Vessel Systems Maintenance Offshore - Engine & Electrical Troubleshooting

Prerequisites

You should hold, or be at the standard of, RYA Coastal Skipper, and ideally have completed the RYA Diesel Engine Course (one day), which covers the basics of how a marine diesel works and routine checks. You should already be able to:

• Carry out daily engine checks (oil, coolant, belt, raw water strainer, fuel level, bilge) and start and stop the engine.
• Locate the seacocks, battery isolators, distribution panel and bilge pumps on your boat.
• Read a battery voltmeter and know which circuits draw significant power.

The Yachtmaster Offshore examiner frequently asks "Why won't the engine start?" and expects you to know the exam boat's systems: filter locations, bleed points, impeller, belt and emergency tiller.

Learning Objectives

• Explain the layout and function of a diesel fuel system and bleed it after a filter change (RYA syllabus: engines and systems).
• Diagnose common diesel faults systematically: no crank, cranks but will not start, overheating, smoke.
• Explain the raw water cooling circuit and replace a water pump impeller.
• Describe a 12V DC system: batteries, distribution, charging, and protection.
• Compare lead-acid and lithium batteries and manage battery state of charge.
• Diagnose an electrical fault with a multimeter.
• Describe charging from alternator, solar and wind, and check charging voltages.
• Describe fresh water, sanitation and steering systems, and rig emergency steering.
• Respond to a through-hull failure.
• Check the gearbox, stern gland, propeller and anodes, and explain the gas system and its safety.
• Follow a planned maintenance schedule and use a structured approach to a charging fault.

RYA Yachtmaster Offshore syllabus items covered: engines and engine failure, diesel fuel and cooling systems, electrical systems and charging, fresh water and sanitation systems, steering failure and emergency steering, gas system safety, planned maintenance, spares and tools, and response to through-hull failure.

Self-Sufficiency

Offshore there is no marina engineer. If the engine stops, the batteries go flat or the steering fails 80 miles from land, the skipper and crew must fix it with the tools and spares aboard, or manage without it. The approach is always the same: understand the system, diagnose logically from the simplest cause, and carry the spares to fix the likely failures.

The Diesel Engine

A diesel needs four things to run: fuel, air, compression and heat. There is no spark; air compressed in the cylinder to around 20:1 becomes hot enough (over 500°C) to ignite fuel injected as a fine mist. To start, it also needs the starter motor to turn it fast enough to create that heat, which requires a healthy battery.

In practice, most yacht engine failures at sea are fuel problems, followed by cooling and electrical problems. Compression problems (worn engines) are rare and cannot be fixed at sea.

The fuel system

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Fuel flows:

1. Tank: pickup tube slightly above the bottom. Water and sediment collect below it. A vent prevents a vacuum.
2. Water separator and primary filter (often combined, around 10 to 30 micron), with a clear bowl and drain to show water and dirt.
3. Lift pump: a low-pressure pump on the engine, usually with a manual priming lever.
4. Secondary (fine) filter on the engine, 2 to 10 micron.
5. Injection pump: raises pressure to several hundred bar and times delivery to each cylinder.
6. Injectors: atomise the fuel into the cylinders. Excess fuel returns to the tank via the leak-off pipe.

Rough seas stir up sediment and water from the bottom of the tank, which is why engines often stop in exactly the conditions when they are most needed. Keep tanks full, use clean fuel, treat for diesel bug if fuel is stored for long periods, and carry several spare filter elements.

Bleeding the fuel system

Air in the fuel system stops a diesel, because the injection pump cannot compress air to the pressure needed. Air gets in after a filter change, running the tank dry, or through a loose fitting on the suction side.

1. Fill the new filter with clean fuel if possible before fitting, to reduce the air to be removed.
2. Open the bleed screw on top of the primary filter. Operate the lift pump lever until fuel free of bubbles flows out. Close the screw.
3. Repeat at the secondary filter's bleed screw.
4. Repeat at the injection pump's bleed screw (if fitted).
5. If the engine still will not start, slacken the injector pipe unions at the injectors one or two at a time, set the throttle to full and crank the engine in short bursts until fuel spurts out, then tighten. Keep hands away from high-pressure fuel: it can penetrate skin.
6. Start the engine and check for leaks.

If the lift pump lever has no effect, the engine may have stopped with the pump cam at its highest point. Turn the engine over slightly by hand (with the decompressor if fitted, or a spanner on the crankshaft pulley) and try again.

Fault-finding: no crank, or cranks but will not start

A healthy 12V lead-acid battery at rest reads about 12.6V or more. While cranking, voltage should not drop below about 10V; if it collapses, the battery is flat or a connection is bad. Limit cranking to around 10 seconds at a time with a 30-second rest, to protect the starter and battery, and do not crank repeatedly without finding the cause, because a wet-exhaust engine that cranks without firing keeps pumping raw water into the exhaust and can flood the engine through the exhaust manifold.

Smoke

Raw water cooling

Most yacht engines have indirect cooling: a closed freshwater/antifreeze circuit cools the engine, and seawater pumped through a heat exchanger cools that circuit, before being injected into the exhaust to cool it.

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Every time you start the engine, check for water coming from the exhaust. If none appears within a few seconds, stop the engine and investigate. If the high-temperature alarm sounds, reduce revs, then stop as soon as safe.

Overheating causes, in order of likelihood:

1. Raw water seacock closed.
2. Strainer blocked with weed or plastic.
3. Impeller failed (vanes broken off), often after running dry.
4. Heat exchanger blocked, often by broken impeller vanes.
5. Low coolant in the closed circuit, or a failed circulating pump or thermostat.
6. Slipping or broken drive belt (which may also drive the water pump and the alternator).

Changing an impeller

1. Stop the engine and close the raw water seacock.
2. Remove the pump cover plate screws and cover. Note the gasket or O-ring.
3. Pull out the impeller with an impeller puller, or carefully with two screwdrivers or pliers, without damaging the housing.
4. Count the vanes. If any are missing, they must be found, usually in the heat exchanger inlet, or they will block it.
5. Inspect the cam and wear plate for scoring.
6. Lubricate the new impeller with washing-up liquid or glycerine (not petroleum grease, which damages rubber). Bend the vanes in the direction of rotation and push it in, twisting in that direction.
7. Refit the cover with a new gasket, open the seacock, start and check water flow at the exhaust.

Practise until you can do it in under 15 minutes. Carry at least two spare impellers and gaskets.

Through-hull failure

Every seacock should have a softwood tapered bung tied to it. If a seacock or hose fails below the waterline, close the seacock if possible; if the fitting has broken off, hammer in the bung. For larger holes, use a collision mat or anything to hand (cushions, sails) held from inside, or drawn over the outside by lines so water pressure holds it in place, while bilge pumps are run. A 25mm hole 0.5m below the waterline can admit roughly 100 litres per minute, more than most manual bilge pumps can handle, so stopping the flow is more important than pumping.

The DC Electrical System

Architecture

Most yachts under about 12m use 12V DC; larger yachts often use 24V, which halves the current for the same power and allows thinner cables.

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A typical yacht has two battery banks: an engine start battery and a larger domestic bank for everything else, with isolator switches and either a split-charge relay, a voltage-sensitive relay or a battery-to-battery charger so both charge from the engine while keeping the start battery isolated from domestic loads. Every positive cable from the battery must be protected by a fuse or breaker close to the battery, sized to protect the cable from overheating. An unfused short circuit can melt a cable and start a fire within seconds.

When working on a DC system, isolate the battery and disconnect the negative terminal first (and reconnect it last), so that a spanner touching the positive terminal and the engine or hull cannot cause a short.

The distribution panel

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Each circuit has its own breaker or fuse rated for its cable and load: for example navigation lights 10A, VHF 5A, windlass 30A or more (often on its own breaker near the battery). All negatives return to a common bus bar and then to the battery negative. If a breaker trips, find out why before resetting it more than once; never fit a larger fuse to stop it blowing.

Batteries

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Lead-acid state of charge from resting voltage (after several hours without charge or load): 12.7V is about 100%, 12.4V about 75%, 12.2V about 50%, 12.0V about 25%. A battery monitor with a shunt, which counts amp-hours in and out, is far more accurate.

Lithium banks charge quickly and are light, but the BMS will disconnect the battery to protect it if voltage, temperature or current go outside limits, which can suddenly kill all power, and a standard alternator can be overloaded charging them. Install them with compatible chargers and a design that protects the alternator. Never mix battery types or ages in one bank.

Charging: the alternator

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The engine drives the alternator by a belt. The regulator controls the output voltage. With the engine running at around 1,500 to 2,000 RPM, the voltage at the battery should be about 13.8 to 14.4V on a 12V system.

• Below about 13.2V with the engine running: not charging. Check the belt (tension: about 10mm deflection under thumb pressure on the longest run; look for glazing and black dust), the alternator connections, the field wire, and the regulator.
• Above about 14.8V: the regulator has failed and is overcharging. Batteries will gas, overheat and be damaged. Stop charging.

Never disconnect the battery while the alternator is running: the voltage spike can destroy the alternator's diodes.

Solar and wind

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Solar panels and wind generators must charge through a charge controller that limits voltage. MPPT controllers extract 10 to 30% more energy than PWM types, especially in cool or cloudy conditions. Wind generators also need a way to stop or brake them in strong winds. Remember that a panel partly shaded by the boom or a sail may lose most of its output.

Electrical Fault-Finding with a Multimeter

A multimeter measures volts (DC), resistance/continuity (ohms) and, on some meters, current. Most faults on a yacht are corroded or loose connections, which add resistance and cause voltage to drop.

Voltage tracing (circuit live)

With the circuit switched on, set the meter to DC volts. Put the black probe on a known good negative (the bus bar or battery negative), then work along the positive side from the battery towards the load, measuring at each point. Where the voltage disappears, the fault lies between that point and the last point where voltage was present.

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Worked example: navigation lights not working

1. Battery: 12.6V. Battery is fine.
2. Output side of the nav light breaker: 12.6V. Breaker is fine.
3. Output of the switch: 12.6V. Switch is fine.
4. Junction box where the mast cable joins: 0V. The fault is between the switch and the junction, probably a corroded crimp or broken wire.
5. Isolate the circuit, remake the connection with a new crimp and heat-shrink, test.

If the positive side reads correctly all the way to the light but the light does not work, check the negative: measure from the light's positive terminal to its negative. If you read 0V there but 12.6V from the positive terminal to the battery negative, the negative return is broken.

Continuity and resistance (circuit dead)

With power disconnected, set the meter to ohms or continuity. A good wire or switch reads close to zero ohms; an open circuit reads infinity (OL). Never measure resistance on a live circuit.

Ohm's law, V = I times R, explains why a corroded joint matters: with a 5A load, a joint with 0.5 ohm of resistance drops 2.5V, enough to stop a VHF transmitting or an autopilot working.

Fresh Water System

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A diaphragm pressure pump switches on when a tap opens and pressure falls. An accumulator smooths pump cycling. A calorifier heats water from the engine coolant or a shore-power element. Faults:

• Pump runs continuously or cycles with taps closed: a leak (check the bilge for fresh water), or air in the system, or the tank is empty.
• Pump runs but no water: tank empty, air lock, blocked strainer before the pump.
• Pump does not run: breaker, pressure switch, wiring.

Turn the pump off at the panel when leaving the boat or in heavy weather; a burst hose could empty the tank into the bilge. Watermakers, carried on many ocean boats, use reverse osmosis membranes that are destroyed by chlorine, oil and prolonged disuse without pickling.

Sanitation System

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Marine toilets pump waste either directly overboard or, via a Y-valve, into a holding tank, which is emptied at a pump-out station or overboard well offshore. MARPOL Annex IV (applying formally to larger vessels, but good practice for all) prohibits discharging untreated sewage within 12nm of land and treated (comminuted and disinfected) sewage within 3nm. Many harbours and countries have stricter local rules.

Faults:

• Toilet hard to pump: blocked outlet, often by scale or paper. Prevent with regular flushing with plenty of water and occasional vinegar treatment.
• Holding tank will not empty: blocked vent filter creating a vacuum, or a blocked outlet.
• Macerator jammed: isolate the power first, then clear.

Close the toilet seacocks when not in use and always in heavy weather: a toilet bowl below the waterline can siphon and flood the boat if the vented loop fails.

Steering Systems and Emergency Steering

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Cable (wire) steering: wires run from the wheel over sheaves to a quadrant on the rudder stock. It gives good feel. Check wire tension, sheave alignment and wear, and look for broken strands near the quadrant.

Hydraulic steering: the wheel drives a pump that moves a ram on the rudder tiller arm. It has no direct feel. Check the reservoir fluid level and hoses for leaks.

Emergency tiller: almost every wheel-steered yacht has an emergency tiller that fits on top of the rudder stock through a deck fitting. For hydraulic systems, a bypass valve must be opened to let the ram move freely. Know where it is stowed, practise fitting it, and steer a course with it. Steering with an emergency tiller is heavy work and needs more anticipation; balance the sails to reduce the load.

If the rudder itself is lost, options include steering by sail balance, towing a drogue or warps from the quarters and adjusting them, or rigging a jury rudder from a spinnaker pole and a floorboard or locker lid.

Transmission, Shaft and Propeller

The engine is only half of the propulsion system. A skipper should be able to check and troubleshoot the rest.

• Gearbox. Check the oil level monthly (on the dipstick, engine stopped) and use the grade the maker specifies; some boxes use engine oil, others automatic transmission fluid. Milky oil means water ingress. Change the oil annually or as per the manual. If the engine revs but the boat does not move, suspect the gear cable, the coupling or a slipping box.
• Gear and throttle cables. Check that the lever positions match the gearbox: the lever at neutral should leave the gearbox in neutral. Sticking or stiff cables are a warning; a snapped cable can leave you with no control. Know how to select gears directly at the gearbox as an emergency.
• Stern gland (shaft seal). A traditional packed gland should drip slowly (a few drips a minute) when the shaft turns, for lubrication and cooling; too tight and it overheats, too loose and it floods. A dripless seal should not drip at all; check the bellows for age and keep a spare. Saildrives have a rubber diaphragm that needs replacing every 7 years or so, whatever its appearance.
• Propeller. A fouled prop is a common cause of loss of power: slime, weed, a lobster-pot line or a polythene bag. Signs are vibration, lack of speed or an overheating engine. Stop the engine, check there is nothing around, and if it is safe go over the side with a mask, or cut the line away with a long knife. Folding and feathering props need greasing to avoid failure to open.
• Anodes. Sacrificial zinc or aluminium anodes protect the propeller, shaft and saildrive from galvanic corrosion by corroding in their place. Replace them when they are about half gone; a shaft anode that is nearly eaten away is a warning sign.

Gas System

Most yachts cook with LPG (butane or propane), which is heavier than air and sinks into the bilge, where a spark can ignite it. Offshore you must manage this carefully.

• Layout. Gas bottles live in a sealed locker that drains overboard, never into the bilge. A regulator on the bottle reduces the pressure and a pipe runs to the cooker. Use approved hose and clips and replace hoses at the maker's interval (usually 5 years).
• Isolation. Turn off the gas at the bottle (or a solenoid valve) after every cooking session, not only at the cooker: the pipe stays pressurised otherwise. Many boats fit a gas detector with an alarm in the bilge.
• Leak test. Brush soapy water on joints: bubbles show a leak. Never use a naked flame to look for one. If you smell gas: no switches or flames, turn off at the bottle, ventilate, and pump the bilge with the manual pump to remove the gas.
• Flame failure. Check that the cooker has a thermocouple flame-failure device that shuts off the gas if the flame blows out.

Shore Power, Corrosion and Hull Fittings

In marinas, a 230 V shore supply brings a risk of electric shock and galvanic corrosion. Use a marine-approved connection with an RCD (residual current device), check that the polarity is correct and never plug in a damaged lead. Stray currents or poor bonding can corrode seacocks and anodes quickly, so do not neglect the annual hull check.

Seacocks should be operated regularly (open and close each one monthly) so they do not seize. Inspect hoses and double-clip every hose below the waterline. A bung of the right size should be tied next to each through-hull fitting. The through-hull failure procedure above applies at any time you find water coming in.

More from the Diesel Engine Handbook

The four-stroke cycle

Nearly all yacht diesels use the four-stroke cycle. On the induction stroke the piston descends and draws in air. On compression the piston rises and squeezes the air to around one twentieth of its volume, heating it above 500 C. Just before the top, fuel is injected, ignites by itself and drives the piston down on the power stroke. On the exhaust stroke the piston rises again and pushes out the burnt gases. Two turns of the crankshaft make one cycle. A diesel is governed by the amount of fuel injected, not by throttling the air, which is why the throttle lever controls the injection pump and why the engine can run on very little air when idling.

Lubrication

Oil is drawn from the sump by a pump, passes through a filter, and is forced to the main bearings, big ends and camshaft. A pressure relief valve protects the system. Check oil level with the engine stopped and level, after a few minutes' drain-back, and keep it between the two marks. Too much oil causes blue smoke and a risk of overload; too little starves the bearings. Oil pressure alarms on most engines mean stop at once. Black oil is normal for a diesel as the oil holds soot in suspension; milky oil means water, and very thin, smelly oil can mean diesel dilution from a faulty lift pump diaphragm or injector.

Change engine oil and filter at the maker's interval, normally every 100 to 250 running hours or once a year, whichever comes first. A used-oil change is easiest by pumping out through the dipstick tube with a small hand pump. Always run the engine until warm first, and catch every drip; never put oil into the bilge.

The cooling system, in more detail

A thermostat keeps the closed circuit at about 75 to 85 C. When the engine is cold the thermostat is closed, which lets the engine warm quickly; when hot it opens to let coolant through the heat exchanger. A thermostat stuck shut makes an engine overheat; stuck open it will never warm up. Antifreeze (typically 33 to 50 percent) protects against both freezing and corrosion and should be changed every two years. Never remove the filler cap from a hot engine; wait until it cools. Clean a blocked heat exchanger by removing the end caps and pushing a rod through the tubes, and flushing out with fresh water and a descaler.

Cold starting

In cold weather, glow plugs or an inlet air heater warm the combustion chambers. Hold the key at the preheat position for the time the handbook states (usually 10 to 20 seconds), then crank. Never use ether starter spray on an engine with a heater, as it can cause an explosion. A cold engine should be warmed gently at about 1,200 rpm rather than raced.

Running hours and service intervals

Good practice for engine use

• Run the engine at about 75 percent of its maximum rated power for a long life; long periods at idle cause glazed bores, carbon in the exhaust and wet stacking.
• Do not sit at tickover for long periods; run under load for the last 10 minutes of a passage to burn off deposits.
• Warm up gently, and let a hot engine idle for a minute or two before stopping.
• Never rev the engine in neutral to charge batteries for long periods.

Electrical Calculations

Power, current and cable size

The three key relationships are:

• Power (W) = volts x amps, so a 60 W anchor light at 12 V draws 5 A.
• Ohm's law: V = I x R, used to find voltage drop along a cable.
• Amp-hours = amps x hours, the capacity or consumption in the battery.

Voltage drop in the wiring is the commonest cause of faults. For navigation equipment the drop should be no more than 3 percent (about 0.4 V at 12 V); for ordinary lights 10 percent is acceptable. A longer cable run needs a thicker wire. As a rule of thumb, copper cable of 2.5 mm squared carries about 20 A over a short distance, and for 10 m runs of 10 A a cable of 2.5 mm squared is generally satisfactory. Always size the cable first, then the fuse to protect the cable (never the other way round).

Worked example. A 100 W fridge-freezer draws 8.3 A. The run is 6 m out and 6 m back (12 m in all) through 2.5 mm squared copper, which has a resistance of about 0.0074 ohm per metre. Resistance = 12 x 0.0074 = 0.089 ohm; drop = 8.3 x 0.089 = 0.74 V, which is 6 percent of 12 V. That is acceptable for a fridge but not for an autopilot or electronics, which need a thicker cable.

Good wiring practice

• Use tinned marine-grade multi-strand copper cable, and crimp (do not twist) each connection with the correct tool and an insulated heat-shrink terminal.
• Keep joints out of the bilge, support every cable with clips, and add a drip loop where cables enter equipment.
• Use red for positive and black or yellow for negative (DC); on 230 V AC the colours are brown live, blue neutral and green/yellow earth.
• Label both ends of every circuit, and keep a wiring diagram aboard.
• Fit a main battery isolator and a fuse close to each battery.
• Never use household twin-and-earth cable on board.

Battery maintenance

Check flooded batteries monthly: top up each cell with distilled water just above the plates (never acid), keep the top clean and dry, and smear terminals with petroleum jelly to prevent corrosion. Use a hydrometer to read specific gravity (fully charged about 1.265); a variation of more than 0.05 between cells indicates a failing cell. Equalise flooded lead-acid banks occasionally. Do not charge a frozen battery. Check battery boxes are well ventilated, because charging produces hydrogen.

Propeller Theory and Handling

The Diesel Engine Handbook and the Yachtmaster exam both cover the propeller as part of the system.

• Pitch and diameter: a propeller screws through the water like a screw through wood, advancing a distance called the pitch for each turn, less the slip. A fixed two-blade prop is simple and cheap but gives high drag under sail; a folding or feathering prop reduces drag at the cost of reliability.
• Direction of rotation: a right-handed propeller (the usual type) throws the stern to port in reverse and, more weakly, to starboard in forward gear. This propeller walk matters when berthing.
• Prop walk is strongest at the start of reverse thrust when the boat is nearly stationary; give a short burst of power, and then let the boat settle.
• Cavitation: if revs are too high or the blades are damaged, vapour bubbles form and thrust drops. A prop that cavitates at normal revs is often fouled or the wrong pitch.
• Over-propped or under-propped: if the engine cannot reach its rated maximum rpm at full throttle, the prop has too much pitch and the engine is overloaded (black smoke); if it overruns the rated maximum, the pitch is too fine.

Worked Example: Overheating Alarm Offshore

You are motoring at 2,200 rpm 40 nm from the harbour when the high-temperature alarm sounds.

1. Reduce revs to tickover and select neutral; do not stop immediately, because an immediately stopped hot engine can seize or crack.
2. Check the exhaust: no water is coming out. The raw-water circuit has failed.
3. After a minute or two, stop the engine. Close the seacock and open the strainer: it is full of weed and a plastic bag. Clear it.
4. Restart with the seacock open and check that a strong flow comes from the exhaust. If it does not, replace the impeller.
5. Allow the engine to cool, then check the coolant level (carefully) in the header tank.
6. Restart; watch the temperature gauge. Run at lower revs until you reach harbour.
7. Record the incident in the maintenance log, and check the strainer more often in this weed-laden water.

If the exhaust had been running well, the likely fault is in the closed circuit (a slipping belt, a low coolant level, or a failed thermostat).

Planned Maintenance Schedule

A skipper who has a written schedule avoids most failures. A typical yacht schedule looks like this.

Keep a maintenance log. It is also useful evidence to the examiner that you take systems seriously and understand running hours, and it makes any problem easier to diagnose because you know what was last done.

Worked Example: A Charging Problem

You are on passage and notice the house battery voltage is 12.1 V at the start of the night watch, even though the engine ran for two hours that afternoon. The voltmeter shows only 13.0 V with the engine running, not the expected 14.2 to 14.4 V.

1. Check the alternator belt tension: press it at its mid-point; it should deflect no more than about 10 mm. A loose belt slips and the alternator does not charge.
2. Check the alternator warning light or output and the connections: loose or corroded terminals at the alternator and battery cause voltage drops.
3. Measure voltage at the alternator output and at the battery. If the alternator shows 14.3 V but the battery sees only 13.0 V, there is a voltage drop in the cable or a poor connection, probably at the isolator or the fuse.
4. Check the battery itself. Old, sulphated batteries accept charge poorly and show low resting voltage soon after charging. If the battery is failing, reduce loads, switch off non-essential equipment and use the starting battery only for engine starts.
5. Report your reasoning and fix. Examiners like to hear: symptom, hypothesis, test, result, action.

Exam Tips: What the Examiner Expects

• You will be asked fault-finding questions such as "the engine will not start" or "the nav lights do not work". Answer in a logical order from the simplest cause (fuel on, isolator on, battery voltage) to the more complex.
• Know where the filters, bleed points, impeller and emergency tiller are on the boat you are examined on.
• Be able to say how often routine checks happen and what you would carry as spares.
• Do not guess: explain how you would test, and what each result would tell you.

Spares and Tools

A minimum offshore spares kit for a typical yacht:

• Fuel filters (primary and secondary) and filter-housing O-rings; a jerry can of clean fuel.
• Two impellers with gaskets; an impeller puller.
• Alternator and water pump belts.
• Engine oil and coolant for a full change; oil filter.
• Fuses of every rating used; spare breaker; crimp terminals, heat-shrink, wire, crimping tool, multimeter.
• Navigation light bulbs (or spare LED units).
• Hose and stainless hose clips of each size; tapered softwood bungs.
• Toilet service kit; pump diaphragm kit.
• Emergency tiller (check it fits).
• Self-amalgamating tape, epoxy putty, duct tape, cable ties.

Worked Example: Engine Failure Near a Shipping Lane

You are 30nm from your destination, motoring across a shipping lane in light winds. The engine slows, surges and stops. A restart attempt runs for 30 seconds and stops again. Wind is Force 3 on the beam.

1. Safety first: get out of the shipping lane. Unroll the genoa and hoist the main, sail clear of the traffic, and keep a lookout. A disabled vessel in a lane is in serious danger; if necessary, call the Coastguard and broadcast a Securité or Pan-Pan.
2. Diagnose: the symptoms (surging then stopping) suggest fuel starvation. Check the primary filter bowl: it shows water and black sediment. Rough weather earlier has stirred up the tank.
3. Fix: drain the separator bowl until clean fuel appears. Change the primary filter element; change the secondary as well. Check the tank vent is clear.
4. Bleed the system from the primary filter through to the injection pump.
5. Restart and monitor: run at moderate revs, keep watching the separator bowl, and carry spare filters to hand. If water keeps appearing, the tank is badly contaminated; consider a temporary feed from a clean jerry can directly into the lift pump suction.
6. Decide: sailing to the destination at 3 to 4 knots will take 8 to 10 hours. If the engine is reliable, motor-sail with sails up as insurance; if not, sail, and plan to enter harbour under sail or request a tow for the final approach.

Common Mistakes

• Cranking repeatedly without finding the fault. It drains the battery and can flood the engine with seawater through the exhaust.
• Not checking for exhaust water after starting. An impeller can fail in minutes without water.
• Not counting impeller vanes. Missing vanes block the heat exchanger.
• Fitting an impeller with vanes the wrong way, or lubricating with petroleum grease.
• Partial bleeding. Work methodically from tank to injectors.
• Fitting a larger fuse to stop it blowing. Find the fault; the fuse protects the cable from fire.
• Disconnecting the battery with the engine running. It can destroy the alternator diodes.
• Measuring resistance on a live circuit. It damages the meter and gives false readings.
• Not knowing where the emergency tiller is. Check during familiarisation and practise fitting it.
• Leaving heads seacocks open in heavy weather.

Summary

• A diesel needs fuel, air, compression and heat; most failures at sea are fuel problems.
• Fuel flows tank, separator and primary filter, lift pump, secondary filter, injection pump, injectors. Bleed in that order after a filter change.
• Check for exhaust water every start; overheating usually means seacock, strainer, impeller or belt.
• Change impellers with vanes counted, correctly bent and lubricated with soap or glycerine.
• 12V systems: separate start and domestic banks, fuse every positive cable near the battery, disconnect the negative first.
• Lead-acid batteries: keep above 50% (about 12.2V resting). Lithium: higher usable capacity but needs BMS and compatible charging.
• Charging voltage with the engine running: 13.8 to 14.4V. Solar and wind always via a charge controller.
• Trace faults with a multimeter from battery to load; the fault lies where voltage disappears.
• Know the fresh water, heads and steering systems; practise fitting the emergency tiller.

Check Your Understanding

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

Answer: Fuel, air, compression and heat (the heat produced by compressing the air). To start, it also needs the starter motor to turn it fast enough, which requires a good battery.

2. Put these in order of fuel flow: secondary filter, injection pump, lift pump, primary filter, tank, injectors.

Answer: Tank, primary filter (with water separator), lift pump, secondary filter, injection pump, injectors.

3. The engine turns over normally but will not fire after you changed the fuel filters. What do you do?

Answer: Bleed the fuel system: open the bleed screw on each filter in turn and operate the lift pump until bubble-free fuel flows, then the injection pump bleed point, and if necessary crack the injector unions while cranking briefly.

4. Shortly after starting, there is no water coming from the exhaust. Give the likely causes in order of checking.

Answer: Seacock closed; strainer blocked; impeller failed; then a blocked heat exchanger or slipping belt. Stop the engine first to prevent overheating and impeller damage.

5. When fitting a new impeller, what three things must you do to prevent problems?

Answer: Find any missing vanes from the old one; lubricate the new impeller with soap or glycerine, not petroleum grease; and fit it with the vanes bent in the direction of rotation.

6. With the engine running at 1,800 RPM, the voltmeter reads 12.4V. What does this tell you and what do you check?

Answer: The alternator is not charging (it should read 13.8 to 14.4V). Check the belt tension and condition, alternator connections and field wire, and the regulator.

7. A 12V lead-acid battery reads 12.2V after resting for several hours. What is its approximate state of charge?

Answer: About 50%, the lowest level it should normally be taken to. Recharge it.

8. The VHF will not power on. Describe how you would find the fault with a multimeter.

Answer: Switch the circuit on. With the meter on DC volts and the black probe on a known good negative, measure at the battery, after the VHF breaker, then at successive connections towards the radio. Where the voltage disappears, the fault lies between that point and the last good one. If the positive is good at the radio, check the negative return. Repair with power isolated.

9. Why should you disconnect the negative battery terminal first?

Answer: If the spanner touches the hull or engine while undoing the positive with the negative still connected, it creates a dead short and can cause sparks, burns or fire. With the negative removed first, there is no complete circuit.

10. Your hydraulic steering fails 50nm offshore. What do you do?

Answer: Slow or heave-to to stabilise the boat; fit the emergency tiller to the rudder stock and open the hydraulic bypass valve so the ram does not lock the rudder; balance the sails to reduce helm load; then look for the leak or failure and refill the fluid if a spare is carried.

11. A stern gland drips steadily when the shaft turns and stops when the shaft stops. Is this a fault?

Answer: No. A packed stern gland should drip a few drops a minute when running for lubrication and cooling. Check that the rate is not increasing and adjust only a little at a time.

12. Why is gas turned off at the bottle and not only at the cooker?

Answer: The pipe between bottle and cooker stays pressurised if you shut only the cooker valve, so a leak in the hose or joints would still release gas, which is heavier than air and collects in the bilge.