33 C.F.R. § 183.520

(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).

(b) Each vent must:

(1) Have a flame arrester that can be cleaned unless the vent is itself a flame arrestor; and

(2) Not allow a fuel overflow at the rate of up to two gallons per minute to enter the boat.