Question 1

How often should a marine diesel be serviced?

Accepted Answer

Use the exact engine manual, and act on whichever limit arrives first: running hours or calendar time. There is no safe universal interval, and an engine that has done very few hours is not exempt. Two things age an engine that is barely used. Oil absorbs combustion by-products and moisture, and those attack bearing surfaces whether the engine runs or not. Coolant, impellers, belts, hoses and anodes degrade on a calendar, not on an hour meter. A yacht auxiliary that does thirty hours a year still needs its annual service. The practical pattern for most recreational owners is an annual service built around oil and filters, a raw-water impeller inspection, a coolant check and a belt, hose and anode inspection, with longer-interval items — coolant replacement, valve clearances, heat exchanger service — scheduled by the manual's hour figures. Record hours, dates, part numbers and what you found each time; the record is what makes the next fault diagnosable.

Question 2

Is a saildrive less reliable than a shaft drive?

Accepted Answer

Not automatically. The two layouts fail in different places, and the difference between a good example and a bad one is far larger than the difference between the types. A saildrive adds a rubber diaphragm sealing the leg to the hull, an aluminium underwater housing, and a corrosion-control system of anodes and approved coatings that must be maintained exactly as the maker specifies. The diaphragm is safety-critical and time-limited — one maker's interval must never be applied to another maker's drive. A shaft drive adds engine-to-shaft alignment, a shaft seal, a cutless bearing and a stern tube. Those wear, and misalignment quietly destroys mounts, seals and couplings over time. Judge the individual installation: documented maintenance, the state of the anodes and coatings, the age and history of the diaphragm or the seal and bearing, and whether the last owner had the work done properly. Condition and records decide reliability, not the stereotype attached to the type.

Question 3

Should I lock the propeller while sailing?

Accepted Answer

Follow the exact instructions for your gearbox, saildrive and propeller. This is one of the few questions where the general advice circulating in marinas is genuinely dangerous to follow. Some gearboxes must be left in gear when sailing, because freewheeling turns the output shaft without the lubrication that only runs when the engine runs. Others specify freewheeling and prohibit locking. Some folding and feathering propellers add their own requirement on top. The consequence of getting it wrong is not immediate — it is bearing wear inside the gearbox that appears as noise or failure much later, at cost. Look the requirement up in the manual for your specific box and drive, write it on a card at the panel, and tell anyone who sails the boat. Drag matters far less than the gearbox does. If drag is the concern, the answer is a folding or feathering propeller chosen for the installation, not a locking practice the maker prohibits.

Question 4

Can I run a diesel at idle to charge batteries?

Accepted Answer

You can, but it is usually a poor way to charge and it is not kind to the engine. Alternator output at idle is well below its rated figure, so the charge rate is slow — and a battery bank's own acceptance rate falls as it fills, which makes the last portion slower still. Hours of idling may return far less energy than owners expect. Extended light-load running is the part that costs you. A diesel run for long periods with almost no load stays cooler than designed, which encourages incomplete combustion, bore glazing, carbon build-up in the exhaust and wet stacking. The engine that spends its life idling at anchor tends to smoke and foul long before an engine that is worked properly. If the boat's energy budget needs regular charging at rest, the answer is usually a larger alternator with a proper regulator, more solar, a bigger house bank, or a generator sized to the real load — not more idling. Where you must idle, run at the manual's recommended charging speed rather than the lowest speed the engine will hold, and follow it with a period under proper load.

Question 5

What causes black diesel smoke?

Accepted Answer

Black smoke is unburned fuel: the engine is being given more fuel than it can burn with the air and load it has. The fault is almost never the fuel system itself, and adjusting fuel delivery to chase the smoke usually makes things worse. Work through the causes in this order. An overloaded or fouled propeller is the most common — if the engine cannot reach its rated maximum rpm at full throttle, it is overloaded, and everything else you check is secondary. A dirty hull adds the same load. Then air: a blocked air filter, a restricted engine-room supply, or a turbo or intercooler fault starves combustion. Then exhaust restriction, which traps what the engine is trying to expel. Injector and pump faults do cause black smoke, but they sit at the end of the list, and confirming them needs test equipment. Establish maximum attainable rpm first — that single measurement separates an overloaded boat from a sick engine.

Question 6

Why does my engine overheat only at high rpm?

Accepted Answer

Overheating that appears only at higher rpm means the cooling system copes at low heat loads and runs out of margin as heat output rises. That narrows the search usefully. Raw-water flow is the first suspect: a partly blocked intake or strainer, a tired impeller, a collapsing suction hose or a fouled heat exchanger will all pass enough water at low load and fall short at high load. Check the volume of water leaving the exhaust, not just that some is leaving. A restricted exhaust does the same thing, as does a heat exchanger or aftercooler with scaled or blocked tubes. On the coolant side, a weak pump, a tired thermostat or trapped air shows up under load first. Finally, consider that the engine may simply be overloaded — a fouled hull or wrong propeller makes it work harder for the same speed, and heat rises with it. Note the maximum rpm you can reach and whether it matches the engine's rating. If it does not, treat the overheating as a symptom of overload until proven otherwise.

Question 7

How much fuel does a yacht diesel use?

Accepted Answer

Published figures are a starting point, not an answer. Consumption depends on the load the engine is actually carrying — hull, propeller, fouling, sea state and the rpm you hold — so two identical engines in different boats will burn quite different amounts. Use the model's fuel curve to get an expected figure at the rpm you cruise at, then measure the real one. The reliable method is to log fuel added against hours run and rpm held over several trips, or to run a measured passage at a steady rpm and refill to the same point. A flow meter or engine data on a multifunction display shortens the work considerably. Once you have litres per hour at a cruising rpm, range follows: usable fuel divided by litres per hour, multiplied by the speed you hold. Usable fuel is not tank capacity — subtract what the pickup cannot reach and hold a reserve, typically a fifth or more for weather, tide and diversion. Recalculate after antifouling, a propeller change or a repower. Those change the numbers more than most owners expect.

Question 8

Can I service a common-rail engine myself?

Accepted Answer

The routine work is owner work if you are competent: oil and filter changes, primary fuel filter and water separator servicing, impeller inspection, coolant checks, belt and hose inspection, anodes, and keeping records. The high-pressure circuit is not, under any circumstances. Common-rail systems hold fuel at pressures capable of injecting through skin and causing life-threatening injury, and pressure can remain after shutdown. Never loosen a high-pressure line, never crack an injector to bleed a common-rail engine the way you would an older mechanical one, and never work on the rail, the high-pressure pump or the injectors. Electronic diagnosis is the other boundary. Fault codes on these engines need the maker's tooling to read properly, and the code names a symptom rather than a cause. An owner can gather excellent evidence — conditions, readings, what changed, what the alarm said — and that evidence makes the technician's visit shorter and cheaper. Bleeding after filter changes also differs from older engines: many common-rail systems self-prime or have a specified procedure. Follow the manual's procedure exactly.

Question 9

Is white exhaust smoke always a head gasket?

Accepted Answer

No, and assuming so has cost plenty of owners an unnecessary repair. Cold vapour is normal on start-up and clears as the engine warms. Unburned fuel from low compression, a cold cylinder or an injector fault also shows white or grey and usually comes with poor running. Steam from water entering the combustion space or the exhaust looks similar again — but on a wet-exhaust marine engine, ordinary cooling water leaving the exhaust produces visible vapour on a cool day with nothing wrong at all. Read the context. Does it clear once warm, or continue? Is coolant level falling? Is there oil in the coolant or coolant in the oil? Does the engine run roughly, and on all cylinders? Is the smoke worse under load? Does the header tank pressurise or blow back? Those answers separate a cheap fix from an expensive one long before anything is dismantled. If coolant is genuinely being lost with no external leak, that is the point to bring in a technician for proper testing.

Question 10

Should I replace an old engine before cruising offshore?

Accepted Answer

Age alone does not decide it. Well-maintained engines of considerable age cross oceans; neglected young ones fail in the first week. Gather evidence before spending. Assess mechanical condition where it is warranted — compression or crankcase pressure testing, and oil analysis trends across several samples rather than one. Inspect the parts owners rarely look at: the exhaust elbow and mixing point, the heat exchanger, the raw-water circuit and everything aluminium below the waterline. Corrosion in the cooling and exhaust path strands more boats than worn bearings do. Then assess supportability. Can you get filters, impellers, belts, gaskets and a starter for this engine in the places you intend to cruise? Parts availability, not condition, is what turns a minor failure into a months-long delay in a remote port. Finally, assess the installation: mounts, alignment, wiring, fuel system, ventilation and the quality of previous repairs. A sound old engine in a poor installation is a worse bet than the engine alone suggests. Where the answers point to a repower, the surrounding systems usually need the budget as much as the engine does.

Question 11

Can I install a larger engine during a repower?

Accepted Answer

Sometimes, but it is rarely the simple upgrade it sounds like, and more power does not usually produce more speed on a displacement hull. A larger engine forces a chain of checks: structure and engine beds for the weight and thrust, gearbox and shaft or drive rating for the increased torque, gear ratio and propeller to convert the power without overloading the engine, exhaust sizing for the increased gas flow, raw-water and coolant capacity for the increased heat, engine-room ventilation for the increased air demand, and fuel supply and tank capacity for the increased consumption. Then the paperwork: the vessel's plate information, registration and survey status where it applies, and your insurer, who may reasonably want to know the installed power has changed. Weight and its distribution deserve particular attention on a sailing yacht, where a heavier engine in the wrong place changes trim and handling. In most repowers the right answer is a modern engine of similar output, correctly propped — that alone often delivers the improvement the owner was chasing.

Question 12

Do I need a marine generator?

Accepted Answer

It depends on two numbers: your peak AC load, and your daily energy consumption. Work both out before shopping, because a generator is heavy, noisy, expensive to install and needs servicing like any other diesel. Peak AC load is what decides whether you need one at all. Air conditioning, an electric galley, a watermaker or workshop tools can exceed what an inverter and battery bank will sensibly deliver. Daily energy decides how long it would run — and a generator that runs lightly loaded for hours is a generator that will foul, so it needs sizing to the real load rather than to the largest number in the brochure. Many cruising boats avoid one entirely with a larger alternator and proper regulator, solar, and a battery bank sized to the actual consumption. That combination is quieter, needs less maintenance and has no exhaust to corrode. The honest test is to log consumption for a few typical days aboard. Boats fitted with generators that turned out to be unnecessary usually skipped that step.

Question 13

Why does my fuel filter keep blocking?

Accepted Answer

Because the filter is doing its job. Something in the tank is producing contamination faster than a filter can hold it, and replacing filters treats the symptom while the source stays aboard. The usual source is water — from condensation, a leaking deck fill, a poor seal or contaminated fuel — sitting at the bottom of the tank. Water supports microbial growth, and the sludge that growth produces is what blocks filters, along with degraded fuel and the rust and debris that lift off the tank floor when the boat moves. Filters block far more often in rough weather because that is when the sediment gets stirred up. The fix works on the tank, not the filter: find and stop the water ingress, drain and remove water from the bottom of the tank, clean or polish the fuel, and inspect the tank interior where access allows. Serious contamination needs a proper tank clean, and a badly corroded tank may need replacement. Once it is clean, keep it that way — keep tanks full to limit condensation, use a water-separating primary filter and drain it on a schedule, buy fuel from busy suppliers, and treat with a biocide only as directed after the water is gone. Biocide added to a wet tank kills growth but leaves the dead matter behind, which blocks filters on its own.

Marine diesel FAQ