Overheat Alarm: What to Do in the First 60 Seconds

As soon as the alarm sounds, ease the throttle back toward idle, if it's safe where you are. Less load means less heat. Glance around for traffic, shallows and the shoreline before you slow down or turn.

Many outboards will cut power on their own when they sense overheating; Yamaha notes that most of its outboards have an RPM reduction mode for this. Don't fight it by pushing the throttle forward.

This is the single most important check:

• Outboard: look at the telltale stream on the side or back of the lower cowl. Yamaha points out that some outboards show little or no stream at idle even when everything is fine, and it appears once RPM rises a bit. So a missing stream at idle is a warning sign, not proof. If it's weak or gone and the alarm is on, treat it as no flow.
• Sterndrive, inboard or diesel: look over the transom at the exhaust outlet. You should see and hear water coming out with the exhaust. White "smoke" that looks like extra exhaust may actually be steam, which BoatUS describes as a sign that not enough water is going through the engine.

If there is no normal water flow, turn the engine off as soon as it's safe to do so. Don't let it idle in neutral "to cool down": that's exactly how the Coast Guard case turned into a fire.

If you're drifting toward danger once the engine is off, anchor, put on life jackets and call for help (step 11). A drifting boat is a problem you can manage; a fire usually isn't.

If cooling water looks normal, the problem could be partial (a worn impeller moving less water), a thermostat, or the gauge itself. Before you decide it's the sensor:

• Look at oil pressure and voltage. BoatUS notes that on some installations a change in voltage at the gauge, such as switching on the panel lights, can nudge the temperature reading up with no real problem.
• Watch the temperature for a minute at idle. If it keeps climbing, shut down anyway. If it settles back to normal, continue to step 10 before running at speed again.

Steam inside the boat usually means a cooling hose or engine leak spraying onto hot metal. Smoke or a hot-plastic smell means something is burning.

If you suspect fire, keep the hatch closed, shut off the engine (and the fuel supply if you have a valve you can reach safely), have the extinguisher ready and call for help. Don't let steam distract you from checking for fire; that is exactly what happened in the Coast Guard case.

Once the engine is off and there are no signs of fire at the hatch, open up and look, carefully:

• The engine compartment, then the compartments aft of it (lazarette, under the cockpit sole, under seats) and anywhere the exhaust hose passes through a bulkhead.
• Look and feel for heat with the back of your hand held near the hose, or use an IR thermometer. A wet exhaust hose and the water jacket of the exhaust parts should stay cool enough to touch in normal running; a hose that is too hot to touch, soft, discolored or smoking has lost cooling water.
• Check what's touching the hose: wood supports, stored gear, cushions or fuel lines.

The Coast Guard stresses this step for any sign of engine trouble, because exhaust and fuel lines often run through spaces forward or aft of the engine room where nobody is looking.

BoatUS warns that the engine temperature often keeps rising after shutdown, because the cooling water has stopped. Wait until it cools before working on it, and don't restart a hot engine: restarting at a high temperature can ruin it.

On a closed-cooled engine (one with a coolant tank or heat exchanger), never open the coolant cap while it's hot. Pressurized coolant can spray out and burn you.

With the engine cool, start where the water comes in and follow it:

• Outboard and sterndrive: tilt up and look for a plastic bag, weeds or mud over the water intake grates on the lower unit. Clear a blocked outboard telltale outlet with a piece of wire. Early in the season, think insects too: BoatUS's spring commissioning checklist warns that bees and wasps build small, easy-to-miss nests in outboard telltale outlets and water intakes over the winter, which is one reason it recommends a test run on flush muffs or in a test tank (never dry) before the first launch.
• Inboard and diesel: make sure the seacock is open. Then close it and check the raw-water strainer for weeds or debris; a plastic bag can also get sucked onto the outside of the hull intake, especially at slow speed. Open the seacock again before restarting.
• Look along the cooling hoses for one that has split or slipped off its fitting. BoatUS notes this can also flood the boat; a split hose may be patched for the trip home with rescue tape and a clamp.

• Sterndrive, inboard and diesel: look for a broken or thrown drive belt. On many engines the belt drives the circulating pump (and sometimes the raw-water pump), so a broken belt means a fast temperature rise. Fit your spare if you have one.
• Impeller: a slow, steady temperature rise over several trips often points to a tired impeller, and a sudden overheat after running in shallow, sandy or weedy water can mean it has just failed. Replacing it on the water is a job only for owners who have done it before at the dock (see the impeller guides).

If you found and fixed something (cleared an intake, cleaned the strainer, replaced a belt), restart and look for cooling water within the first few seconds. On many boats, shifting briefly into reverse at idle helps blow debris off an intake. If water doesn't appear quickly, shut down again: running the pump dry, even briefly, damages the impeller.

If flow and temperature return to normal, head in at a slow, steady speed and keep watching the gauge. BoatUS notes that partly clogged exhaust passages often show up as overheating only at higher speed or under load, so don't jump straight back to cruising speed.

If you can't find the cause, or the temperature climbs again, stay shut down. Anchor if you're drifting toward shallows, rocks or traffic, and call TowBoatUS or Sea Tow. Use VHF channel 16 or 911 if anyone is in danger, the boat is taking on water, or you see smoke or fire.

Before the next trip, find out why it happened: a worn impeller, a clogged strainer or intake, a broken belt, scale in the exhaust parts or a failed thermostat. Then:

• Follow your engine maker's schedule for the raw-water pump impeller. BoatUS insurance claims show overheating as a real cause of exhaust fires and recommend changing the impeller every other year, after a grounding, or after running in very dirty water (typical; confirm in your manual).
• Have the exhaust hoses and the exhaust parts checked if the engine overheated badly.
• Keep the raw-water strainer clean. The Coast Guard points out that an impeller's life depends on it: a strainer clogged with weeds, plastic or growth starves the pump and wears the impeller out early.
• On closed-cooled engines, ask your mechanic about the heat exchanger (and the oil cooler, if fitted). The Coast Guard notes they need periodic service so they don't foul or leak.
• Practice this routine with your crew. The Coast Guard encourages engine casualty drills, not just fire and man-overboard drills, so everyone knows what to do without thinking.