Standard naval vessels carry several types of fire extinguishers, each matched to specific fire hazards found onboard. The most common portable units use potassium bicarbonate dry chemical (known as PKP or Purple-K Powder), while larger fixed systems rely on carbon dioxide, foam, or clean gaseous agents to protect engine rooms and machinery spaces. The type you need depends entirely on what’s burning: fuel, electrical equipment, or ordinary combustibles.
PKP: The Go-To Portable Extinguisher
Potassium bicarbonate powder, called PKP aboard ship, is the standard portable extinguisher for flammable liquid fires on naval vessels. It comes in 18-pound and 27-pound sizes. The 18-pound unit has an effective range of 19 feet, while the 27-pound version reaches 21 feet. PKP works by breaking the chemical chain reaction that sustains a flame, knocking down fire quickly on contact.
PKP is designed for immediate response to small, isolated fuel fires covering less than 10 square feet. It’s meant to be used by whoever is closest to the fire when it starts, before the situation gets out of control. If a pool fire grows beyond that 10-square-foot threshold, portable PKP extinguishers alone are unlikely to put it out.
There are two important limitations. PKP has no cooling capability, so hot metal surfaces or electrical arcs can reignite the fuel after the powder settles. It also doesn’t leave a lasting blanket of inert atmosphere over flammable liquids the way foam does. For these reasons, PKP is a first-strike tool, not a sustained suppression method. It should also be kept away from electrical cabinets with sensitive relays and contacts, since the powder can damage delicate electronics.
AFFF for Fuel and Flight Deck Fires
Aqueous Film-Forming Foam, or AFFF, is the primary agent for large-scale flammable liquid fires, especially those involving jet fuel. AFFF works by spreading a thin film across the surface of burning liquid, cutting off oxygen and preventing reignition. This makes it far more effective than PKP for sustained suppression of fuel fires that have spread beyond what a portable extinguisher can handle.
On aircraft carriers, AFFF is a critical component of flight deck firefighting. Twin Agent Units (TAUs) combine PKP and AFFF into a single system: the PKP knocks down the visible flame instantly, and the AFFF follows up with a sealing layer that prevents the fire from coming back. TAUs require at least two firefighters to deploy and a third to manage hose when the distance exceeds 20 feet. Testing by the Naval Research Laboratory found that using water instead of AFFF solution in hand lines increased the time needed to control and extinguish fires by as much as 50%, which underscores why foam is the preferred agent for fuel fires at sea.
Carbon Dioxide for Enclosed Spaces
Carbon dioxide systems are the standard fixed suppression method for engine rooms, paint lockers, and flammable liquid storage areas on both Navy and commercial ships. CO2 extinguishes fire by displacing oxygen in an enclosed space. A typical naval engine room of about 250,000 cubic feet uses roughly 10,000 pounds of carbon dioxide in its suppression system, with some large marine engine rooms requiring as much as 20,000 pounds.
Because CO2 displaces breathable air, strict safety protocols govern its use. U.S. Coast Guard regulations require that systems holding more than 300 pounds of CO2 include a delayed discharge of at least 20 seconds after the alarm sounds, giving personnel time to evacuate. Two separate manual controls must both be activated before CO2 releases into a protected space, preventing accidental discharge. International maritime rules under SOLAS similarly prohibit automatic release of gaseous suppression agents in most situations and require audible warnings before any discharge.
Portable CO2 extinguishers are also carried aboard for electrical fires. CO2 leaves no residue, making it safe for use around electronics, switchboards, and navigation equipment where PKP powder would cause damage.
Clean Agent Systems Replacing Halon
For decades, Halon 1301 was the preferred gaseous flooding agent in naval machinery compartments. It was extremely effective and left no residue, but its production was banned under international environmental agreements because it damages the ozone layer. The Navy has been transitioning to replacement agents, with heptafluoropropane (HFC-227ea) as a leading substitute. The Naval Research Laboratory has tested this agent extensively in ship compartment configurations, evaluating its performance against fires involving low-flash-point liquids. These clean agent systems work similarly to CO2 by flooding a sealed space, but they’re less toxic to personnel and require lower concentrations to suppress fire.
Class D Agents for Metal Fires
Combustible metal fires, classified as Class D, require specialized dry powder agents. These fires involve metals like magnesium or titanium, which burn at extremely high temperatures and react violently with water. The correct extinguishing agent is a dry powder formulated specifically for the metal involved. Class D fires are most relevant on vessels that handle aviation components or ordnance containing reactive metals. Standard water, foam, or CO2 extinguishers will not work on metal fires and can make them significantly worse.
Lithium-Ion Battery Fires
As naval vessels increasingly carry equipment powered by lithium-ion batteries, a newer fire risk has emerged. Battery thermal runaway produces intense heat, toxic gases, and can reignite repeatedly even after flames appear to be out. Standard CO2 suppression systems have limited effectiveness against these fires. Water applied in large volumes remains the most practical cooling method for containing battery thermal runaway, since the primary challenge is removing heat from cells that generate their own oxygen as they decompose. This is an evolving area of maritime firefighting where existing extinguisher types don’t offer a clean solution.
How Extinguisher Types Map to Fire Classes
- Class Alpha (ordinary combustibles like wood, paper, fabric): Water is the primary agent, providing both flame suppression and cooling.
- Class Bravo (flammable liquids and fuels): PKP for small initial fires under 10 square feet. AFFF for larger or sustained fuel fires. Twin Agent Units combining both for flight deck scenarios.
- Class Charlie (electrical equipment): Portable CO2 extinguishers for localized electrical fires. CO2 or clean agent flooding systems for larger electrical spaces.
- Class Delta (combustible metals): Specialized dry powder matched to the specific metal burning.
Every gasoline- and diesel-powered boat in naval service is required to have fire extinguishers in place and charged. These are listed as standard outfit items alongside boat hooks and other essential gear, and they must be removed before any vessel is turned in for maintenance or disposal. The specific number and size of extinguishers carried depends on the vessel’s size, mission, and the types of fuel and equipment aboard.