Tricky Marine Engineering interview questions

75 questions with answers (MAR-426 to MAR-500). Try each one aloud before reading the answer.

Ship Construction & Naval Architecture

MAR-426 · How can a steel ship float when steel is denser than water?

Trap: Assuming the material's density decides whether something floats. The hollow hull encloses a large volume of air, so it displaces a weight of water equal to the ship's total weight before the deck reaches the waterline.

MAR-427 · Gross tonnage tells you how heavy the ship is, right?

Trap: Treating "tonnage" as weight. Gross tonnage is a dimensionless measure of enclosed volume. Weight is given by displacement, and carrying capacity by deadweight.

MAR-428 · Deadweight is the same as cargo capacity, isn't it?

Trap: Ignoring non-cargo loads. Deadweight includes fuel, lube oil, fresh water, ballast, stores, crew and effects, so cargo capacity is always less than deadweight.

MAR-429 · Does thicker plating anywhere make the hull stronger in bending?

Trap: Thinking all steel adds equal strength. Longitudinal bending strength depends on section modulus, so material far from the neutral axis (deck and bottom) matters most. Steel near the neutral axis adds weight but little bending strength.

MAR-430 · Do more bulkheads always make a ship safer after damage?

Trap: Assuming more subdivision is always better. Longitudinal bulkheads can cause asymmetric flooding, a large list and capsize. Subdivision must be planned so flooding stays symmetric or is cross-flooded quickly.

MAR-431 · In hogging, the bottom is in tension, right?

Trap: Reversing the stresses. In hogging the middle bends upward, so the deck is in tension and the bottom is in compression. In sagging the deck is in compression and the bottom in tension.

MAR-432 · Is a finer hull form (low Cb) always better?

Trap: Looking only at resistance. A fine form reduces wave resistance at high speed but carries less cargo for its size. Slow bulk carriers and tankers use full forms because wave resistance is small at their low Froude numbers.

Ship Stability

MAR-433 · Is a very large GM good?

Trap: Thinking more GM always means a safer ship. A large GM makes the ship "stiff", with a short, violent roll that stresses cargo lashings, structure and crew. Too small a GM makes it "tender" and risks capsize, so GM should be moderate.

MAR-434 · Does a ship float higher in seawater or fresh water?

Trap: Assuming draft is the same in all water. Seawater is denser, so less volume is needed to support the ship and it floats higher. Moving into a river, it sinks deeper by about the fresh water allowance.

MAR-435 · Does adding any weight always reduce stability?

Trap: Ignoring where the weight goes. Weight loaded below the ship's centre of gravity lowers G and increases GM. Only weight placed above G reduces it.

MAR-436 · Does a fuller slack tank cause more free surface effect?

Trap: Linking free surface effect to liquid quantity. FSE depends on the free surface's dimensions (i = l·b³/12) and liquid density, not how much liquid is in the tank. A tank pressed completely full has no free surface effect.

MAR-437 · When cargo is lifted by the ship's crane, does its weight still act at its stowage position?

Trap: Forgetting suspended weights. Once lifted clear, the weight acts at the crane jib head, so the ship's G rises immediately. This can sharply reduce GM during heavy lifts.

MAR-438 · If GM is positive, is the ship safe at any heel angle?

Trap: Using GM beyond small angles. GM only describes initial stability. Safety at large angles depends on the full GZ curve, its range, maximum value and area, which can be poor even with good GM.

MAR-439 · Is a list always corrected the same way as an angle of loll?

Trap: Confusing the causes. A list comes from an off-centre G and is corrected by shifting weight to the opposite side. Loll comes from negative GM; shifting weight to the high side can make the ship roll to a worse angle the other way, so G must be lowered instead.

Resistance & Propulsion

MAR-440 · Why do ships have bulbous bows? Do they help at all speeds?

Trap: Thinking a bulb always helps. The bulb makes its own wave that partly cancels the bow wave, cutting wave-making resistance near design speed and draft. At other speeds or light drafts it can add resistance, which is why slow-steaming ships sometimes retrofit bows.

MAR-441 · Are bigger propellers always better?

Trap: Ignoring constraints. A larger, slower propeller is usually more efficient, but its size is limited by draft, hull clearance, ballast-condition immersion, vibration and the engine's rpm range.

MAR-442 · Can propeller slip be negative?

Trap: Thinking negative slip is impossible. Apparent slip uses ship speed, not actual inflow speed. In a following current, wind or sea, ship speed can exceed pitch × rpm, giving negative apparent slip, even though real slip stays positive.

MAR-443 · If you double the ship's speed, does it need double the power?

Trap: Assuming a linear relation. Power is roughly proportional to V³, so doubling speed needs about eight times the power, and even more near the hump in wave resistance.

MAR-444 · Doesn't a hull efficiency above 1 break energy conservation?

Trap: Treating ηH as a full energy balance. It exceeds 1 because the propeller works in the wake and recovers energy the hull gave to the water. The overall propulsive coefficient stays below 1.

MAR-445 · Is propeller cavitation caused by water heating up and boiling?

Trap: Linking boiling only to heat. Cavitation happens because local pressure drops below vapour pressure at normal water temperature. Lower pressure, not higher temperature, causes the vapour bubbles.

MAR-446 · Are more propeller blades always better?

Trap: Assuming more blades means more thrust or efficiency. More blades lower pressure pulses and vibration, but add friction, reduce optimum diameter and often slightly reduce efficiency. Blade number is chosen to avoid hull and shaft resonances.

Marine Diesel Engines

MAR-447 · Are two-stroke engines less efficient than four-stroke engines?

Trap: Generalising from small two-stroke petrol engines. Large slow-speed two-stroke marine diesels are the most efficient heat engines in common use, reaching around 50% thermal efficiency.

MAR-448 · Is a higher cylinder oil feed rate always safer?

Trap: Thinking more oil is always better. Excess cylinder oil causes deposits on piston crowns and rings, scavenge space fires and higher cost, and excess BN can cause liner polishing. The feed rate should match load, sulphur and liner condition.

MAR-449 · Can you open the crankcase straight after an oil mist alarm to find the hot spot?

Trap: Rushing to inspect. Opening a hot crankcase lets fresh air into an oil mist atmosphere and can cause an explosion. The engine must be stopped and allowed to cool for the time specified before the doors are opened.

MAR-450 · Does a large two-stroke engine need a gearbox to go astern?

Trap: Assuming every engine turns only one way. Slow-speed two-stroke engines are directly reversible: the starting air and fuel timing are shifted so the engine starts and runs in the opposite direction.

MAR-451 · A cylinder shows much lower exhaust temperature than the others. Is it running better?

Trap: Thinking a cooler cylinder is a healthier one. A low exhaust temperature usually means that cylinder is getting less fuel or misfiring, for example from a blocked injector or faulty fuel pump. It must be investigated.

MAR-452 · Is a turbocharger driven by the crankshaft?

Trap: Confusing turbochargers with mechanical superchargers. A turbocharger is driven by exhaust gas energy. At low loads its air supply is not enough, so electric auxiliary blowers start on two-stroke engines.

Marine Boilers & Steam Systems

MAR-453 · If the gauge glass shows a full glass, the boiler has plenty of water, right?

Trap: Trusting the glass without checking it. A choked steam cock can give a false high level, and a completely full glass can mean dangerous overfilling. Blowing through the glass confirms the true level.

MAR-454 · Does higher boiler water alkalinity always give better protection?

Trap: Thinking more treatment is always better. Too high an alkalinity causes caustic embrittlement, gouging and foaming. Alkalinity must be kept within the recommended band.

MAR-455 · Does steam at a higher pressure carry more latent heat?

Trap: Assuming latent heat rises with pressure. Latent heat of vaporisation falls as pressure rises and is zero at the critical point. Total enthalpy changes only a little over the usual boiler range.

MAR-456 · Is pure distilled water safe as boiler feed without any treatment?

Trap: Thinking pure water cannot corrode. Distilled water dissolves oxygen and carbon dioxide and has little buffering, so it is aggressive to steel. It still needs pH control and oxygen scavenging.

MAR-457 · A boiler is producing black smoke. Is there too much air?

Trap: Reversing the cause. Black smoke means too little air or poor atomisation, leaving unburnt carbon. White smoke can mean too much excess air or water in the fuel.

MAR-458 · Since an exhaust gas boiler has no burner, can it always run dry safely?

Trap: Assuming no burner means no fire risk. Unless designed and approved for dry running, the tubes can overheat, and soot deposits can ignite. The maker's rules on circulation and soot blowing must be followed.

Auxiliary Machinery (Pumps, Purifiers, Compressors, FWG)

MAR-459 · Should every pump be started with the discharge valve open?

Trap: Treating all pumps the same. A centrifugal pump is usually started with the discharge closed to cut starting power, but not run that way for long. A positive displacement pump must never be started against a closed discharge, or pressure will build until the relief valve lifts or something fails.

MAR-460 · Does denser fuel need a larger gravity disc in the purifier?

Trap: Getting the relationship backwards. The denser the oil, the smaller the gravity disc inner diameter needed to hold the interface in the right place.

MAR-461 · Does increasing FWG vacuum always increase output?

Trap: Thinking more vacuum is always better. Too high a vacuum causes violent boiling and priming, carrying salt water into the distillate. There is an optimum vacuum for output and purity.

MAR-462 · Does adding detergent to the bilges help the oily water separator?

Trap: Thinking cleaner bilges mean cleaner discharge. Detergents form stable emulsions that the OWS cannot separate and can fool the oil content meter, risking illegal discharge.

MAR-463 · Can you control a gear pump's flow by throttling its discharge valve?

Trap: Treating it like a centrifugal pump. A positive displacement pump delivers a fixed volume per revolution, so throttling only raises pressure until the relief valve lifts. Flow is controlled by speed or a bypass.

MAR-464 · Air receivers hold compressed air, so do they really need draining?

Trap: Thinking compressed air is dry. Moisture condenses as air cools after compression, and oil carries over. Undrained receivers corrode, and oil deposits can create an explosion risk in starting air lines.

Marine Electrical Systems

MAR-465 · An earth fault on a ship's insulated system trips the breaker immediately, doesn't it?

Trap: Applying earthed-system thinking. With an insulated neutral, one earth fault only shows on the earth lamps and does not trip. A second fault on another phase makes a phase-to-phase short circuit, so the first must be traced and cleared quickly.

MAR-466 · Is generator power factor controlled by the governor?

Trap: Mixing up the controls. The governor controls fuel and so active power (kW). Reactive power (kVAr) and power factor are controlled by excitation through the AVR.

MAR-467 · Should you close the breaker when the synchroscope is spinning fast through 12 o'clock?

Trap: Thinking only the phase position matters. A fast-moving pointer means a large frequency difference. Close when it is turning slowly in the "fast" direction, just before 12 o'clock.

MAR-468 · Does a hot motor with lower insulation resistance mean the insulation has failed?

Trap: Ignoring temperature. Insulation resistance falls sharply as temperature rises. Readings must be compared at similar temperatures or corrected, and trends matter more than one value.

MAR-469 · For shore power, is matching voltage and frequency enough?

Trap: Forgetting phase sequence. A wrong phase sequence makes motors run backward, which can damage pumps and fans. Phase sequence must be checked before connecting.

MAR-470 · Can the emergency generator be in the engine room for easy maintenance?

Trap: Ignoring its purpose. It must work when the machinery space is lost to fire or flooding. SOLAS requires it outside the machinery spaces, above the uppermost continuous deck and with its own fuel supply.

Refrigeration & AC on Ships

MAR-471 · Does a refrigeration plant "produce cold"?

Trap: Treating cold as a substance. A refrigerator removes heat from the cold space and rejects it, plus compressor work, at the condenser.

MAR-472 · Does adding more refrigerant always improve cooling?

Trap: Thinking more charge means more capacity. Overcharging floods the condenser, raises head pressure and power, and can cause liquid floodback to the compressor.

MAR-473 · Does the thermostatic expansion valve control the room temperature?

Trap: Confusing its function. The TXV controls superheat at the evaporator outlet. Room temperature is controlled by a thermostat acting on the solenoid valve or compressor.

MAR-474 · Heavy frost on the compressor suction line means the system is working well, right?

Trap: Taking frost as a good sign. Frost reaching the compressor usually means low superheat and liquid floodback, which can damage the compressor.

MAR-475 · Can you cool the galley by leaving the provision room door open?

Trap: Forgetting where heat goes. The plant rejects the heat it removes plus compressor work at the condenser. With the condenser in the same area, the net effect is heating, and the plant overworks and ices up.

MAR-476 · R134a has zero ozone depletion potential, so it has no environmental impact?

Trap: Confusing ozone depletion with global warming. R134a is ozone-safe but has a high global warming potential, which is why HFCs are being phased down in favour of low-GWP refrigerants.

Fuels, Lubrication & Bunkering

MAR-477 · Does flash point show how easily fuel ignites in the engine?

Trap: Confusing a safety property with ignition quality. Flash point relates to storage and fire safety. Ignition quality in the cylinder is shown by cetane index for distillates and CCAI for residual fuels.

MAR-478 · Is heating fuel more always better for combustion?

Trap: Thinking lower viscosity is always better. Overheating gives too low a viscosity, causes gassing in the fuel system, pump wear and coking. Fuel is heated only to the target injection viscosity.

MAR-479 · Is high-BN cylinder oil safest when burning low-sulphur fuel?

Trap: Thinking more base is always safer. With little sulphur acid to neutralise, excess BN leaves calcium deposits that polish liners and cause scuffing. Low-sulphur fuel needs lower-BN oil.

MAR-480 · The bunker barge soundings look right, so the quantity received is correct?

Trap: Trusting soundings alone. Air blown into the fuel (the "cappuccino effect") swells the volume temporarily. Check for foam and bubbles, take soundings after settling, and compare with the BDN and samples.

MAR-481 · Dark lube oil is always degraded and must be changed?

Trap: Judging oil by colour. Detergent oils go dark quickly because they hold soot in suspension, which is their job. Condition is judged by analysis of viscosity, BN, water and wear metals.

MAR-482 · IMO 2020 allows 0.50% sulphur fuel worldwide, so it is fine in an ECA too?

Trap: Forgetting stricter areas. Inside a SOx Emission Control Area the limit is 0.10%, unless an approved scrubber is used.

Maritime Regulations (SOLAS, MARPOL, STCW, ISM, ISPS)

MAR-483 · Under MARPOL, can 15 ppm bilge water be discharged anywhere at any time?

Trap: Thinking the 15 ppm limit is the only condition. The ship must also be en route, use approved filtering equipment with an alarm and automatic stop, and comply with special area rules. Discharge is not allowed in the Antarctic area.

MAR-484 · Is the ISM Code about ship security?

Trap: Mixing up ISM and ISPS. ISM is about safety management and pollution prevention. Security against threats is covered by the ISPS Code.

MAR-485 · Can the 10 hours of rest per day be split into five 2-hour breaks?

Trap: Counting only the total hours. STCW allows rest in no more than two periods, one of which must be at least 6 hours.

MAR-486 · Is a classification certificate the same as a statutory certificate?

Trap: Treating class and flag as one. Class certificates confirm compliance with class rules. Statutory certificates (SOLAS, MARPOL, Load Line) are issued by or on behalf of the flag state, often by a class society acting as recognised organisation.

MAR-487 · Can plastics be discharged at sea if shredded?

Trap: Thinking grinding makes plastics acceptable. MARPOL Annex V prohibits discharge of all plastics everywhere, shredded or not. They must be kept for shore facilities or approved incineration.

MAR-488 · With a scrubber fitted, can the ship burn high-sulphur fuel everywhere?

Trap: Ignoring local rules. A scrubber is an approved equivalent under Annex VI, but some ports and coastal states ban open-loop washwater discharge, so the ship may need closed-loop mode or compliant fuel there.

Safety, Firefighting & Survival

MAR-489 · CO₂ is harmless because we breathe it out, so is the flooded space safe to enter?

Trap: Assuming it is non-toxic at any level. At flooding concentrations CO₂ causes unconsciousness and death quickly, both by displacing oxygen and by its own toxic effect. Never enter without breathing apparatus.

MAR-490 · Can an EEBD be used for firefighting?

Trap: Treating every breathing device as the same. An EEBD gives only a short air supply for escape. Firefighting needs SCBA with protective clothing.

MAR-491 · Is it a good idea to ventilate an enclosed space with oxygen to make it safe?

Trap: Thinking more oxygen is always safer. Oxygen enrichment greatly raises fire and explosion risk. Spaces are ventilated with fresh air only.

MAR-492 · Can a survivor in a liferaft drink a little seawater to extend water supplies?

Trap: Thinking small amounts help. Seawater speeds up dehydration and kidney damage. Only fresh water rations, rainwater or desalted water should be drunk.

MAR-493 · Water can never be used on electrical equipment fires?

Trap: Taking the rule as absolute. Water jets must not be used on live equipment, so power is isolated first. Once it is dead, water may be used, and approved water mist systems protect machinery spaces with electrical equipment.

MAR-494 · Once the flames are out, is the fire danger over?

Trap: Ending the response too early. Hot surfaces and smouldering material can reignite. Boundary cooling, a fire watch and checks for hot spots continue until temperatures are safe.

Engine Room Operations & Watchkeeping

MAR-495 · On a UMS ship, is nobody responsible for the engine room at night?

Trap: Taking "unmanned" literally. A duty engineer is always responsible and on call, with alarms relayed to the cabin and bridge, and must respond to every alarm.

MAR-496 · If no alarms are sounding, is the machinery fine?

Trap: Trusting the alarm system completely. Sensors can fail or drift, and leaks or noises may not be monitored. Regular rounds and trend checks are still needed.

MAR-497 · Does the turning gear matter only during maintenance?

Trap: Ignoring its interlock. Engaged turning gear blocks starting, and forcing a start can damage the gear and engine. It must be disengaged and checked before manoeuvring.

MAR-498 · Should the engineer always stop a faulty main engine, even if the bridge needs propulsion?

Trap: Ignoring that ship safety comes first. In confined waters, stopping can endanger the ship. The engineer must tell the bridge about the fault and risk at once, and the Master decides, while automatic shutdowns still protect against critical faults.

MAR-499 · Can a mistake in the Oil Record Book be erased and rewritten?

Trap: Treating official logs as drafts. Entries must not be erased. A mistake is struck through with a single line, initialled and dated, and the correct entry written, so the original remains readable.

MAR-500 · Modern engines can be started cold, so is warming up unnecessary?

Trap: Thinking start capability equals safe operation. Starting and loading a cold engine causes thermal stress, poor combustion and cold corrosion. Jacket water is preheated and load is increased gradually.

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