A boat bilge pump is easy to ignore when the bilge is dry. Then rain gets past a hatch, a hose starts to drip, or spray finds its way below. A good pump moves that water out before it becomes a bigger mess. Yet the number on the box is only a starting point. A bilge pump may reach its full rated flow through a short, flat hose in a lab, while your boat adds lift, bends, valves, and wire resistance. Each one can slow the flow. This guide covers built-in automatic pumps, a high-flow pump with a separate switch, a remote diaphragm pump, and a hand pump. None removes the risk from a major leak. The full system still needs sound wiring, safe plumbing, a clear inlet, and regular checks.
Quick picks
Bilge pump comparison
| Pump | Control and power | Published pump capacity |
|---|---|---|
| Rule-Mate RM1100B | Built-in sensor, 12V | 1,100 gallons per hour at 0 ft; 806 GPH at 3.3 ft |
| Attwood Sahara Mk2 S1200 | Built-in float, 12V | 1,214 GPH open at 13.6V; 709 at 3.3 ft with valve |
| Johnson Ultima Bilge 1250 | Built-in sensor, 12V | 1,250 GPH straight; 800 at 3 ft |
| Whale Supersub Smart 1100 | Built-in sensor, 12V | 1,050 GPH open; 750 at 3 ft |
| Johnson Heavy-Duty 2200 | Separate switch, 12V | 2,060 GPH straight; 1,900 at 3 ft |
| Whale Gulper 320 | Separate switch, 12V | Up to 300 GPH |
| Beckson Thirsty-Mate 124PF | Hand powered | Up to 10 GPM at the maker's stated stroke rate |
Pump capacity and installed flow: head height matters
Bilge pump flow is shown in gallons per hour, or GPH. The largest number is usually open flow, with little lift and hose load. A pump may also be tested at 13.6 volts, close to a charging system. A resting 12V battery can give less installed flow.
- Head height: Head is the vertical rise from the pump to the outlet. More head means less flow. Long or small hoses, tight bends, and check valves add resistance. The Attwood S1200 moved 1,214 GPH with no head or valve, but 709 GPH with its valve and 3.3 feet of head.
- Boat size: There is no sound one-line rule that matches pump capacity to boat length. Measure the rise and hose path, count the fittings, then read the pump curve at the right voltage.
- Water paths: Bilge water may collect in more than one place as a vessel heels. Some vessels need a small pump at each low point instead of one huge pump. Watch the water level under way and at rest.
- The source: Trace each leak before asking a pump to manage it. Fix leaking water lines, deck drains, stuffing boxes, and hull problems first. A pump removes excess water; it does not stop water from entering the vessel.
- The full system: Secure hoses and wires, keep each inlet clear, and check mounting holes for water paths. The pump curve, fuse, bilge pump switch, outlet, and regular maintenance all affect safety.
A pump is only one layer
A bilge pump clears normal water from rain, spray, and small leaks. This helps manage water that can affect a vessel's buoyancy and stability, but the pump does not stop a leak. It may not keep up with a split hose or hull damage. Stop the water at its source. Add a high-water alarm and a separate backup where the risk calls for them.
How to choose electric bilge pumps and controls
- Pump type: Most electric bilge pumps use a submersible pump with a centrifugal impeller. They move a lot of clean water for their size. A remote diaphragm pump gives easier service and better dirt handling, but its pump capacity is lower. Manual bilge pumps add a power-free layer.
- Water control: For automatic operation, a bilge pump may use a built-in float switch, a sealed water-level sensor, or an external float switch that acts as the bilge pump switch. Make sure gear and dirt cannot block the control.
- Power: For a 12V pump, check current draw, fuse size, and wire length. A weak battery or long wire can cut installed flow.
- Installed flow: Read flow at your real head height. Look for test voltage and note whether a valve was fitted. That tells you more about how fast the pump can clear excess water.
- Hose path: The discharge hose carries water overboard. Match its size to the pump outlet. Short, wide hoses with gentle bends waste less water flow.
- Care and cover: A strainer helps keep debris from clogging the inlet. Check how it comes off, whether the impeller is easy to reach, and when the warranty starts. A clogged filter, strainer, or inlet can cut flow.
- Materials: Read the maker's material notes and corrosion resistance claims. Saltwater can expose weak fasteners and poor wire joints.
- Different jobs: Baitwell pumps may look similar, but they serve a different water path. Do not use one as a bilge pump unless its maker documents that use.
1. Rule-Mate RM1100B
The Rule-Mate RM1100B combines a centrifugal pump and electronic water sensor. It handles automatic operation and accepts manual control from a panel switch.
Key facts
Specs and control: Draw is 3.8 amps at 12V and 4.8 amps at 13.6V; the fuse is 7.5 amps. The sensor starts near 2.5 inches of water and stops near 5/8 inch.
Flow and fit: The current manual lists 1,100 GPH at 0 feet, 806 at 3.3 feet, and 554 at 6.7 feet. The included check valve fits 1-inch and 1-1/8-inch outlets. Rule calls for flexible hose. Its current installation manual says the valve is not an anti-siphon device.
Warranty and limits: After more than 15 seconds with no useful load, the pump checks again every 2.5 minutes. This helps when air or debris fools the sensor. The three-year limited warranty starts on the manufacture date, so check older stock.
2. Attwood Sahara Mk2 S1200
The Sahara Mk2 S1200 pairs a built-in mechanical float with an X-Air impeller made to push out trapped air. Its body lifts from the base for strainer access.
Key facts
Specs and control: This 12V pump draws about 4.4 amps at open flow and uses a 6-amp fuse. It starts near 1-3/4 inches and stops near 3/4 inch.
Flow and fit: Part 5512-7 has 1-inch and 1-1/8-inch straight and 90-degree fittings plus a non-return valve. Flow is 1,214 GPH at 13.6V with no valve or head, 709 GPH with the valve and 3.3 feet of head, and 296 GPH at 12V with the valve and 2 meters of head.
Warranty and limits: The three-year consumer warranty covers qualifying pleasure-boat use, not racing or commercial work. Valve drag, head height, and voltage mean the “1200” label is not installed flow.
3. Johnson Ultima Bilge 1250
The Ultima Bilge 1250 uses a sealed field sensor instead of a moving float. A third wire adds manual control.
Key facts
Specs and control: This 12V pump draws 3 amps and uses a 5-amp fuse. The sensor starts near 2 inches and stops below 3/4 inch.
Flow and fit: At 13.6V, flow is 1,250 GPH straight and 800 GPH at 3 feet. Threaded ports are 1-1/8 inch and 1-1/4 inch; the basket, cover, and check valve come off. Mount it on a 3/4-inch block, clear both sensor faces, and keep wire joints above the bilge.
Warranty and limits: The maker lists three years, while one current store page says one year. Confirm the packed terms. Straight flow is not installed-hose flow.
4. Whale Supersub Smart 1100
The low Supersub Smart fits where a tall round pump may not. Its solid-state sensor has no float arm to jam.
Key facts
Specs and control: This 12V pump draws 4.3 amps. It waits about five seconds to start, then pumps about 25 seconds after the water falls.
Flow and fit: Flow is 1,050 GPH open and 750 GPH at 3 feet, despite the “1100” name. Its 9.5-by-2.1-by-3-inch body has a stepped 1-inch and 1-1/8-inch outlet and non-return valve.
Warranty and limits: A current catalog calls for a 5-amp fuse; an older sheet says 7 amps. Follow the packed label and guide. Whale now lists a three-year warranty, while older guides say one year.
5. Johnson Heavy-Duty 2200
This Johnson pump has no water sensor. Pair it with a rated float or electronic switch, or use a manual panel switch.
Key facts
Specs and control: This 12V pump draws 7.5 amps and uses a 12-amp fuse. Its separate control may be a float, electronic, or manual switch.
Flow and fit: At 13.6V, flow is 2,060 GPH straight and 1,900 GPH at 3 feet. Outlets are 1-1/8 inch and 1-1/4 inch. The pump is about 7 inches tall, 4.25 inches wide, and nearly 5 pounds. Mount it low, route the hose uphill without a sag, and do not run it dry.
Warranty and limits: The model says 2200, but the table says 2,060 GPH straight. The limited warranty is three years. Allow room for the switch, hose, and firm mounting block.
6. Whale Gulper 320 BP2052
The Gulper 320 is a remote diaphragm pump. It can sit dry while a pickup hose reaches a dirty or hard-to-reach bilge.
Key facts
Specs and control: This 12V pump draws 7.5 to 8 amps, uses a 10-amp fuse, and needs an external switch. It can run dry and pass small debris.
Flow and fit: Output is up to 19 liters per minute, or about 300 GPH. Ports are 3/4 inch or 1 inch, and the pump self-primes up to 3 meters.
Warranty and limits: The “320” is not a match for a 1,100 or 2,000 GPH centrifugal pump. The manual warns about fast water entry. Whale now says three years, while an older guide says one year; confirm US terms.
7. Beckson Thirsty-Mate 124PF
The manual Thirsty-Mate needs no wire, fuse, or battery. Its plastic body is nonconductive.
Key facts
Specs and control: The body is 24 inches long with a 1-3/4-inch cylinder. Its foot valve comes out for cleaning, and the working limit is 120°F. There are no electric controls.
Flow and fit: Beckson lists 10 GPM at six strokes per gallon. That equals 600 GPH only while a person keeps pace, so it is not a steady electric rating. The main chart says a 24-inch hose is included; the store page says no outlet hose. Check the box.
Warranty and limits: No published warranty was found. A person must work the pump, so keep it within reach rather than under gear.
Automatic bilge pump controls and float switches
An automatic bilge pump with a built-in float switch saves space and keeps the parts together. A mechanical float switch is easy to understand, but dirt or loose gear can block it. A sealed electronic sensor has no moving arm, though oil, soap, or a wet surface can still affect automatic operation. An external float switch takes more room and wire, but it lets you place the pump and switch where each works best. This can suit a large pump such as the Johnson Heavy-Duty 2200. Use a bilge pump switch that can carry the pump's start and steady load, add manual control, and test the real water level at which the system starts and stops.
Bilge pump installation: safe wiring and plumbing
Start with the maker's guide. Use its fuse size, not a guess. Put the fuse near the battery feed. Size marine wire for the full round-trip route and the pump load. A long, thin wire can cut voltage and flow. Use sealed marine joints above the highest water level you expect. Secure the wires so they cannot rub on a sharp edge.
- Electrical safety: The U.S. Coast Guard Boatbuilder's Handbook explains federal electrical rules, including ignition protection where fuel vapor may collect. The ABYC standards list points to H-22 for bilge pumps and E-11 for boat electrical systems. Standards words on a package do not prove that the full setup is safe; the wire, fuse, switch, hose, and outlet all count.
- Mounting and hoses: Mount a submersible bilge pump on a firm block at the lowest point, not straight through a thin hull skin. Keep the filter or strainer clear. Use the full outlet size and route hoses in a smooth rise with few bends. Smaller hoses can cut flow.
- Siphon control: Keep the discharge above the heeled waterline or use the safe layout named in the guide, which may include a vented loop and seacock. A check valve can slow backflow, but it is not an anti-siphon loop. It adds drag and can clog.
- Keep water out: Rainwater is easier to stop than to pump. A sound boat cover can keep much of it out while the boat sits, but it does not replace a working pump.
Bilge pump maintenance and pre-departure checks
Test the bilge pump before each trip and after work on the boat. Lift the float or wet the sensor as its guide directs. Then add clean water and watch the full cycle. A motor sound alone does not prove that excess water reaches the outlet.
- Clean the inlet and strainer. Remove hair, leaves, cable ties, and grit.
- Inspect the discharge hose for kinks, soft spots, cracks, and loose clamps.
- Look for hot, green, loose, or wet wire joints.
- Inspect the through-hull and outlet for nests, mud, ice, and other blocks.
- Find the cause if the pump starts often. Do not treat the pump as the cure.
Bilge pump FAQ
- Should an automatic bilge pump stay powered when I leave?
- That depends on the boat, battery plan, dock rules, and maker's wiring plan. Many boaters keep the automatic feed live through a fused circuit. Do not leave a weak battery, dirty sensor, or known leak for the pump to manage. Ask a qualified marine electrician if the circuit is not clear.
- Is a larger bilge pump GPH number always better?
- No. A large pump may need more wire, a larger fuse, wider hoses, and more space. Its real water flow may fall hard in a poor hose path. Choose from the installed-flow curve and make sure every part of the path can pass that water.
- Do I need a check valve?
- Not always. It can stop water in the hose from falling back into the bilge, which may cut short cycling. Yet it also adds drag and another place for dirt to lodge. It never takes the place of safe anti-siphon plumbing. Follow the pump and boat maker's layout.
- Can one pump serve as both the main pump and backup?
- One pump is still one point of failure. A second pump on its own fuse and switch adds another layer. A hand pump also works when power is gone, but only while someone can use it. The right mix depends on where the boat travels and how fast help can arrive.
- Why does my bilge pump work but move little water?
- Look for low voltage, a blocked strainer, an air pocket, a pinched hose, too much lift, a clogged valve, or a worn impeller. Shut off power before reaching into the bilge. If fuel or strong fumes are present, leave the space and deal with that hazard first.