Which vessels are required to have backfire flame arrestors?

Backfire flame arrestors are important safety devices designed to prevent the spread of fire from the engine compartment to the rest of the boat. This is particularly relevant for powerboats, as they typically have internal combustion engines that can produce a backfire.

The requirement for backfire flame arrestors applies to all powerboats except those that are equipped with outboard engines. This is because outboard motors are designed in such a way that the risk of backfire spreading into the boat is significantly reduced, given that the engine and fuel system are mounted externally. However, powerboats with inboard engines or those with engines that are installed within the boat must have these devices to provide an additional layer of safety.

In contrast, the requirements do not extend to sailboats or fishing boats as a general category without regard to their engine type, which is why those choices do not apply. The delineation is based on the risk associated with the type of engine and the vessel's design. Thus, the correct answer highlights the specific safety measure that is mandated for a significant portion of powerboats, aligning with recognized safety standards in boating.