Which type of fire extinguisher should be on board a vessel with a permanently installed fuel tank?

A Type B fire extinguisher is the appropriate choice for a vessel with a permanently installed fuel tank because it is specifically designed to extinguish fires involving flammable liquids, such as gasoline or oil. In a boating environment, where such fuels are commonly used, having a suitable fire extinguisher is crucial for safety. Type B extinguishers use substances that can effectively snuff out flames caused by burning fuels, minimizing the risk of fire spreading and ensuring the safety of both the vessel and its occupants.

The other types of extinguishers cater to different types of fires: Type A is for ordinary combustibles like wood and paper, Type C is for electrical fires, and Type D is for combustible metals. While these types have their specific uses, they are not designed for dealing with fuel-related fires, making Type B the essential choice for situations involving flammable liquids on boats.