Gasoline vapour is heavier than air and settles in the bilge. One spark from a starter motor can light it. The engine compartment blower pulls that vapour out before the engine starts, and on US-built gasoline boats it's a federal requirement.
What the rule asks
33 CFR 183.610 applies to each compartment with a permanently installed gasoline engine with a cranking motor. Unless the compartment is open to the atmosphere, it must be ventilated by an exhaust blower system. The rule sets:
- Capacity. By net compartment volume: a blower rated at 50 cfm for compartments under 34 cubic feet, rising with the volume above that.
- The intake. Each intake duct in the lower one-third of the compartment and above the normal level of bilge water.
- The label. Near each ignition switch, in plain view, warning that gasoline vapours can explode and to operate the blower for 4 minutes and check the bilge for vapours before starting.
Our 33 CFR 183 ventilation page covers the rest of subpart K.
How a surveyor checks it
Switch the blower on and feel for a strong flow at the outlet. Follow the duct from the blower to its intake: it should be connected and uncrushed, with its intake low in the compartment, above the bilge water. Look for the label at each ignition switch. Sniff the bilge before anything else is switched on.
What decides the grade
Whether the vapour can be cleared. No working blower in an enclosed gasoline engine compartment is an A. A weak blower, a loose or crushed duct, or an intake in the wrong place is a B. A missing label is a C.