Guide to Marine Fire Extinguishers

Boat fires pose can spread fast and force you to abandon ship. Because fire is such a severe threat, federal laws and the United States Coast Guard require most boats to have fire extinguishers onboard. 

Before choosing what type of fire extinguisher to get for your boat, you should understand how both fires and extinguishers are classified. Here’s our guide to choosing marine fire extinguishers. 

Types of Boat Fires

Boat fire types are classified based on the combustible that fuels and feeds them. 

There are five types of marine fire, classified as follows:

• Class A: Involve solid combustibles including trash, paper, wood, fabric and many plastics
• Class B: Involve liquid and gaseous combustibles including fuel, solvents, propane and butane
• Class C: Involve energized electrical equipment that could shock a person, including wiring and live electrical power 

NOTE: Removing the live power reverts a Class C fire to one of the other fire classes.

• Class D: Involve metal combustibles including magnesium, potassium, sodium and lithium
• Class K: Involve cooking combustibles including oils, fats and greases

Because recreational boats are unlikely to experience Class D and K fires, we’ll focus solely on Class A, B and C fires. 

Classes of Fire Extinguisher

Effective firefighting requires the use of an extinguisher suited to the type of fire it’s fighting. For example, water is ideal for extinguishing burning paper (Class A fire), but using water on burning gasoline (Class B fire) will make the fire worse, and using water on an electrical fire (Class C fire) can result in an electric shock.

Various fire extinguishers exist to combat different types of fires, classed according to the types of fires they’re made to fight:

• Class A: An extinguisher suitable for use on burning solids
• Class B: An extinguisher suitable for use on burning liquids
• Class C: An extinguisher suitable for use on electrical equipment

Most marine fire extinguishers are suitable for use on more than one type of fire, so they’re classed accordingly (AB, BC, or ABC). The fire extinguisher on your boat should be at least a Class BC for gasoline/diesel fires and electrically charged fires, which are the most common boat fires. However, you should invest in a Class ABC fire extinguisher for use on virtually any type of onboard fire.

Marine Fire Extinguisher Effectiveness

Boat fire extinguishers also have numbers before the letters that classify them. These numbers are UL ratings that indicate the effectiveness of the extinguisher at putting out of each type of fire.

• The number before A indicates the effectiveness of the extinguisher against a Class A fire, as measured in units of 1.25 gallons of water
• The number before B indicates the effectiveness of the extinguisher against a Class B fire, as measured in square feet

For example, a 3A40BC fire extinguisher is capable of extinguishing the same amount of Class A fire that 3.75 gallons of water would, or of extinguishing an area of 40 sq/ft of Class B fire. 

Of course the higher the UL rating, the bigger the extinguisher. The fire extinguishers on your boat should strike a balance between physical size and extinguishing capability. 

Marine Fire Extinguishers and Federal Law

The federal laws pertaining to recreational boats are as follows:

• Any boat up to 26 feet in length must carry one B-I rated fire extinguisher

NOTE: Boats powered by outboard motors, with no permanent fuel tanks and no spaces for flammable gasses to collect are exempt from carrying a fire extinguisher.

• Any boat between 26-40 feet in length must carry two B-I rated fire extinguishers, or one B-II rated fire extinguisher
• Any boat over 40 feet in length must carry three B-I rated fire extinguishers, or one B-II rated fire extinguisher plus one B-I rated fire extinguisher

So what are B-I and B-II fire extinguishers? Unlike the UL ratings that are based on the extinguisher’s actual performance, the USCG classifies fire extinguishers based on the amount of extinguishing agent contained within.

• B-I fire extinguishers contain 1.25 gallons of foam, or 4lbs of CO2, or 2lbs of dry chemicals, or 2.5lbs of halon
• B-II fire extinguishers contain 2.5 gallons of foam, or 15lbs of CO2, or 10lbs of dry chemicals, or 10lbs of halon

We hope you never have to use a marine fire extinguisher, so be careful when refueling; regularly check the motor’s cooling system to prevent overheating; make sure the bilge is well ventilated; and never have naked flames on board.