Boat Fires

Understanding where, when and how most fires start is the first step in reducing the risk of an onboard fire.

By Beth A. Leonard

Photo of damage from a neighboring boat's fire
The boat above was a casualty of a neighboring boat's fire — when
one boat burns, it puts at risk nearby boats, property, and people.

It takes two things to start a fire: something that will burn and something that will start it burning. Every fire must have, in fire-investigator parlance, a fuel source and an ignition source. When BoatUS Marine Insurance turns a claim over to a fire investigator, his or her task is to find both. They don't always succeed — about eight percent of the time, no cause can be assigned. But it should come as no surprise that, in those cases where a cause can be determined, at least half of all boat fires originate around the motor. That's because, when it comes to boats, fuel and ignition sources are most likely to come together in the engine room or under the cowl of an outboard.

Figure 1: Causes Of Fires On
Recreational Boats, 2009-2013
Causes of fires on recreational boats graph
Source: BoatUS Marine insurance Claim files

What might come as a bigger surprise is how often the boats BoatUS insures are casualties of someone else's fire. More than a quarter of the time, our insured's boat burns when something else goes up in flames: The marina, the storage facility, the house, the garage, the barn, the neighbor's house (Figure 1). In more than 70 percent of those cases, it's the marina that burns. While the cause of those fires cannot always be assigned, based on what we do know, a high percentage start on someone else's boat. That means that every boat owner has a responsibility to prevent fires on board — not just to keep the boat safe, but also to keep anyone aboard safe, and to keep the marina where the boat is kept, and the people who work there, safe.

So what can you do to protect your boat from fire? The Seaworthy editors analyzed five years of BoatUS Marine Insurance claim files to understand the major causes of fires aboard boats to determine what owners can do to minimize the chances of a fire. Understanding the causes of fires aboard begins by understanding where exactly in the boat the fire originated. We found that the places where fires originated, and the specific causes as to how they started, depend upon whether your boat has an outboard or an inboard engine. In this article, we'll focus on fires on inboard and inboard/outboard (I/O or sterndrive) boats. We'll take a look at outboard boats in our next issue.

Figure 2: Causes Of Fires On Inboards and I/Os
Excluding Off-Boat Sources, 2009-2013

Causes of fires on inboards and I/Os excluding off-boat sources graph
Source: BoatUS Marine insurance Claim files

Inboard boats tend to be larger than outboards, and the areas around the engine, where most of the action takes place, are not visible and accessible in the way that an outboard engine is. These larger boats are also more likely to be stored at marinas, which makes them more vulnerable to the marina fires that are a major source of BoatUS's total fire losses. Whether underway or tied up to a dock with no one aboard, it's easier for a fire that gets started on a larger, inboard boat to gain some serious momentum before somebody notices it. Because, as can be seen in Figure 2, most of the fires can be traced back to maintenance issues in the DC electrical system, the AC electrical system, and the engine (particularly the engine cooling system), a regular maintenance schedule combined with attention to critical components in each of these systems can have a huge impact on reducing the incidence of fire aboard inboard boats. So what exactly causes the fires in these different areas?

DC Electrical Fires

Thirty-five percent of the fires that originated on a BoatUS-insured boat were caused by problems in the 12-volt DC electrical system (Figure 2). But the fire that starts in the wiring under the headliner or behind a panel in the galley is the exception, not the rule. More than half of DC electrical fires, or 19 percent of all fires originating on BoatUS inboard-powered 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 some heat. In addition, starting the engine and charging the batteries generate significantly higher amperages than those in most other areas of the boat. These higher 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.

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 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.

Beyond good maintenance, there are three other steps owners can take to prevent DC electrical fires on inboard boats. Wiring harnesses and starters account for the majority of DC electrical fires on boats 25 years old or older. If you have an older boat and the starter and wiring harness are original, consider replacing them. Another problem area has to do with hooking up the batteries at the beginning of the season. Every year, we see cases where the battery cables were reversed or the batteries were hooked up in parallel instead of in series. See Alerts for some suggestions on ways to avoid this slap-yourself-in-the-head mistake.

Photo of battery cables incorrectly connected causing a fire
Reconnecting battery cables incorrectly can cause all kinds of havoc
in the electrical system, including a fire.

Outside of the engine room, there is no single area where most of the remaining DC electrical fires originate. Locations are pretty much equally spread across electrical panels, instrument panel gauges, bilge pumps, lights, and equipment of various types, including air conditioners, windlasses, and winches. Most of these fires could have been prevented if adequately sized, marine-grade wiring had been used and loose or corroded connections had been located and addressed.

AC Electrical Fires

To have 120-volt, alternating current (AC) to run our air conditioners, our refrigerators, and our water heaters aboard, we usually 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 shorepower system are vulnerable to dirt, corrosion, and moisture, any of which can cause arcing that damages the contacts and eventually leads to increasing resistance and heat buildup. Seaworthy has explored the hazards of shorepower electrical connections before (see "When Your Shorepower Loses Its Cool," July 2010 [PDF 1.19M]). In addition to the cord itself, the data pinpointed a particularly vulnerable link in the shorepower chain: the shorepower inlet on the boat. And not 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.

Electric heaters, another hazard Seaworthy has addressed before (see "The Boater's Guide to Winterizing" [PDF 2.57M]), 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, they still draw a great deal of power, and any corrosion in the shorepower system (or worse, household extension cords powering heaters) will tend to build up heat somewhere that can result in a fire. BoatUS continues to recommend not using heaters in lieu of winterizing, and never to leave a heater running if there is no one aboard.

Photo of a fire started by an auto battery charger
Automotive-style battery chargers don't have the safeguards to protect your boat while the battery is charging, especially long-term.

Finally, battery chargers are much like heaters, but in addition to the demands they make on the shorepower cords and connections, using an automotive battery charger instead of a proper 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 multi-stage charging regimes and temperature sensors to make sure the batteries get just the right amount of current at each stage of the charging cycle.

Other Engine-Related Fires

Fifty-six percent of non-electrical engine fires result from failures in the cooling system, while problems in the exhaust system account for another 20 percent. Engines overheat when not enough water circulates to keep the engine at its proper operating temperature. Reduced water flow usually happens for one of three reasons: debris blocks the water intake or sediment enters the cooling system, scale or marine growth restricts water flow in the heat exchanger or hoses, or the water pump ceases to circulate adequate water, most often due to an impeller failure. Impeller failure can result from sediment in the raw water, so it pays to replace your impeller if you know you were running in particularly dirty water. Otherwise, replacing impellers every other year and flushing cooling systems every season will help to prevent these kinds of fires. If the engine ever overheats, before getting underway again, take the time to check the engine room to make sure the engine is at operating temperature and everything is normal.

Photo of an overheated exhaust system
Overheated exhaust systems are usually caused by lack of
cooling water — another reason to replace impellers every two years.

Exhaust-system fires almost always result when something flammable comes into contact with something hot on the exhaust. That can happen when the cooling system fails so that no water is available to cool the hot engine gases. But they can also occur when cooling water is still circulating, such as when a backfire displaces the flame arrestor, or where the water-cooling injection system in the exhaust fails. Inspecting the exhaust system regularly and replacing the exhaust manifold every five years will help you avoid most of these fires.

Other Causes Of Fire

As Figure 2 shows, of the fires that originated on a BoatUS-insured boat in the claim files, no cause was assigned to 10 percent of our total fires. A quarter of the fires in Figure 2 fall into the categories of fuel and other. While no single cause stands out in these categories, there are a few takeaways:

  • Fuel. The majority of fuel fires come from fuel leaks due to failures of hoses or hose clamps. Wiping down hoses with a rag and smelling it can alert you to a slow leak that hasn't yet caused a serious problem. If the rag smells of gasoline or diesel, either you have a leak or the hose is old enough that it is becoming porous and needs to be replaced.
  • Stoves. The incidence of fires due to stoves has decreased with the gradual replacement of alcohol stoves with propane stoves and electric ranges. Two percent of fires were caused by stoves, more than half resulting from problems with lighting alcohol stoves. Given how few alcohol stoves there are on boats these days, they are significantly more dangerous than those that use other fuel sources. If you still have an alcohol stove on board, you may want to consider upgrading. Most people agree that they don't heat very well, anyway.
  • Shrinkwrapping. BoatUS gets a few fires every year associated with installing shrinkwrapping. These can be particularly problematic because the fires can spread quickly from boat to boat in a crowded marina hardstand area. This is a job that we'd prefer you leave to professionals.

Detection and Suppression

Preventing fires is just one leg of a three-legged stool that can minimize fire damage. Professionals label these three legs prevention, detection, and suppression. If prevention fails, you need to know that a fire has started and then you need to be able to put it out as quickly as possible. As the statistics from the BoatUS claim files show, fires that spread are a huge hazard to other boats. Even more, a fire that gets out of control in a marina can easily endanger lives, including those of the firefighters called to put the fire out.

Detection: Smoke detectors on larger boats with living quarters can provide the kind of early warning that could save a boat and possibly a marina. However, a detector that goes off in the engine room isn't enough because the chances are it will not be heard if you're in the cockpit underway or if the boat is closed up at the dock with no one aboard. Connecting a smoke detector to an alarm and a light at the helm station will alert you if you are underway. In the marina, the smoke detector can be hooked up to an alarm on deck. Or, if you're into gadgets, you can buy a security system that will send an alarm to your phone. These require some kind of connectivity to work and may not function if shore power is cut, so they're not as reliable as something that depends only on ship's power.

Suppression: If prevention fails, you don't just want to be alerted to the fire; you need to have a way to put it out. The Coast Guard requires that you carry a certain number and size Type B extinguishers aboard, depending upon the length of your boat (Tables 1 and 2). Type B extinguishers are designed to put out fuel-based fires, while A are for non-fuel combustibles (paper, wood), and C are for electrical fires. However, by choosing a Type ABC extinguisher, you won't have to think about what type of fire it is when you pick it up — you can just point and shoot.

Table 1. Fire Extinguisher Sizes

TypeFoam (gallons)CO2 (pounds)Dry Chemical (pounds)
Source: A Boater's Guide to the Federal Requirements for Recreational Boats, United States Coast Guard
B-I (Type B, Size I)1.7542
B-II (Type B, Size II)2.51510

Table 2. Fire Extinguisher Minimum Carriage Requirements

Vessel LengthNo Fixed SystemWith Approved Fixed System
Source: A Boater's Guide to the Federal Requirements for Recreational Boats, United States Coast Guard
Less than 26 feet1 B-I0
26 Feet to Less Than 40 Feet2 B-I or 1 B-II1-BI
40 Feet or 65 Feet3 BI or 1 B-I and 1-B-II2 B-I or 1 B-II
For more information about fire extinguishers, visit

When it comes to fire, you don't want to carry just the minimum size or quantity, especially on a larger boat. A B-I extinguisher will last for only about 10 to 15 seconds before you'll be left holding an empty canister. You'll want an extinguisher within reach in the engine room, in the galley, and near a diesel or propane heater if you have one aboard. We'd also recommend having one in each sleeping compartment so you can escape if a fire breaks out in the main cabin. A fire blanket for a stove can also be an effective way to put out a small fire without having to spray corrosive powder all over the galley.

Given the high incidence of fires in the engine area, the engine compartment should be fitted with a fire port so that you can suppress the fire without having to open the compartment, introducing more oxygen and further feeding the fire. Larger boats with real engine rooms should be equipped with automatic fire-suppression systems.

Ten Takeaways

Fire prevention comes down, above all, to maintenance. Here are 10 things you can do to reduce your risk of fire aboard your inboard boat:

  • Inspect all electrical connections associated with the engine-starting and charging systems at least once a month during the 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, replace both, if possible.
  • Inspect shorepower cords, and replace them if pitting or corrosion is visible on the plug's blades or if the cord itself shows any signs of wear, including kinked or pinched areas. Replace cords that are over 10 years old. Black electrical tape is no substitute for intact insulation!
  • Inspect your boat's shorepower inlet every five years, and address any corrosion in the terminal or the wires that attach to it. Replace the inlet every 10 years.
  • Never use an electric heater when no one is aboard.
  • Use only high-quality marine battery chargers on your boat.
  • Flush your cooling system annually.
  • Change your impeller every other year or anytime the engine has ingested debris.
  • Replace your exhaust manifold every five years or at the first sign of pinhole corrosion.

If every inboard-powered boat owner checked off every item on this list, not only would the risk of fire on any given boat decrease by more than half, but every boat that spends time in a marina would be safer.End of story marker

Published: October 2015

Seaworthy, the damage-avoidance newsletter, is brought to you by the BoatUS Marine Insurance Program. For an insurance quote, please call 1-800-283-2883 or apply online at

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