Rule's non-automatic cartridge series bilge pumps offer traditional operation activated by a float or panel switch. fully automatic bilge pumps feature computerized operation on the principle of impeller resistance and do not require float switches. Bilge pumps are used to remove bilge water (the bilge is the lowest compartment inside the hull of a ship, where the two sides meet at the bottom). Fuel can mix with the bilge water, therefore, electric bilge pumps are designed to be non-sparking. Electric bilge pumps are often fitted with float switches which turn on the pump automatically when the bilge fills to a set level. Because bilge pumps can fail, a backup pump is recommended. The primary pump should be located at the lowest point of the bilge, while the secondary pump should be located somewhat higher. This activates the secondary pump only when the primary pump is overwhelmed and keeps the secondary pump free of the debris in the bilge that tends to clog the primary pump.