What are the classes of fire and which extinguishers are used on ships?

Fires are classed by the fuel involved because the extinguishing agent must match the fuel; the wrong agent can spread the fire or electrocute the firefighter. The classes used at sea are:

• Class A: ordinary solid combustibles such as wood, paper, bedding, rope and accommodation furnishings.
• Class B: flammable liquids, fuel oil, diesel, lube oil, paint and grease, the dominant machinery-space risk.
• Class C: flammable gases such as LPG, acetylene from welding sets, or cargo vapours on gas carriers.
• Class D: combustible metals such as magnesium or aluminium swarf, rare but needing special dry powders.
• Class F (K in US practice): hot cooking oils and fats in the galley.

Electrical fires are treated as a separate hazard: once the equipment is isolated the fire is fought according to what is actually burning, but while it is live it needs a non-conducting agent such as CO2 or dry powder.

Agent selection follows from the fire triangle of fuel, heat and oxygen. Water extinguishers cool Class A fires well but must never go onto live electrics or burning oil, where a jet scatters the oil and a hot fat pan can erupt violently with steam. Foam forms a smothering blanket over liquid fires and resists reignition, making it first choice for oil spills and save-alls. CO2 displaces oxygen and leaves no residue, ideal for electrical panels and machinery, but it gives little cooling so reflash is possible, and discharging it in a confined space asphyxiates; never use CO2 portables in a small unventilated compartment without a clear escape path. Dry powder knocks down Class A, B and C fires fast and is safe on live gear, but visibility drops to nothing and it gives no cooling. Wet chemical saponifies burning fat and is the only safe agent for deep-fat fryers.

Practical discipline matters as much as theory. Know the location and type of every extinguisher on your muster route, check pressure gauges and seals during weekly rounds, and remember a portable gives roughly 10 to 60 seconds of discharge, enough for a small fire only. If the first extinguisher does not kill it, close the door, raise the alarm and let the fire party fight it properly. SOLAS drills exist so the choice of agent is automatic when the smoke is real.