Depending on the position of the fuel tank relative to the engine, the USCG requires the use of anti-siphon valves in the fuel (to engine or generator) supply lines. In the event of a break in the fuel line, these valves prevent the fuel in the tank from being siphoned out of the tank and into the boat.
Pressure Relief Valves
Most pressure relief valves are normally closed. They are set to open automatically when a predetermined pressure level is reached. The tolerance requirements on the accuracy of opening at the correct pressure can affect the cost of the valve with tighter tolerances resulting in increasingly more expensive valves. Pressure relief valves can be mounted directly onto the tank, installed as in-line valves in a section of hose or even built right into a fuel cap.
These vent system devices are used to provide either full or partial ullage (air) space outside of the fuel tank. Tanks designed to provide full ullage will be approximately 6% of the volume of the fuel tank. They can provide airspace without sacrificing range and can lessen problems associated with providing access to fuel system components as required by USCG regulations. There are trade-offs in packaging space and total system costs.