33 CFR §183.520

1. (a)Each fuel tank must have a vent system that prevents pressure in the tank from exceeding 80 percent of the pressure marked on the tank label under § 183.514(b)(5).
2. (b)Each vent must:
   1. (1)Have a flame arrester that can be cleaned unless the vent is itself a flame arrestor; and
   2. (2)Not allow a fuel overflow at the rate of up to two gallons per minute to enter the boat.