A check valve may be installed in the discharge line of a bilge pump. It can prevent cycling of the pump. (Water in the hose when the pump shuts off runs back into the bilge, floats the switch and turns the pump back on.) A check valve can also prevent back-flooding through the discharge fitting if the fitting becomes submerged as it might when launching a boat off a trailer at a steep ramp, or if aground and listing. But check valves can clog with salt and debris, so they must be accessible, and boaters should make it a habit to inspect these for proper operation regularly.