Frustrated by the bilge partially refilling after the pump stops? Adding a check valve on the pressure side of the pump can help reduce the “backwash”. See the photo. (Many boaters believe that check valves are not favored in bilge pump systems. They reduce flow, are notorious for failing and the added obstruction increases the chance of a stoppage. If you install one, inspect it frequently. – Ed. Note) Another alternative is to replace the OEM pump with an 'automatic' pump (Rule has one) that has no float switch and mounts directly in the bilge (minimizing backwash by eliminating the bilge to pump hose run). They work by performing 'check runs' every few minutes; if the impeller senses resistance (water) the pump continues to run and pump out the bilge. No impeller resistance means no water, so the pump shuts off within a couple seconds. The Rule pump I bought and refer to is rated at 500 gph, draws 1.9 Amps, and is much quieter than the OEM pump.
Warren Updike, Warr De Mar, #62 and Bob Seastream, Intuition, # 906