Marine Engine Checks and Maintenance - Diesel Troubleshooting

Prerequisites

Before this lesson you should:

• Have completed RYA Competent Crew or equivalent experience, and be familiar with the layout of a cruising yacht below decks.
• Know where the engine, seacocks, bilge pump and fuel shut-off are on a typical yacht, and the basics of starting and stopping the engine as crew.
• Understand basic battery and 12 volt electrical safety.

The Day Skipper practical syllabus requires you to know the engine checks to carry out before starting, and to be able to deal with common problems. The RYA Diesel Engine Course (one day) is an excellent companion to this lesson but is not a prerequisite.

Learning Objectives

By the end of this lesson you will be able to meet the engine and maintenance items of the RYA Day Skipper syllabus (engine checks before starting, fuel, cooling and electrical systems, basic fault finding, routine maintenance, spares and tools):

• Calculate fuel consumption and range, and apply the one-third rule (RYA syllabus: passage planning and victualling).
• Describe the propeller, shaft and gearbox arrangement and recognise a stern gear fault (RYA syllabus: handling under power).
• Explain how fuel contamination ("diesel bug") arises and how to prevent and cure it.

• Explain how a marine diesel works and what it needs to run: fuel, air and compression.
• Describe the fuel, cooling, exhaust and electrical systems on a typical yacht engine.
• Carry out the daily pre-start checks (the RYA WOBBLE checks).
• Start, warm up and stop the engine correctly, and check it is running properly.
• Operate seacocks and manage a battery bank and changeover switch safely.
• Diagnose and deal with the common faults: failure to start, overheating, loss of power, fouled propeller, charging failure.
• Change a raw water impeller, change a fuel filter and bleed the fuel system.
• Draw up a sensible spares and tools list.

Why the Skipper Must Know the Engine

On most cruising yachts the engine is used for every harbour entry and exit, for punching a foul tide, for charging batteries and as the main tool for man overboard recovery. When it fails, it fails at the worst moments: in a narrow channel, off a lee shore, or when the wind has died and the tide is carrying you towards a headland. Most engine failures are caused by a few simple things: dirty fuel, air in the fuel, a failed raw water impeller, a slipping belt or a flat battery. All of these can be prevented or fixed by the skipper with a few minutes of routine checks and basic tools. The RYA practical course notes make the point bluntly: engine problems are the most common cause of lifeboat call-outs, and most of them are preventable by regular checks. A daily engine check is the best way to spot a problem before it stops you.

How a Marine Diesel Works

A diesel engine has no spark plugs. Air is drawn into the cylinder and compressed by the piston to about one twentieth of its volume. Compression heats it to well over 500 degrees C. Fuel is then sprayed in by the injector, and it ignites spontaneously in the hot air.

So a diesel needs just three things:

1. Clean fuel, free of air, delivered at the right moment.
2. Air, through an unblocked intake and with an unblocked exhaust.
3. Compression, from a mechanically sound engine turning fast enough.

When a cold engine struggles to reach ignition temperature, glow plugs (heaters in each cylinder) help. Almost every problem you can fix at sea is a fuel or air problem. Compression problems need an engineer.

The fuel system

Fuel travels: tank, shut-off valve, primary filter and water separator (usually a bowl you can see), lift pump (with a hand priming lever), secondary fine filter on the engine, high-pressure injection pump, injectors. Diesel is intolerant of water and dirt, and air in the system stops it completely, because the injection pump cannot pump air bubbles at high pressure.

Fuel quality matters. Diesel absorbs moisture from condensation inside a part-empty tank, and water at the bottom of the tank lets bacteria and fungi grow at the interface, forming a black slime known as diesel bug. The slime blocks filters and starves the engine, typically in rough weather when the sludge is stirred up from the tank bottom. Prevention: keep the tank full when laid up (less air, less condensation), buy fuel from busy suppliers, drain the water separator regularly, use a biocide treatment if the boat is used infrequently, and carry spare filters. Cure: drain and clean the tank, or have the fuel polished (circulated through a fine filter) by a marine engineer, and replace all filters.

Fuel consumption and the one-third rule

A small auxiliary diesel burns roughly 0.25 to 0.3 litres per horsepower-hour at full load, much less at cruising revs. As a guide, a 30 hp engine at 2,500 rpm in a 35 ft yacht burns about 2 to 3 litres per hour, and at low revs or in light wind about 1 litre per hour. Know your own figures, from the engine log, by noting engine hours against litres added.

Worked example: you plan to motor 24 miles into a light headwind at 5 knots. Time is 24 / 5 = 4.8 hours. At 2.5 litres an hour this uses 12 litres. Apply the one-third rule: use no more than a third of the usable fuel for the outward leg or the planned run, keep a third for the return or alternative plans, and a third in reserve. So you would want at least 36 litres of usable fuel on board for this leg alone, plus an allowance for the engine to charge batteries and to manoeuvre. Remember to dip the tank (gauges lie), and that the last few litres cannot be used because of the sump of the tank.

The cooling system

Most yacht engines are indirectly cooled, with two circuits:

• Fresh water (coolant) circuit: a mixture of fresh water and antifreeze circulates round the engine block, driven by a belt-driven circulating pump, and is controlled by a thermostat. You check its level in the header tank.
• Raw water circuit: sea water is drawn in through a seacock and strainer by the raw water pump, which contains a flexible rubber impeller. It passes through the heat exchanger, cooling the fresh water, and is then injected into the exhaust to cool it and is expelled over the stern with the exhaust gas.

Some older or smaller engines are directly cooled: sea water goes straight round the block. They have no header tank.

The exhaust system

The diagram shows how hot exhaust gas and raw cooling water meet in the exhaust elbow, pass through a water-lift muffler and leave at the stern.

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This is why the first check after starting is to look over the stern for water coming out with the exhaust. No water means the raw water pump is not pumping: stop the engine at once before the exhaust hose melts or the engine overheats. Exhaust colour also tells you a lot:

Over years the exhaust elbow (where water meets gas) can clog with carbon and salt, restricting flow and reducing power. This is a workshop job, but it is worth knowing as a cause of gradual power loss.

Ventilation

A diesel needs a lot of air, and the engine space must be ventilated to remove heat and fumes. The diagram shows the principle.

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Never stow bags, sails or fenders over engine vents. A starved engine loses power and produces black smoke, and a hot, unventilated compartment shortens the life of hoses, belts and wiring. A smell of diesel in the compartment means a leak: find it before you go to sea.

Transmission, Shaft and Propeller

Power passes from the engine through the gearbox (which gives ahead, neutral and astern), along the propeller shaft and out through the hull to the propeller. There are three common arrangements.

• Shaft drive with stern gland: the shaft passes through a bearing or stuffing box (the stern gland) near the keel. Check the gland as described in Other Routine Checks below.
• Saildrive: a leg through the hull with a rubber diaphragm seal. The diaphragm must be replaced on schedule (often every seven years or as specified) because failure floods the boat. Check the oil in the leg for a milky colour.
• Outboard or pod drives: small yachts and tenders, with separate checks (fuel mix, tilt and lock).

The propeller may be fixed, folding or feathering. A folding or feathering propeller reduces drag under sail but may not open or give full astern power if it is fouled with growth. Propellers are sensitive to marine growth; a clean propeller and hull can improve speed under power by a knot or more.

Gearbox rules: change gear only at idle revs, pause in neutral when changing from ahead to astern, and follow the handbook on what to do with the gearbox when sailing: some gearboxes must be left in gear to stop the shaft turning (and be lubricated), others must be left in neutral, and a freewheeling propeller can damage some gearboxes. Propeller walk is the sideways force of the propeller at low speed, especially astern. For most right-handed fixed propellers the stern kicks to port when going astern, and this is the key to controlling a yacht in marinas. Know the direction for your own boat and use it to manoeuvre (for example, a short burst astern swings the stern to port when backing out of a berth).

Seacocks

A seacock is a valve where a pipe passes through the hull below or near the waterline. The engine cooling intake is one; others serve the toilet, sinks and cockpit drains.

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• Handle in line with the pipe: open. Handle across the pipe: closed.
• The engine cooling seacock must be open before you start. A closed intake is a common cause of overheating and a ruined impeller within a minute or two.
• If a hose below the waterline splits, closing the seacock is the quickest way to stop the flooding. Every crew member should know where the seacocks are.
• Exercise each seacock regularly so it does not seize. Keep a softwood bung tied to each one, sized to fit the hole, in case the seacock itself fails.
• Hose clips below the waterline should be stainless steel, and ideally doubled.

The Daily Engine Checks: WOBBLE

The RYA teaches the mnemonic WOBBLE for the checks before starting each day. Some books use a similar list, WOFBLT (Water, Oil, Fuel, Belts, Leaks, Temperature); the content is the same.

Also check fuel: the tank level (dip it, gauges lie) and the water separator bowl for water or sediment, draining it off if necessary.

The RYA course notes add a few items to the daily and weekly routine:

• Use the daily check to look for oil, fuel or water leaks, loose belts, signs of wear and the lubricating oil level, topping up if required. Oil that has turned a milky colour means cooling water and oil are mixing: call an engineer.
• Keep the fuel tank full to avoid condensation and water in the fuel; a low level also disturbs the sediment in the tank, which can block the filters.
• Check the gearbox dipstick weekly or every 30 hours, and the fresh water level in the expansion tank or heat exchanger header.
• Clean the seawater intake strainer weekly and check that the engine seacock is open.
• Check the alternator and other drive belts for tension and wear, referring to the handbook for the correct tension.
• Carry a toolkit plus spare fuel filters, an impeller and belts.

The checks take about five minutes. Note anything unusual in the engine log. Trends matter: a belt that needs tightening every day, or oil going down faster than last month, is a warning.

Starting, Running and Stopping

Starting procedure

1. Complete the WOBBLE checks.
2. Make sure the gear lever is in neutral, and there are no lines over the side that could foul the propeller.
3. Battery switch on. Turn the key to the "on" position: the oil pressure and charging warning lights should come on and the alarm sound.
4. Use the glow plugs (heat position) for the time in the manual, typically 10 to 20 seconds when cold.
5. Open the throttle slightly (or as the manual says) and crank. Release the key as soon as the engine fires. Never crank for more than about 10 seconds; wait at least 30 seconds between attempts to let the starter motor cool and the battery recover.
6. Once running, the oil pressure light and alarm should go out within a few seconds. If they do not, stop the engine.
7. Check for cooling water coming out at the stern within about 30 seconds, and look again after another 30 seconds to be sure the flow is steady (the RYA sequence: check for cooling water from the exhaust, recheck after 30 seconds).
8. Check that the fuel gauge and other instruments are registering, and that the voltmeter shows the battery charging (about 13.8 to 14.4 V).
9. Let the engine warm up at fast idle for a few minutes before putting it under load.

While running

Glance at the instruments regularly: temperature, oil pressure (or warning light), charging light, revs. Listen for changes in sound and smell for hot rubber, burning or diesel. Check the stern for cooling water every so often. On a long motoring passage, look in the engine compartment every hour or two for leaks.

Stopping

Return to idle in neutral for a minute or two to let temperatures settle, then stop the engine with the stop control (on many engines a pull cable or button that cuts off fuel) and only then turn the key off. On some engines turning the key off before the engine has stopped can damage the alternator or leave the stop solenoid energised. Know your engine.

Batteries and Electrics

Most yachts have at least two battery banks: an engine start battery and a domestic (house) battery for lights, instruments, VHF and fridge. The alternator charges them while the engine runs. The diagram shows a typical arrangement.

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Key rules:

• Keep the start battery for starting only, so it is always charged. A flat domestic bank is an inconvenience; a flat start battery may mean no engine.
• Never turn the battery switch to OFF while the engine is running. Disconnecting the alternator under load destroys its diodes in an instant. Moving between 1, BOTH and 2 is fine on a make-before-break switch, but never via OFF.
• If the start battery is flat, select BOTH (or use an emergency parallel switch) to start from the domestic bank, then switch back once running.
• A healthy 12 V battery at rest reads about 12.6 to 12.8 V when full, about 12.2 V at half charge. Below about 12.0 V it is effectively flat. With the engine running, expect about 13.8 to 14.4 V.
• Charging warning light on while running means the alternator is not charging: check the belt first.

Batteries: capacity and charging arithmetic

Battery capacity is given in amp-hours (Ah). Never discharge a lead-acid battery below about 50 percent (12.2 V at rest); doing so repeatedly shortens its life. A 100 Ah bank therefore gives about 50 Ah usable.

Worked example: a yacht has 200 Ah of domestic batteries, so about 100 Ah usable. Overnight the fridge (about 3 A average), the instruments and VHF (about 1 A), the anchor light (about 0.5 A for an LED) and cabin lights (about 1.5 A) draw a total of about 6 A. Over 12 hours that is 72 Ah, so the bank is at about 64 percent next morning. A 60 A alternator will not deliver 60 A for long, as the output tapers as the battery fills; expect to run the engine for one to two hours to put back most of the 72 Ah, but the last 20 percent takes much longer, so do not rely on short engine runs to top up a bank. Isolate any loads that you do not need. A solar panel or wind generator reduces the engine hours needed.

Other electrical notes: a flat battery can be started from another bank by selecting BOTH; jump leads should be connected positive to positive then negative to the engine block of the dead boat, away from the battery. Keep terminals clean and greased, and check that all connections are tight and the starter cable is not hot after a start. Shut off the engine room extractors if there is a fire. Wiring chafe is a common cause of electrical fire: check cable runs, fuses and the ratings of cables to loads.

Fault Finding

Engine will not turn over

The starter does not crank, or only clicks.

1. Battery switch on and correct bank selected?
2. Gear lever in neutral? Many engines have a neutral safety switch.
3. Battery voltage: if low, try BOTH or the other bank.
4. Connections: check battery terminals and the heavy starter cable for looseness or corrosion.
5. A single click with a good battery often means a sticking starter solenoid or starter motor. A sharp tap with a spanner sometimes frees it.

Engine turns over but will not start

This is almost always fuel or air:

1. Fuel in the tank? Dip it.
2. Fuel shut-off valve open?
3. Stop control pushed fully back in?
4. Glow plugs used long enough?
5. Water in the separator bowl? Drain it.
6. Air in the fuel (after running low, a filter change or a loose joint)? Bleed the system.
7. Blocked air filter or something stuffed in the intake?

Do not keep cranking. You will flatten the battery and, on many engines, flood the exhaust with sea water: the raw water pump keeps filling the water-lift muffler, but without exhaust pressure it cannot push it out, and water can back up into the engine. If several attempts fail, close the raw water seacock while you investigate, and open it again the moment the engine fires.

Overheating

A temperature gauge rising or a high-temperature alarm needs action now. Work through the diagnostic tree below.

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1. Reduce revs immediately and, if it is safe to do so, stop the engine. Get sail up or anchor if you need to hold position.
2. Is water coming from the exhaust? If not, the raw water side has failed.
3. Raw water side: seacock open? Strainer blocked with weed or a plastic bag? Impeller failed (most common)? Intake blocked outside the hull?
4. If raw water is flowing: check coolant level in the header tank (only when cool; opening a hot pressure cap can spray scalding coolant). Then belt tension (the belt often drives the fresh water pump). Then suspect the thermostat.
5. Fix the cause before restarting, and check the strainer and impeller housing for pieces of broken impeller vane, which can lodge in the heat exchanger.

Loss of power or vibration

• Sudden heavy vibration, or the engine labouring and revs dropping, often means a rope, net or plastic round the propeller. Put the engine in neutral and stop it immediately. Running in gear can rip out the shaft coupling, the P-bracket or the stern gland. Try going briefly astern at idle (it sometimes unwinds), otherwise it may need a diver or drying out.
• Gradual loss of power with black smoke: overload or air starvation (fouled hull, blocked air filter, carbonised exhaust elbow).
• Engine hunting, coughing or dying: fuel starvation (blocked filter, air leak, dirty fuel stirred up by rough seas). Change the filters and bleed.

Charging failure

Charging light on or volts below about 13 V with the engine running: check the alternator belt (slipping, broken), then the wiring at the alternator, then fuses. Conserve battery power by switching off non-essential loads.

Basic Maintenance a Day Skipper Should Be Able to Do

Changing the raw water impeller

The diagram shows a good and a worn impeller and the replacement steps.

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1. Stop the engine and close the raw water seacock.
2. Remove the screws holding the pump cover plate and take off the cover and its gasket or O-ring.
3. Pull out the old impeller with an impeller puller or two screwdrivers padded with cloth, levering gently and evenly.
4. Count the vanes. If any are missing, find the pieces: look in the pump housing, hoses and the inlet of the heat exchanger.
5. Clean the housing and check for scoring.
6. Lightly smear the new impeller with washing-up liquid or glycerine (not petroleum grease, which damages some rubbers).
7. Push it in while turning it in the direction of rotation, so the vanes bend backwards, trailing the direction of rotation.
8. Fit a new gasket, replace the cover and screws evenly.
9. Open the seacock, start the engine and check for water at the exhaust and for leaks at the pump.

Replace impellers annually or about every 200 to 300 hours, and always carry at least one spare. Impellers fail most often after the engine has been run with the seacock shut, or after long periods unused.

Changing a fuel filter and bleeding the system

1. Turn off the fuel at the tank valve.
2. Put a container and absorbent pads under the filter.
3. Unscrew the filter or remove the element. Fill the new filter with clean fuel if the design allows, lightly oil the new seal, and fit hand-tight plus the amount stated (often a quarter to half a turn).
4. Open the fuel valve.
5. Bleed, working from the tank towards the engine. Open the bleed screw on the primary filter, operate the lift pump priming lever until fuel comes out free of bubbles, close the screw. Repeat at the secondary filter, then at the injection pump bleed screw if the manual says so. If the lift pump lever feels dead, the cam may be at the top of its stroke: turn the engine over by hand a little.
6. Wipe up all spilled diesel and dispose of rags properly.
7. Start the engine. If it fires then dies, there is still air: bleed again. Some engines need the injector pipes cracked open while cranking; follow the manual.

Other routine checks

• Stern gland: a traditional stuffing box should drip a few drops a minute when the shaft is turning and little or none at rest. Dripless seals should be dry; check for weeping.
• Anodes: the zinc anodes on the shaft or saildrive corrode instead of the propeller and leg. Replace when about half wasted.
• Oil and filter: change at the hours or interval in the manual, typically every 100 to 200 hours or annually.
• Engine log: record engine hours each day, fuel added, and any maintenance or faults.

Quick fault finding table

The skipper's maintenance schedule

Winterising matters because water left in the raw water circuit can freeze and crack the block or heat exchanger. Flush the raw water side with a mix of antifreeze before laying up if the manufacturer advises, and always follow the engine handbook. If the engine is to be left unused for a long period, run it once a month under load for at least twenty minutes to evaporate condensation.

Spares and Tools

A day skipper should check before each passage that essential spares are on board and their location is known.

Know your limits. Injector servicing, gearbox faults, internal engine work and major electrical faults are jobs for a marine engineer.

Worked Example

Scenario

You are motoring out of a harbour into a 2 knot foul stream when the high-temperature alarm sounds. You are 4 cables from a rocky shore with a light onshore breeze.

1. Safety first: reduce revs. Is the boat safe if you stop the engine? Not yet: you are close to rocks. You unroll the genoa and bear away into open water under sail. Once well clear, you stop the engine.
2. Check the exhaust: before stopping you looked over the stern: steam, no water. Raw water failure.
3. Seacock: open.
4. Strainer: you close the seacock, open the strainer: it is packed with weed. You clean it, refit it, open the seacock.
5. Impeller: with weed blocking the intake, the impeller has run dry. You close the seacock and remove the pump cover: three vanes are missing. You fit the spare impeller, find two vane pieces in the pump outlet hose and the third at the heat exchanger inlet, and refit.
6. Restart: water at the exhaust within 20 seconds. You let the engine idle and watch the temperature fall, then motor slowly back to harbour, noting in the log "1415 high temp alarm, strainer blocked weed, impeller replaced, 1 spare impeller now to replace".

Total time: about 40 minutes. Without the spare impeller and a skipper who knew the system, it would have been a tow and possibly a cracked cylinder head.

Worked Example: Engine Will Not Start in a Marina

You are leaving a marina at 0800 with the crew aboard. You turn the key and hear a rapid clicking but no cranking.

1. Stay calm. You are in a safe berth. Tell the crew there will be a delay.
2. Check the battery switch. It is on 1, the start battery. The voltmeter shows 11.4 V, which is very low.
3. Think about why. The fridge and cabin lights ran overnight on the BOTH setting, so the start battery has been pulled down with the domestic bank. This is a reminder to keep the start bank separate.
4. Select the other bank (the domestic bank reads 12.3 V) for starting. The engine turns but is slow and starts after eight seconds.
5. Check on running: oil light out, water from the exhaust, charging voltage 13.9 V.
6. Let the engine charge for at least 30 minutes before stopping, then return the switch to its normal setting (start bank for starting, domestic bank for loads).
7. Log it: "0805 flat start battery after overnight loads on BOTH, started on domestic. Recommend isolating start battery overnight and investigating if the start battery will not hold charge."

A bank that sags to 11.4 V overnight may be ageing. Test it after a full charge with a load, and replace it if needed.

Examiner's Expectations

The Day Skipper examiner will often ask you to demonstrate the checks before starting the engine, to identify the seacock, strainer and impeller housing and to explain what you would do for a given symptom ("water is not coming out of the exhaust"). They will expect you to use the engine log, to know your fuel consumption and range, and to say what you would do if the engine failed in a tight channel (hoist sail, anchor, call for help). The Yachtmaster examiner will expect these answers to be quick and practical. Keep your hands clean and tools tidy: showing respect for the engine room is part of seamanship.

Common Mistakes

The diagram lists the classic engine mistakes.

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• Not checking for water at the exhaust after starting. Look every time.
• Over-cranking. Ten seconds maximum, then wait; repeated cranking floods the exhaust.
• Starting with the seacock shut. Make the seacock part of the routine and tell crew not to close it without telling you.
• Skipping the daily checks because "it was fine yesterday."
• Turning the battery switch to OFF with the engine running. It wrecks the alternator.
• Changing a filter and not bleeding, or forcing the throttle while it is full of air.
• Running with a rope round the prop. Neutral and stop immediately.
• Opening a hot header tank cap. Scalding coolant; wait until cool.
• Carrying no spares, or spares that do not fit this engine.

Summary

• A diesel needs clean, air-free fuel, air and compression; most at-sea faults are fuel or air.
• Learn the fuel, cooling (raw water and fresh water), exhaust and electrical systems of your boat.
• Do the WOBBLE checks every day: Water, Oil, Belts, Bilges, Leaks, Electrics, plus fuel level and separator.
• After starting, check oil pressure light out and water at the exhaust.
• Never switch batteries to OFF with the engine running; keep the start battery for starting.
• Overheating: reduce revs, make the boat safe, check exhaust water, strainer, impeller, coolant, belt, thermostat.
• Rope round the prop: neutral and stop at once.
• Be able to change an impeller, change a fuel filter and bleed the system.
• Carry the right spares and tools, and keep an engine log.

Check Your Understanding

1. What three things does a diesel engine need to run?

Answer: Clean fuel (free of air and water), air, and compression (to heat the air enough to ignite the fuel).

1. What does WOBBLE stand for?

Answer: Water, Oil, Belts, Bilges, Leaks, Electrics.

1. What is the first thing you check once the engine has started, and what do you do if it is wrong?

Answer: That the oil pressure warning light has gone out and cooling water is coming out with the exhaust at the stern. If not, stop the engine immediately.

1. Your engine turns over well but will not fire. You recently ran the tank low. What is the likely cause and remedy?

Answer: Air in the fuel system (possibly with sediment drawn in). Fill the tank, drain the separator, change filters if needed, and bleed the system from the tank towards the engine.

1. Why must you never turn the battery switch to OFF with the engine running?

Answer: Disconnecting the alternator while it is charging causes a voltage spike that destroys its diodes.

1. Name four possible causes of overheating.

Answer: Closed seacock, blocked raw water strainer or intake, failed impeller, low coolant, slipping or broken belt, stuck thermostat.

1. When fitting a new impeller, which way should the vanes be bent?

Answer: Trailing the direction of rotation: rotate the impeller in its normal direction as you push it in.

1. The engine suddenly vibrates heavily and slows after you motor through a line of pot buoys. What do you do?

Answer: Put the engine in neutral and stop it immediately to avoid damage; the propeller is probably fouled. Get sail up or anchor to stay safe, then try a brief burst astern at idle or clear it when it is safe to do so.

1. How much should a traditional stern gland drip?

Answer: A few drops a minute while the shaft is turning, little or nothing at rest.

1. Your engine burns about 2.5 litres an hour and you plan to motor for 6 hours. How much usable fuel should you have on board by the one-third rule?

Answer: The run needs 15 litres. Applying the one-third rule means carrying about three times that, 45 litres of usable fuel, so that a third is for the run, a third for alternatives and a third as reserve. Dip the tank to check, as gauges are unreliable.

1. Your 200 Ah domestic bank has run down to 12.0 V at rest. Is it safe to keep using it, and what should you do?

Answer: No. 12.0 V is effectively flat, below the recommended 50 percent discharge, and repeated deep discharge ages the battery. Run the engine to charge, reduce loads, keep the start battery separate, and expect the last 20 percent of charge to take much longer than the first.

1. What is diesel bug, how does it show itself and how can you prevent it?

Answer: It is a growth of bacteria and fungi at the water and fuel interface, forming slime that blocks filters, often shown by the engine dying in rough weather when the tank is stirred up. Prevent it by keeping the tank full when laid up, draining the water separator, using biocide and buying fuel from a busy supplier. Cure it by cleaning the tank or polishing the fuel and changing filters.