The boat engine will not start

An engine that refuses to start just as you are about to leave the harbour is a boating classic. Before taking anything apart, work out which case you are in: the starter does not turn at all, it turns but the engine does not “catch”, or the engine starts and then stalls. Each case has its own causes, and most can be found without tools. This guide is mainly about inboard diesels in sailboats and motorboats; the reasoning partly applies to a petrol engine or an outboard. Your engine manufacturer’s manual always has the last word.

1. What you observe

• Nothing happens when you turn the key or press the button: no sound, a single sharp “clack”, or a rapid clicking. It is almost always electrical.
• The starter turns slowly, laboriously, and the engine never reaches the speed it needs to fire: weak battery or poor connection.
• The starter turns normally, but the engine does not start: most often the fuel, air in the fuel system or the glow plugs.
• The engine starts and then stalls after a few seconds or a few minutes: often a fuel supply that cannot keep up.
• The starter strains, stops dead or makes a knocking noise: this is the case to take seriously, see below (water in the cylinders).

2. Safety first

In the harbour, an engine that will not start is an annoyance. At sea, near a coast, a breakwater or in a channel, it is first of all a safety matter: before looking for the fault, get the boat out of danger — sails up if it is a sailboat and there is room, anchor ready to drop, crew informed. If you are drifting towards a hazard, call for help on VHF (channel 16) without waiting to find the cause.

• Do not keep cranking. Short attempts (around ten seconds, or what the manual says), with a pause between them. The starter overheats and the battery drains.
• Several failed attempts: close the raw-water seacock. On an engine with a wet exhaust, the pump sends seawater into the exhaust at every attempt, but as long as the engine is not running, the exhaust gases do not push it out: it builds up and can rise all the way to the cylinders. Seacock closed during the attempts, and reopened as soon as the engine runs — otherwise it overheats within minutes.
• No starting spray on a diesel fitted with glow plugs: risk of an explosion inside the engine and serious damage.
• Petrol engine: ventilate the engine bay before every start, and do not start if you can smell petrol.
• To touch terminals or cables, engine stopped and key removed.

3. Likely causes, from most to least common

The starter does not turn, or turns badly

• Battery switch — set to “off”, or to the house battery instead of the engine battery.
• Flat engine battery — a light or a fridge left on, an old battery, an alternator that no longer charges. See also batteries that keep going flat.
• Neutral safety switch — many engines only start with the lever in neutral; a lever barely engaged is enough to block everything.
• Loose or corroded terminal, on the battery, the starter or the engine ground: not enough current gets through to turn the starter.
• Fuse, circuit breaker or key switch on the engine panel.
• Relay or the starter itself — pinion seized by damp, worn brushes. Less common, but a classic after a long lay-up.

The starter turns, the engine does not start

• Stop control left pulled out — on engines stopped by a pull cable, it cuts the fuel until it is pushed fully back in.
• Not enough preheating in cold weather, or faulty glow plugs.
• Tank empty or nearly so — inaccurate gauge, or heel uncovering the fuel pick-up.
• Fuel valve closed.
• Air in the fuel system — after a filter change, running dry, or a fitting that lets air in.
• Clogged filter — water-separating pre-filter or fine filter, often after a rough sea has stirred up the sediment at the bottom of the tank.
• Water in the fuel or “diesel bug” — micro-organisms that grow between the water and the diesel and form a black sludge that blocks the filters.
• Less common: lift pump, injection pump, injectors, or compression too low on a very worn engine.

The engine starts and then stalls

• Fuel that cannot keep up — a partly blocked filter, a small air leak: the engine starts on what is left in the pipes, then runs out of breath.
• Blocked tank vent — the small pipe that lets air in as the fuel is used; if it is blocked (insects, salt), the tank goes into vacuum and the fuel stops arriving.
• Idle speed too low or throttle control out of adjustment.
• Propeller fouled by a rope — the engine stalls as soon as you put it in gear.

The starter stops dead

• Water in the cylinders — after long series of attempts, a perforated exhaust elbow, an overfull exhaust muffler or an installation with no protection against water flowing back. Water does not compress: cranking on can bend an internal engine part. Leave it alone and call a mechanic.
• Seized engine after a very long lay-up — rare.

4. What you can safely check yourself

In this order, from the simplest to the longest:

• The lever in neutral and the stop control pushed fully in.
• The battery switch in the right position (engine battery, or both combined if your installation allows it).
• The engine battery voltage, on a voltmeter or the display, engine stopped: a charged lead-acid battery shows about 12.6 V at rest; at around 12.2 V it is half empty and struggles to turn a diesel. A lithium battery reads differently: follow its instructions.
• The terminals, key removed: tight, clean, with no green or white deposit.
• The fuse or circuit breaker on the engine panel.
• The preheating: the time given in the manual, longer in cold weather.
• The actual fuel level (sight glass, dipstick) and the fuel valve open.
• The water separator bowl: water or sludge at the bottom? Drain it into a container, with rags under the filter.
• The tank vent: if the engine stalls, open the filler cap for a moment. A rush of air, then an engine that starts again, point to a blocked vent.
• Bleeding the air from the fuel system with the manual priming pump and the bleed screws, if the manual describes the method and you have done it before. Never loosen the injectors’ high-pressure pipes with the engine running: the jet of diesel can pierce the skin.
• If the engine stalls when put in gear: a look at the propeller, in the harbour, engine stopped and key removed, and only if you can do it safely.

5. When to stop and call a professional

Stop there if the starter stops dead or knocks, if the oil level has risen or the oil looks milky (water or diesel has got into the engine), if the starter smokes or smells hot, or if the engine still will not start after the checks above. The starter, the alternator, the wiring beyond the terminals, the injection pump, the injectors, compression testing and, of course, water in the cylinders are a job for a marine mechanic — or an electrician for the electrical side. To help them, give the make and model of the engine, its hours, when it started (first start of the season, after refuelling, after a rough sea, after a filter change), exactly what you observe (silence, clack, slow starter, engine stalling) and what you have already checked, with the results: voltage measured, state of the water separator, bleeding done or not. A short video with sound helps a lot.

6. Stopping it from happening again

• Change the fuel filters on schedule and drain the water separator several times a season; keep a set of filters on board.
• Lay the boat up with a full tank: less air, less condensation, less water in the fuel.
• Keep the engine battery separate from the house battery, test it every season, and protect the terminals.
• Run the engine under load from time to time, rather than long idling sessions at the pontoon.
• Have the exhaust elbow checked on schedule: once perforated, it lets water reach the cylinders.

The detailed due dates are in the guide to maintaining an inboard engine, and getting going again after winter in the one on laying up and recommissioning. Once the fault is fixed, record it in the Hullteam log: the cause, the parts replaced, the date and the engine hours. It is this record, kept up over the years, that will give the boat its value at resale — see which proof really counts.