On a gasoline boat the tank vent breathes both ways: vapour out while the tank is filled, air in as fuel is used. Under the US boat building rules, its outlet must stop a flame getting back into the tank and keep overflowing fuel out of the boat.
What the rule requires
33 CFR 183.520 says each fuel tank must have a vent system that keeps the pressure in the tank below 80% of the pressure on the tank label. Each vent must:
- have a flame arrester that can be cleaned, unless the vent itself acts as one
- keep fuel overflowing at up to 2 gallons a minute from entering the boat
The Coast Guard's Boatbuilder's Handbook explains that salt spray and debris collect on the arrester, so it must be possible to clean it from inside or outside, or by removing the fitting. Where overflow goes depends on the deck, the coaming, nearby air vents and hawseholes.
Signs of a blocked vent
The handbook says blow-back through the fill will occur if the vent is plugged. A vent line that's too small or restricted can also cause it at normal pump rates of 9 to 12 gallons a minute. It gives a 9/16 in inside diameter vent line, with fittings no smaller than 7/16 in, as generally enough, and stresses that the line must have no low points that trap fuel.
How to check
Find the vent fitting and look at its screen for salt or paint. Trace where fuel coming out of it would run, and follow the vent hose from the tank for sags. Ask the owner whether the tank blows back when filled.