Heating an Engine Room or Bilge in Winter (Boats Kept in the Water)

A heater makes sense when:

• You still use the boat in winter and can't fully winterize between trips, or
• The boat is winterized and you want extra protection for gear that can't be drained.

It does not make sense as the only protection on an unattended boat. If you won't visit the boat for weeks, winterize it. Our winterizing guides walk you through it for each engine type.

Federal rules (33 CFR 183.410) require electrical parts on gasoline boats to be ignition protected, meaning they won't ignite fuel vapor around them, unless they're isolated from fuel sources. An engine room is the last place to cut corners. Look for a heater made for engine compartments and labeled as meeting USCG ignition protection and UL 1500. For example, Caframo says its Pali engine-compartment heater meets those standards, uses a self-regulating element that can't overheat, and turns itself on and off with a sealed thermostat.

Household heaters, even "marine" cabin heaters, are not built for this. Diesel boats have less vapor risk, but the fire risks from tipping, lint, oil-soaked rags and cords are the same, so use an engine-room heater there too.

Engine-room heaters are rated for a compartment size and outside temperature. Measure your engine compartment (length x width x height) and choose a heater rated for it. A big cruiser may need two smaller units rather than one large one. Bigger is not better: more watts means more load on the shore power cord and inlet, the place where most dockside electrical fires start.

A heater runs for hours, which stresses every connection between the pedestal and the outlet.

• Use a marine shore power cord only. Replace it if the plug blades are pitted or darkened, the jacket is cracked, or the plug feels warm in use.
• Check the boat's shore power inlet. BoatUS fire claim analysis found that on boats over 10 years old, the back of the inlet, where the boat's wiring connects, is a high-risk spot. If the inlet is original on an older boat, have it inspected or replaced.
• Never plug the heater into a household extension cord or power strip. Caframo, for one, says not to use an extension cord with its heater.

Water and 120V power together can kill. Plug the heater into a boat outlet protected by a GFCI, and if possible, make sure the whole boat has an ELCI (a whole-boat leakage detector, now required by ABYC on new boats with shore power). Test the GFCI with its button before each cold season. If the GFCI or ELCI trips, don't keep resetting it: something is leaking current to ground. BoatUS's electric shock drowning advice applies here too: keep people out of the water around docks with shore power.

• Mount the heater with the maker's bracket so it can't tip over from a wake or wind.
• Keep it off the bilge floor and above the highest water level the bilge ever reaches. Caframo warns against placing its heater low where water collects.
• Keep it clear of fuel lines, hoses, wiring bundles, insulation, rags, cardboard and anything else that can melt or burn. Follow the clearances in the heater manual.
• Aim the airflow at the parts most likely to freeze: the raw-water side of the engine, the strainer, the water heater and plumbing.

Run the heater cord so it can't be pinched by a hatch, chafed on a sharp edge or soaked in bilge water. Don't run it through a door or hatch that closes on it. Coil any extra loosely. Check that the plug is fully seated in the outlet; a loose plug overheats.

On a cool day, turn on shore power and confirm the heater cycles on. Know your heater's built-in thermostat point first: Caframo says its Pali's sealed thermostat switches on at about 45°F, so on a mild afternoon it may sit idle and look "dead" when it's working as designed. Test when the engine room is below the switch-on point, or follow the maker's test procedure. Use a thermometer or a wireless sensor to check the engine room actually stays above freezing. Smell for hot plastic and feel the cord and plug after an hour of running: they should be barely warm, never hot.

Assume the power will fail in the worst storm of the year. Your backup plan:

• Winterize what you can. Drain or add antifreeze to anything you don't need, such as the water heater, washdown and head plumbing.
• Remote monitoring. A boat monitor can text you if shore power drops, the temperature falls or the bilge water rises.
• Someone close by. Arrange for a marina staffer or neighbor to check the boat and close seacocks or drain the engine if a freeze is coming and power is out.
• Keep batteries charged so bilge pumps keep working if the shore power fails.

Check the boat after every hard freeze and every storm: heater running, cord cool, no burned smell, bilge dry, pumps working, lines and fenders in place. Look for condensation and mold too. BoatUS notes heaters can increase mildew by causing condensation on cold surfaces such as metal portlights and hatches, so keep some ventilation open.

When you finally winterize fully or haul out, unplug and remove the heater. Store it dry and check it for damage before next season.