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A course such as this should be based on fact, rather than gut feeling. To this end your lecturer went back to his old hire fleet (before it closed) and analysed the recorded breakdowns. He then applied his knowledge of the general conditions of privately owned boats to modify the analysis to something more representative.


Despite glaring examples to the contrary most hire fleets are well inspected and maintained. In fact they may well be inspected by experienced engineers twice a week. Despite this a number of breakdowns occur.





Big end gone Engine run dawn to dusk for 12 days without checking oil
5 Blocked sea inlets


Impeller failure. One nearly sunk boat because plastic silencer melted.
2 Raw water pump failures Late season wear. Match stuck in pump grooved impeller
1 Hose failure Plastic hose fell off
2 Heat exchangers blocked (1 engine & 1 gearbox) Customers read the instruments and got help before damage was done.
4 Filled tank with water Drain tank, replace injector pump, clean/replace filters.
1 Run out of fuel Another dawn to dusk over 14 days.
1 Pipe fracture Engine run away – internal copper fuel pipe fracture late in season.
1 Air leak Loose union.
1 Stopped charging Alternator brushes worn.
1 Starter caught fire Mariniser’s design fault. Solenoid in bilge.
2 Central heating failures 1 Glow plug burnt out

1 Low battery charge.

6 run out of water Prime water pump.
2 Shower pumps Blocked with hair etc.
2 Fridge failures 1 Re-light, 1 GBH to evaporator
1 Holding tank will not pump out Remove chicken carcass from tank outlet
1 Recirc. flap jammed open. Boat had hit lock displacing screen which then stopped flap from closing.
3 Will not engage gears 2 Push button back in.

1 Replace nipple in control.

1 Steering faults Inadequate steering push-pull cable broken (Teleflex)
2 Props fouled 1 trousers (blown off rear cabin roof and reversed over)

1 length of chicken wire.

1 Prop bent Tow to slip & replace prop (later sent for refurbishment)
1 Rudder bent Reversed against bank at high speed.
1 Coupling fallen off shaft Usual problem with shrunk on parallel couplings.


Boat sunk Transom corner swung against concrete holing it.


The above shows that the majority of faults resulted from some form of human inadequacy, rather than mechanical faults.

As you own or in some other way take a great deal more care of your boat most of the problems will not occur. On a like for like basis, your lecture estimates the fault tally would look like this.





No faults  


3 Sea inlets blocked  


1 Alternator brushes worn  


5 Unspecified Much more complex systems breed unreliability.


1 Nipple broken  
1 Coupling off  
1 Propeller fouled The trousers would never had been put there in the first place.


Unfortunately, it would appear to your lecturer that many private boats are poorly maintained, often by the "professionals" entrusted with their care and by owners being badly advised.

The following table shows the extra faults your lecturer would expect from his experience.






1 Cracked block Drained instead of using anti-freeze so some water left behind blocked drain-tap froze.
1(+) impeller broken up. Not changed regularly.
1 Raw water pump end plate bowed Not drained - frozen
1 Loose pipe on suction side. Over heating


Water in fuel Two previous engineers had not correctly diagnosed this condensation fault and had not cleaned the tank.


Including domestic A long list – see later in course

We now have macerators & vacuum pumps etc.


Seized cable End been left dangling in bilge over winter.


Knock from under boat Worn cutlass bearing and misaligned engine.



The conclusion can be drawn that the diesel engine is NOT the main cause of unreliability – people are. The more informed the owner the more they can foresee problems arising and the better they will be able to judge the quality of work being done on their boat.




If you want a long and efficient life from your engine only four needs require addressing:

A clean, cool, and adequate supply of air.
An adequate supply of clean oil.
An adequate supply of clean fuel, with no water in it.
cooling that is resistant to freezing and causing corrosion.

Items 3 and 4 are dealt with later, under their individual topic areas.



Ensuring a cool air supply has to be considered during the building of the boat and may well involve fitting ducting to the intake.

CLEANLINESS is ensured by fitting an air filter. This will also tend to silence intake roar. There are two types in common use:

Paper element - similar to those used on cars, but simpler in construction.

Wire gauze types - similar to those used on some rally cars.

Paper element filters are serviced by replacing the paper element and removing the dead flies etc. from the case.

Wire gauze filters are serviced by giving them a thorough wash in paraffin or white spirit and letting them drain/dry out. The gauze should then be flooded with engine oil and allowed to drain before refitting. The oil makes the gauze sticky so it traps the smaller dust particles.


The air enters the cylinder via the engine's inlet valve(s). Most marine diesels operate the valves by rockers that do require occasional adjusting. This is only slightly harder than average DIY job, but is more time consuming than difficult once the procedure has been mastered. Any other form of valve operation (usually on very modern automotive derived engines) is probably best left to experts.



A light tapping noise from the very top of the engine is likely to indicate too large valve clearances, this is not particularly harmful, but the engine will suffer an slight drop in efficiency, the louder the tap the greater the loss. A well adjusted valve clearance should be nearly silent in operation, unfortunately a valve which has NO CLEARANCE and is this being held slightly open, will also be silent.

In the later case the burning mixture will start to burn and destroy the valve, and in the very worst case might eventually be hit by a piston. Both outcomes will be expensive. I am very happy if I can hear a slight taping from the rockers, I know the valves are not being burnt. To be sure, it would be a good idea to check the clearances every couple of years, even if you do not feel confident enough to adjust them. The following description of procedure is intended as a memory jogger for skills learnt on the course, not a full set of instructions.

Get the clearance data from your engine manual or distributor, that is the gap required and whether it is adjusted hot or cold.

Identify the inlet and exhaust valves (only if they have different clearances) by seeing which valves are located closest to the relevant manifolds.

Ensure the engine is at correct temperature and remove rocker cover (top engine cover).

Turn engine over by hand until one valve is fully down.

Chalk mark front pulley or flywheel (anything directly connected to the crankshaft) to mark position. The crankshaft is usually located close to the bottom of the engine and central within the block.

Turn engine ONE COMPLETE REVOLUTION, until the mark is back in the starting position and adjust that valve.

NOTE - on a few proper marine engines the camshaft is used to drive the pulleys. This will be well up the engine and offset to one side. In this case proceed as described above, but only turn the engine HALF A REVOLUTION.

Repeat for all the other valves

Special note for those experienced in car valve adjustment - your procedures fine for "normal" straight 4,6, or 8 cylinder engines. The above procedure works for any engine.



This topic needs considering under three headings:

Gettingthe correct oil

Changingthe oil

Changingthe filter.


The cost of oil (at any one time) is small when compared with the cost of a worn out engine. Beware of purchasing oil that is re-processed/ re- cycled. This may meet its stated specification when put in the can, but how well it maintains that specification in use is another matter.

The only way the average person can tell how "good" and oil is, is to buy: brand names, oil marketed by a manufacturer, oil marketed by someone with a lot to loose if it is wrong, or by making sure the back of the can shows a lot of unintelligible spec. standards. These are stings of letters and numbers starting with something like a vehicle manufacturer's name or letters life GM MIL etc.

Most people contemplating changing their oil will have some idea about what the SAE viscosity spec. means :- 20w50 10w40 etc.

Just use the one recommended by your engine manufacturer. It might be permissible to move to on using larger numbers if the engine has done an extremely high number of hours, but this will mask, rather than cure problems.

All oil should also have an API specification. This consists of letters and should be on the back of the can. Again your engine manufacturer will specify what is required. There is a major problem here for marine use.

API (American Petroleum Institute) regularly update their specs and say that their latest spec. supersedes the previous ones. In automotive use this is probably true, in leisure marine use it is not.

I am indebted to Lister Petter for a whole sheath of academic studies, all of which, have allowed me to understand a little of the problems resulting from running marinised engines under light load and also with the pack of chemicals added to modern oils. It is NOT wholly a marketing ploy for engine manufacturers to sell their own brand of oil.

Depending upon engine design and use I have become convinced that there is an arguable case that if the engine manufacturer specified an API grade of CC/CD, then using a modern oil of (say) API CE/CF may well cause oil burning problems and cylinder bore glazing. If your engine specifies a CC/CD oil then use one, either use an engine manufacturer's (liable to be sold at a premium), one from a national, reputable motor factors (Partco.), or one form a specialist oil supplier (Morris).

Do not buy oil solely by sae rating (20w50) because diesel engines need more detergent in their oil (NOT fairy liquid) than a petrol engine. Also use the API spec stated by the engine manufacturer.


Oil change intervals.

The only realistic advice is to observe your engine manufacturer's recommendations. I would also advocate changing the engine oil and filter at the close of the cruising season. That way most of the acidic products of combustion are removed from the engine before it is stood for long periods.

If your engine does not have an oil draining pump fitted I would advise fitting one, Using a glorified garden sprayer to suck oil from the dipstick hole needs thin, hot oil, and often quiet a wait.


Changing the filter.

Filters mainly come in two types. Paper (felt) element and "spin on".

Paper element types require care to remove a large "washer" like object and spring from the bottom of the old filter element, these have to be re-used. The metal bowel should be washed out with something like white spirit, and dried. You should also change the sealing rings (an old hat pin is good for hooking the old ones out), but whilst you gain confidence, using the old one for a time or two is unlikely to cause problems.

Spin on types simply require the engine block to be cleaned, a thin film of oil spread over the sealing ring, and the tightened until it touches the block, plus a further 1/2 to 3/4 of a turn. This topic will be covered on the course, but please make sure you physically check for leaks, with the engine running, after you have fitted the filter and re-filled with oil.


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