The U.S. Coast Guard requires all recreational boats that use inboard or inboard/outboard engines for electrical generation, mechanical power, or propulsion to be equipped with a ventilation system. Gasoline vapors can collect in the bilge and explode, which is why you also need to keep the bilge clean and free of trash.
Types of Ventilation Systems
There are two types of ventilation systems: natural and powered. A natural ventilation system has a supply opening located on the exterior surface of the boat. The air comes in through the opening as the boat moves through the water. As the air passes through the bilge, it carries any fumes from the engine out through an exhaust duct into the atmosphere.
Natural ventilation system preventing vapors from collecting in the bilge
If your boat was built after 1980, every compartment that has a permanently installed gasoline engine with a remote starter will have a powered ventilation system. This ventilation system has at least one exhaust blower that carries the engine fumes outside of the boat.
Ventilation System Use
If your boat has a gasoline-powered inboard or inboard/outboard engine with a powered ventilation system, you must run the blower before starting the engine. Let the blower run for at least 4 minutes, and then check the engine compartment bilge for gasoline vapors. If the blower has run long enough, you won't be able to smell any gasoline vapors. Only then is it safe to start the engine. Electronic sniffers which detect explosive gasses are a highly recommended safety item.
If your boat has an outboard gasoline engine and a built-in fuel tank, the same safety precautions used for inboard or inboard/outboard engine must be followed. Any spark, such as that from a bilge pump or other electric device can cause an explosion of flammable gasses lingering in the bilge.
Be especially careful after refueling, since the fueling operation may have introduced vapors from around the boat into the bilge via the ventilation system.
Diesel-powered or electrically-powered propulsion systems do not have any explosive vapors.
If the galley has a propane or other form of fuel-powered stove, similar explosion precautions should be followed. Always turn the fuel supply on at the storage tank. The fuel storage tank should always be located in an outboard, well-ventilated location, and you should always check for combustible odors after using. Explosive gas detectors are again highly recommended.
Ventilation System Maintenance
You are responsible for making sure the ventilation in your boat is working properly. Keep all openings free of obstructions, and be sure the ducts are not blocked or torn. If your boat also has a powered ventilation system, check the blowers for correct operation, and replace worn components with equivalent marine-type parts.
Make certain all electrical connections in the bilge (and everywhere in the boat) are secure to avoid any sparks at loose wiring connections. Do not use any open frame brush-type DC motors, since they always generate sparks where the brushes contact the rotating armature; these motors are inappropriate for marine use.