What class of fire extinguisher should be onboard a vessel with a permanently installed fuel tank?

A vessel with a permanently installed fuel tank requires a Class B fire extinguisher because this type of extinguisher is specifically designed to combat fires involving flammable liquids, such as gasoline or oil. Since vessels commonly use fuels that are categorized as flammable liquids, having a Class B extinguisher on board is essential for safely addressing potential fire hazards associated with fuel spills or engine fires.

Class A extinguishers are suitable for ordinary combustibles like wood and paper, Class C extinguishers target electrical fires, and Class D extinguishers are for combustible metals, which are not relevant to typical marine fuel scenarios. Thus, a Class B extinguisher is the appropriate choice for addressing the specific risks associated with a vessel's fuel system.