The Environmental Protection Agency (EPA) and the American Yacht and Boat Council (AYBC) are requiring devices that prevent foam and splash during fueling and operation from reaching the carbon canister. If sea water did reach the fuel filter, it could destroy filter effectiveness and compromise venting capacity.
The most common prevention device in the U.S. is the Raycor Lifeguard fuel/air separator. Suited to diesel or gasoline, this pint-sized unit installs vertically in the vent line above the tank and uses a series of baffles and a float valve to prevent fuel from exiting though the vent. Many boats are factory outfitted with these valves.
On smaller boats, builders often use P-traps instead to prevent splash and following seas from entering the fuel tank. Like the air/fuel separators, they use baffles to stop liquid flow. Generally very small, they are integrated into the through-hull fitting. They are not designed to block flow due to sustained immersion (like while heeling), so a high loop and a mounting location above any heeled waterline is still required.
On larger boats, a high vertical loop is simpler and more common, often combined with a splash-protected through-hull fitting. The Coast Guard requires a flame arrestor on gasoline tank vent lines, and this is often incorporated into the through-hull fitting.
On small powerboats and automobiles, high vent loops are not universal, and carbon cartridges may be installed below the fuel tanks. Such mounting is poor practice, in our opinion.
Air/fuel separators make the most sense to us, as foaming and overfills are all too common at fuel docks.