What to do if your engine overheats at sea

Throttle back to idle immediately, assess whether you can safely stop, then shut down. A marine diesel destroys itself within minutes of running hot — a distorted head, a failed gasket or a seized turbocharger — and the repair costs many times more than a tow. The engine is almost never worth saving the trip.

The exception is a lee shore or a shipping channel, where losing propulsion is more dangerous than damaging the engine. Judge that first.

In the first minute

1. Reduce to idle. Load is what generates the heat, and idling buys you time to think.
2. Assess your situation before shutting down. Sea room, traffic, wind direction, anchoring depth. Losing way in the wrong place is worse than an engine.
3. Get the anchor ready if the water is shallow enough to use it.
4. Tell someone. A position report before you lose power is worth far more than one afterwards.
5. Shut down once it is safe. Do not keep running to “get a bit closer”.

What not to do

Do not open the coolant cap. A hot marine engine’s coolant system holds pressure, and the contents are well above boiling. Opening it is one of the more common serious injuries in boating.

Do not immediately add cold water to a hot engine even once the cap is safe to open. Rapid cooling can crack a head.

Do not assume it will be fine once cooled. An engine that has been properly hot has probably suffered, and running it home may finish the job.

Once it has cooled

Work the raw water circuit from the sea inwards. Most overheats at sea are intake-side, and several are fixable aboard:

1. Check the exhaust for water flow on a brief restart. No water means do not run it.
2. Check the seacock is open.
3. Clear the strainer. A plastic bag, weed or a jellyfish over the intake is the single most common offshore cause and the easiest fix.
4. Check the impeller. Carry a spare and the cover gasket — this is the repair most often completed at sea, and how to change an impeller is the procedure.
5. Check the belt to the raw water pump where fitted.
6. Check the coolant level once genuinely cool, and look for an obvious leak.

If you find and fix an intake blockage, restart and watch the temperature closely at low revs before loading the engine.

If you cannot fix it

Do not run the engine. Options in order:

• Sail, if you have rig
• Anchor, if the depth allows, and wait for conditions or assistance
• Call for assistance early rather than late. A tow arranged in daylight in moderate conditions is a different event from one at night in a rising sea

Preventing the offshore version

The faults that strand people offshore are almost all visible beforehand:

• An impeller past its interval
• A heat exchanger never cleaned — how to tell if yours is blocked
• A strainer not checked before departure
• A cooling system running “slightly warm” and ignored for a season

Frequently asked questions

Should I keep running to get home if the engine overheats? No. An overheating marine diesel damages itself within minutes, and the repair costs far more than a tow. The exception is when losing propulsion is itself dangerous.

Why must I not open the coolant cap? The system holds pressure and the coolant is above boiling point. Opening a hot cap causes serious scald injuries.

What is the most common offshore overheat? A blocked intake or strainer — weed, a plastic bag, or a jellyfish. It is also the easiest to fix aboard.

What spares make this fixable at sea? A spare impeller and cover gasket, plus the tools to fit them, and a means of clearing the strainer.

Can I restart after fixing a blockage? Yes, but watch the temperature closely at low revs before loading the engine.