A bilge pump is the boat component you spec once, install once, and ignore until the day water comes in. The day always arrives: a heavy rain at the slip, a stuffing box that started weeping, a wave that washed over the transom. The right pump moves water fast enough that you find a soggy bilge instead of a sunken boat. After looking at 17 current 12 volt bilge pumps from the major marine brands, these seven stood out for real GPH at 3.3 feet of head, switch design, and long-term motor life in salt. The lineup covers small runabouts, fishing boats, sailboats, and small yachts.
Quick comparison
| Pump | Rated GPH | GPH at 3.3 ft | Switch | Warranty |
|---|---|---|---|---|
| Rule 1100 GPH | 1100 | 660 | External float | 3 years |
| Johnson Aqua Void 750 | 750 | 480 | Electronic | 3 years |
| SeaFlo 1100 | 1100 | 580 | None (add float) | 2 years |
| Attwood Sahara S500 | 500 | 320 | Built-in float | 3 years |
| Rule-Mate 2000 | 2000 | 1250 | Built-in electronic | 3 years |
| Whale Supersub 650 | 650 | 410 | Built-in electronic | 5 years |
| Jabsco 36600 Diaphragm | 240 | 220 | Pressure switch | 2 years |
Rule 1100 GPH, Best Overall
Rule has owned the bilge pump category for decades and the 1100 GPH model is the default for a reason. Centrifugal pump, 1100 GPH rated, around 660 GPH at 3.3 feet of real head, and a separate external float switch that mounts away from the pump so debris in the bilge does not jam it. The motor housing is sealed and the impeller is replaceable.
The real strength is parts availability. Every marine supply store from Florida to the Pacific Northwest stocks the 1100 GPH and its replacement parts. Lose an impeller mid-season and you have it the next day. The 3 year warranty is solid and Rule honors it on motor failure.
Trade-off: the external float switch is a separate purchase and a separate failure point. It is the right design (better than built-in floats for debris tolerance), but it adds 25 dollars and a second wire run.
Johnson Aqua Void 750, Best Electronic Switch
The Aqua Void replaces the mechanical float switch with an electronic water sensor that detects water contact and switches on the pump. No moving parts to jam, no float arm to break, and no oily film coating that prevents float activation. The pump draws around 2.5 amps in operation and 5 mA in standby.
For a sailboat or any boat that sits in the slip for weeks at a time, electronic switching is the right pick. The pump activates within seconds of water touching the sensor and shuts off cleanly when the bilge is dry, which prevents the dry-running motor wear that plagues some built-in float pumps.
Trade-off: the 5 mA standby draws a small but constant current. On a boat with no shore power, plan on a battery top-up every 4 to 6 weeks.
SeaFlo 1100, Best Budget
SeaFlo offers a 1100 GPH centrifugal pump at roughly half the price of the Rule equivalent. Real flow at 3.3 feet of head is around 580 GPH, slightly behind Rule but within striking distance. The build is good for the price; sealed motor, replaceable impeller, and a 2 year warranty that has held up.
For a freshwater inland boat, weekend warrior boats, or a backup pump in a two-pump install, SeaFlo is the practical pick. No float switch included; budget for a Rule 35A float switch separately.
Trade-off: parts availability is regional. Easy to find online but harder to walk into a local marina store and grab. Plan ahead for spares.
Attwood Sahara S500, Best for Small Runabouts
The S500 packages a 500 GPH centrifugal pump with a built-in mechanical float switch in a single compact unit. For a 14 to 17 foot runabout or fishing boat, this is the right size and form factor. Real flow at 3.3 feet runs around 320 GPH, which clears rainwater fast enough that you find a dry bilge after a storm.
The integrated float switch keeps the install simple: two wires (positive and negative) and the pump handles everything else. The base mounts with two stainless screws.
Trade-off: built-in float switches sit in the bilge with the pump, which means debris that floats around the bilge can jam them. Keep the bilge clean and inspect the float every season.
Rule-Mate 2000, Best for Larger Boats
For boats 22 feet and up or any boat carrying significant ballast or fuel weight, the Rule-Mate 2000 brings serious capacity. Built-in electronic switch (no float arm), 2000 GPH rated, around 1250 GPH at 3.3 feet of head. The pump auto-cycles every 2.5 minutes to sample for water and turns on if it finds it.
The auto-cycle is the differentiator. No mechanical float to jam, no constant electronic draw (the unit sleeps between cycles), and a pump that finds water within 2.5 minutes of intrusion. Standby current is 1 mA.
Trade-off: the auto-cycle pump moves a small amount of air on each sample, which can be audible from inside the cabin. Mount it under floorboards or in an enclosed bilge compartment to keep noise down.
Whale Supersub 650, Best Low-Profile
The Whale Supersub is the slimmest serious bilge pump on the market. The pump body is 2.5 inches tall, which means it fits under floorboards, in shallow bilges, and in tight engine compartments where a standard cylindrical pump will not. Electronic switch, 650 GPH rated, around 410 GPH at 3.3 feet, and a 5 year warranty that no one else matches.
The low profile is the reason to buy it. On a sailboat with a shallow keel sump or a runabout with a flat bilge floor, the Supersub fits where Rule and Attwood pumps do not.
Trade-off: pricier than other 650 GPH pumps and parts are British-sourced, so impeller replacements take a week if not in stock locally.
Jabsco 36600 Diaphragm, Best for Dry Bilge
The 36600 is a diaphragm pump rather than a centrifugal, and the reason to buy one is dry-priming capability. The pump can run dry without damage, lifts water from up to 10 feet vertically, and pumps the bilge down to almost zero water. Self-priming pressure switch.
The Jabsco is the pump to use in a boat where you cannot tolerate any standing water (fiberglass deck above the bilge, electronics mounted low) and you need the bilge truly dry. Flow rate is low at 240 GPH but the pump finishes the job.
Trade-off: 4 to 5 times the cost of a centrifugal pump and lower flow rate. Use it as a complement to a high-flow centrifugal pump, not as a replacement.
How to choose
Real flow at head, not rated GPH
Rated GPH is at zero lift, which never happens on a real boat. Real performance at 3.3 feet of head is 50 to 65 percent of rated. Size up one bracket from what you calculate.
Switch reliability matters more than flow
The pump never fails; the switch does. A mechanical float with no debris guard has the highest failure rate. Electronic switches without moving parts are the most reliable but draw a small standby current. External floats are the most debris-tolerant but require a separate install.
Two pumps for any boat over 18 feet
A primary pump near the bilge low point handles routine water. A secondary pump mounted 4 to 6 inches higher kicks in only when the primary fails or cannot keep up. Two separate fuses, two separate floats, two separate motors.
Tinned marine wire and heat-shrink crimps
Standard copper wire corrodes within a year in a salt environment. Tinned marine wire and heat-shrink crimp connectors at every junction are the right install. Pay the extra 20 dollars; redoing the install at year 3 costs much more.
For related boat gear, see our guide on marine VHF radio basics and the breakdown in boat anchor types by bottom. For details on how we evaluate marine equipment, see our methodology.
A bilge pump is cheap insurance against an expensive problem. The Rule 1100 GPH is the default for most boats, the Johnson Aqua Void is the right call for slip-stored boats, and the Attwood S500 covers small runabouts cleanly. Wire it correctly, fuse it, and add a high-water alarm for the days you are not on board.
Frequently asked questions
What GPH rating do I need?+
GPH ratings on the box are measured at zero head (no vertical lift), which is not how the pump operates in a boat. Real performance at 3.3 feet of head, which is typical bilge to gunwale lift, is 50 to 65 percent of the rated number. A 750 GPH pump pushes about 450 GPH in the boat. Size up by one bracket from what you calculate: a 16 foot runabout that needs 500 GPH should get a 1100 GPH unit so it actually delivers 600 to 700 at real head.
Automatic or manual switch?+
Both. Run an automatic float switch on the primary pump so it kicks on while you are away from the boat, and wire a manual override switch at the helm so you can run the pump even if the float fails or is debris-blocked. Some pumps have a built-in electronic switch that activates on water contact; these have no moving parts to fail but draw a small constant current. For long storage periods, a mechanical float draws zero standby current and is the safer pick.
Do I need two pumps?+
On any boat over 18 feet or any boat you leave at a slip overnight, yes. The primary pump handles routine rainwater and minor leaks. A secondary pump mounted higher in the bilge runs only when the primary fails or cannot keep up with a major intrusion. Two pumps with separate float switches and separate fuses provide actual redundancy. The cost of a 1100 GPH backup is far less than the cost of recovering a sunk boat.
Submersible or diaphragm?+
For most boats, submersible centrifugal pumps win on flow rate per dollar and simplicity. They sit in the bilge water and pump as long as water keeps coming. Diaphragm pumps are self-priming, can run dry without damage, and handle small debris, but they cost two to three times more and move less water. For dewatering a flooded bilge, submersible centrifugal is the right pick. For pumping the last half inch dry, diaphragm wins.
What size fuse and wire?+
Match the fuse to the pump's amp draw plus 25 percent. A 1100 GPH pump pulls around 4 amps, so use a 7.5 amp fuse. Wire size depends on run length: under 10 feet use 16 AWG, 10 to 20 feet use 14 AWG, over 20 feet use 12 AWG. Use tinned marine wire, not standard automotive, because copper corrosion in salt environments destroys connections within a year. Heat-shrink crimp connectors at every junction.
Priya Sharma
Priya Sharma writes for The Tested Hub.