The ideal bilge-pump system does not exist. Boats come in all sorts of sizes and are put to all sorts of uses in a variety of different environments. However, after decades of boating I have developed a few prejudices, offered here as a sort of buyer’s guide to bilge pumping.
I recommend the following total-bilge-pump capacities as rules of thumb. Boats less than 20 feet: 1,000 gph; 20 to 25 feet: 2,500 gph; 25 to 32 feet: 4,000 gph; 32 to 36 feet: 6,000 gph; 37 to 45 feet: 8,500 gph.
The number embossed on the pump is optimistic. There are formulas for calculating actual gph versus rated gph based on hose diameter, the height that the water has to be lifted (the "head") and voltage. I simply count on an actual capacity of 60 percent the advertised rating.