Marine fuel backflow preventer and over-fill alarm system

A marine fuel backflow preventer and overfill alarm for use with marine vessels includes a housing provided for in-line installation in the fuel fill inlet line and directs fuel entering the a fuel filling inlet generally directly to a fuel filling outlet such that the fuel passes freely through the back flow preventer while being directed to the vessel fuel tank. The housing further functions as a reservoir for containing fuel which has accumulated therein due to over-filling and/or that has been carried into the housing via the fuel tank vent line. By providing a fuel containing reservoir disposed in-line the present invention thus functions to prevent unintended fuel spillage into the environment. The back flow preventer further includes an electro-mechanical float switch that automatically activates an alarm upon sensing the presence of fuel within the housing. The alarm provides the fuel filling attendant a prompt warning that an overflow condition is being experienced such that the flow of fuel may be terminated prior to spillage into the environment.