canal boats are indirect injection engines with a higher cooling requirement. Also dry exhaust boats add their manifold cooling and oil cooler to the engine cooling load, rather than using raw water for the job.

The formula below was produced by one of our Physicist/Engineers. It is given as a guide for use in the absence of more authoritative information on skin tank size.

It assumes steel construction and cooling on ONLY ONE side of the tank, so in reality the tank can be a bit smaller. (Copper, brass or bronze keel cooling pipes would give a lower area required).

Please note there are about 746 Watts in a Horse Power.



This gives a tank area of about 0.5sq ft per "cooling hp". It is often stated that about 0.3 sq ft per hp is sufficient. I regret that experience causes me to doubt the lower figure.

The "cooling hp of an engine will be about 65 to 70% of its rated hp, so something like a 33hp BMC is likely to require a skin tank area of 11 sq. ft.

The tank size on inland boats normally only becomes an issue when "punching" a stream or during prolonged high-speed use.

In an emergency "dumping" heat to domestic water on a calorifier equipped boat by turning a hot tap on may mitigate the too small skin tank, as will turning on the central heating pump with a twin coil calorifier.

Over cooling will not be an issue on any decent engine because the thermostat will keep the engine at its optimum operating temperature.

Better overlarge, than too small and boiling in a strong stream!

Skin tanks should have some method of allowing air to be vented from the top of the tank, either by pipework running "uphill" away from the tank or by the fitting of a bleed screw. They should also contain internal baffles and take hot water in at the top and pass the cooled water out of the bottom.

There is NO MERIT in making a tank thicker it just gives more scope for the hot water to avoid the "cool" side of the tank. It should also be considered bad practice to install horizontal skin tanks unless there is a VERY GOOD reason saving a bit of cash by using the engine beds as the tank sides is not a good reason!



When preparing the cooling system for winter your main aim should be to stop it freezing.

At this time keel/tank cooled systems with dry exhausts appear attractive, and almost worth the noise they tend to make. These systems should already have antifreeze in them so apart from checking the strength, nothing needs to be done.

The same goes for the actual engine on wet exhaust, heat exchanger or keel/tank cooled boats. It does not go for direct cooled engines or the raw water circuits on any other type.

There are two ways to prepare these engines for winter, one cheap but potentially difficult with the possibility of a partial failure which could be expensive, the other easy, involving some cost and a degree of pollution. Each is described below.



Conventional winterisation.

Locate all drain plugs and taps on the engine, oil coolers, heat exchanger, and exhaust manifold.

Remove each plug/ turn on each tap and using a piece of wire ensure the water is free to drain away and the hole does not become blocked. On old, poorly maintained engines this can be difficult and might involve removing taps and using a hand drill to drill through years of sediment until the water drains. Do not drill through the inner wall.

Put the plugs in a labelled tin or box (margarine tub?).

Turn off seacock and remove Jabsco impeller. Place screws and cover in the box above. Mark the impeller to show direction of rotation and store in water.

Inspect all hose runs and disconnect/lift any which form a water trap to drain them.

You might want to run the engine at idle for a couple of minutes to dry the internal surfaces.

Place a notice over engine start control to warn that the engine is inoperative.

Failure to remove water from any metal parts might result in a split if the water freezes.

Some manufacturers also recommend methods of using an oil to coat the internal engine surfaces.


Unconventional method

Obtain a gallon of antifreeze and make up about half a gallon to 50% strength. (the other half gallon is a reserve and may well be needed on a direct cooled engine).

Put mixture in a large jug or easily controlled watering can.

Turn off seacock and remove the cap. Place a funnel in the filter if required.

Start engine and run on fast idle, pour the antifreeze mixture into seacock so it is pumped around the system.

When the exhaust water turns blue/green/red/yellow (depending on colour of antifreeze) continue for a short while and then stop engine. You have now filled the system with antifreeze mixture - so it should be safe. (In England, a 25% antifreeze mixture will protect a stationary engine from freezing, so you have already allowed for some water to dilute the mixture).

Leave seacock turned off.

Remove Jabsco impeller and store as detailed above.

Place a notice over the engine start control to warn that the engine is inoperative.

The antifreeze will help prevent internal corrosion so there should be little need to add corrosion inhibiting oil.


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