BOAT-COMPARATOR Guide

Marine Heatwaves and Motorboats: The Overheating Risk, and How to Avoid It

Unusually warm sea water cools engines less efficiently: what a marine heatwave changes for a rental boat's engine, and the right precautions to take.

During a marine heatwave, surface water temperatures can run well above the seasonal norm. For a motorboat, that's not just a comfort issue on board: sea water also cools the engine, and warmer water cools it less effectively.

How a marine engine cools itself

On a RIB or a day cruiser, the cooling circuit draws in sea water through an intake under the hull to carry heat away from the engine, before discharging it (often visible at the exhaust outlet). The warmer that water is to begin with, the less efficient the heat exchange — pushing the engine closer to its limits, especially during prolonged low-speed running, idling at anchor, or slow manoeuvring in a crowded harbour.

Warning signs to watch for

A weaker or hotter-than-usual cooling water jet at the exhaust, a temperature alarm triggering sooner than expected, or an unusual smell are signals to take seriously: cut the engine, check that the water intake isn't blocked by seaweed or a plastic bag, and contact the rental operator before continuing.

Good habits in hot weather

• Avoid long idling periods at anchor; switch the engine off once anchored rather than leaving it running.
• Favour early morning or late afternoon outings, when surface water is slightly cooler and the area less crowded.
• Check the cooling water intake before starting up if the boat has been anchored near seagrass or an algae-prone spot.
• Report any alarm or smell to the rental operator immediately rather than pushing through on the way back.

A phenomenon that affects the crew too

Heat is as hard on people as it is on the mechanics: our guide on sun and heat protection on board complements this advice, and our guide on where to sail when the sea is too warm helps you pick a cooler spot if the heatwave drags on.

Renting with the full picture

Professional operators maintain their fleets, but a quick check before you leave costs nothing: ask whether the cooling circuit has been checked recently, especially on older models. Then compare the available offers on Boat-Comparator, which shows Boataround, SamBoat, Click&Boat and GlobeSailor listings in one search.

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