Best Bilge Pumps: Ranked by Real Output at Head Height
Best bilge pumps ranked by delivered output at head height, not nameplate GPH — six picks from Rule, Attwood, Johnson Pump and SEAFLO, read from the manufacturers' own manuals.
9 min readGear & Accessories
The number on the box is the most misleading figure in boating. A bilge pump's headline GPH is its open flow — zero lift, no hose, usually at a charging system's 13.6 volts. Your boat gives it a resting battery, a metre of lift and feet of hose — output falls by a quarter to a half. The best bilge pumps are the ones whose makers publish what happens after that fall. This ranking reads the manuals instead of the boxes: six pumps, judged on delivered flow at head height.
How These Bilge Pumps Were Ranked
This is a research ranking, not a bench test: nobody's boat was flooded for it. Every figure comes from the manufacturers' own documents — Xylem's installation manuals for the Rule and Rule-Mate ranges, SPX FLOW's Johnson Pump catalogue, Attwood's Sahara selection guide and catalogue data, and SEAFLO's pump manual. Ranking ran first on published output at one and two metres of head, then on switch design, draw, outlet size and serviceability. Makers publishing no at-head figure were marked down for it. All six picks are ignition-protected, built to the ISO 8849 standard.
Why the Best Bilge Pumps Are Judged at Head Height
Head is the vertical distance from pump to outlet, and every foot costs flow: a submersible bilge pump is a centrifugal impeller trading flow for pressure along a curve: the higher it lifts, the less it moves. Voltage bends the curve too: 13.6-volt ratings describe a running engine's charging system; a boat at anchor lives nearer 12. Test rigs differ too — Johnson publishes one range under two standards that score it differently.
Xylem's standard-pump manual is the honest worked example: testing at nominal voltage — a battery at rest, not a charger — it turns the 2000's nameplate 2,000 GPH into 1,480 open, 1,220 at one metre and 1,035 at two:
| Pump | Nameplate GPH | Output at 1 m head | Output at 2 m head |
|---|---|---|---|
| Rule 2000 | 2,000 | 1,220 | 1,035 |
| Rule 3700 | 3,700 | 2,300 | 2,050 |
| Rule-Mate 1100 | 1,100 | 806 | 554 |
| Johnson Aqua Void 1100 | 1,100 | 870 | 585 |
| SEAFLO 1100 automatic | 1,100 | 865 | 433 |
| Attwood Sahara Mk2 S800 | 800 | Not published | Not published |
Two cells say "not published": Attwood publishes open flow only for the Mk2, though its previous Sahara had a full curve (970 GPH at 3.3 feet, 750 at 6.7). And under ISO 15083 these systems serve an intact boat's normal water — spray, rain, seepage, spillage — not damage control, as Xylem warns in both manuals.
Best Bilge Pumps for Most Boats: The Six Picks
Six pumps made the cut — the winners are the ones whose manuals survive the metre test. It also works in reverse when shopping the boats for sale.
Rule 2000 — the best bilge pump for most boats
The benchmark manual pump — pair it with a separate float switch. Xylem's manual tests it at nominal voltage and still records 1,480 GPH open, 1,220 at a metre and 1,035 at two — the shallowest fall of any 2,000-class pump here. It draws 8.4 amps at 12 volts (12 at 13.6) through a 1-1/8-inch hose. Owners like it: the switch can be replaced alone.
Rule-Mate 1100 — the best automatic bilge pump
Rule's automatic uses a solid-state level sensor: Xylem's Rule-Mate user guide has it on at about 2-1/2 inches of water and off at 5/8 inch. A backup program guards it: fooled into running dry for 15 seconds, the pump drops to a check cycle every two and a half minutes. Output: 806 GPH at one metre, 554 at two, on a 7.5-amp fuse, drawing 4.8 amps at 13.6 volts, with interchangeable 1-inch and 1-1/8-inch outlets.
Attwood Sahara Mk2 S800 — the best compact automatic
Attwood's Sahara Mk2 selection guide rates the S800 at 830 GPH open flow (816 with its check valve fitted). Its X-Air impeller vents trapped air that air-locks other pumps (Attwood: the only automatic with the feature), and its SteadySwitch resists motor-killing rapid cycling. External feet, a basket-mounted outlet and a one-handed cartridge release suit tight bilges. Outlets are 3/4-inch and 1-inch.
Johnson Aqua Void 1100 — the easiest bilge pump to service
Its motor cartridge twists out by hand, leaving the hoses attached, on a plug-in harness. SPX FLOW's Johnson Pump catalogue publishes the 1100 at 870 GPH at one metre and 585 at two under ABYC H-22 at 13.6 volts (780 and 560 under ISO at 12), with 14.4 feet of maximum head, drawing at most 5.3 amps on a 7.5-amp fuse through a 1-1/8-inch port.
SEAFLO 1100 — the best budget bilge pump
The value pick — and a surprise: SEAFLO publishes a full curve for its automatic 1100 in its pump manual, holding 865 GPH at one metre, level with the Johnson. The fall comes later — 433 GPH at two metres, the steepest fade here, with a four-metre maximum head: a pump for shallow bilges. Add a built-in switch, snap-off strainer base, 3.0-amp draw and 1-1/8-inch outlet.
Rule 3700 — the best heavy-duty bilge pump
For bigger boats and high-mounted backup duty: a 3,700 GPH nameplate that Xylem's nominal-voltage test turns into 3,000 open, 2,300 at one metre and 2,050 at two — more delivered water at two metres than most pumps claim at zero. It draws 15.5 amps at 12 volts (20 at 13.6), needs a 25-amp fuse and a 1-1/2-inch hose. Johnson's Heavy Duty 2200 is the alternative: 2,060 straight, 1,900 at three feet.
Automatic vs Manual: The Switch Is the System
A manual pump plus a float switch separates the two failure points: floats jam, pumps burn out — each replaceable without the other. An automatic pump integrates the sensing — no float to jam, but electronics to fool: a leaf or wet paper towel can trick the Rule-Mate's sensor, which is exactly why its backup program exists. Johnson's Ultima Switch is the third route — solid-state, no moving parts, sold separately, on at about 50mm of water, off at about 20mm. Whichever route, budget switch, panel and alarm as one system — see the boat accessories essentials list.
Check valves earn a warning. They stop discharge water running back, but they tax flow: Attwood's guide prices the Sahara Mk2 S1200's check valve at 354 GPH of open flow (1,214 falling to 860). Xylem allows one only against backflow cycling — never in a manifolded discharge.
Sizing and Installing the System
Size by delivered flow at your boat's lift, not by nameplate. Attwood's guide steps classes by boat length — 500 class to about 18 feet, 800 into the low twenties, 1200 to about 26 — but a tall lift can demote any of them a class. Then double it: an automatic primary low in the bilge, a larger pump higher up, on its own circuit, with a high-water alarm. Xylem's manual states the matching ABYC rule: enclosed-accommodation boats need an audible high-water alarm.
Two ownership notes. A pump is only as good as its bank: Xylem sets the Rule-Mate's minimum voltage at 10.5 volts, and a weak house battery derates every figure here — see the best marine batteries guide. Inspecting a used boat? Pump and switch go on the checklist next to the seacocks — our used boat inspection checklist shows where.
Installation protects output. Xylem's flow killers are all discharge-side: hose length, bends, rough interiors, and restrictions like check valves and undersized thru-hulls. Run smooth-bore hose short and straight, with two stainless clamps at the barb. The thru-hull belongs at least eight inches above the heeled waterline; lower, and the manual's answer is a seacock plus a vented loop — never a check valve against siphoning. Use the maker's fuse exactly (7.5 amps for both 1100-class picks); Xylem advises against circuit breakers here altogether. Keep wire runs honest — 16-gauge to about 15 feet, 14-gauge beyond, per the Rule-Mate guide, because voltage drop is output drop — and make every connection above the highest bilge water level, sealed.
Keeping It Alive: Five-Minute Checks
Bilge pumps die of neglect, not work: pull the pump, clear the strainer and impeller, inspect the check valve. Test the switch, not just the motor — press and hold the Rule-Mate's button for five seconds. Listen for short-cycling: backflow or a fouled sensor. At lay-up, remove the check valve and drain the line — the pump belongs on the boat maintenance checklist round, and on the drain-and-prove list in our guide to winterizing a boat.
Best Bilge Pump FAQs
What size bilge pump does my boat need?
Judge by delivered flow at your lift, not the maker's boat-length class alone: an 800-class pump holding 673 GPH at one metre (Rule-Mate 800) out-pumps a fading 1,100 nameplate. Whatever the size, fit two: an automatic primary low in the bilge, a larger backup higher up, separately circuited, with a high-water alarm.
Why does my 2,000 GPH pump move so much less water?
Because 2,000 GPH is open flow at zero lift. Add a metre of head and Xylem's manual puts the Rule 2000 at 1,220 GPH; test at a resting battery's voltage, not a charger's 13.6, and it starts from 1,480 GPH before any lift.
Is an automatic bilge pump better than a manual pump with a float switch?
For the primary pump, usually: integrated sensing removes the float that jams, sticks or sinks, and adds a backup program against fouled sensors. Manual-plus-float still wins on serviceability and price — either half is replaceable alone. Many installations run one of each.
Can a bilge pump save a boat that is holed?
No — the manuals say so. ISO 15083 bilge systems serve an intact boat's normal water: spray, rain, seepage, spillage. A breach below the waterline admits water faster than any pump here removes it; the pump buys time for crew, radio and repair — it does not win.
Conclusion
The honest spec was in the manual all along: a best bilge pumps shortlist is a list of curves, not nameplates. Match the pump to your boat's actual lift, wire it to the maker's numbers, give it a second pump and an alarm, and it will move the water the manual promised. Ready to put that system in a different hull? The listings at BoatDealsDirect.com are where the search starts.
