Bilge Pump Buying Guide: What Size You Need, Auto vs Manual and Real GPH
Quick answer: Choose an automatic bilge pump sized for your boat (roughly 500 to 1,100 GPH under 18 feet, 1,100 to 2,000 GPH for 18 to 24 feet, 2,000 GPH and up for larger boats) and expect it to move only about 60 to 70 percent of its rated GPH once hose, lift and real battery voltage are in play. Add a second, higher-capacity pump mounted a little higher as a backup, and wire the automatic switch so it works even when the battery switch is off.
A bilge pump is the safety gear most boaters never think about until water is already coming in. A good setup keeps rainwater out of the bilge at the dock, handles spray and washdown underway, and buys you critical time if a hose pops or a fitting fails. A bad setup, an undersized pump or a dead float switch, is how boats sink at the slip. This guide covers sizing, automatic versus manual, switches, plumbing and wiring.
Step 1: Size by boat length, then size up
| Boat length | Primary pump (rated GPH) | Recommended backup |
|---|---|---|
| Up to 18 ft | 500 to 1,100 | Manual pump or second small automatic |
| 18 to 24 ft | 1,100 to 2,000 | Second automatic, mounted higher |
| 24 to 32 ft | 2,000 to 3,500 | High-capacity automatic plus high water alarm |
| 32 ft and up | 3,500 or more, often multiple pumps | Pumps in each compartment plus alarm |
These are starting points. Boats with large open cockpits, livewells, or lots of rain exposure should go bigger.
Step 2: Understand real-world output
Pump GPH ratings are measured with no hose, no lift and full charging voltage. On your boat, every foot of vertical rise, every foot of hose, every bend and fitting, and a battery at 12 volts instead of 13.6 cuts real flow. Losses of 30 to 40 percent are normal. If you need 1,000 GPH of real pumping, buy at least a 1,500 GPH pump.
Step 3: Automatic, manual or both
- Automatic pumps turn on by themselves using a float switch, an electronic water sensor, or a timed sensing cycle. This is what protects an unattended boat at the dock.
- Manual pumps only run when you flip the helm switch. Good as a backup or for drying the bilge on demand.
- Best setup: an automatic primary in the lowest point of the bilge, plus a larger second pump mounted a few inches higher, and a high water alarm so you know when the backup kicks on.
Step 4: Plumb it so it can actually pump
- Match the hose to the pump outlet, commonly 3/4, 1-1/8 or 1-1/2 inch. Never neck down to a smaller hose.
- Use smooth-bore bilge hose to reduce friction, and keep runs short with gentle bends.
- Discharge through a thru-hull well above the waterline, and route the hose up in a loop so water cannot siphon back in.
- Avoid check valves in bilge discharge lines. They clog and cut flow.
- Double-clamp with all-stainless clamps.
Step 5: Wire it to work when it matters
- Wire the automatic float switch circuit directly to the battery with its own fuse, so the pump works even when the main battery switch is off.
- Use tinned marine wire sized for the pump's amp draw and run length, and heat shrink connections, especially in the bilge.
- Use a three-way helm switch (Auto/Off/Manual) for control and testing.
Our wire and terminal guide and fuse guide cover the details.
Step 6: Test it every trip
Lift the float switch or pour water into the bilge and confirm the pump runs and water exits the thru-hull. Clean the strainer base of hair, zip ties and debris. Plan on replacing pumps every few years or as soon as flow drops or the motor runs hot.
Common bilge pump mistakes
- Buying on rated GPH alone and ignoring hose, lift and voltage losses.
- Running one pump with no backup and no alarm.
- Automotive wire in the bilge. It corrodes fast. Use tinned marine wire and sealed connections.
- Float switch in the wrong spot, where sloshing triggers it constantly or debris jams it.
- Never testing it. Many owners find out their pump is dead only when they need it.
Our picks by boat size
All in stock at Boat Supplies Direct.
Small boats and skiffs
Rule Rule-Mate 500 Fully Automated Bilge Pump or the Attwood Sahara S500 Automatic.
Center consoles and bowriders (18 to 24 ft)
Rule 1500 GPH Automatic Bilge Pump or the Johnson Pump 2200 GPH Auto with Mag Switch.
Larger boats and backup pumps
Rule 2000 GPH Automatic, the Rule 3700 Automatic, or the Rule 4000 Non-Automatic paired with a float switch.
Switches, alarms and panels
Johnson Pump Automatic Float Switch, Rule Hi-Water Bilge Alarm, and the Attwood 3-Way Auto/Off/Manual Switch.
Hose
Trident Marine 1-1/8 in Bilge and Livewell Hose.
Frequently Asked Questions
What size bilge pump do I need for a 20 foot boat?
A 1,100 to 2,000 GPH automatic pump is a solid choice for most 20 foot boats, plus a backup pump or a manual pump.
Why does my bilge pump move less water than its rating?
Ratings are measured with no hose, no lift and full charging voltage. Hose length, fittings, height and battery voltage often cut real output by 30 to 40 percent.
Do I need an automatic bilge pump?
Yes for any boat left in the water. An automatic pump protects the boat when nobody is aboard.
Should a bilge pump be wired to the battery switch?
The automatic circuit should be wired directly to the battery with its own fuse so it works even when the main switch is off.
Where should a bilge pump be mounted?
At the lowest point of the bilge for the primary pump, with a backup pump mounted a few inches higher.
How often should I replace my bilge pump?
Test it every trip and replace it every few years, or sooner if flow drops, it runs hot, or the float switch sticks.
This guide is general information. Follow your boat builder's and pump manufacturer's instructions and ABYC standards, and use a qualified marine technician for electrical work.
