A bilge pumping system is used to remove water from a ship's inner hull. This is important, as water in the hull is a leading cause of sinking (hmm...) Bilge pumps have a capacity ranging from 2500 GPH (gallons per hour) for a small sailboat, to 10,000 GPH or more for a larger 50-60ft boat.

Bilge pumps may be manually activated, or set to trigger at a certain water level. It's generally a good idea to have a bilge pump in each hull compartment that has no free flow of water to another compartment (i.e., may fill up with water and weigh the boat down).

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Information from David H. Pascoe's writing on the subject at http://marinesurvey.com/yacht/bilge_pumps.htm