How to Size a Bilge Pump for Your Boat (Real-World GPH Explained)
The number on the box is the open-flow rating, measured with no hose, no lift and full voltage. In a real installation the pump pushes water up and through a hose and a thru-hull, often on a battery that isn't fully charged. Real-world output is commonly half or less of the rating. That's why sizing up is the smart move.
What reduces real flow
- Head height: every foot the water is lifted cuts flow.
- Hose length and bends: long runs, corrugated hose and tight bends add resistance. Use smooth-bore hose and the shortest run you can.
- Voltage drop: undersized wire or a weak battery slows the motor. Wire to ABYC standards and fuse it properly.
- Check valves: generally avoid them on bilge discharge lines unless required, since they restrict flow and can stick.
Rule-of-thumb sizing
| Boat | Primary pump | Notes |
|---|---|---|
| Jon boats, skiffs, bass boats under 18 ft | 500–750 GPH | Automatic pump keeps rain water out at the dock |
| Center consoles, bay boats 18–24 ft | 750–1100 GPH | Consider a second, higher-mounted pump |
| 24–30 ft offshore boats | 1100 GPH+ primary | Add a high-water backup pump and alarm |
These are general guidelines. Your hull's design, cockpit drainage and builder's recommendation come first.
Automatic pump or pump plus float switch?
All-in-one automatic pumps like the Rule-Mate series sense water electronically, so there's no mechanical float to jam with debris. They're our top pick for most small to mid-size boats.
Rule-Mate 500 GPH Automatic Bilge Pump, 12V$69.99, view details →
Rule-Mate 750 GPH Automatic Bilge Pump, 12V$84.99, view details →
Rule-Mate 1100 GPH Automatic Bilge Pump, 12V$109.99, view details →
A separate pump and switch is easier to service in pieces. If the switch fails you replace the switch, not the pump. Electronic switches like the Johnson Ultima avoid sticking floats entirely.
Attwood Sahara 1100 GPH Bilge Pump, 12V$59.99, view details →
Johnson Pump Ultima Electronic Bilge Switch, 12/24V$54.99, view details →
Installation tips from our shop
- Mount the pump at the lowest point of the bilge, secured so it can't tip.
- Wire an automatic float-switch circuit directly to the battery through a fuse, so it works even when the battery switch is off.
- Test monthly: lift the float or add water.
- Keep the strainer clean. Most "dead" pumps are just clogged.
Blue Sea Systems ST Blade Fuse Block, 12 Circuits with Negative Bus$49.99, view details →