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Analyzing Electrical Fires On Boats

Your electrical system gives you lights, communications, cold drinks, and much more. But it's also the place that most boat fires start.

Extinguishing boat fire

Photo: BoatUS Marine Insurance

We analyzed five years of BoatUS Marine Insurance claim files and found that the boat's DC electrical system — batteries, lights, wiring, and so on — cause more than a third of all fires. The boat's AC shore power system contributes to another 9 percent. More than half of DC electrical fires, or 19 percent of all fires originating on boats, were associated with either the engine or the batteries, both of which tend to be in the engine room. That's because there are so many things that can burn in the engine room — fuel, oil from a slow leak, or even, in the case of a gasoline engine, gas fumes — once a DC wiring problem creates enough heat. In addition, that's where a lot of high-load devices are — things like the engine starter and battery chargers. These higher amperage loads create more heat where there are undersized wires, loose or corroded connections, or intermittent shorts. Finally, the vibration from the engine increases the likelihood of chafe in such vulnerable areas as the wiring harness and connections to the alternator and the starter.

Causes Of Boat Fires, Excluding Off-Boat Sources

Causes of Boat Fires chart

Preventing these fires comes down to good electrical maintenance on every component of the DC system associated with the engine and the batteries. Regular maintenance — on a monthly basis during the boating season — should include ensuring all connections are tight, especially those from the batteries to the starter to the alternator, making sure wiring is supported and secured to minimize the impact of vibration, keeping battery terminals clean, and inspecting wiring for signs of chafe. If you've just bought a used boat, check the wiring especially carefully for twist-on connectors and improper house wire. Boats require crimp connectors and more flexible stranded wire.

Damaged wires and hoses

Wires have to be protected from anything that can chafe or burn them. All wires, especially in the engine room, should be properly supported away from hot surfaces.

Having 120-volt alternating current (AC) power to run air conditioners, refrigerators, and water heaters usually means we have to plug in to shore power. Even if you don't have any of these luxuries on your boat, you quite likely still plug in to charge the batteries. In the marine environment, the plugs and inlets/outlets in the shore-power system are vulnerable to dirt, salt, corrosion, and moisture, any of which can cause arcing that damages the contacts and eventually leads to increasing resistance and heat buildup.

Shore power cable damage

Check shore-power cords frequently for burned or discolored ends.

In addition to the cord itself, the data pinpointed a particularly vulnerable link in the shore-power chain: the shore-power inlet on the boat — and not necessarily the entire inlet, but the terminals at the back of the inlet where the boat's wiring is connected. These inlets are particularly vulnerable to water intrusion, and the connections are subject to vibration and corrosion and are often surrounded by material that ignites easily. They should be pulled out and inspected at least every five years. If there's any sign of corrosion, replace them. Also, if there's any discoloration on the shore-power connections, it means they've been hot in the past and should be replaced.

Electrical heater fire

Every year, electric heaters cause boat fires.

Electric heaters continue to be a major source of AC electrical fires aboard. While safer heaters have been developed that are less prone to being tipped over or to igniting anything combustible that falls on them, many still draw a great deal of power, and any corrosion in the shore power system (or worse, household extension cords powering heaters) will tend to build up heat somewhere that can result in a fire. Never use heaters in lieu of winterizing, and never leave a heater running if there is no one aboard.

Exploded boat battery

Automotive-style battery chargers can overcharge batteries over time, which can damage them and even start a fire.

Finally, battery chargers make big demands on shore-power cords and connections. Using an automotive battery charger instead of a marine battery charger can easily lead to fires aboard. High-quality marine battery chargers are not only designed for the marine environment with potted components that resist water intrusion, but they also use multistage charging regimes and temperature sensors to make sure the batteries get the right amount of current at each stage of the charging cycle. Automotive chargers can easily overcharge batteries, boiling away the electrolyte eventually starting a fire.

6 Ways To Prevent Boat Fires

Here are six things you can do to reduce your risk of fire aboard your boat:

  • Inspect all electrical connections associated with the engine-starting and -charging systems at least once a month during boating season. Tighten loose connections, replace corroded wire ends and terminals, secure and support all wiring to protect it from chafe and vibration, and replace battery switches that are more than 15 years old.
  • Inspect battery wiring, switches, and terminals at least once a month during the boating season. Clean terminals and connectors at the beginning and end of the season.
  • If your boat is more than 25 years old and the wiring harness and starter are original, consider replacing both. Lots of fires begin due to these old components.
  • Inspect shore power cords, and replace them if pitting, corrosion, or burning is visible on the plug's blades or if the cord itself shows any signs of wear, including kinked or pinched areas. Black electrical tape is no substitute for intact insulation!
  • Inspect the rear of your boat's shore power inlet every five years, and address any corrosion in the terminal or the wires that attach to it.
  • Never use an electric heater when no one is aboard.

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Charles Fort

Contributing Editor, BoatUS Magazine

Charles Fort is BoatUS Magazine's West Coast Editor. He often writes local news items for BoatUS Magazine's Waypoints column and contributes to Reports, in-depth tech features in every issue written to help readers avoid accidental damage to their boats. He is a member of the National Association of Marine Surveyors, he's on ABYC tech committees, and has a 100-ton U.S. Coast Guard license. He lives in California.