Marine Engines, Electrical Systems and Maintenance
The engineering domain checks that a skipper can keep a small diesel running and diagnose it when it stops: the four-stroke cycle, cooling, fuel and lubrication systems, exhaust smoke as a diagnostic clue, and the 12/24 V DC and 230 V shore systems on board. Questions are usually symptom-based - what does white smoke mean, why did the engine overheat at high rpm.
The diesel cycle and its systems
Four strokes: induction, compression (roughly 30–40 bar and 500–700 °C), power with injection just before top dead centre, exhaust. Air, fuel and cooling must all be right for combustion. Raw water is drawn through a strainer by an impeller pump, passes the heat exchanger and is injected into the exhaust elbow - check for water at the exhaust within seconds of starting. Fuel runs tank, water separator, pre-filter, lift pump, secondary filter, injectors; air in the line means bleeding. Lubrication: check oil level, filter and pressure alarm.
Reading the symptoms
- Overheat at high rpm: blocked intake or strainer, worn impeller, fouled heat exchanger, low coolant, stuck thermostat, coked exhaust elbow.
- White smoke: unburnt fuel or coolant ingress; black smoke: overload, fouled propeller, blocked air filter; blue smoke: lube oil burning.
- Vibration: fouled propeller, bent shaft, worn mounts, misalignment.
Electrical systems
Batteries rated in Ah and CCA; fuses and isolation switches close to the battery; cable size chosen for voltage drop. Charging by alternator and shore charger. Ventilate battery boxes - charging releases hydrogen - and never make sparks nearby. On 230 V shore supply check polarity, use the RCD, and keep galvanic isolation and sacrificial anodes in good order to stop electrolytic corrosion.
Worked example
A 12 V fridge draws 4 A at a 50% duty cycle over 24 hours: 4 × 24 × 0.5 = 48 Ah. Navigation lights take 1 A for 6 hours = 6 Ah, so the daily draw is 54 Ah. A 2 × 100 Ah bank gives 200 Ah, but only 50% is usable for lead-acid, i.e. 100 Ah - leaving a 46 Ah margin. Reasoning: leave headroom for cold starts and ageing, and recharge daily by engine or charger.
Traps that cost marks
- Running the engine without checking the raw-water strainer or confirming water at the exhaust.
- Over-cranking a diesel that will not start, flooding the exhaust with water.
- Trusting the temperature gauge alone instead of checking coolant and water flow.
- Mixing battery types or ignoring ventilation.
- Reverse polarity on the shore inlet.
- Ignoring a worn impeller until the engine overheats offshore.