Fire Extinguishers

Almost all boats with engines, except certain small ones, must carry a minimum number of USCG-approved fire extinguishers (as described in 33 CFR 175e). The exceptions are boats that have no risk of fire from fumes or flammables, particularly inside enclosed spaces. A boat does not need a fire extinguisher only when all of the following are true:

Boat is under 26 ft
Has no inboard engine
Uses portable fuel tanks
Has no enclosed spaces that trap vapors or hold flammable items

What
type?

Extinguishers must be marked as USCG-approved Marine Type. They must be Class B (for flammable liquids like gasoline) and meet current UL standards (since 2022): 5-B or 10-B or 20-B. The number, 5 or 10 or 20, shows the square feet of flammable liquid fire that a trained operator can put out. It is also about twice the pounds of agent. A 10-B has twice the capacity of a 5-B while a 20-B has twice that of a 10-B. Older boats, pre-2018, may still use B-I or B-II type extinguishers if they are serviceable.

Sometimes the class name will have a ":C" appended. The C means that the extinguishing agent is nonconductive and can be used on energized electrical equipment. On most sailboats, a 20-B:C extinguisher is perfect, being much larger and more capable than the small units commonly found on runabouts and trailerable boats, while still small enough to be practical.

How
many?

Most powered boats need at least one extinguisher. Boats under 26 ft need only one. Boats over 26 ft need two. Boats over 40 ft need three. If the boat has an installed fire suppression system—called a fixed system—that counts as one extinguisher.

Fixed
system

Unlike a handheld extinguisher, a fixed system is mounted in place, usually in the engine room. It has a pre-installed discharge nozzle and it activates automatically, via heat, or manually by means of a pull cable or button at the helm. When activated, it floods the compartment with an extinguishing agent.
Most fixed systems use clean fluorocarbon agents like FM-200 or FE-241 or plain CO₂ (carbon dioxide). When a fire raises the temperature, a heat-sensitive element triggers the system, the agent is released, and it smothers the fire by removing oxygen or interrupting combustion. Many systems also shut down the engine and turn off blowers and fans to keep the agent inside the compartment.

Condition
and maintenance

Extinguishers must be readily accessible—not buried in a locker—and in good, serviceable condition, and not expired. Disposable units expire after 12 years. They must be properly charged with the gauge in the green, the pin in place, and no corrosion.